Water-quality assessment of the Rio Grande Valley, Colorado, New Mexico, and Texas: Occurrence and distribution of selected pesticides and nutrients at selected surface-water sites in the Mesilla Valley, 1994-95

The Rio Grande Valley study unit of the U.S. Geological Survey
National Water-Quality Assessment Program conducted a two-phase
synoptic study of the occurrence and distribution of pesticides
and nutrients in the surface water of the Mesilla Valley, New
Mexico and Texas. Phase one, conducted in April-May 1994 during
the high-flow irrigation season, consisted of a 6-week time-
series sampling event during which 17 water-column samples were
collected at 3 main-stem sites on the Rio Grande and a synoptic
irrigation-run sampling event during which 19 water-column
samples were collected at 7 main-stem sites, 10 drain sites, and 2
sites at the discharges of wastewater-treatment plants. Three
samples are included in both the time-series and irrigation-run
events. Phase two, conducted in January 1995 during the low-flow
non-irrigation season, consisted of a non-irrigation synoptic
sampling event during which 18 water-column samples were
collected at seven main-stem sites, nine drain sites, and two
sites at the discharges of wastewater-treatment plants and a bed-
material sampling event during which 6 bed-material samples were
collected at six sites near the mouths of drains that discharge to
the Rio Grande.

The 51 water-column samples were analyzed for 78 pesticides
and metabolites and 8 nutrients along with other constituents.
The six bed-material samples were analyzed for 21 pesticides and
metabolites, gross polychlorinated biphenyls, and gross
polychlorinated naphthalenes.

The presence of dissolved pesticides in the surface water of
the Mesilla Valley is erratic. A total of 100 detections of 17
different pesticides were detected in 44 of the water-column
samples. As many as 38 percent of these detections may be
attributed to pesticide use upstream from the valley or to
nonagricultural pesticide use within the valley. There were 29
detections of 10 different pesticides in 17 samples during the
irrigation run and 41 detections of 13 pesticides in 16 samples
during the non-irrigation run. Nine pesticides were detected
during both phases of the study.

The most commonly detected pesticides in the water-column
samples were DCPA, which was detected in 29 samples, and
metolachlor, which was detected in 17 of the samples. DCPA was
detected throughout the Mesilla Valley, whereas metolachlor was
detected mainly in the northern and central parts of the valley.
The maximum pesticide concentration found during the study was
0.75 microgram per liter of carbofuran, which was detected at the
East Side Drain site during the irrigation run. No water-column
pesticide concentration exceeded U.S. Environmental Protection
Agency's drinking-water standards or any applicable Federal or
State criteria or guidelines.

A total of 21 occurrences of six pesticides and metabolites
were found in the bed-material samples. Chlordane, diazinon, and
methyl parathion were detected once each, whereas DDD, DDE, and
DDT were detected at all six bed-material sites.

Water-column samples for the analysis of nutrient
concentrations were collected at all sampling sites during both
phases of the study. The concentrations of each nutrient ranged
from at or below the individual minimum reporting level to as much
as two or three orders of magnitude larger than the minimum
reporting level. The concentration of each nutrient was left
skewed with most of the values toward the lower end of the range.
The larger concentrations of each nutrient, except dissolved
nitrite plus nitrate, were associated with wastewater-treatment-
plant sites 4 and 16. The larger concentrations of dissolved
nitrite plus nitrate were generally associated with the non-
irrigation run; however, the largest concentration was at site 4
during the irrigation run.

During this study, the Mesilla Valley as a unit was a source
of nutrients to the Rio Grande. Wi