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#           Water-quality assessment of the Rio Grande Valley, Colorado, New Mexico and Texas: Ground-water quality in the Rio Grande flood plain, Cochiti Lake, New Mexico, to El Paso, Texas, 1995      

      September 1, 1997    

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From March to May of 1995, water samples were collected from30 wells located in the flood plain of the Rio Grande betweenCochiti Lake, New Mexico, and El Paso, Texas. These samples wereanalyzed for a broad host of constituents, including fieldparameters, major constituents, nutrients, dissolved organiccarbon, trace elements, radiochemicals, pesticides, and volatileorganic compounds. The main purpose of this study was to observethe quality of ground water in this part of the Rio Grande Valleystudy unit of the U.S. Geological Survey National Water-QualityAssessment program. The sampling effort was limited to the basin-fill aquifer beneath the above-defined reach of the Rio Grandeflood plain because of the relative homogeneity of thehydrogeology, the large amount of ground-water use for publicsupply, and the potential for land-use activities to affect thequality of ground water. Most of the wells sampled for the studyare used for domestic purposes, including drinking water. Depthsto the tops of the sampling intervals in the 30 wells ranged from10 to 345 feet below land surface, and the median was 161.5 feet;the sampling intervals in most of the wells spanned about 10 feetor less. Quality-control data were collected at three of thewells.

     A significant amount of variation was found in the chemicalcomposition of ground water sampled throughout the study area,but the water generally was found to be of suitable chemicalquality for use as drinking water, according to currentenforceable standards established by the U.S. EnvironmentalProtection Agency (EPA). Nutrients generally were measured atconcentrations near or below their method reporting limits. Themost dominant nutrient species was nitrite plus nitrate, at amaximum concentration of 1.9 milligrams per liter (as N). Onlyeight of the trace elements analyzed for had medianconcentrations greater than their respective minimum reportinglevels. Water from one well exceeded the lifetime health advisoryestablished by the EPA for molybdenum; water from a different wellexceeded the proposed EPA maximum contaminant level for uranium.Gross alpha and gross beta particle activities generally appearedto strongly correlate with quantities of uranium and potassium,respectively, detected in ground water. However, water from onewell exceeded the EPA maximum contaminant level for gross alphaparticle activity and may exceed the EPA maximum contaminantlevel for beta particle and photon activity, although currentdata on gross beta particle activities are not conclusive on thispoint. Radon concentrations did not appear to directly correlatewith uranium concentrations. The herbicide prometon was the onlysynthetic organic compound detected in ground water in the studyarea, and was detected in only one well, at a concentration of0.038 microgram per liter. This well is shallow and is not usedfor drinking water. With the exception of the one detection ofprometon, no strong evidence was found of effects on ground-waterquality from human activities. Therefore, most of the watersampled probably recharged at the margins of the alluvial basinsor recharged through the flood plain before human developmentbegan.

     With respect to major constituents, the concentrations ofdissolved solids ranged from 209 to 3,380 milligrams per liter,and the median concentration was 409.5 milligrams per liter.There is evidence that the overall chemical composition of groundwater in the study area may be affected by several processes,including cation exchange, feldspar weathering, calcitedissolution and precipitation, dissolution of volcanic glass, andmicrobial activity. Several chemical constituents in ground watershowed relatively distinct spatial patterns that appear to berelated to one or more of these processes.

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