RIO GRANDE NAWQA

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Rio Grande Valley Study--Unit Description

Although the Rio Grande NAWQA study unit is delineated primarily along the surface-water drainage of the Rio Grande and its tributaries (fig. 1 and pl. 2), the boundaries of the ground-water flow systems in the study unit do not conform to the drainage boundaries. The boundaries of the ground-water systems are controlled by geology and location of recharge to and discharge from the ground-water systems. Many different ground-water flow systems at many different scales are found in the study unit. A large number of these ground-water flow systems are connected and ultimately discharge into the Rio Grande, which is the master stream in much of the study unit. For uniformity throughout this section, the term "basin" is used in the context of a structural basin, rather than in the context of a topographic basin or a valley.

Structural Geologic Setting

Two main structural settings can be identified in the Rio Grande NAWQA study unit: alluvial basins and bedrock basins. The alluvial-basins setting is typified by a basin partially or entirely surrounded by highlands composed of rocks older than mid-Tertiary. Erosion of the highlands adjacent to these basins has resulted in the deposition of relatively large thicknesses of mid-Tertiary or younger basin-fill deposits in these basins. Many of the alluvial basins in the study unit are in a tectonically active area referred to as the Rio Grande rift (pl. 2). The Rio Grande rift is an area delineated on the basis of high heat flow, late Quaternary faults, late Pliocene and younger volcanoes, and deep basins (Seager and Morgan, 1979, p. 88). The basins in the Rio Grande rift contain a larger thickness of basin-fill deposits than the basins outside of the rift; however, the basins outside of the rift are hydrologically similar to the basins in the rift.

The bedrock basins in the study unit--San Juan and Chama Basins--differ from the alluvial basins because they contain many layers of sedimentary rocks ranging in age from Mississippian to Quaternary. The hydrology of these basins is much different than that of the alluvial basins because the many layers of sedimentary rocks are distinct, separate aquifers. These distinctions are significant to the understanding of the complexity of the ground-water flow systems in the study unit.

Alluvial Basins

A large number of the alluvial basins in the study unit are in the Rio Grande rift, a dominate structural feature in the study unit. A discussion of the geologic history of the Rio Grande rift has been included because the rift affected the configuration of the bounding uplands, which affect precipitation, ground-water recharge, source material of the basin-fill deposits, aquifer characteristics, and ground-water quality. The alluvial basins in the Rio Grande rift are the San Luis, Espanola, Santo Domingo, Albuquerque-Belen, La Jencia, Socorro, San Marcial, Engle, Palomas, Mesilla, San Agustin, and Jornada del Muerto Basins. This discussion of the Rio Grande rift is primarily from Chapin (1979).

Rifting began between 32 and 27 million years (m.y.) ago when regional extension reactivated uplift of the southern Rocky Mountains, a major north-trending zone of crustal weakness. By 26 m.y. ago, the crust along the developing rift had sagged sufficiently to form broad, shallow basins in which mafic volcanic flows and volcanic ash beds were intercalated with sedimentary basin-fill deposits. As the rift continued to sag, highland areas adjacent to the rift eroded and the rift continued to fill with sediments. Volcanism along the Rio Grande rift increased slowly from 20 to 13 m.y. ago with most of the activity near Socorro, New Mexico, and the Jemez Mountains. With the exception of the Taos area, early-rift magmatism was sparse and late-rift volcanism was voluminous. Prior to about 4 m.y. ago many of the basins in the Rio Grande rift were closed with respect to surface-water drainage. Lakes or playas were near the center or topographic low point in these basins. About 4 m.y. ago, increased runoff from newly uplifted regions resulted in an integrated drainage to form the ancestral Rio Grande.

The ancestral Rio Grande is thought to have followed the same general course as the present Rio Grande although the deposits of the ancestral Rio Grande covered a wider area than the present deposits of the Rio Grande. The ancestral Rio Grande is thought to have drained into a large lake in the northern end of the Mesilla Basin and the southern end of the Jornada del Muerto Basin. Widespread geomorphic surfaces, graded to the ancestral Rio Grande and its tributaries, became mantled with basalt flows. Headward erosion by a river, which drained into the Gulf of Mexico, resulted in the capture of the ancestral Rio Grande near El Paso. Following this capture, the lowering of base levels, and subsequent increase in stream gradient, resulted in dissection of basins along the Rio Grande drainage and isolation of basalt-capped mesas. In the Albuquerque area, the Rio Grande has down cut more than 400 feet through the basin-fill deposits. Epeirogenic uplift has continued since 4 m.y. ago, at a reduced rate, and rifting continues today.

The western part of the Mimbres, Playas, and Hachita Basins are located west of the Rio Grande rift. These basins are similar to the basins in the Rio Grande rift; however, the deposits filling these basins are generally older and, in some cases, the basin-fill deposits are not as thick as the deposits in the Rio Grande rift. These basins have been filling with basin-fill deposits from early- to mid-Tertiary and thus the mountains surrounding these basins have eroded more and have less topographic relief than some of the mountains surrounding the basins in the rift.

Bedrock Basins

The San Juan and Chama Basins are the bedrock basins in the study unit. These basins contain sedimentary rocks of Mississippian to Quaternary age. The total thickness of rocks can be large in these basins. The rocks consist of material deposited in many different environments, resulting in many different rock types in these basins. This layering of rock types results in many different distinct aquifers that are separated by confining beds.

Hydrologic Setting

A ground-water flow system can be defined as "a region within saturated earth materials where there is a dynamic movement of ground water from a source to a sink" (Mifflin, 1968, p. 3). At the source area, water moves from the atmosphere through the unsaturated zone to the zone of saturation. This is generally referred to as recharge, and the area this occurs in is referred to as the recharge area. The saturated earth materials are generally referred to as the aquifer. The permeability, or ability to transmit water, of an aquifer can vary considerably. In some areas rocks with small permeability are considered aquifers because water can be obtained from wells completed in these rocks; however, in many other areas rocks with small permeability would not be considered aquifers because of the low water yield. Thus the importance of an aquifer is not based solely on permeability values but must be viewed in the context of aquifers available in an area. At the sink area of a flow system, water moves out of the saturated materials to the atmosphere, a surface-water drainage system, lakes, plants, or the unsaturated zone. This area is generally referred to as the discharge area. To define an aquifer system one needs to identify the source or recharge area and the sink or discharge area and be able to demonstrate that water flows through the aquifer between these two points. This seems relatively simple, but it is difficult to precisely define a ground-water flow system in most areas because of insufficient hydrologic information. Flow systems can be defined in a general sense by using known geologic and hydrologic information. Knowledge of the geology and structural controls in an area may be used to identify recharge or discharge areas to help define general boundaries of the flow systems. Knowledge of the hydrologic properties of the rock types in an area is useful in determining if rocks have sufficient permeability to transmit significant quantities of water. In this report a flow system is considered to be movement in a specific aquifer from a recharge area to a discharge area. The recharge and discharge areas include movement to or from the zone of saturation and also include subsurface transfer of ground water between different aquifers.

Although many scales of flow systems are in the study unit, the larger and most important flow systems can be grouped into two major types: alluvial basin flow systems and bedrock basin flow systems. The principal aquifers in the alluvial basin flow systems are basin-fill deposits. The aquifers in the bedrock basin flow systems are the permeable sedimentary rocks. Most major population centers and major water-use areas in the study unit are located on and withdraw ground water from the basin-fill deposits. Presently (1992), ground-water withdrawal is minimal from the bedrock aquifers. From 1955 to 1991, large amounts of ground water were withdrawn from bedrock aquifers in the Grants area to dewater uranium mines. With the closure of the last operating mine in 1991, the mine dewatering in this area has been suspended.

