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ALTERNATIVE MONITORING STRATEGIES FOR ESTIMATING PHOSPHORUS CONCENTRATIONS AND LOADINGS FOR A TMDL
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.orgCitation: Pp. 429-434 in Total Maximum Daily Load (TMDL) Environmental Regulations: Proceedings of the March 11-13, 2002 Conference, (Fort Worth, Texas, USA) 701P0102.
Authors: Anne McFarland, Larry Hauck
Keywords: Water quality monitoring, TMDL, monitoring strategies, sampling, NPS pollution
Data collection efforts generally represent a compromise between statistical confidence and available resources. Defining an appropriate data collection strategy for monitoring TMDL implementation involving nonpoint source contributions is, thus, a formidable task. Our objective was to define the sampling needed to adequately assess phosphorus (P) concentrations and loadings on an annual basis for sites along the North Bosque River in central Texas. Two TMDLs were conducted for segments on the North Bosque River to decrease soluble reactive P loadings and concentrations by about 50 percent. Our evaluation involved subsampling intensive storm and base flow monitoring data collected between 1995 and 1999. Preliminary results using routine sampling clearly show an increase in statistical confidence for annual average concentrations with increasing sampling frequency from quarterly to monthly to biweekly. Average annual P and TSS loadings were generally underestimated by biweekly sampling, using an integration approach, as compared to ‘true” loads as estimated from a full data set of storm and grab samples. Assuming that only a limited number of storm events could be monitored per year, storm sampling strategies evaluated included intensive monitoring of the first four storm events in a year or monitoring the first storm event in each quarter of a year. Flow-weighted event mean concentrations for storm events were associated with periods of elevated flow for loading calculations in conjunction with biweekly grab data representing base flow conditions. Intensive monitoring of the first four storm events in a given year appeared to give the most promising results for annual loading calculations for sites within the North Bosque River watershed. For sites where P or TSS is transported predominately by nonpoint source runoff, storm sampling using automated samplers is recommended to complement routine sampling.