Quality of water from shallow wells in the rice-growing area in southwestern Louisiana, 1999 through 2001

In 1999-2001, the U.S. Geological Survey installed and sampled 27 shallow wells in the

rice-growing area in southwestern Louisiana as part of the Acadian-Pontchartrain Study Unit of

the National Water-Quality Assessment Program. The purpose of this report is to describe the

waulity of water from shallow wells in the rice-growing area and to relate that water quality to

natural and anthropogenic activities, particularly rice agriculture. Ground-water samples were

analyzed for general ground-water properties and about 150 water-quality constituents, including

major inorganic ions, trace elements, nutrients, dissolved organic carbon (DOC), pesticides,

radon, chloroflourocarbons, and selected stable isotopes.

Dissolved solids concentrations for 17 wells exceeded the U.S. Environmental Protection Agency

secondary minimum containment level of 500 milligrams per liter (mg/L) for drinking water.

Concentrations for major pesticides generally were less than the maximum contaminant levels for

drinking water. Two major inorganic ions, sulfate and chloride, and two trace elements, iron

and manganese, had concentrations that were greater than the secondary maximum containment

levels. Three nutrient concentrations were greater than 2 mg/L, a level that might indicate

contamination from human activities, and one nutrient concentration (that for nitrite plus

nitrite as nitrogen) was greater than the maximum contaminant level of 10 mg/L for drinking

water. The median concentration for DOC was 0.5 mg/L, indicating naturally-occurring DOC

conditions in the study area. Thirteen pesticides and 7 pesticide degradation products were

detected in 14 of the 27 wells sampled. Bentazon, 2, 4-D, and molinate (three rice herbicides)

were detected in water from four, one, and one wells, respectively, and malathion (a rice

insecticide) was deteced in water fromone well. Low-level concentrations and few detections of

nutrients and pesticides indicated that ground-water quality was affected slightly by

anthropogenic activities. Quality-control samples, including field blanks, replicates, and

spikes, indicated no bias in ground-water data from collection on analysis.

Radon concentrations for 22 of the 24 wells sampled wer at or greater than the U.S.

Environmental Protection Agency proposed maximum contaminant level of 300 picocuries per liter.

Chlorofluorocarbon concentrations in selected wells indicated the apparent ages of the ground

water varied with depth water level and ranged from about 17 to 49 years. The stable isotopes

of hydrogen and oxygen in water molecules indicated the origin of ground water in the study area

was rainwater that originated near the study area and that few geochemical or physical processes

influenced the stable isotopic composition of the shallow ground water.

The Spearman rank correlation was used to detemrine whther significant correlations existed between physical properties, selected chemical constituents, the number of pesticides detected, and the apparent age of water. The depth to ground water was positively correlated to the well depth and inversely correlated to dissolved solids and other constituents, such as radon, indicating the ground water was under unconfined or semiconfined conditions and more dilute with increasing depth. As the depth to ground water increased, the concentrations of dissolved solids and other constituents decreased, possibly because the deeper sands had a greater transmittal of ground water, which, over time, would flush out, or dilute, the concentrations of dissolved solids in the natural sediments. The apparent age of water was correlated inversely with nitrite plus nitrite concentration, indicating that as apparent age increased, the nitrite plus nitrite concentration decreased. No significant correlations existed between the number of pesticides detected and any of the physical or chemica