NUTRIENT-LOSS TRENDS FOR VEGETABLE AND CITRUS FIELDS IN WEST-CENTRAL FLORIDA .2. PHOSPHATE

Vegetable and citrus production in west-central Florida has come under suspicion as a hazard (with respect to NO3-N and ortho-P) to local groundwater and surface-water bodies, including a 33 000-ha drinking-water supply reservoir near Bradenton in Manatee County. Using a combination of multilevel samplers in the shallow (surficial) aquifer beneath selected vegetable fields and citrus groves, coupled with piezometric wells around each field's periphery to assess depth-integrated solute concentrations and direction and rate of groundwater flow, ortho-P levels have been assessed at 10 sites for three vegetable-production seasons during 1990 and 1998. Some ortho-P movement from vegetable production beds to surface waters and shallow groundwater appears likely, but ortho-P concentrations also are elevated at a native range site that has not received P fertilizers, and in both man-made and natural surface-water retention ponds plus nearby intermittent streams throughout the area. Naturally occurring phosphatic clays appear to be introducing considerable P into local shallow groundwater and associated surface-water bodies. Regulatory strategies requiring sizeable retention ponds for tailwater-return flow capture may be contributing to P loadings of the surface water, whenever pond construction intercepts phosphatic clay materials.