Water quality and benthic macroinvertebrate bioassessment of Gallinas Creek, San Miguel County, New Mexico, 1987-90

Upper Gallinas Creek in north-central New Mexico serves as the
public water supply for the City of Las Vegas. The majority of
this 84-square-mile watershed is within national forest lands
managed by the U.S. Forest Service. In 1985, the Forest Service
planned to conduct timber harvesting in the headwaters of
Gallinas Creek. The City of Las Vegas was concerned about possible
effects from logging on water quality and on water-supply
treatment costs. The U.S. Geological Survey began a cooperative
study in 1987 to (1) assess the baseline water-quality
characteristics of Gallinas Creek upstream from the Las Vegas
water-supply diversion, (2) relate water quality to State water-
quality standards, and (3) determine possible causes for spatial
differences in quality. During 1987-90, water-quality
constituents and aquatic benthic macroinvertebrates were
collected and analyzed at five sampling sites in the watershed.

Specific conductance, pH, total hardness, total alkalinity,
and calcium concentrations increased in a downstream direction,
probably in response to differences in geology in the watershed.
The water-quality standard for temperature was exceeded at the
two most downstream sites probably due to a lack of riparian
vegetation and low streamflow conditions. The standards for pH
and turbidity were exceeded at all sites except the most upstream
one. Concentrations of nitrogen species and phosphorus generally
were small at all sites. The maximum total nitrogen concentration
of 2.1 milligrams per liter was at the mouth of Porvenir Canyon;
only one sample at this site exceeded the water-quality standard
for total inorganic nitrogen. At each of the sites, 10 to 15
percent of the samples exceeded the total phosphorus standard of
less than 0.1 milligram per liter. Except for aluminum and iron,
almost all samples tested for trace elements contained
concentrations less than the laboratory detection limit. No
trace-element concentrations exceeded the State standard for
domestic water supplies. Suspended-sediment concentrations
appeared to increase with distance downstream; suspended sediment
increased significantly from the uppermost site to the second
site near the national forest boundary, most probably caused by
runoff from the unpaved forest road adjacent to Gallinas Creek.
The aquatic macroinvertebrate assessment indicated that the three
upstream sites had good biological conditions and were
nonimpaired, whereas the two downstream sites had lowered
biological conditions and were slightly impaired. The water-
quality and biological assessments provided similar results.