WSP 2381

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German, E.R., 1996, Analysis of Nonpoint-Source Ground-Water Contamination in Relation to Land Use: Assessment of Nonpoint-Source Contamination in Central Florida: U.S. Geological Survey Water-Supply Paper 2381, 63 p.


Ground-water quality in central Florida is affected by land-use practices, such as the urbanization of karstic terrain with accompanying stormwater disposal through drainage wells, citrus cultivation with accompanying application of large quantities of fertilizers and pesticides, and mining and processing of phosphate ores into fertilizers. Stormwater entering drainage wells in urban areas can directly affect the Upper Floridian aquifer, the upper part of the Floridan aquifer system, which is used as a source of potable water throughout the area. Leachate from fertilizers and pesticides, and activities associated with phosphate mining are most likely to affect the surficial aquifer system. Although water from the surficial aquifer system is not used extensively as a source of public water supply, it does ultimately recharge the Floridan aquifer system.

Ground-water quality in three developed areas with different land uses was compared to ground-water quality in an undeveloped control area. Statistical tests were used to determine if differences in ground-water quality among the areas were significant. A probability level of 5 percent was used to indicate significant differences.

The primary study areas were an undeveloped area referred to as the control area, an urban area, a citrus production area, and a phosphate-mining area. The control area is an undeveloped part of the Ocala National Forest, where only low-density recreational activities and periodic logging occur. The urban area is in Orlando, where hundreds of drainage wells convey stormwater to the Upper Floridan aquifer. The citrus area is near Windermere, west of Orlando, where citrus has been cultivated for at least 25 years. The mining area is near Bartow, Fla., in an area that supplies as much as one-fifth of the phosphate used in manufacturing fertilizer worldwide. In addition to the primary study areas, two other areas were studied to test transferability of the findings from the primary urban and citrus study areas. These were a citrus area near Lake Wales and an urban area in Ocala.

Concentrations of most of the major constituents, nitrogen species, phosphorus, and organic compounds in water from the Upper Floridan aquifer underlying the urban area were significantly greater than concentrations in water from this aquifer in the control area, possibly as the result of drainage-well inflow. Trace elements and volatile organics were detected in ground water in the control area as frequently as in ground water in the urban area.

Most major constituents and nitrogen species were present in greater concentrations in water from the surficial aquifer system underlying the citrus area than in water from the surficial aquifer system in the control area. Nitrate-nitrogen concentrations exceeded the U.S. Environmental Protection Agency primary maximum-contaminant level for drinking water (10 milligrams per liter) in water from more than half the 33 wells sampled in the citrus area. The pesticide bromacil was detected in water from more than half of the 19 wells in the citrus area sampled for pesticide analysis. Concentrations of bromacil exceeded 20 micrograms per liter in water from some wells in the citrus area.

Study results indicate that the most mineralized ground water is in the mining area. Concentrations of all major constituents, nitrogen species, and phosphorus were significantly greater in water from the surficial aquifer system underlying the mining area than in ground water from the control area. The numbers and concentrations of organic compounds detected in ground water also were greater in the mining area than in the control area. Arsenic was more commonly detected in ground water in the mining area than in the control area and was the only trace element that was detected significantly more frequently in any developed area than in the control area.

Ground-water quality was determined in a second urban area (the Ocala area) to test the transferability of the study results to other urban areas. Ground-water quality in the Upper Floridan aquifer beneath the Ocala area differed from that in the undeveloped area as did ground-water quality in the Orlando urban area. In both of these urban areas, stormwater is disposed of in drainage wells or sinkholes. However, the specific effects of stormwater on ground-water quality may not be consistent from one urban area to another.

Ground-water quality also was determined in a second citrus area to test the transferability of study results to other citrus areas. A comparison of the water-quality data for the two areas indicated that citrus cultivation had affected the quality of water in the surficial aquifer system in both areas. Nitrate concentrations in water from the surficial aquifer system in both citrus areas generally exceeded the maximum-contaminant level for drinking water. Also, bromacil was detected in concentrations exceeding 20 micrograms per liter in ground-water samples from several wells in each of the two citrus areas.

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