Determining Nonpoint-Source Contamination by Agricultural Chemicals in an Unconfined Aquifer, Dade County, Florida: Procedures and Preliminary Results

Abstract

South Dade County, Florida, is one of the few areas in the continental United States where winter fruits and vegetables are grown. About 85% of the agricultural area is underlain by Rockdale soil composed of crushed limestone and organic debris. The remainder of the agricultural area is underlain by Perrine marl. Both soils must be irrigated and require frequent applications of fertilizer, micronutrients, and pesticides. The agricultural area overlies permeable limestone of the unconfined Biscayne aquifer, which is designated as the sole source of drinking water for southeast Florida. The unsaturated zone in the agricultural area ranges in thickness from 0.3 to 4.6 m. An evaluation of nonpoint-source contamination from agricultural chemicals began in March 1985. Representative fields from the 30 000 ha under cultivation were selected for ground-water quality monitoring. Agricultural production is comprised of row crops, seasonal tropical vegetables, groves, and commercial wholesale nurseries. Five test plots underlain by both Rockdale and Perrine marl soils were selected for intense evaluation. Wells ranging in depth from 2.4 to 15.7 m were installed in areas hydraulically upgradient and downgradient and near the center of these plots. Additional wells were installed in the shallow mixing zone within and adjacent to the agricultural area to determine background and baseline water-quality conditions. The ground-water monitoring program was designed to leave in place a permanent well network and sampling stratagem that can be adapted to future water-quality concerns. Wells are drilled by clean water rotary and left open hole in the sampling interval so that organic, inorganic, or biological samples could be taken without contamination by drilling fluids, grouting material, or well casing and screens. A quality assurance program involving 10 to 20% of all samples was used to check field and laboratory procedures and to establish the accuracy of analytical data. Preliminary results of water-quality sampling indicate that agriculture has had a minimal effect on ground-water quality. Of the constituents sampled, only nitrate and potassium from fertilizers are found at elevated concentrations beneath farmed areas. Elevated concentrations of some nutrients, major ions, and trace metals are found beneath areas used for storage and disposal of agrichemicals.