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Lake Wales Ridge Pesticide Monitoring

Abstract: A litmus area for transport of agricultural chemicals into ground water and lakes, central Florida

Abstract published in the American Water Resources Association 2003 Spring Specialty Conference “Agricultural Hydrology and Water Quality”, May 12-14, 2003, Kansas City, Mo., Proceedings, AWRA Technical Publication Series No. TPS-03-01.


A.F. Choquette, D.K. Moore, E.C. Dehaven, J.D. Haber, and R.M. Turner *

ABSTRACT: Lake Wales Ridge, in central Florida, is one of the most vulnerable regions in the nation for the transport of agricultural chemicals to the subsurface. The propensity for leaching of agricultural chemicals on the Ridge is due in part to extensive citrus land use on well-drained sandy soils deficient in organic matter. Rates of annual herbicide and insecticide application in citrus groves are among the highest of any U.S. agricultural land use. To evaluate pesticide transport into ground-water systems, a consortium of state and federal agencies have established a regional-scale water-quality monitoring network currently consisting of 31 wells tapping the unconfined, surficial aquifer system. The water is acidic, with pH typically ranging from 4 to 6. Ground-water monitoring includes long-term records (1989-present) and repeat quarterly sampling during 1999 through 2001. Recent detections of pesticides and pesticide degradates in ground water from the wells include: norflurazon (81% of the wells), desmethylnorflurazon (65%), simazine (55%), bromacil (42%), diuron (42%), aldicarb sulfoxide (23%), aldicarb sulfone (23%), deisopropylatrazine (desmethyl simazine) (19%), metalaxyl (6%), thiazopyr monoacid (6%), imidacloprid (3%), and aldicarb (3%).

Target chemicals include nutrients and 28 pesticides (and degradates), most of which are currently labeled for use in Florida citrus groves. Concentrations of nitrate (as N) have been as high as 89 mg/L and have exceeded 10 mg/L at 81% of the wells. Five pesticides or degradates have exceeded state or federal human health guidance levels for concentrations in drinking water. Health guidance exceedances have occurred one or more times at 8 of the 31 network wells. However, the incidence of health guidance exceedances for single pesticides or degradates, considered separately, was limited to a maximum of three wells, and the exceedances typically were not persistent over time. In terms of chemical mixtures or co-occurring pesticides in ground water on Lake Wales Ridge, 24 wells (77%) have yielded three to as many as eight pesticides and degradates at concentrations exceeding laboratory detection limits and relatively high nitrate concentrations, typically in the range of 10 to 25 mg/L. In a comparison with other national ground-water data, maximum concentrations of nitrate and six pesticides or degradates from the Ridge network exceeded maximums from the USGS National Water Quality Assessment (NAWQA) network of approximately 4,000 wells, by factors of 1.2 to 130 times the NAWQA maximums.

Documentation of areal and temporal variability in concentrations along with the incidence of co-occurring compounds provides a framework for sampling strategy optimization, chemical fate and transport modeling, and agricultural chemical management. Evaluation of relations between concentrations of agricultural chemicals and environmental factors, such as thickness of the unsaturated zone above the water table, depth of sampling within the saturated zone, antecedent precipitation, and dominant background water chemistry, will provide insight into conditions associated with the transport and fate of these compounds in the subsurface. Planned expansion of study to the numerous seepage lakes (ground-water fed), which characterize this region, will increase understanding of the potential for pesticide transport between subsurface and surface water systems.

KEY TERMS: monitoring networks, citrus agriculture, pesticides, nutrients, nitrate, sandy soils

*Respectively, U.S. Geological Survey, Florida Integrated Science Center (FISC), Tallahassee, FL 32301 [850.942.9500 X3031; Fax: 850.942.9521; E-Mail:]; Bureau of Pesticides, Florida Dept. of Agriculture and Consumer Services, Tallahassee, FL 32399; Southwest Florida Water Management District (WMD), Tampa, FL 33637; Southwest Florida WMD, Tampa, FL 33637; U.S. Geological Survey, FISC, Tallahassee, FL 32301.

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