Concepts Applicable to Alluvial-basin Flow Systems

The alluvial basins in the study unit include the San Luis Basin, Espanola Basin, Santo Domingo Basin, Albuquerque-Belen Basin, Socorro Basin, La Jencia Basin, San Agustin Basin, San Marcial Basin, Engle Basin, Palomas Basin, Jornada del Muerto Basin, Mesilla Basin, Mimbres Basin, Hachita Basin, and Playas Basin (pl. 2). There are two general types of basins: those through which surface streams flow and those with a closed surface-water drainage system. Most of the basins are drained by a through-flowing stream; however, the northern part of the San Luis Basin and San Agustin, Jornada del Muerto, Mimbres, Hachita, and Playas Basins are closed with respect to surface-water drainage. In a strict sense these basins include only the area underlain by basin-fill deposits; however, the mountainous areas adjacent to the basin-fill deposits have been included in the discussion of the ground-water flow systems in these basins.

In the mountainous areas that bound the basin-fill deposits are localized ground-water flow systems in the alluvial channels and relatively impermeable rocks forming the mountains. These flow systems generally are of limited areal extent in comparison to the flow systems in the alluvial basins and are not discussed in detail in this report. However, their importance is as a source of recharge to the basin-fill deposits.

The basic principles of recharge, discharge, and movement of ground water in all alluvial basins in the study unit are similar. Discussion of these basic principles will be in reference to a "generic" alluvial basin. This generic alluvial basin is bounded by mountains on the east side and a bedrock basin on the west side (fig. 12). On the north and south, the basin is bounded by adjacent alluvial basins. A perennial stream drains the basin and the adjacent alluvial basins. In the following discussion, examples from specific alluvial basins will be used to illustrate these basic principles.

Throughout the report the term "basin-fill deposits" is used in reference to the principal aquifer in alluvial basin flow systems. It is recognized that this term includes many different deposits representing different depositional environments and ages. Many of these deposits have been mapped on a local or regional basis and are formally recognized in the literature. However, the subsurface correlation throughout the study unit is complex, difficult, and in some areas questionable, and in almost all cases is not crucial to understanding the shallow ground-water flow systems. In forthcoming publications, the ground-water resources of selected areas may necessitate a detailed discussion of the geology that will require identification of basin-fill deposits by their formal geologic nomenclature; for this report, however, that detail is not warranted.

Rio Grande Valley NAWQA study unit

The basin-fill deposits include sedimentary and volcanic deposits and are Tertiary or younger in age. The thickness of these deposits ranges from a feather edge at the basin margins to about 19,000 feet in the San Luis Basin (Leonard and Watts, 1989). Throughout the Rio Grande rift the thickness is generally several thousand feet; however, hydrologic data are available for only the upper several hundred feet. Coalescent-fan, alluvial-fan, and piedmont deposits are found along the margins of the alluvial basins that are bounded by mountains. These deposits grade into or intertongue with fine-grained sediments. In many of the basins, ancient playa deposits of fine-grained material can be found interbedded in the basin fill. Axial river deposits consisting primarily of clay, silt, sand, and gravel are found along the present channel of the Rio Grande as well as along the ancestral course. Throughout much of the study unit, and particularly along the western side of the San Luis Basin, and in the Jemez Mountains, extensive deposits of volcanic flows, volcaniclastic rocks, and tuffaceous material are found in great thicknesses at the surface or interbedded in the basin-fill deposits. In the deeper parts of the rift, the basin-fill deposits consist of semiconsolidated deposits of clay, silt, sand, and gravel.

As would be expected from diverse depositional environments, these deposits vary significantly with respect to their permeability. The fine-grained playa deposits and clay layers have relatively small permeability and thus may not transmit much water or may form confining layers. The alluvial-fan deposits are generally very poorly sorted; thus the permeability of these deposits is significantly less than that of the well-sorted axial river deposits. The volcanic-related deposits vary from dense basalts having minor secondary permeability to very permeable tuffaceous material. In many areas the volcanic rocks are more permeable than the basin-fill deposits.

Recharge to the alluvial basin ground-water flow systems can occur by several different processes: (1) mountain-front recharge, (2) infiltration of water from the Rio Grande and major tributaries, (3) infiltration of applied irrigation water, (4) arroyo channel recharge, (5) direct recharge, and (6) subsurface recharge. Along the mountainous areas adjacent to the basin-fill deposits, mountain-front recharge is important. In many of the alluvial basins mountain-front recharge contributes in the majority of recharge to the basin-fill aquifer. Mountain-front recharge can be divided into two main types: subsurface inflow of ground water from the mountains and mountain-stream channel recharge. Subsurface inflow from the mountains is ground-water flow from the aquifers in the mountains into the basin-fill aquifer. In the mountains thin alluvium along the stream channels and the relatively impermeable bedrock are aquifers. Although the bedrock is relatively impermeable, the potentiometric gradients in the bedrock aquifers can be large, which can result in the movement of large amounts of ground water into the basin-fill aquifer (fig. 13). Mountain-stream channel recharge is infiltration through the stream channel bed into the basin-fill deposits. In the mountainous areas there are many perennial streams because the rocks in the mountains are relatively impermeable and mountainous areas generally receive more precipitation than the alluvial basins because they are higher in altitude. Water in these perennial streams infiltrates rapidly into the basin-fill deposits after discharging from the mountains. In many areas, the channels of the streams in the alluvial basins lose their identity because the flow infiltrates and no overland flow occurs.

Infiltration of water from the Rio Grande and its major tributaries is also a major source of recharge to the basin-fill deposits. In many areas of the study unit, the bottom of the river channel is higher than the water table in the surrounding aquifer, and surface water recharges the ground-water flow system.

Infiltration of applied irrigation water also results in recharge to the basin-fill deposits. This is generally restricted to a relatively small area along the Rio Grande which coincides with the floodplain in most of the study unit, however, this process of recharge has historically been very important in parts of the San Luis Basin.

Arroyo-channel recharge is the infiltration of water through the beds of arroyo channels in response to runoff from areas adjacent to the arroyos during intense storm events. Arroyo-channel recharge is localized (along arroyos) and occurs infrequently.

Direct infiltration of precipitation is another source of recharge to the basin-fill deposits. The amount of direct recharge varies considerably in the study unit due to variations in the amount and rate of precipitation and evapotranspiration. In most of the study unit direct recharge is not an important source of recharge because of the small amount of precipitation, short duration of precipitation, large evapotranspiration rates, and the relatively impermeable material at the surface. A study to determine the amount of direct recharge occurring near Santa Fe indicated that no direct recharge occurs over large areas. Direct recharge could be important in areas of sand dunes or where permeable volcanic rocks occur at the land surface.

Subsurface recharge to the basin-fill deposits occurs in areas where ground water discharges from bedrock basin flow systems into the basin-fill deposits. Several of the alluvial basins are partially bounded by bedrock basins, and ground water is known to move from the bedrock basin flow systems into the basin-fill deposits. Subsurface recharge also occurs between alluvial basins along the Rio Grande. In general, this flow is from north to south from basin to basin.

The discussion of recharge to the basin-fill deposits emphasizes the fact that land use is not the controlling factor in determining the quality of ground water throughout most of the study unit. Because of little or no direct recharge and large depths to water in many areas, little or no flux of water moving downward through the unsaturated zone reaches the water table to affect ground-water quality. An understanding of this concept is critical when evaluating the effects of land use on the quality of ground water. Only in those areas where direct recharge or infiltration of irrigation water occurs can land use be related to the quality of ground water.

