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Water-quality assessment of the Rio Grande Valley, Colorado, New Mexico, and Texas; shallow ground-water quality of a land-use area in the San Luis Valley, south-central Colorado, 1993

May 1, 1997

This report describes the quality of shallow ground water in
an agricultural area in the San Luis Valley, Colorado, and
discusses how natural and human factors affect the quality of
shallow ground water. Thirty-five wells were installed, and
water samples were collected from these wells and analyzed for
selected dissolved common constituents, nutrients, trace
elements, radionuclides, and synthetic organic compounds.

The San Luis Valley is a high intermontane valley that is
partially drained by the Rio Grande. The San Luis Valley land-use
study area was limited to a part of the valley where the depth to
water is generally less than 25 feet. The area where the 35
monitor wells were installed was further limited to the part of
the study area where center-pivot overhead sprinklers are used to
irrigate crops. Precipitation, runoff from adjacent mountainous
areas, and ground-water inflow from the adjacent mountainous
areas are the main sources of water to the aquifers in the San
Luis Valley. Discharge of water from the shallow, unconfined
aquifer in the valley is mainly from evapotranspiration. The
dominant land use in the San Luis Valley is agriculture, although
nonirrigated land and residential land are interspersed with
agricultural land. Alfalfa, native hay, barley, wheat, potatoes,
and other vegetables are the main crops.

Dissolved-solids concentrations in shallow ground water
sampled ranged from 75 to 1,960 milligrams per liter. The largest
median concentration of cations was for calcium, and the largest
median concentration of anions was for bicarbonate in shallow
ground water in the San Luis Valley. Calcium concentrations
ranged from 7.5 to 300 milligrams per liter, and bicarbonate
concentrations ranged from 28 to 451 milligrams per liter.
Nitrite plus nitrate concentrations ranged from less than 0.1 to
58 milligrams per liter as N; water from 11 wells had nitrite plus
nitrate concentrations greater than 10 milligrams per liter as N.
With the exception of the following trace elements--aluminum,
barium, iron, manganese, molybdenum, and uranium--the
concentrations of trace elements were less than 10 micrograms per
liter in 90 percent of the samples. All trace-element
concentrations measured were below the maximum contaminant levels
set by the U.S. Environmental Protection Agency. Five samples
exceeded the proposed maximum contaminant level of 0.02 milligram
per liter for uranium. All samples collected exceeded the
proposed maximum contaminant level for radon-222. The volatile
organic compound methyltertbutylether was detected in one sample
at a concentration of 0.6 microgram per liter. Of the pesticides
analyzed for, one or more were detected in water from 5 of the 35
wells sampled. Metribuzin was the most commonly detected
pesticide and was detected in water from three wells at
concentrations ranging from an estimated 0.005 to 0.017 microgram
per liter. Metolachlor (detected in one sample at a concentration
of 0.072 microgram per liter), prometon (detected in one sample
at a concentration of 0.01 microgram per liter), and p,p'-DDE
(detected in one sample at an estimated concentration of 0.002
microgram per liter) were the other pesticides detected. The U.S.
Environmental Protection Agency lifetime health advisory for
metolachlor, metribuzin, and prometon is 100 micrograms per
liter, which is much larger than the concentrations measured in
the shallow ground water sampled for this study.

The elevated nitrite plus nitrate concentrations in shallow
ground water are indicative of leaching of fertilizers from the
land surface. This conclusion is consistent with conclusions made
in other investigations of the San Luis Valley. On the basis of
areal distribution and range of trace-element concentrations,
human activities have not caused widespread trace-element
contamination in the shallow grou

Citation Information

Publication Year 1996
Title Water-quality assessment of the Rio Grande Valley, Colorado, New Mexico, and Texas; shallow ground-water quality of a land-use area in the San Luis Valley, south-central Colorado, 1993
DOI 10.3133/wri964144
Authors S.K. Anderholm
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Water-Resources Investigations Report
Series Number 96-4144
Index ID wri964144
Record Source USGS Publications Warehouse