RIO GRANDE AQUIFER SYSTEM

Gayla R. Corley Spring, 2006
GO 571 Geohydrology

Dr. Marcia K. Schulmeister


Table of Contents


Introduction

The Rio Grande Aquifer supplies water to several municipal areas of Texas and New Mexico and water for irrigation. This principal aquifer is located in the Rocky Mountains and Basin and Range Physiographic Provinces in southern Colorado, central New Mexico, and western Texas providing water for a 70,000 square mile area (USGS, WWW), Fig. 1.

Fig. 1 Map of Rio Grande aquifer.
Image taken from: United States Geological Survey, capp.water.usgs.gov/gwa/ch_c/C-text4.html,
map modified from capp.water.usgs.gov/gwa/ch_c/jpeg/C052.jpeg.

The climate of the region is arid evaporation rates are high in the San Luis Valley in Colorado, pan evaporation rates per month may be 10 times greater than the amount of precipitation received in a one month period (USGS, WWW), Fig. 2. The San Luis Valley is considered to be high mountain desert. These evaporation rates are high because of low humidity, wind, and high temperatures in the summer.

Fig. 2 Graph illustrating the monthly pan evaporation is greater than the precipitation in the San Luis Valley.
From: United States Geological Survey, capp.water.usgs.gov/gwa/ch_c/C-text4.html,
graph taken from capp.water.usgs.gov/gwa/ch_c/jpeg/C053.jpeg.

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Geology

The main geologic feature of the region is the Rio Grande Rift. The Rio Grande, a perennial river, flowing through the San Luis basin and into a steep canyon for about 100 miles (USGS, WWW). When it leaves the canyon it moves through a series of small basins and south of Santa Fe, NM, flows through wide basins and narrow valleys into Texas. This rift is a graben, Fig. 3, with the sides of the rift uplifted and the middle blocks dropped and rotated.

Fig. 3 Diagram of the basin fill and bedrock of the Rio Grande Rift,
from: United States Geological Survey, capp.water.usgs.gov/gwa/ch_c/C-text4.html,image taken fromcapp.water.usgs.gov/gwa/ch_c/jpeg/C056.jpeg

Sediments in the rift valley are deep and in the San Luis Valley it is estimated the fill is more than 30,000 feet and at Albuquerque, NM, 20,000 feet (USGS, WWW). The fill thins to the south and at El Paso, TX it is approximately 2000 feet thick. The fill is Oligocene-Holocene age (Aber, WWW).The bedrock is Precambrian gneiss, schist, quartzites and granite and the valley fill is unconsolidated clays, gravels, sands and rhyolitic and andesitic lavas, breccias and tuffs (USGS, WWW). The fill in the lower valley is Quaternary gravels, clays, and sands.

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Aquifer Recharge

Recharge of the aquifer occurs from runoff of precipitation and melting snow from the surrounding mountains. The runoff spreads across permeable alluvial fans and percolates into the aquifer or is evaporated. Some of the precipitation in the mountains flows into the bedrock aquifer through fractures and permeable layers in the bedrock. Water from the bedrock aquifers is discharged into the basin fill and known as \ldblquote mountain front recharge\rdblquote occurring where the mountains front the Rio Grande valley (USGS, WWW). Approximately half of the 2,000,000 acre-feet per year of runoff from the San Juan Mountains enters the San Luis Valley through the bedrock aquifers. The Sangre de Cristo drainage basin contributes about 250,000 acre feet of water to the aquifer and is basically all stream flow (USGS, WWW). Precipitation falling in the valleys does not contribute to recharge of the aquifers as most of it is lost to evaporation and transpiration. Fig. 4 illustrates recharge and discharge by underflow.

Fig. 4 Diagram illustrating the recharge and discharge of the Rio Grande aquifer system,
from: United States Geological Survey, capp.water.usgs.gov/gwa/ch_c/C-text4.html,
image taken fromcapp.water.usgs.gov/gwa/ch_c/jpeg/C058.jpeg

The water table may be 100 feet or more below the surface in the upper reaches of the valley. Near the Rio Grande and in the center of most closed basins the water table level is near ground surface. Recharging the aquifer from the stream bed is limited by this high water level where water is close to the land surface (USGS, WWW). When the water level declines near the streams from groundwater removal recharge of the aquifer may take place from the stream. Recharging also occurs with irrigation return and from water seeping into the soils from the irrigation canals.