Discharge from alluvial basin flow systems occurs by discharge to surface-water systems, evapotranspiration, subsurface ground-water flow to adjacent alluvial basins, and pumpage of ground water (fig. 13). The Rio Grande and several other rivers are known to gain flow in certain reaches as the result of ground-water discharge. In many of the alluvial basins along the Rio Grande, ground water discharges to the Rio Grande in the southern part of each basin. This occurs because in many of the basins a reduction in the cross-sectional area of the basin-fill deposits causes the gradient of the potentiometric surface to increase and upward movement of ground-water flow (convergence), resulting in discharge to the Rio Grande.

Discharge from the ground-water system as the result of evapotranspiration is important in areas where ground water is near the land surface. The rate of evapotranspiration increases as the depth to ground water decreases. The largest rate of evapotranspiration occurs where the ground water is at land surface. In irrigated areas having a very shallow water table, generally along the Rio Grande, a large volume of ground water will be discharged as the result of evapotranspiration. In most areas in the alluvial basins the depth to ground water is greater than 50 feet; therefore, no discharge from the ground-water system will occur as the result of evapotranspiration in these areas.

Discharge of subsurface ground-water flow to adjacent alluvial basins through the basin-fill deposits occurs between some basins. It is particularly important within the basins along the Rio Grande.

Pumpage of ground water in the study unit is for agricultural, municipal, domestic, stock, and industrial use with the majority of pumpage for agricultural and municipal use. Part of the water pumped for irrigation evapotranspires and part of the water infiltrates recharging the ground-water system. The pumpage for municipal use occurs in localized areas coincident with the population centers scattered throughout the study area. Part of the water pumped for municipal use is discharged to the Rio Grande or its tributaries as treated sewage.

Ground water in the alluvial basins generally flows from the basin margins toward the center of the basins. In many of the basins ground water also moves southward through the basins. The majority of recharge occurs along the basin margins and the majority of discharge occurs near the center of the basin or in the subsurface to an adjacent alluvial basin. In the alluvial basins drained by the Rio Grande, the Rio Grande and the irrigated area along the Rio Grande are important discharge areas for the ground-water system. Movement of ground water from the recharge areas to the discharge areas can take thousands of years because of the distance traveled and the aquifer characteristics.

In irrigated areas along the Rio Grande in the alluvial basins south of the San Luis Basin, small-scale-flow systems are superimposed on the large-scale flow systems because of the recharge and discharge that are due to human-related activities. The number and extent of these localized flow systems are a function of the geometry of the sources of recharge and areas of discharge resulting from the irrigation network (fig. 14). The main sources of recharge in these areas are the fields where irrigation water is applied and the canals and laterals that transport the water to the fields. The main sources of discharge are the drains that have been constructed to intercept the ground-water system to maintain water levels below land surface, evapotranspiration from the ground-water system, and wells that are used to supply irrigation water. The Rio Grande may be a source of recharge or an area of discharge depending on the river stage and the altitude of the water table. The interaction of all of these sources of recharge and areas of discharge is in a constant state of flux during the year, especially during the irrigation season when significant amounts of water recharge the ground-water system resulting in rising water levels in the irrigated areas. The rise in water levels increases gradients near the drains, thus increasing ground-water discharge to the drains. During the nonirrigation season, the trend is that the localized ground-water system is drained and water levels are lowered. As a result, water-quality changes in the localized ground-water system are extremely time dependent, which is an important factor when developing a ground-water sampling and monitoring program.

Characteristics of Alluvial Basins

Recharge

The recharge processes for alluvial basins, discussed in detail in a previous section of this report, are listed again for review. They are: (1) mountain-front recharge, (2) infiltration of water from the Rio Grande and major tributaries, (3) infiltration of applied irrigation water, (4) arroyo channel recharge, (5) direct recharge, and (6) subsurface recharge. The following discussion will present available recharge data and interpretations from previous investigations for the alluvial basins in the study area.

In the San Luis Basin several investigations have been conducted that provide estimates or discuss recharge to the ground-water system for various parts of the basin (Powell, 1958; Emery and others, 1971; Crouch, 1985; Hearne and Dewey, 1988; and Leonard and Watts, 1989. The recharge values are derived in a number of different ways and represent estimates for different time frames since ground water was initially used for irrigation around 1890.

Recharge in Sunshine Valley is primarily from losses in perennial streams and related irrigation ditches and from precipitation. At least 20,000 acre-ft per year recharges the valley: in excess of 10,000 acre-ft from surface water and at least 10,000 additional acre-ft from precipitation (Wilkins, 1986, p. 21). Most of the recharge is to the alluvial sediments.

Bjorklund and Maxwell (1961, p. 52) estimated 30,000 acre-ft of Rio Grande underflow moves into the Albuquerque-Belen Basin.

Discharge

In the Sunshine Valley water in the lava discharges to the Rio Grande. Total discharge to the Rio Grande and Red River between the Lobatos gage on the Rio Grande in Colorado and the Confluence of the Red River is about 80,000 acre-ft per year (Wilkins, 1986, p. 22).

Ground-water flow systems

On a regional scale, ground-water flow in the alluvial basins drained by the Rio Grande (San Luis, Espanola, Santo Domingo, Albuquerque-Belen, Socorro, San Marcial, Engle, Palomas, and Mesilla Basins) is from basin margins toward the Rio Grande and southward from one basin to the next (pl. 2). There is a bedrock high covered by a thin veneer of basin-fill deposits at the southern end of the Mesilla Basin and little ground water flows out of the Mesilla Basin through the basin-fill deposits (Slichter, 1905). Most ground water discharges at the southern end of the Mesilla Basin to drains, evaporation, and transpiration (Wilson and others, 1981). Therefore, ground-water flow out of the study unit in the basin-fill deposits along the Rio Grande is minimal.

Ground-water flow in the alluvial basins not along the Rio Grande is into basins along the Rio Grande, into alluvial basins west of the Continental Divide, or out of the study unit into Mexico (pl. 2). Ground water flows from the Magdalena Mountains east and northward into the Socorro Basin. Ground-water flow in the San Agustin Basin is from the northern basin margin southward and eastward into the southern end of the Engle Basin along Alamosa Creek. Ground water flows from the east, west, and north margins of the northern Jornada del Muerto Basin southward and westward. Some ground water flows from the Jornada del Muerto into the northern end of the San Marcial Basin. Ground water also flows southward out of the southern Jornada del Muerto Basin into the northern end of the Mesilla Basin. Ground water in the Mimbres Basin flows southeastward from the Cobre and Mimbres Mountains into Mexico. Ground water in the Hachita Basin flows from the east and west margins toward the axis of the basin and southeastward into Mexico. Ground water in the Playas Basin flows from basin margins toward the basin center and northward out of the study unit.

Ground-water flow system in the San Luis Basin

Ground-water development in the San Luis Basin began in 1890 with the installation of hundreds of flowing wells in the confined system that were used to irrigate crops and pasture land. This confined system is one of three flow systems in the basin-fill deposits, and combined with the other two represents a significant percentage of the ground-water use in the study unit. Because of the complex nature of these flow systems and large amount of ground-water use, the flow systems in this basin will be discussed in some detail. A ground-water divide separates the northern part of the basin, a closed ground-water basin, from the southern part of the basin where flow is to the Rio Grande (fig. 15). In part of the basin a confining layer of clay separates two permeable sections of basin-fill deposits, creating a confined flow system that does not coincide with the closed ground-water basin but extends to the San Luis Hills.

