Metal mobilization from municipal biosolids stockpiles: The role of dissolved organic matter

This project involves field observations and laboratory experiments to assess the mobilization potential of trace metals from municipal biosolids stockpiles. We propose to study the concentration, speciation, and lability of trace metals released from the biosolids both in surface runoff and in groundwater leachate. Field observations include monitoring of metals concentrations in the solid-phase, and in the biosolids dissolved organic matter (DOM), at our field site in Maine. Laboratory experiments will be conducted using the biosolids DOM collected from the field to determine the stability and the electrochemical lability of trace metal-DOM complexes. Both field and laboratory studies will be done using the runoff and the leachate as a function of the age of the biosolids stockpile.

The loss of metals from biosolids stockpiles is of great concern in Maine because of the potential for contamination of surface and ground water, especially in rural areas where residents rely on groundwater for drinking water. Metals in biosolids are usually linked to organic and inorganic (oxides and clays) components. The models used for the development of the USEPA Part 503 rule to assess the risk of groundwater contamination by biosolids are based on simplistic assumptions with respect to metals migration into surface and ground waters. Metal mobility is greatly enhanced through binding with the biosolids dissolved organic matter (DOM), competition with NH4+ for adsorption sites, and dissolution of Fe and Mn (hydr)oxides by microbes under anaerobic conditions. Our preliminary field observations suggest significant dissolved metal concentrations that are complexed by the DOM in the runoff and leachate from the biosolids stockpiles. The conventional models do not account for biosolids DOM-facilitated transport, and as such, underestimate the potential for contamination of the surrounding water bodies by metals.
 

Information on the mobilization potential of metals from biosolids stockpiles is currently limited. The existing information is largely based on site-specific observations, and as such, may not be used as a basis for evaluating stockpiling risks and regulations. The results from this study will provide a fundamental understanding of metal mobilization potential, and the role of biosolids DOM in facilitated metal transport and stability. Our findings can, therefore, provide a basis for the assessment of likely risks to water quality posed by stockpiling biosolids, and for evaluating stockpiling rules under Maine 06-096 CMR Chapter 419 of the solid waste rules.