Characterization of Compost Leaching Caused By Intermittent Rainstorms.

Stormwater runoff from impervious surfaces such as rooftops, parking lots, and roadways can pose a threat to receiving waters through transport of dissolved and particulate contaminants. Bioretention systems are natural systems that rely on vegetation and mixtures of soil, sand, and compost to filter stormwater runoff by removing contaminants and modulating discharge. Studies have shown, however, that bioretention systems can leach metals and nutrients, and compost is thought to be responsible for this leaching. In order to safely implement bioretention systems for stormwater management, it is crucial to determine the composition of compost leachate. We quantified and characterized the leachate composition of compost following intermittent storm events. Columns of 6- and 24-months-old compost were constructed in triplicate, and irrigated with deionized water to simulate a 6-month, 24-hour storm in the Seattle-Tacoma region. Each sample was analyzed for water quality parameters including pH, EC, particulate concentration, dissolved organic carbon, and surface tension. In addition to these parameters, composite samples from selected storms were analyzed for total Kjeldahl nitrogen, nitrate/nitrite, total and ortho phosphorus, and total and dissolved copper concentrations. Results indicate a decreasing trend in water quality parameters over the course of individual storms. Results also indicate a decreasing trend in nutrient and copper concentrations in the leachate over the course of repeated storms. A mass balance of copper indicates that less than 1% of the original copper content leached out after eight storm events, but nonetheless, Cu concentrations exceeded water quality standards.