The northern part of the basin is called the San Luis Closed Basin because of a low topographic divide that prevents surface-water drainage to the Rio Grande. This topographic divide also coincides very closely with the southern edge of the ground-water closed basin where a ground-water divide has been created as the result of recharge to the unconfined flow system by leakage from irrigation canals (Powell, 1958, p. 61). The location of the ground-water divide moves north and south in response to changes in the amount and location of recharge in this area (Crouch, 1985). Flow in the ground-water closed basin is from the basin margins, where large amounts of recharge occur, toward the center of the basin where discharge occurs as the result of evapotranspiration and pumpage from irrigation wells. Recharge also occurs throughout the ground-water closed basin from infiltration of excess applied irrigation water. The depth to water in the ground-water closed basin has varied several feet since irrigation began as a result of changes in irrigation practices. The primary changes have been: (1) withdrawal of ground water from the confined aquifer to supplement surface-water irrigation, (2) withdrawal of ground water from the unconfined aquifer by large capacity wells to supplement surface-water irrigation, and (3) change from gravity or flood irrigation to the use of sprinkler irrigation methods (Hearne and Dewey, 1988, p. 11).

South of the closed basin, the southern boundary of the unconfined flow system of the basin is the San Luis Hills. Flow in the unconfined system in this area is from the basin margins toward the Rio Grande. Recharge occurs along the basin margins as mountain-front recharge and in irrigated areas as infiltration of excess applied irrigation water. Discharge is due to evapotranspiration and discharge of ground water to the Rio Grande. Hearne and Dewey (1988, p. 49) estimated that discharge from both the open and closed unconfined system from evapotranspiration is 3,900 ft3/s . Hearne and Dewey (1988) indicated that little or no ground water flows (discharges) southward from the San Luis Basin into the Taos Plateau area.

The confined aquifer is present below the uppermost blue clay or uppermost layer of volcanic rock below land surface. The blue clay layer is from 50 to 130 feet below land surface and varies from 10 to 80 feet thick. Ground-water flow in this aquifer is from the recharge areas around the edge of the basin toward the center with a southerly component along the trend of the rift. Along the north side of the San Luis Hills, faulting has provided an avenue for discharge from the confined aquifer, creating numerous springs at the surface. McIntire Spring discharges at the base of the San Luis Hills and is one of the larger springs in the area. Powell (1958, p. 37) measured an average discharge of 18.7 ft3/s from McIntire Spring. The confined aquifer has been supplying water to wells since the early 1890's when hundreds of flowing artesian wells were drilled to supply irrigation water. As a result, significant amounts of ground water are suspected to leak from this aquifer into the overlying unconfined aquifer because of faulty casing. There is also natural discharge of water from this flow system upward through the confining layer into the unconfined flow system (Hearne and Dewey, 1988).

• Ground water--Surface water interactions
• Depth to water
• Aquifer characteristics
• Water-level fluctuations
• Water use

Irrigation is the primary use of water in the San Luis Valley. In 1990, an estimated 1,580 million gallons per day (Mgal/d) was used for irrigation. This included 100% of the surface-water withdrawals (898 Mgal/d) and 98.4% of the ground-water withdrawals (682 Mgal/d).

Irrigation practices and factors that affect those practices greatly influence water use in the San Luis Valley. In parts of the valley conjunctive use, or the practice of using both surface and ground water, and the availability of surface water, influence the amount of ground water withdrawn in any given year. The recent trend to convert to center-pivot irrigation systems has increased the efficiency and less water is actually needed to irrigate the same number of acres. There are 1,926 pivots in the valley and about 90% of these systems are currently using ground water. In some areas of the valley, surface water is not used directly to irrigate, but is used to recharge the aquifer. This practice, along with seepage from the canals and irrigated land, serves to raise the water table and provide water for the crops through subirrigation. The location of canals in the valley and their condition (lined vs. unlined or recently dredged) affects the amount of water that is lost in conveyance. For the entire valley, about 36% of the diverted surface water (322 Mgal/d) was lost in conveyance. This includes the amount of water that was intentionally recharged to the aquifer as well as the water that seeped from the numerous canals in the valley.

Approximately 644,750 acres were irrigated in 1990. The irrigated crop land averages about 400,000 acres and has remained steady over the past decade. The amount of irrigated pasture, however, is dependent upon availability of surface water in a given year.

Concepts Applicable to Bedrock-basin Flow Systems

The two main bedrock basin flow systems in the study unit are in the San Juan Basin and the Chama Basin (pl. 2). There are other bedrock flow systems in the study unit but they are very localized in areal extent and minimal data is available to define the flow systems. Consequently they will not be discussed in theis report. The San Juan Basin and Chama Basin are similar with respect to stratigraphy and structural geology. The basins are structural basins that contain sedimentary rocks ranging in age from Mississippian to Quaternary. The rocks generally dip toward the center of the basins from the margins and the rocks are younger toward the center of the basins. In some areas along the margins of the basins the dips are greater than 45 degrees. The bedrock in these basins was deposited in a wide range of depositional environments ranging from deep water marine to arid continental and thus there is a large range in the permeability of the rocks in these basins. Many water-yielding units or aquifers are in these basins and generally a separate flow system in each aquifer. Localized ground-water flow systems in the Quaternary alluvium have been deposited along many of the streams and valleys eroded in the bedrock.

Characteristics of Bedrock Basins

Recharge

Recharge results from the same general processes as discussed in the section on alluvial basin flow systems. Mountain-front recharge and infiltration of water from the major streams in the basins are the most important sources of recharge to these basins. Direct recharge could also be important in these basins in areas that receive more than 12 inches per year of precipitation. The main types of discharge from the different bedrock units are discharge to surface-water systems, leakage through confining beds to adjacent aquifers, and flow from the bedrock aquifers into the basin-fill deposits. The flow is generally from the recharge or highland areas along the basin margins toward the center of the basins. In the San Juan Basin ground-water movement in rocks Jurassic age and younger is generally out of the study unit to the north. In the southern San Juan Basin, ground-water movement in the rocks older than Jurassic age is from the Zuni Mountains eastward toward the Albuquerque-Belen Basin. The direction of ground-water flow in the rocks in the Chama Basin is not known because of the lack of data in the area.

• Discharge
• Ground-water flow systems
• Depth to water
• Aquifer characteristics
• Water-level fluctuations
• Water use

Ground-water Quality in Alluvial Basins

Ground-water quality in basins having a through-flowing river generally is acceptable for human consumption, although there are local exceptions, especially in the southermost basins. Saline water tends to be flushed out of the shallow ground-water system by ground-water discharge to drains, canal leakage, and exchange of water with the through-flowing river. However, there commonly is a deep ground-water-flow system that causes an upwelling of mineralized water at the lower end of the basins. The volume of this upwelling may be small, however, depending on basin dimensions and hydraulic properties of the basin-fill deposits.

In basins with closed surface drainage and little, if any, ground-water outflow, dissolved minerals are generally concentrated in ground water near the center of the basin. Shallow ground-water levels in parts of these basins have caused and, in some basins, still cause large losses to evapotranspiration. Evapotranspiration, without a mechanism to flush the remaining salts, results in a body of brackish or saline ground water near the topographically low areas of the basin. Fresh groundwater may only occur near the margin of the basin where mountain front recharge occurs. However, if there is ground-water outflow from the basin, some of the dissolved salts may be flushed from the basin.