Discharge of water from the aquifer occurs from wells, discharge from stream flow, underflow, and evapotranspiration. Evapotranspiration annually removes approximately 2,800,000 acre feet of water from the San Luis Valley\rquote s 1,700,000 acre area. This amounts to about 20 inches of water per year from an area receiving 10 inches of precipitation per year (USGS, WWW). The southern valley receives less precipitation, approximately 8 inches per year, and loses more than 20 inches per year to evapotranspiration.

In 1985 the major withdrawal of water from the Rio Grande aquifer system was from wells. The majority of the water from this withdrawal, 90 percent, was used for irrigation. Most municipalities receive their water from this aquifer and this is a small portion of the total withdrawal. Fig. 5 identifies the use of water from the Rio Grande aquifer in 1985.

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Hydrologic Properties

Ground water flows to the Rio Grande from open basins where the water levels are highest around the margins of the basins. This water discharges into the river, pumping wells, drains, or as evapotranspiration. Closed basins also recharge the groundwater and moves by underflow into areas where pumping occurs or as evapotranspiration in nearby shallow playas (USGS, WWW).

Confining beds of impermeable clay, silt and unfractured volcanic rock occur extensively and retard vertical movement of water. Where wells are drilled through these confining layers water may rise above the land surface and flow as artesian wells. One example of such a confining unit occurs in the San Luis Valley where the layer of silt, sand, clay and unfractured volcanic rock exceeds the thickness of 1200 feet near the center of the valley (USGS, WWW). Wells drilled in the confined water layer have water levels higher than wells in the unconfined water levels lying from zero to 100 feet below the surface. Fig. 6 illustrates confined and unconfined aquifers with an artesian well.

Confining beds of impermeable clay, silt and unfractured volcanic rock occur extensively and retard vertical movement of water. Where wells are drilled through these confining layers water may rise above the land surface and flow as artesian wells. One example of such a confining unit occurs in the San Luis Valley where the layer of silt, sand, clay and unfractured volcanic rock exceeds the thickness of 1200 feet near the center of the valley (USGS, WWW). Wells drilled in the confined water layer have water levels higher than wells in the unconfined water levels lying from zero to 100 feet below the surface. Fig. 5 illustrates confined and unconfined aquifers with an artesian well.

Fig. 5 Illustration of the unconfined and confined aquifers and an flowing or artesian well,
from: United States Geological Survey,capp.water.usgs.gov/gwa/ch_c/C-text4.html, image taken fromcapp.water.usgs.gov/gwa/ch_c/jpeg/C061.jpeg>

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Water Resources

The Rio Grande aquifer is the source of domestic water for a population of approximately 3 million people living in Colorado, New Mexico, Texas, and Mexico. El Paso, TX, has a population of 700,000 people and across the border in Ciudad, Juarez, Mexico, 1.3 million live making these two bordering cities the largest population center of any international border in the world (El Paso Information and Links, WWW). Sixty thousand acre-feet if water (over 19 billion gallon) is allotted to Mexico each year for use in Ciudad (Robert). The upper basin in Colorado\rquote s population was 45, 400 in 2000 (Colorado Water Conservation Board, WWW). New Mexico has a large population on the Rio Grande aquifer including the population center of Albuquerque, population 690,000 in 2000 and Dona Ana County, population 175,000. Largest consumer of water is the agriculture industry. Fig. 6 illustrates water usage in 1985.

