Effects on ground-water quality of seepage from a phosphatic clayey waste settling pond, north-central Florida

Abstract

Water samples were taken from test wells drilled near an inactive phosphatic clayey waste storage settling pond, from the settling pond and its perimeter ditch, and from an active settling pond near White Springs, Hamilton County, in north-central Florida. The purpose was to document the seepage of chemical constituents from the inactive settling pond and ditch into the adjacent surficial groundwater system, and to assess the potential for movement of these constituents into the deeper Floridan aquifer system which is the major source of public supply in the area. The study area is underlain by at least a 2,500-foot-thick sequence of Coastal Plain sediments of Early Cretaceous to Holocene age. The rocks of Tertiary and Quaternary age that underlie the test site area can be grouped into three major hydrogeologic units. In descending order, these units are: surficial aquifer, Hawthorn confining unit, and Floridan aquifer system. Phosphate deposits occur in the upper part of the surficial aquifer. Water in the active settling pond is a calcium magnesium sulfate type with a dissolved-solids concentration of 250 milligrams per liter and contains greater amounts of phosphorus, iron, aluminum, barium, zinc, and chromium than the other surface waters. Water in the perimeter ditch is a calcium sulfate type with a dissolved-solids concentration of 360 to 390 milligrams per liter and contains greater amounts of calcium, sulfate, nitrogen, and fluoride than other surface waters. Water from the inactive settling pond is a calcium magnesium bicarbonate type with a dissolved-solids concentration of 140 milligrams per liter and contains more bicarbonate than the other surface waters. Large amounts of the chemical constituents in the phosphate wastedisposal slurry are apparently trapped in the sediments of the settling ponds. The quality of water in the upper part of the surficial aquifer from wells within 200 to 400 feet of the inactive settling pond shows no signs of chemical contamination from phosphate industry operations. The horizontal groundwater velocity calculated for this aquifer between, the ditch surrounding the settling pond and the test wells is between 100 to 2,000 feet per year, which is enough time for water to have reached the test wells in the 6 years the pond has been operating. The groundwater velocity beyond the test wells to a distance of 1 mile is much less. The water in the lower part of the surficial aquifer is almost identical in quality to the water in a distant background uncontaminated well.