Ground-water quality in all the basins may also be influenced by geothermal activity or by ground-water inflow from adjacent areas, regardless of the surface-water/ground-water relation. These two sources of quality problems are almost always associated with the deeper ground-water flow systems.

The areal variation of ground-water quality in the basin-fill deposits throughout the individual basins in the study unit is complex but the processes that affect the areal variation of water quality are well understood. These processes can be divided into two categories: natural processes and human-induced processes.

Natural processes

The natural processes that affect the distribution of ground-water quality are differences in the quality of recharge water, chemical reactions that occur as ground water moves along a flow path, and evapotranspiration from naturally occurring vegetation where the ground water is close to land surface. The majority of recharge to the basin-fill deposits occurs along the margins of the basin-fill deposits and is the result of ground-water inflow from the mountains and infiltration of streamflow derived from the mountainous areas adjacent to the basin-fill deposits. In a large part of the study unit, the amount of ground-water recharge due to direct infiltration is insignificant. The quality of water in the basin-fill deposits in the areas of the study unit where basin-fill deposits are adjacent to mountainous areas is a function of the quality of the recharge water. The quality of this recharge water is generally very good (dissolved-solids concentrations less than 200 mg/L) because this water is derived from the mountainous areas which receive relatively large amounts of precipitation, the rock in these mountainous areas is relatively unreactive, and the contact time of the water with the rocks in the mountainous areas is short. Subsurface inflow of ground water to the basin-fill deposits from adjacent bedrock units is another source of recharge that has an effect on the ground-water quality. In several areas this inflow water is saline and the mixing of this water with other recharge water and water in the basin-fill deposits results in poor water quality having specific-conductance values as high as 45,000 uS/cm (Anderholm, 1988).

Infiltration of surface water from the Rio Grande and its tributaries mixes with shallow ground water in the basin-fill deposits and can improve or degrade the quality of this water. In the northern part of the study unit, the quality of the infiltrating water is similar to the quality of water in the shallow basin-fill deposits; thus, there is little effect on the quality of water. In the Albuquerque area and southward, the quality of the infiltrating water is not as good as the quality of water in the basin-fill deposits and the mixing of these waters results in the degradation of the quality of water in the basin-fill deposits. The amount of degradation of the quality of water in the basin-fill deposits is a function of the volume of infiltrating water.

Ground-water movement in the basin-fill deposits is away from the recharge areas or margins of the basin-fill deposits so chemical reactions of the recharge water with the basin-fill deposits will result in changes in the water quality as the water moves along a flow path. The most common chemical reactions that have been observed are dissolution of gypsum and calcium/sodium ion exchange (calcium in water exchanged for sodium on clays). Weathering of the sediments to clay minerals is a reaction that could be important in the evolution of water chemistry along a flow path.

Evapotranspiration by native or natural vegetation occurs in the San Luis Basin and along the Rio Grande and its tributaries where the water levels are close to land surface. Evapotranspiration results in the concentration of solutes in the ground water, causing a degradation in the quality of water. The magnitude of the degradation is a function of the amount of evapotranspiration and thus the amount of concentration of solutes. In some areas in the San Luis Basin, evapotranspiration has resulted in significant degradation of water quality (dissolved-solid concentrations greater than 31,000 mg/L) (Emery and others, 1972).

Human-induced processes

The human-induced processes that affect the distribution of water quality can be grouped into agricultural, pumpage, urbanization and mining. As ground water and surface water are used for irrigation, solutes tend to concentrate in the irrigation water by evapotranspiration. Recharge of excess applied irrigation water can result in the degradation of the quality of water in the upper parts of the aquifer. The application of agricultural chemicals to irrigated areas has also adversely affected the quality of water in the basin-fill deposits. Dissolution of these compounds in applied irrigation water can result in the transport of these compounds downward to the ground water.

Pumpage affects ground-water quality by changing ground-water gradients, which results in the mixing of waters of different quality. Pumping from deeper in the aquifer can result in the downward movement of the shallow, higher salinity water to deeper parts of the aquifer. The effects of recharge of excess applied irrigation water that has a larger salinity resulting from evapotranspiration, and movement of this water as the result of pumping have been documented in the Las Cruces area. The results of this work will be presented to demonstrate the complex nature of the areal and temporal variations in water quality that result from irrigation. It is important to stress that irrigation affects the shallow ground-water quality throughout the study unit, but the magnitude and extent of the effects have not been documented in most of the study unit.

In the Las Cruces area, sections of wells have been installed to monitor water quality and water-level changes resulting from irrigation practices. Water in four clusters of wells along a section show a large variation in specific conductance horizontally and vertically (fig. 16). Along this section, the specific conductance of water in shallow wells tends to increase with distance from the Rio Grande and the specific conductance generally decreases with depth. There are also temporal variations in specific conductance at a specific depth in the aquifer as shown in figure 17. This cluster of wells is also located in the Las Cruces area approximately 5 miles northwest of the above-mentioned section of wells. The specific conductance ranged from about 500 to 1,200 uS/cm in ground water from the perforated interval 31 to 35 feet. There are similar differences in the trends in specific conductance of water in other parts of the aquifer during this same period of investigation. Water levels, especially from the deeper parts of the aquifer, vary during the year (fig. 18). Decreases in water levels of as much as 13 feet in the deeper parts of the aquifer probably are in response to ground-water withdrawals (pumping). The water levels are relatively constant from November until mid- to late March during the nonirrigation season. The water levels also indicate downward gradients; thus the shallow ground water moves downward in the aquifer. This results in the degradation of the quality of water in the deeper parts of the aquifer.

Urbanization in the study unit has also had an effect on the quality of water. Large areas of range and agricultural lands have been converted to residential use. Infiltration of septic tank effluent has resulted in degradation of the quality of water in these areas. Increases in nitrate and dissolved trace-element concentrations have been documented in the Albuquerque area; however, the problem is probably much more widespread. In conjunction with the conversion of range and agricultural lands to residential use, various industries and service businesses have located in these areas. Contamination of ground water by industrial solvents, trace elements, and gasoline have been documented in several localized areas within the study unit (New Mexico Water Quality Control Commission, 1990).

Characterization of quality of water

The quality of ground water in the San Luis Valley has been studied from the early 1900's to the present. Many of the previous investigations have focused on describing the quality of water in both the unconfined and confined aquifers using inorganic constituents with an emphasis on major ions, dissolved solids, and nitrogen. Results from the previous studies indicate the water in the unconfined aquifer around the perimeter of the valley is primarily a calcium bicarbonate type, typical for waters in recharge areas. As the water in the aquifer moves toward the center of the valley, an area known as the sump, the water changes to a sodium bicarbonate type. Approximately 1,045 square miles (mi ) of the unconfined aquifer have been classified predominately as a calcium bicarbonate water type and 556 mi have been classified as a sodium bicarbonate water type (Williams and Hammond, 1989). Water types in the confined aquifer were also classified with about 723 mi classified as a calcium bicarbonate water type and 876 mi classified as a sodium bicarbonate water type.

Following is a summary of the background water-quality described by Powell (1958) and of the some of the studies that have been done since the early 1970's for which data are available in the U.S. Geological Survey WATSTORE data base (Emery and others, 1972, 1973; Edelmann and Buckles, 1984; Williams and Hammond, 1989). Data that are available from other sources but have not yet been analyzed are also presented.

