Quality of water in domestic wells in the Chicot and Chicot equivalent aquifer systems, southern Louisiana and southwestern Mississippi, 2000-2001

In 2000-2001, water-quality data were collected from 60 randomly selected domestic wells in the

Acadian-Pontchartrain Study Unit, as part of the National Water-Quality Assessment Program. The

data were collected from wells screened in shallow sands (less than 350 feet below land surface)

in two major aquifer systems--the Chicot aquifer system in southwestern Louisiana and the Chicot

equivalent aquifer system in southeastern Louisiana and southwestern Mississippi. The Chicot

equivalent aquifer system is part of the Southern Hills regional aquifer system, and both the

Chicot aquifer system and the Southern Hills regional aquifer systems are designated as

sole-source aquifers by the U.S. Environmental Protection Agency (USEPA).

The well depths ranged from 40 to 340 feet below land surface with a median depth of 120 feet.

The ground-water-quality data included 5 physiochemical properties, dissolved solids, 9 major

inorganic ions, 24 trace elements, 6 nutrients, dissolved organic carbon, 109 pesticides and

degradation products, and 85 volatile organic compounds (VOC's); and a subset of the wells were

sampled for radon, chlorofluorocarbons, and stable isotopes.

Water from 35 of the 60 domestic wells sampled had pH values less than the USEPA Seconday

Maximum Contaminant Level (SMCL) range of 6.5 to 8.5 standard units. Specific conductance

ranged from 17 to 1,420 microsiemens per centimeter at 25 degrees Celsius. Dissolved-solids

concentrations in water from two wells exceeded the SMCL of 500 mg/L (milligrams per liter); the

maximum concentration was 858 mg/L. Sodium and calcium were the dominant cations, and

bicarbonate and chloride were the dominant anions. One chloride concentration (264 mg/L)

exceeded the SMCL of 250 mg/L. One arsenic concentration (55.3 micrograms per liter) exceeded

the USEPA Maximum Contaminant Level (MCL) of 10 micrograms per liter. Iron concentrations in

water from 22 wells exceeded the SMCL of 300 micrograms per liter; the maximum concentration was

8,670 micrograms per liter. Manganese concentrations in water from 26 wells exceeded the SMCL

of 50 micrograms per liter; the maximum concentration was 481 micrograms per liter. Health

Advisories have been established for six of the trace elements analyzed; no concentrations were

greater than these nonenforceable standards. Radon concentrations in water from 9 of 50 wells

sampled were greater thanthe proposed USEPA MCL of 300 picocuries per liter.

Concentrations of ammonia, ammonia plus organic nitrogen, and nitrite plus nitrate in water from

four wells were greater than 2 mg/L, a level that might indicate anthropogenic influences. The

median dissolved organic carbon concentration was an estimated 0.30 mg/L, which indicated

naturally occurring dissolved organic carbon conditions in the study area. Eight pesticides and

two degradation products were detected in water from five wells. Twenty-four VOC's were

detected in water from 44 wells. All concentrations of pesticides and VOC's were less than

USEPA drinking-water standards.

Quality-control samples, which included field-blank samples, replicates, and field and

laboratory spikes, indicated no bias in ground-water data from collection procedures or

analyses. VAriance between the environmental sampls and he corresponding replicate samples was

typically less than 5 percent, indicating and acceptable degree of laboratory precision and data

collection reproducibility.

The Mann-Whitney rank-sum test was used to compare depth to top of screen and selected physicochemical properties and chemical constituents between six groups of wells. Values for selected physicochemical and chemical constituents were typically greater in wells located in the Chicot aquifer system than in the Chicot equivalent aquifer system. Values for specific conductance, pH, calcium, sodium, bicarbonate, chloride, dis