Natural Attenuation Strategy for Groundwater Cleanup Focuses on Demonstrating Cause and Effect

In the 20 years since cleanup of contaminated groundwater has been a high priority in the United States, recognition of both the scope of the problem and the technical difficulties involved has grown steadily. Estimates of the number of hazardous waste sites where groundwater may be contaminated vary between 300,000 and 400,000 nationwide [NRC, 1994]. Legislation passed in the 1980s by Congress and the states generally required that groundwater in contaminated aquifers be restored to background or drinking water standards. Unfortunately, attempts to meet these goals using conventional methods, such as pump and treat systems, frequently have been unsuccessful [NRC, 1994]. Moreover, constructing and operating engineered groundwater cleanup systems is very expensive, with total spending on environmental remediation in the United States estimated at $9 billion in 1996 alone [NRC, 1997]. Over the last 5 years, the high costs of engineered cleanup systems and their disappointing performance has spurred both a search for alternative remediation strategies and rethinking of remediation goals and time frames. In this climate, interest in the capabilities of a remediation strategy known as monitored natural attenuation has skyrocketed. The Environmental Protection Agency (EPA) defines natural attenuation as "a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants" [EPA, 1999] (Figure 1). Use of monitored natural attenuation as a remediation strategy involves filing a formal regulatory application to allow natural biological, chemical, and physical processes to treat groundwater contaminants, and conducting ongoing monitoring to verify that these processes are effective. In many cases, the feasibility of a natural attenuation strategy depends on whether the regulatory goal is to clean up the contaminant plume to drinking water standards or whether a less stringent,