Determining the Statistical Sensitivity of the Water Quality Monitoring Program in the Taylor – Nubbin Slough, Florida Project

ABSTRACT

Reducing agricultural nonpoint source (NPS) pollution by implementing best management practices (BMPs) throughout entire watersheds' is being studied in 20 Rural Clean Water Program (RCWP) projects throughout the United States. The Taylor Creek – Nubbin Slough RCWP Project area is located directly north of Lake Okeechobee in southern Florida. The watershed covers 110,000 acres of which 63,109 were identified as critical agricultural sources of phosphorus entering Lake Okeechobee. These sources were primarily improved pastures and dairies. The project has an extensive pre-BMP water quality data base for statistical comparison with post-BMP data. In addition, most of the BMP implementation occurred in 1985, 1986, and 1987, allowing for four to five years post-BMP water quality monitoring before the end of the project. Therefore, this project should be able to document land treatment effects on water quality. Water quality monitoring data from in-stream sampling was analyzed to determine the magnitude of measured concentration change (minimum detectable change, MDC) in total phosphorus (TP) and or-thophosphate-phosphorus (OP) required to indicate that the change is real. High variability in the hydrologic system contributed to a high MDC. The impact of adjustments for precipitation, seasonality, upstream concentrations, and groundwater levels on reducing the MDC were investigated. The MDC for total phosphorus ranges from 10 to 59 percent over nine years of monitoring after adjustments for available covariates. MDC was found to be a function of subwatershed size and variability in covariates such as antecedent precipitation, groundwater levels, season, and upstream concentrations. The RCWP land treatment in the watershed emphasizes stream protection, animal waste management, vegetative cover, and grazing land protection. A significant decreasing trend for total phosphorus in three subwatersheds and at the outflow from the project area was found. These trends appear to be related to RCWP land treatment and to dairy closures.