Powell (1958) described the water in the unconfined aquifer using chemical analyses from 271 wells located primarily in the closed basin but included wells located south of the Rio Grande River in Alamosa and northern Conejos Counties. The dissolved-solids concentrations in the unconfined aquifer were smallest on the west side of the valley where there is a considerable amount of recharge, and largest, in the sump area. Dissolved solids ranged from 52 to 13,800 ppm and percent sodium ranged from 1 to 100. Powell concluded that the large concentration of dissolved solids and large percent sodium that occurs in the sump area was due to dissolution of minerals in transit and concentration due to evaporation. The quality of the water in the confined aquifer was described using analyses from 41 wells. Dissolved-solids concentrations ranged from 70 to 437 ppm and percent sodium ranged from 2 to 35. The percentage of sodium increased as water moves fromt he west side of the valley towards the east and reached a maximum in the sump area of the closed basin.

More recently, Emery and others (1972, 1973) concluded that the chemical quality of water in the unconfined aquifer was suitable for irrigation in the western part of the valley but high salinity and alkali hazards existed in the central valley. They based their conclusions on analyses from 206 wells in the unconfined aquifer and 188 wells in the confined aquifer that were distributed throughout the entire valley. Their samples were collected in 1968-69 and analyzed for major ions, nitrate, specific conductance, pH, and temperature. In the unconfined aquifer, specific conductance, an indicator of dissolved solids, was generally less than 250 micromhos per centimeter (umhos/cm) around the rim of the valley and increased up to 32,000 umhos/cm in the sump area. Dissolved nitrate as nitrate concentrations as high as 65 milligrams per liter (mg/L) were documented near Center, north and east of Monte Vista in Rio Grande and Alamosa Counties and in southern Saguache County. Nitrate concentrations were generally low for the rest of the San Luis Valley. Water in the confined aquifer near the edges of the valley had specific conductance generally less than 200 umhos/cm but increased to greater than 2250 umhos/cm in the central valley. Dissolved nitrate as nitrate concentrations were very low, median value was 0.5 mg/L.

Edelmann and Buckles (1984) described the quality of ground water in the agricultureal area of the San Luis Valley which includes parts of Saguache, Rio Grande, Alamosa, and Conejos Counties using chemical analyses of water from 57 wells completed in the unconfined aquifer and 25 wells completed in the confined aquifer. The samples were collected in March and July of 1981 and analyzed for major ions, nitrogen species, dissolved solids, specific conductance, pH, and temperature. Edelmann and Buckles (1984) reported nitrite plus nitrate as nitrogen concentrations in the unconfined aquifer as high as 33 mg/L in the Center area and generally were less than 3.5 mg/L for the rest of the study area. Nitrite plus nitrate, as nitrogen concentrations were lower in the deeper part of the unconfined aquifer just above the confining layer and may be due to upward leakage of water from the confined aquifer or adsorption, or a combination of both. Specific conductance (representing dissolved solids) in the unconfined aquifer ranged from 144 to 6,000 umhos/cm and the median value was 382 umhos/cm. High dissolved solids and sodium hazards generally occurred in the central valley. Water from wells completed in the confined aquifer contained concentrations of less than 1 mg/L of nitrite plus nitrate, as nitrogen and the area of high specific conductance (750-2250 umhos/cm) was limited to an area between Hooper and Alamosa.

Although the areas of study varied slightly, similar patterns for nitrate concentrations and salinity hazard (specific conductance) were observed by Emery and others (1973) and Edelmann and Buckles (1984). Median values for nitrate, as nitrogen by land-survey range were highest in R 8 E., an area that includes Monte Vista and Center. The median specific conductance increased from west to east and was highest in R 10 E. (1,007 umhos/cm) for analyses from the Edelmann and Buckles study. The highest median value for the Emery and others study was in R 11 E. (2310 umhos/cm). Both of these land-survey ranges include portions of the sump area of the valley.

As part of the Southwest Regional Aquifer System Analysis, Williams and Hammond (1989) summarized selected water-quality characteristics of ground water in the San Luis Basin (including northern New Mexico and the Conejos River subbasin). They evaluated chemical analyses of water from 99 wells and 19 springs, using data available from the U.S. Geological Survey WATSTORE data base. The samples were collected between 1940 and 1985 and included analyses for pH, specific conductance, hardness, major ions, SAR, silica, nitrogen species, phosphorus species, trace elements, and dissolved organic carbon. Samples collected from 2 springs in the Conejos River subbasin also were analyzed for tritium and indicate the springs represent water from the confined aquifer. Well depth and an ionic balance within 10 percent were used as criteria for selecting analyses for evaluation. Wells less than 100 feet deep were assumed to be completed in the unconfined aquifer and those greater than 100 feet deep were assumed to completed in the confined aquifer. This may not be an accurate assumption for parts of the San Luis Basin (see Emery and others, 1973, plate 2). In general, Williams and Hammond's (1989) conclusions were similar to those by Emery and others (1973) and Edlemann and Buckles (1984).

The U.S. Geological Survey WATSTORE data base contains no data more recent than 1985. Many of the wells included in the data base have no information on well depth or screened interval -- data crucial in determining which aquifer was sampled. There are 81 wells in the data base that contain total well depth. These include 32 in Alamosa County, 13 in Conejos County, 19 in Rio Grande County, and 17 in Saguache County. No well depth data is available for Costilla County. Well depth and screened interval information may be available through other sources but it is not easily obtained or matched to wells that are in the current data base.

Several other sources of ground-water quality data are available for the San Luis Valley. These include the Bureau of Reclamation Closed Basin Project, a cooperative project between the Soil Conservation Service and Bureau of Reclamation, and Dr. Deanna Durnford of Colorado State University in conjunction with the Colorado Department of Health.

The Bureau of Reclamation Closed Basin Project has water quality information for approximately 170 salvage wells completed in the unconfined aquifer and an additional 70 observation wells within or adjacent to the boundaries of the project (see attached figure). Analyses of water from these wells began in 1981 and will continue on a regular basis. Detailed well-completion data are available for each well. Information includes analyses of dissolved solids, specific conductance, major ions, selected pesticides, nutrients and trace elements. To date, these data have not been evaluated by either the Bureau of Reclamation or the U.S. Geological Survey. Because many of the wells are in close proximity to each other, an initial assessment of the data is planned to evaluate areal variability. Following this evaluation, additional information on selected wells will be requested from the Bureau of Reclamation.

In July, 1987, the Bureau of Reclamation, working with the Soil Conservation Service, collected samples from 18 wells in Alamosa, Rio Grande, and Saguache Counties. Well depth information was not known for 6 of the wells but the remaining wells were 100 feet deep or less and the pump intakes were generally 60 to 90 feet deep. The analyses include specific conductance, dissolved solids, pH, major ions, nitrates, and some trace elements. Nitrate as nitrogen ranged from 0.7 to 13.6 mg/L with the highest concentrations near Center, agreeing with previous investigations by Emery and others (1972, 1973) and Edelmann and Buckles (1984). Dissolved solids and percent sodium increased toward the central part of the valley.

Water-quality data for 34 irrigation wells sampled in the summer of 1990 in the most intensely irrigated region of the San Luis Valley are available through Dr. Deanna Durnford and the Colorado Department of Health. Thirty of these wells are located in the Center area, encompassing approximately 225 mi . Two wells were sampled near Blanca, and two near Antonito. These data include pesticides, major ions, and nitrates. Durnford and others (1990) concluded that the ground water has nitrate concentrations above drinking water standards in some areas and may contain low concentrations of pesticides. They also state that the results from the sampling program are inconclusive due to possible sample contamination, well bore problems, samples taken at high velocities causing volatilization of the pesticides, or the length of the screened interval masking potential aquifer contamination. Their study included an in-depth pesticide index listing nomenclature, use, pesticide usage practices in the San Luis Valley, chemical properties, physico-chemical properties, toxicological properties, and soil transport properties for 25 pesticides used in the San Luis Valley. These data have been requested from the Colorado Department of Health but to date, exact locations of the wells sampled in this study are not known.