Fig. 6 Graph illustrating the use of water in the Rio Grande aquifer system in 1985,
from: United States Geological Survey,capp.water.usgs.gov/gwa/ch_c/C-text4.html, image taken fromcapp.water.usgs.gov/gwa/ch_c/jpeg/C065.jpeg>

The Middle Rio Grande aquifer studies show groundwater of the Santa Fe Group aquifer is being depleted rapidly (Bexfield and Anderholm). The sole water source for Albuquerque, NM, is the Middle Rio Grande aquifer. Flow patterns of the aquifer have changed from northeast to southwest and now flow from all directions to wells where the most water is being removed from the aquifer. These cones of depression underlie the eastern side of Albuquerque, the western side of the city, and beneath Rio Rancho a city north of Albuquerque (Robert). The decrease in the available groundwater to supply the cities growing population is of concern. Depletion of this aquifer puts the cities of Las Cruces, NM, El Paso, TX, and Ciudad, Juarez, Mexico, at risk of not having a dependable water supply in the future. Presently in the Middle Rio Grande Aquifer there is a 55,000 to 70,000 acre-feet per year deficit of water exaction over recharge of the aquifer (Robert).

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Groundwater Quality

The processes leading to contamination of aquifers natural and human. Natural processes make a difference in the quality of the water and include minerals present in the soil and naturally occurring vegetation. Evapotranspiration by the native grasses and plants help concentrate the solutes in the ground water and the ground water is degraded (USGS, WWW). In the Rio Grande aquifer the most common chemicals dissolved in the groundwater are calcite and dolomite (calcium and magnesium carbonates), gypsum (calcium sulfate), and halite (rock salt) (USGS, WWW).

Human processes can be divided into four categories pumpage, agricultural, urbanization and mining (USGS, WWW). Irrigation can concentrate solutes in groundwater by evapotranspiration. These concentrated solutions are carried into the groundwater with recharge, degrading water quality. Other contaminates carried into the groundwater with recharge of irrigation water include nitrates from fertilizers and insecticides. Agriculture affects ground water as chemicals are used on crops, large concentrations of livestock increase the possibilities of fecal contamination of groundwater, and there is an increased need to use water with large livestock operations and irrigation.

Pumpage creates a mixing of aquifer waters from different levels and different qualities. Irrigation water has a higher salinity and when recharged into the aquifer increases the salinity of the shallow aquifers. Pumpage draws this high saline water into the deep aquifers (USGS.WWW). The impact of quality of groundwater by urbanization is a problem as rangeland and agricultural land is converted into residential and industrial use. There is an increased risk of fecal contamination; petroleum products particularly from leaking underground storage tanks, trace elements, industrial solvents and nitrates may contaminate the groundwater (USGS, WWW).

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Conclusions

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References

Aber, James S. San Luis Valley, Colorado. World Wide Web URL: http://academic.emporia.edu/aberjame/field/rocky_mt/zapata.htm Date accessed 30 April, 2006.

Bexfield, Laura M. and Scott K. Anderholm. Estimated Water-Level Declines in the Santa Fe Group Aquifer System in the Albuquerque Area, Central New Mexico, Predevelopment to 2002. World Wide Web URL: http://nm.water.usgs.gov/publications/abstracts/mapreport02-4233.pdf Date accessed 29 April, 2006.

Colorado Water Conservation Board. Rio Grande Basin Facts.World Wide Web URL: http://cwcb.state.co.us/Fact_Sheets/RioGrande/page0001.htm Date accessed 30 April, 2006.

El Paso Information and Links. World Wide Web URL: http://www.agiweb.org/geotimes/may04/feature_riogrande.html Date accessed 30 April, 2006.

United States Geological Survey. Ground Water Atlas of the United States, Arizonia, Colorado, New Mexico, Utah, HA 730-C, World Wide Web URL: http://capp.water.usgs.gov/gwa/ch_c/C-text4.html Date accessed 29 April, 2006

United States Geological Survey. NWAQA Study Unit. World Wide Web URL: http://nm.water.usgs.gov/nawqa/sudescription.html Date accessed 30 April, 2006.

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This webpage was created to meet the requirements of GO 571 Geohydrology at Emporia State University, Emporia, Kansas, and is the property of the author. For more information, contact the author, Gayla R. Corley, rextheterrible@yahoo.com

Created 30 April, 2006, from the Earth Science Department, Emporia State University:http://www.emporia.edu/earthsci/, at Emporia State University, Emporia, KS htpp://www.emporia.edu/.