Phosphorous Sorbing Material (PSM) Filters Effective At Reducing Phosphorus Loading Rates From Agricultural Ditches.

Nutrient loading continues to be a water quality issue in the Chesapeake Bay and current practices are not sufficient to reduce phosphorus (P) additions from agricultural sites to local waterways. This study was conducted to determine whether previously lab-tested phosphorus sorbing materials (PSMs) could be effective at P removal in field environments. Two different PSMs, gypsum and steel slag, were each tested at three different sites on the Eastern Shore of Maryland for a two year period. These filters were placed into agricultural ditches and flow control structures were used to limit the amount of water bypassing the filter. Water samples were taken before entering and after leaving the filter and the amount of water flowing through and bypassing the filter was quantified. Over the two year period, the gypsum filters performed with an average system efficiency of 20%, while the slag filters averaged 35%. In addition to the overall efficiencies, we were able to determine which storm event conditions allowed each type of filter to reach its highest removal potential. The gypsum filters removed up to 50% of the incoming P when water percolated through the filter at a slow, constant rate. The steel slag filters could remove up to 65% P when frequent heavy storm events caused water to flow quickly through the filters and recede, allowing the filter to dry. Both the PSM filter types were effective in removing significant loads of P that would otherwise have traveled to the Chesapeake Bay. Large scale implementation of these types of filters can reduce the impact of agriculture on nutrient pollution.