Hydrogeology and ground-water quality of the chromic acid pit site, U.S. Army Air Defense Artillery Center and Fort Bliss, El Paso, Texas

The Chromic Acid Pit site is an inactive waste disposal site
that is regulated by the Resource Conservation and Recovery Act of
1976. The 2.2-cubic-yard cement-lined pit was operated from 1980
to 1983 by a contractor to the U.S. Army Air Defense Artillery
Center and Fort Bliss. The pit, located on the Fort Bliss military
reservation, in El Paso, Texas, was used for disposal and
evaporation of chromic acid waste generated from chrome plating
operations. The site was certified closed in 1989 and the Texas
Natural Resources Conservation Commission issued Permit Number
HW-50296 (U.S. Environmental Protection Agency Permit Number
TX4213720101), which approved and implemented post-closure care
for the Chromic Acid Pit site. In accordance with an approved
post-closure plan, the U.S. Geological Survey is cooperating with
the U.S. Army in evaluating hydrogeologic conditions and ground-
water quality at the site. One upgradient and two downgradient
ground-water monitoring wells were installed adjacent to the
chromic acid pit by a private contractor. Quarterly ground-water
sampling of these wells by the U.S. Geological Survey began in
December 1993.

The Chromic Acid Pit site is situated in the Hueco Bolson
intermontane valley. The Hueco Bolson is a primary source of
ground water in the El Paso area. City of El Paso and U.S. Army
water-supply wells are located on all sides of the study area and
are completed 600 to more than 1,200 feet below land surface. The
ground-water level in the area of the Chromic Acid Pit site has
declined about 25 feet from 1982 to 1993. Depth to water at the
Chromic Acid Pit site in September 1994 was about 284 feet below
land surface; ground-water flow is to the southeast.

Ground-water samples collected from monitoring wells at the
Chromic Acid Pit site contained dissolved-solids concentrations
of 442 to 564 milligrams per liter. Nitrate as nitrogen
concentrations ranged from 2.1 to 2.7 milligrams per liter;
nitrite plus nitrate as nitrogen concentrations ranged from 2.3
to 3.0 milligrams per liter. Nitrate concentrations are
abnormally high in the Old Mesa well field located about 5,000
feet southwest of the Chromic Acid Pit site. Volatile and
semivolatile organic compounds in water samples were analyzed for
the first sampling round; no confirmed volatile or semivolatile
organic compounds were detected above the laboratory reporting
limits. Total chromium concentrations ranged from 0.0099 to 0.092
milligram per liter; dissolved chromium concentrations ranged
from 0.0068 to 0.0094 milligram per liter. Overall, water-quality
characteristics in water from the chromic acid pit ground-water
monitoring wells are similar to those in the surrounding area.
Detected chemical concentrations in water from the chromic acid
pit monitoring wells during the four sampling periods were below
U.S. Environmental Protection Agency-established maximum
contaminant levels for public drinking water supplies.

Statistical analyses were performed on 39 of the chemical
constituents analyzed for in ground water from the chromic acid
pit monitoring wells. Concentrations of chloride and fluoride
were significantly less in water from the downgradient wells than
in water from the upgradient well, whereas concentrations of
nitrate as nitrogen, nitrite plus nitrate as nitrogen, and
dissolved solids were significantly greater in water from the
downgradient wells than in water from the upgradient well.
Concentrations of nitrate as nitrogen were significantly
different in water from the two downgradient wells. Differences
detected through statistical analysis of chemical constituents of
water in the chromic acid pit monitoring wells did not appear to
indicate a release of hazardous chemicals from the chromic acid
pit. There was no indication of ground-water contamination in
either downgradient well.