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

From March to May of 1995, water samples were collected from
30 wells located in the flood plain of the Rio Grande between
Cochiti Lake, New Mexico, and El Paso, Texas. These samples were
analyzed for a broad host of constituents, including field
parameters, major constituents, nutrients, dissolved organic
carbon, trace elements, radiochemicals, pesticides, and volatile
organic compounds. The main purpose of this study was to observe
the quality of ground water in this part of the Rio Grande Valley
study unit of the U.S. Geological Survey National Water-Quality
Assessment program. The sampling effort was limited to the basin-
fill aquifer beneath the above-defined reach of the Rio Grande
flood plain because of the relative homogeneity of the
hydrogeology, the large amount of ground-water use for public
supply, and the potential for land-use activities to affect the
quality of ground water. Most of the wells sampled for the study
are used for domestic purposes, including drinking water. Depths
to the tops of the sampling intervals in the 30 wells ranged from
10 to 345 feet below land surface, and the median was 161.5 feet;
the sampling intervals in most of the wells spanned about 10 feet
or less. Quality-control data were collected at three of the
wells.

A significant amount of variation was found in the chemical
composition of ground water sampled throughout the study area,
but the water generally was found to be of suitable chemical
quality for use as drinking water, according to current
enforceable standards established by the U.S. Environmental
Protection Agency (EPA). Nutrients generally were measured at
concentrations near or below their method reporting limits. The
most dominant nutrient species was nitrite plus nitrate, at a
maximum concentration of 1.9 milligrams per liter (as N). Only
eight of the trace elements analyzed for had median
concentrations greater than their respective minimum reporting
levels. Water from one well exceeded the lifetime health advisory
established by the EPA for molybdenum; water from a different well
exceeded the proposed EPA maximum contaminant level for uranium.
Gross alpha and gross beta particle activities generally appeared
to strongly correlate with quantities of uranium and potassium,
respectively, detected in ground water. However, water from one
well exceeded the EPA maximum contaminant level for gross alpha
particle activity and may exceed the EPA maximum contaminant
level for beta particle and photon activity, although current
data on gross beta particle activities are not conclusive on this
point. Radon concentrations did not appear to directly correlate
with uranium concentrations. The herbicide prometon was the only
synthetic organic compound detected in ground water in the study
area, and was detected in only one well, at a concentration of
0.038 microgram per liter. This well is shallow and is not used
for drinking water. With the exception of the one detection of
prometon, no strong evidence was found of effects on ground-water
quality from human activities. Therefore, most of the water
sampled probably recharged at the margins of the alluvial basins
or recharged through the flood plain before human development
began.

With respect to major constituents, the concentrations of
dissolved 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 ground
water in the study area may be affected by several processes,
including cation exchange, feldspar weathering, calcite
dissolution and precipitation, dissolution of volcanic glass, and
microbial activity. Several chemical constituents in ground water
showed relatively distinct spatial patterns that appear to be
related to one or more of these processes.