United States Department of Transportation
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Experimental evaluation of compost leachates.
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2015-09-01
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Abstract:Compost is often used in raingardens, 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. We quantified and characterized the leachate composition
of compost following intermittent, simulated storm events. We 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.
Our 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. Our 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 our 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.
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