In summary, there is a reasonable amount of major ion and nutrient data with good spatial coverage in the closed basin portion of the San Luis Valley. The data are more limited in the southern part of the basin. Pesticide data are limited and available from sources other than the U.S. Geological Survey. This data has not yet been evaluated.

At this stage of the project, sufficient information is available to provide a detailed description of ground-water quality in only three basins--the San Luis, Albuquerque-Belen, and Mesilla. The following discussion of water quality is based on the most recent data published for these basins.

The quality of ground water in the San Luis Valley has been studied from the early 1900's to the present. Many of the previous investigations have focused on describing the quality of water in both the unconfined and confined aquifers using inorganic constituents with an emphasis on major ions, dissolved solids, and nitrogen. In the San Luis Basin, the perimeter areas of the unconfined aquifer contain water that is primarily a calcium bicarbonate type, having a specific conductance generally less than 250 uS/cm. The central part of the unconfined aquifer contains water that is primarily a sodium bicarbonate type, having a specific conductance of more than 250 to as much as 30,000 uS/cm in shallow wells in the sump area northeast of Alamosa in the closed basin.

The median dissolved-solids concentration in the unconfined aquifer of the San Luis Basin is about 315 mg/L. The confined aquifer has the same general configuration of water types as the unconfined, a calcium bicarbonate type near the perimeter and sodium bicarbonate type near the center. The dissolved-solids concentrations in the confined aquifer are generally less than 500 mg/L, with a median of about 185 mg/L. Water-quality samples from the unconfined aquifer contain concentrations of sulfate greater than 476 mg/L in 10 percent of the samples and contain concentrations of dissolved iron greater than 300 ug/L in 25 percent of the samples (U.S. Geological Survey, 1988). Concentrations of nitrite plus nitrate as nitrogen were greater than 10 mg/L, which is the primary (maximum enforced contaminant level) drinking-water standard (U.S. Environmental Protection Agency, 1986), and were detected in several samples of water from the unconfined aquifer in an area east of Del Norte and northeast of Monte Vista.

Ground water in the upper reach of the Conejos River is predominantly a calcium carbonate type water, and in the lower reach is a sodium bicarbonate type water. There is minimal data on dissolved-solids concentration of water from the unconfined aquifer; water from two wells had concentrations of dissolved solids less than 500 mg/L. Water from wells completed in the confined aquifer had concentrations of dissolved solids less than 300 mg/L.

Ground water in the Albuquerque-Belen Basin is of several chemical types and has a wide range of specific conductance. The basin has been divided into seven zones on the basis of water type (fig. 19). In zone 1, the water is a calcium bicarbonate type and the specific conductance is generally less than 500 uS/cm. In zone 2, the water is a magnesium calcium sodium sulfate type and the specific conductance generally is greater than 1,000 uS/cm. In zone 3, the water is of two types, sodium sulfate and calcium sodium sulfate chloride, and specific conductance generally is greater than 2,500 uS/cm. Water from springs near the basin boundary in zone 3 is a sodium chloride type, and has specific conductance of about 35,000 uS/cm. The water types in the other zones of the basin are sodium bicarbonate sulfate, sodium bicarbonate, sodium sulfate, calcium sodium bicarbonate, and calcium sodium sulfate chloride types. The specific conductance ranges from less than 500 to more than 2,500 uS/cm.

Albuquerque-Belen Basin

Anderholm, in Frenzel and Kaehler (1990), divided the Mesilla Basin into three areas for the discussion of water quality: (1) west of the Mesilla Valley, (2) Mesilla Valley, and (3) east of the Mesilla Valley. The Mesilla Valley was defined as the inner valley of the Rio Grande. The division was based on differences in ground-water flow systems, water quality, and chemical processes in the three divisions. The density of water-quality data in the basin is sparse and is not evenly distributed. Therefore, the description of water quality for some areas is based on chemical analysis of water from a single well.

The water quality of the ground water in the area west of the Mesilla Valley is controlled by subsurface inflow water from the Mimbres Basin. In the northwestern part of the area, the water entering the basin has a specific conductance between 1,400 and 2,310 uS/cm. Sulfate is the dominant anion and sodium is the dominant cation. Subsurface inflow along the southwestern margin of the basin can be divided into two types, one type having a specific conductance less than 2,000 uS/cm (sodium and bicarbonate are dominant ions) and the second type having a specific conductance of about 7,000 uS/cm (sodium and chloride are dominant ions). The ground water in the area west of the Mesilla Valley generally has a specific conductance less than 900 uS/cm, with sulfate being the dominant anion. Water samples from a well sampled at several depths indicate that the specific conductance decreases and the percentage of bicarbonate increases with depth.

The quality of ground water in the Mesilla Valley varies areally and vertically, due to the effects of excess applied irrigation water mixing with the water in the aquifer. Shallow water generally has larger dissolved-solids concentrations than deeper water in the aquifer. The location of the transition zone between the shallow more saline water and the deeper water probably is transient and moves in response to ground-water withdrawals and recharge.

In the area east of the Mesilla Valley are many types of ground water. In the northeastern part of the area, the percentage of sulfate plus chloride and the specific conductance are greater in recharge water from the San Andres Mountains than in recharge water from the Organ Mountains. Subsurface inflows of geothermal water have large concentrations of chloride, silica, and potassium. In the southeastern part of the area, the zone of water that has dissolved solids greater than 3,000 mg/L is shallow and thin.

Sources of water-quality problems

Water-quality problems in the study area result from both natural and human-induced processes. The natural processes generally result in a degradation of the quality of water in one area by indlow of poorer quality water. This may be poorer quality water that is high in only one or a few constituents or it may bne high in several constiturents. Degradation of ground water from humam-induced processes can be categorized into two types--non-point source or point source.

Non-point source contamination is caused by diffuse sources such as large numbers of small septic tanks spread throughout an area, residual minerals from evapotranspiration, urban runoff, widespread application of agricultural chemicals. Point source contamination is caused by oil and gas production practices; from petroleum products such as leaking underground storage tanks; solvents; metals and/or minerals caused by mining and milling; nitrate from dairies, sewage treatment plants, explosivies manufacturing or handling facilities, other industrial facilities, and septic tanks; and landfills.

Non-point sources

The first water-quality issue is the effects of agricultural chemicals on shallow ground water in the San Luis Closed Basin. The San Luis Closed Basin is a highly intensive agricultural area that depends on irrigation for crop growth. The water table in the unconfined aquifer is relatively shallow, usually less than 15 feet (Crouch, 1985; and Edelmann and Buckles, 1984). Edelmann and Buckles (1984) reported concentrations of nitrite plus nitrate as nitrogen as high as 33 mg/L in shallow ground water from wells near Center, Colorado. They identified a large area of nitrite plus nitrate as nitrogen contamination (greater than 3.5 mg/L). Other agricultural chemicals were not included in the study by Edelmann and Buckles. Pesticides and nutrients are known to be applied to the crops, either directly or by application in irrigation water.

