Toxic Substances Hydrology Program
Organic Contaminants Stored on Sediments Can Slow Down Groundwater Restoration
The amount of organic material stored on an aquifer's sediments can slow down natural processes that are often relied upon to restore groundwater quality in a wastewater-contaminated aquifer. U.S. Geological Survey (USGS) scientists report in Chemical Geology that analysis of sediment cores is critical to obtaining reliable predictions of the rate of restoration. These predictions are often the basis for decisions about the use of monitored natural attenuation at contaminated sites.
This conclusion is supported by data from detailed sampling of water and sediment at a facility on Cape Cod, Massachusetts, where treated municipal wastewater was disposed to the ground for 60 years. In the 15 years since disposal was stopped, dissolved oxygen has not returned to pristine levels near the disposal site, even though conservative tracers of the wastewater such as chloride and boron have been flushed from the area. Laboratory measurements of oxygen demand and organic carbon and nitrogen concentrations were made on sediment cores collected from the disposal site and used in computer models of the restoration process. The results indicate that the return of dissolved oxygen to the aquifer, even near the disposal site, could take decades depending on the amount of biodegradable organic compounds from the past disposal that remain stored on the sediments.
The return of dissolved oxygen to the aquifer affects the fate and transport of many contaminants, such as nitrate, ammonium, dissolved iron, and some toxic metals, which are often present at sites contaminated by mixed wastes (for example, landfill leachate and wastewater). These sites include the several million septic systems in the United States. Reliable predictions of cleanup rates hinge on predictions of re-oxygenation, which in turn depend greatly on an accurate characterization of the adsorbed organic carbon and nitrogen compounds that are stored on the aquifer's sediment surfaces in the contaminated zone.
Smith, R.L., Repert, D.A., Barber, L.B., and LeBlanc, D.R., 2013, Long-term groundwater contamination after source removal--The role of sorbed carbon and nitrogen on the rate of reoxygenation of a treated-wastewater plume on Cape Cod, MA, USA: Chemical Geology, v. 337-338, p. 38-47, doi:10.1016/j.chemgeo.2012.11.007.