Experimental evaluation of compost leachates.

Compost is often used in rain gardens, roadsides, and bioretention systems, not only because of its beneficial properties on soil quality, but also because compost improves water infiltration and retains stormwater contaminants. However, when compost is freshly applied, materials from compost can leach out when rain or stormwater seep through compost or compost-amended bioretention media. The goal of this research project was to experimentally evaluate and characterize leachates from compost. The authors quantified and characterized the leachate composition of compost following intermittent, simulated storm events. The authors used municipal compost of different ages and different source materials. Compost was filled into columns and then irrigated with deionized water and stormwater. Stormwater was spiked with copper to increase copper concentrations. Six-month, 24-hour rain storms were applied every two weeks for a total of up to 18 rain storms. Outflow was analyzed for pH, electrical conductivity (EC), particulate concentration, surface tension, dissolved organic carbon (DOC), nitrogen, phosphorus, and copper. Particulate and dissolved organic matter was further characterized for molecular structure by Nuclear Magnetic Resonance (NMR) and Fourier-Transformed Infrared (FTIR) Spectroscopy. The results showed that concentrations of dissolved and particulate organic carbon and electrical conductivity in the leachate where high at the onset of each storm, but decreased as the storm progressed. However, each new storm released another peak of constituents. Leaching from mature compost was less a function of age and type, but more a function of number and frequency of storms and leaching history. Concentrations of nitrogen, phosphorus, and copper were high in the initial few storms, and then decreased. Only a small fraction (3%) of the copper present in the compost leached out, but nonetheless, the copper concentrations in the leachate remained above the regulatory waste discharge standards. NMR and FTIR analyses revealed that dissolved organic carbon consisted mainly of aliphatic and aromatic components typical of fulvic and humic acids. The results show that municipal compost can serve as a sustained source of leaching of nutrients and metals, regardless of compost age and source. However, the copper concentrations observed in the experiment are likely not toxic due to the presence of elevated DOC, as DOC readily forms complexes with dissolved copper thereby reducing its toxicity to aquatic organisms. The DOC plays a dual role in terms of copper fate and transport: DOC can enhance leaching of copper by forming aqueous complexes, but at the same time, reduces toxicity of copper because copper-DOC complexes are less bioavailable.