The third water-quality issue is nitrate and organic contamination and (or) low-oxygen concentration in the shallow ground water in the Rio Grande flood plain near Albuquerque. The New Mexico Water Quality Control Commission has determined that there is a large area of nitrate contamination along the Rio Grande flood plain near Albuquerque. In one area of the flood plain the nitrate levels have doubled and tripled due to septic tanks during the period 1977-90. Several areas of organic contamination were also present in the Rio Grande flood plain in Albuquerque, generally on the east side of the Rio Grande. In 1980 two Albuquerque supply wells were shut down due to detection of several chlorinated solvents in the water. One adjacent industrial area has been selected as a Superfund site. A large area of low- oxygen concentration in ground water was found in the same area, which also is attributed to septic tanks (New Mexico Water Quality Control Commission, 1990).

Historically, ground-water contamination has been limited to depths of 100 feet or less below land surface. Withdrawals from deep wells may have drawn the contamination to greater depths. At one location, hazardous substances were found at a depth of 220 feet below land surface. This vertical migration of contaminants may present a long-term threat to all deep wells located in the Rio Grande flood plain, including City of Albuquerque municipal wells. In addition, pumpage of deep wells east of the Rio Grande flood plain may have resulted in a reversal of the natural ground-water flow direction. Traditionally, ground water flowed from the basin boundaries to the Rio Grande. At present (1992), there is concern that the ground water in the Rio Grande flood plain may be moving east toward the Albuquerque municipal well fields.

Point sources

Several areas of organic contamination are also present in the Rio Grande flood plain in the Albuquerque South Valley, generally on the east side of the Rio Grande. In 1980 two Albuquerque supply wells were shut down due to detection of several chlorinated solvents in the water. One adjacent industrial area has been selected as a Superfund site (New Mexico Water Quality Control Commission, 1990). Contamination of gasoline from leaking underground storage tanks (LUSTs) occurs along virtually all of the major streets in the Albuquerque South Valley (Gallaher, 1988). Many instances of LUSTs have been documented in the shallow aquifer along the Rio Grande Valley.

Another major concern is the potential for nitrate contamination from unlined or manure-lined holding ponds used for the disposal of dairy wastes. The total nitrogen concentration of dairy effluents can range from 100 to 600 mg/L, thus seepage from these holding ponds present a major threat to shallow underlying ground water (New Mexico Water Quality Control Commission, 1990). A study conducted in 1984 by the Environment Department at seven dairies in the Mesilla and Rincon Valleys documented that state ground-water standards for nitrate was exceeded at three of the seven sites (Dye and others, 1984).

Another source of contamination is leachate from landfills overlying shallow ground water. Constituents known to occur in the leachate are chloride, nitrogen species, solvents, and a large number of other organic contaminants. Large quantities of septage (solids and liquids pumped from septic tanks) have been discharged to unlined pits at several landfills.

Mining operations in the study unit has affected the quality of ground water in localized areas. The most notable is in the Grants Mineral Belt where uranium mining and milling operations have resulted in several areas of contamination from seepage of inactive mill tailings ponds, unregulated discharges, and abandoned spoil piles.

A total of five sites in the study unit are on the Superfund National Priorities List and one site is proposed. Contamination of ground water at these sites results from industrial sources, uranium mills, PCBs, lead from old batteries, and a mill site heavily contaminated with lead, silver, zinc, copper, and arsenic.

Selected References

Anderholm, S.K., 1987, Reconnaissance of hydrology, land use, ground-water chemistry, and effects of land use on ground-water chemistry in the Albuquerque-Belen Basin, New Mexico: U.S. Geological Survey Water- Resources Investigations Report 86-4174, 37 p.

____ 1988, Ground-water geochemistry of the Albuquerque-Belen Basin, central New Mexico: U.S. Geological Survey Water-Resources Investigations Report 86-4094, 110 p., 2 pls.

Bjorklund, L.J., and Maxwell, B.W., 1961, Availablility of ground water in the Albuquerque area, Bernalillo and sandoval Counties, New Mexico: New Mexico State Engineer Technical Report 21, 117 p.

Borton, R.L., 1974, General geology and groundwater conditions in the Truchas-Espanola-Velarde area of Rio Arriba County, New Mexico, in Guidebook of Ghost Ranch: New Mexico Geological Society, 25th Field Conference, p. 351-354.

Bryan, Kirk, 1938, Geology and ground-water conditions of the Rio Grande depression in Colorado and New Mexico, in Regional Planning, Part 6, Upper Rio Grande: U.S. National Resources Commission, p. 197-225.

Bushman, F.X., 1963, Ground water in the Socorro Valley, in Guidebook to Socorro Region: New Mexico Geological Society, 14th Field Conference, p. 155-159.

Chapin, C.E., 1979, Evolution of the Rio Grande rift - A summary, in Rieker, R.E., ed., Rio Grande Rift--Tectonics and Magmatism: American Geophysical Union, Washington, D.C., 438 p.

Crouch, T.M., 1985, Potentiometric surface, 1980, and water-level changes, 1969-1980, in the unconfined valley-fill aquifers of the San Luis Basin, Colorado and New Mexico: U.S. Geological Survey Hydrologic Investigations Atlas HA-683, scale 1:250,000, 2 sheets.

Edelmann, P.F., and Buckles., D.R., 1984, Quality of ground water in agricultural areas of the San Luis Valley, south-central Colorado: U.S. Geological Survey Water-Resources Investigations Report 83-4281, 24 p.

Emery, P.A., Boettcher, A.J., Snipes, R.J., and McIntyre, H.J., Jr., 1971, Hydrology of the San Luis Valley, South-Central Colorado: U.S. Geological Survey Hydrologic Investigations Atlas 381, scale 1:250,000, 2 sheets.

Emery, P.A., Snipes, R.J., and Dumeyer, J.M., 1972, Hydrologic data for the San Luis Valley, Colorado: Colorado Water Conservation Board Basic-Data Release No. 22, 146 p.

Emery, P.A., Snipes, R.J., Dumeyer, J.M., and Klein, J.M., 1973, Water in the San Luis Valley, south-central Colorado: Denver, Colorado Water Conservation Board Water-Resources Circular 18, 26 p.

Emery, P.A., Patten, E.P., Jr., and Moore, J.E., 1975, Analog model study of the hydrology of the San Luis Valley, south-central Colorado: Colorado Water Resources Circular 29, 21 p.

Frenzel, P.F., and Kaehler, C.A., 1990, Geohydrology and simulation of ground-water flow in the Mesilla Basin, Dona Ana County, New Mexico, and El Paso County, Texas, with a section on Water quality and geochemistry, by S.K. Anderholm: U.S. Geological Survey Open-File Report 88-305, 179 p.

Gries, R.R., 1989, San Juan sag--Oil and gas exploration in a newly discovered basin beneath the San Juan volcanic field, in Lorenz, J.C., and Lucas, S.G., eds., Energy Frontiers in the Rockies, AAPG-SEPM-EMD Rocky Mountain Section Meeting, October 1-4, 1989, Albuquerque, N. Mex., p. 69-78.

Hearne, G.A., 1981, Mathematical model of the Tesuque aquifer system underlying Pojoaque River basin and vicinity, New Mexico: U.S. Geological Survey Open-File Report 80-1023, 181 p.

Hearne, G.A., and Dewey, J.D., 1988, Hydrologic analysis of the Rio Grande Basin north of Embudo, New Mexico, Colorado and New Mexico: U.S. Geological Survey Water-Resources Investigations Report 86-4113, 244 p.

Kernodle, J.M., 1992, Summary of U.S. Geological Survey ground-water-flow models of basin-fill aquifers in the Southwestern Alluvial Basins Region, Colorado, New Mexico, and Texas: U.S. Geological Survey Open-File Report 90-361, 80 p.

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