Leaching of nutrients and copper from compost in bioretention systems by intermittent rainstorms

Jessica M. Mullane

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 engineered 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 contribute to this leaching. In order to safely implement bioretention systems for stormwater management, it is crucial to determine the composition of compost leachate. This thesis reports on the leaching of chemical constituents from compost following intermittent rainstorms. The specific objectives of this thesis were to (1) characterize the chemical properties of compost leachate during a single rainstorm as a function of time, (2) characterize the chemical properties of compost leachate during sequential rainstorms, and (3) compare leachate from mature composts of two different ages. 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, electrical conductivity (EC), particulate concentration, dissolved organic carbon (DOC), 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 EC, DOC, nitrogen, phosphorus, and copper in the leachate following successive storms. Each new storm event caused an initial peak in EC and DOC concentrations in the leachate. A mass balance of copper indicates that less than 3% of the original copper content from both composts leached out after nine storm events. Nonetheless, copper concentrations exceeded regulatory discharge standards. Although not always significant, the 6-months compost leached higher amounts of constituents, including dissolved organic carbon and copper, than the 24-months compost. The major results of this thesis show that compost serves as a sustained source of leaching, regardless of age.

Leaching of nutrients and copper from compost in bioretention systems by intermittent rainstorms

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