Ground-water quality, water year 1995, and statistical analysis of ground-water-quality data, water years 1994-95, at 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 closed in 1989, and the Texas Natural
Resources Conservation Commission issued permit number HW-50296
(U.S. Environmental Protection Agency 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
monitoring and evaluating ground-water quality at the site. One
upgradient ground-water monitoring well (MW1) and two
downgradient ground-water monitoring wells (MW2 and MW3),
installed adjacent to the chromic acid pit, are monitored on a
quarterly basis. Ground-water sampling of these wells by the U.S.
Geological Survey began in December 1993.

The ground-water level, measured in a production well located
approximately 1,700 feet southeast of the Chromic Acid Pit site,
has declined about 29.43 feet from 1982 to 1995. Depth to water at
the Chromic Acid Pit site in September 1995 was 284.2 to 286.5
feet below land surface; ground-water flow at the water table is
assumed to be toward the southeast.

Ground-water samples collected from monitoring wells at the
Chromic Acid Pit site during water year 1995 contained dissolved-
solids concentrations of 481 to 516 milligrams per liter. Total
chromium concentrations detected above the laboratory reporting
limit ranged from 0.0061 to 0.030 milligram per liter; dissolved
chromium concentrations ranged from 0.0040 to 0.010 milligram per
liter. Nitrate as nitrogen concentrations ranged from 2.1 to 2.8
milligrams per liter; nitrite plus nitrate as nitrogen
concentrations ranged from 2.4 to 3.2 milligrams per liter. Water
samples from wells MW1 and MW2 were analyzed for volatile organic
compounds for the first quarter; no confirmed volatile organic
compounds were detected above laboratory reporting limits.
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. Overall,
water-quality characteristics of water from the chromic acid pit
ground-water monitoring wells are similar to those of other wells
in the surrounding area.

Statistical analyses were performed on 56 of the chemical
constituents analyzed for in ground water from the chromic acid
pit monitoring wells. Concentrations of chloride, fluoride,
sulfate, and potassium were significantly less in water from one
or both downgradient wells than in water from the upgradient well.
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, chloride,
and potassium were significantly different in water from the two
downgradient wells. Statistical analysis of chemical constituents
in water from 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.