Grass and Leaf Decomposition and Nutrient Release Study under Wet Conditions

A significant source of nutrient input to water bodies is from grass clippings and leaves (yard debris) washed into drainage systems during storms. Brevard County Surface Water Improvement conducted a study to determine the nutrient release rates from grass clippings and leaves in order to better understand the chemistry and resultant pollutant loading mechanisms. Sixty-gram samples of mixed freshly cut St. Augustine yard grass (Stenolaphrum secundalum) and oak leaves (Quercus sp.) were placed into opaque containers. Coarsely filtered storm/ditch water was added to fill the containers to the 8-liter marks. Samples were allowed to go anaerobic, typical of wet sump best management practice (BMP) structure conditions, and tested periodically after soaking and processing. At intervals of: 0, 1, 5, 9, 14, 22, 34, 50, 70, 130, and 180 days, triplicate bucket sets were agitated to simulate mixing from stormwater influx, then poured through a Number 35, US Standard Soil Sieve, and the liquid analyzed. The solids that remained in or on the sieve were analyzed, and the results compared to those of the corresponding liquid phase. The results presented depict "typical" east-central Florida lawn and leaf litter decomposition and nutrient release rates. This information may be useful in the selection or site design of BMP's for treating nutrients in stormwater, and determining cleaning frequency.