Managing Runoff Water Quality from Clay Settling Areas Used for Intensive Agricultural Production
Clay settling areas from phosphate mining in central Florida contain fertile, highly productive agricultural soils. However, gently sloped (2% grade) beds intended to improve surface drainage, combined with increased soil disturbance associated with intensive crop production, could lead to greater runoff volumes and sediment loads. The objective of this work was to examine surface water quality discharged from agriculturally utilized phosphatic clay settling areas. Two preliminary replicated small plot (1 m x 1.5 m) field trials were used to describe the characteristics of runoff from bare, bermudagrass-covered, or ryegrass-covered phosphatic clay at either 2% or 8% slope. Results from the first small plot experiment were used to calibrate the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model with respect to sediment and runoff predictions. Water quality measurements included runoff and sediment amounts and N and P concentrations. The calibrated GLEAMS model was successfully used to predict runoff and sediment loading observed in a second small plot experiment. The GLEAMS model also predicted N loading in runoff and sediment, but over-predicted P. Sensitivity analysis showed that the current GLEAMS model does not have appropriate mechanisms for addressing the high mineral-P concentration of phosphatic clays. Sediment loading varied with slope, and bare phosphatic clay generated greater than nine times as much sediment as ryegrass-covered small plots at the 2% slope. The amounts of N and P found in the runoff were more than values reported in the literature for native prairies, but much less than values observed for agricultural soils receiving P fertilization. P was strongly correlated with sediment. Large field plots (approximately 1.2 ha each) were established in bermudagrass or corn followed by wheat. Both fields were landformed into macrobeds with 2% slopes. Flumes and automated water sampling equipment were installed at the discharge end of each macrobed. A weather station was used to collect rainfall amount, temperature, and wind data pertaining to the field plots. The corn/wheat rotation field produced more runoff events than the bermudagrass field. While all measures of N were low, measures of sediment and P from the cornfield were 2 to more than 6 times the concentration found in runoff from the bermudagrass field.