Nutrient‐Loss Trends for Vegetable and Citrus Fields in West‐Central Florida: II. Phosphate

Abstract

AbstractVegetable 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 depthintegrated 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 1991. 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‐wateretention 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.