Decades Required for Natural Processes to Clean Wastewater-Contaminated Ground Water

These infiltration beds on Cape Cod, MA, were used for 60 years to dispose of wastewater from a sewage treatment plant. In 1995 their use was discontinued and a large plume of contaminated ground water was allowed to begin to naturally restore itself. USGS investigations of the natural restoration have estimated that it will take decades for the ground water to return to pristine conditions.
(Larger version)

The natural restoration of ground water contaminated by a plume of wastewater on Cape Cod, Massachusetts, is predicted to take decades after the 1995 discontinuation of the release of wastewater to the subsurface, according to a team of U.S. Geological Survey (USGS) scientists. The team has been studying the fate and transport of wastewater in ground water at a research site on the Massachusetts Military Reservation (MMR), Cape Cod. For more than 60 years treated wastewater from a sewage treatment plant was discharged to rapid infiltration beds. This practice created a plume of wastewater more than 6 kilometers (approximately 4 miles) long that contained sewage derived compounds that the treatment process did not remove. After the disposal of wastewater was discontinued in 1995 the major question was: how long will it take natural processes to clean the contaminated ground water?

The Question: How Long Will it Take Natural Processes to Clean the Contaminated Ground Water?

Monitored natural attenuation – using Nature’s natural restorative properties to cleanse ground water ­– is an increasingly common choice for the restoration of contaminated ground water. In practice, however, regulatory requirements for the use of monitored natural attenuation as a cleanup option often limit water-quality monitoring to a small set of toxic constituents. As a result, little information is available regarding the natural restoration of all the chemical constituents of wastewater contaminant plumes following the removal of the contaminant source. The timing and sequence of such a restoration are unknown, their prediction uncertain, and subject to speculation.

Natural Restoration Status

The wastewater contaminant plume on the MMR is comprised of many dissolved chemicals. Each of these chemicals interacts with aquifer sediments (adsorption to sediments for example) and with each other to different degrees. The more mobile chemicals move along at close to the rate of movement of the ground water, while other less mobile chemicals interact and lag well behind the flowing ground water. As of 2004, uncontaminated ground water has flushed the trailing edge of the wastewater's more mobile dissolved constituents (boron, for example) more than 0.6 kilometers (approximately 0.4 miles) downgradient from the infiltration beds (see plume cross sections). The concentrations of nutrients, such as dissolved nitrogen have diminished substantially, but nutrients have not moved nearly as far away from the infiltration beds as the mobile constituents. It appears that the aquifer's sediments have adsorbed a large amount of material from the wastewater over time, and ground water flowing through these sediments is leaching out less mobile wastewater constituents. The result of this process is that the "reservoir" of wastewater material in the sediments is serving as a continuing source of dissolved constituents (such as nitrogen), which contributes to the long-term persistence of the overall plume. Consequently, dissolved oxygen concentrations in the plume have remained virtually unchanged due to biological degradation of wastewater constituents by subsurface microorganisms beneath the infiltration beds. The oxygen in ground water flowing into the reservoir of adsorbed wastewater constituents from upgradient locations is quickly used up by microorganisms that are degrading the material leaching out of the sediments (see plume cross sections).

Vertical cross sections along the longitudinal axis of the wastewater plume on Cape Cod, Massachusetts. Each cross section gives a snapshot of the concentration of dissolved oxygen (upper three, blue) and boron (lower three, green) during 1996, 2000, and 2004. Concentrations are given in micromolar (µM, micromoles/liter). These cross sections show that mobile constituents, such as boron, have been flushed away from the infiltration beds by fresh ground water (flow is from left to right) while dissolved oxygen concentrations have remained low. Black dots show the positions of well screens and multilevel-sampler ports. The cross sections are a modified version of figure 3 from Repert and others, 2006. (click image for a larger version)

Predictions and Trends

Based on a wealth of data collected at the site, USGS scientists have developed statistical extrapolations that predict the time needed for natural processes to clean wastewater-contaminated ground water. Using these predictive tools and the observed trends in the concentration of dissolved oxygen in the plume, the scientists predict a return to oxygen levels characteristic of pristine aquifers in the first 0.6 kilometers of the contaminant plume by 2021 to 2028, more than 30 years after the wastewater disposal practice was discontinued. In contrast, the more mobile constituents, such as boron, were flushed from the first 0.6 kilometers of the plume in less than 8 years.

Application

The results of this study have application to understanding the impacts of residential septic systems, water reclamation facilities that use treated wastewater, and the land application of wastewater from sewage treatment plants through infiltrations beds and spray irrigation fields. The predictive tools and the knowledge gained from this study will help resource managers:

• Understand the long-term impacts to ground water of disposing wastewater through septic systems and infiltration beds.
• Provide better estimates of how long the natural restoration of ground water impacted by wastewater could take.
• Develop sound policies regarding ground-water protection, best practices for septic systems, and wastewater disposal and reuse.

Reference

• Repert, D.A., Barber, L.B., Hess, K.M., Keefe, S.H., Kent, D.B., LeBlanc, D.R., and Smith, R.L., 2006, Long-term natural attenuation of carbon and nitrogen within a groundwater plume after removal of the treated wastewater source: Environmental Science and Technology, v. 40, no. 4, p. 1154-1162.