Toxic Substances Hydrology Program
Zinc and Phosphate Come Back to Haunt Aquifer
Contaminants such as zinc and phosphate can be released (desorbed) rapidly from contaminated sediments to groundwater, report a team of U.S. Geological Survey (USGS) scientists in Water Resources Research. Despite being “attached” (adsorbed) to the sediments for decades, the right change in chemical conditions can mobilize the contaminants, allowing them to enter and move with groundwater. These results are part of a long-term study of the fate of contaminants in a subsurface plume of treated wastewater at a Cape Cod, Massachusetts, research site.
The plume being studied by the scientists has an overabundance of zinc and phosphate that is adsorbed to subsurface sediments. This large reservoir of zinc and phosphate can pose a long-term threat to groundwater quality. The scientists wanted to determine the ability of these adsorbed contaminants to be released back into groundwater in response to changing chemical conditions (such as pH) caused by the cessation of release of the wastewater. The findings are applicable to other scenarios that can cause a change in chemical conditions, such as contaminant cleanup activities, or a change in land use.
The scientists conducted groundwater tracer experiments in which they injected a solution of chemical tracers into the subsurface. The solution was designed to create a small subsurface zone, or plume, that had a more acidic pH. The scientists observed that as the plume moved through the subsurface, zinc and phosphate were rapidly released from the subsurface sediments despite having been adsorbed to the sediments for many decades. These results can help environmental professionals, land managers, and regulators develop sound policies to protect the quality of groundwater.
Kent, D.B., Wilkie, J.A., and Davis, J.A., 2007, Modeling the movement of a pH perturbation and its impact on adsorbed zinc and phosphate in a wastewater-contaminated aquifer: Water Resources Research, v. 43, no. 7, W07440, doi:10.1029/2005WR004841.