Migration of Contaminated Ground Water Under Control at High-Priority Cleanup Sites

The EPA Superfund and Resource Conservation and Recovery Act (RCRA) Programs conduct a number of activities to address the nations most severely contaminated lands. The programs investigate and collect data on potentially contaminated sites to determine whether they are contaminated and require cleanup. When a potentially hazardous waste site is reported to EPA, trained inspectors determine whether the site presents a hazard to human health and the environment. Sites that pose the greatest threat are placed on the National Priorities List (NPL) or RCRA Cleanup Baseline. For RCRA, "sites" are more commonly referred to as RCRA Corrective Action "facilities."

One of the priorities for both the NPL and RCRA Cleanup Baseline sites is preventing the continued spread of contaminated ground water, often referred to as plumes of contaminated ground water. Protecting the ground water is especially important in areas where it is the primary source for drinking water and irrigation, or a potential source for future water supplies.

EPA and state officials determine that the migration of contaminated ground water is under control (i.e., not continuing to spread in concentrations above levels of concern) when ongoing monitoring shows that the contaminant plume is not expanding or negatively impacting surface waters (U.S. EPA, 1999, 2004, 2012, 2015c). Further migration of contaminated ground water may be prevented by an action taken, such as installation of a pump-and-treat or subsurface barrier system, or by natural attenuation of the contaminants. A determination of whether migration has been prevented is based on monitoring data (usually from hundreds of analytical samples) collected from ground water wells within and surrounding the spatial extent of the ground water plume (U.S. EPA, 1999, 2004, 2012, 2015c).

This indicator describes the percentage of NPL Indicator Baseline sites and RCRA Cleanup Baseline facilities where government officials have determined that contaminated ground water is not continuing to spread in concentrations above levels of concern (e.g., that exceed the appropriate drinking water standards). This indicator covers NPL Indicator Baseline sites, and all 3,779 RCRA Cleanup Baseline facilities. The percentage of sites where ground water contamination continues to spread is also noted, as well as the number of sites where there are insufficient data to make a finding. The intention of the indicator is not to capture an action or administrative determination on the part of EPA, but to convey the underlying pressure on the environment and potential for human health effects resulting from contaminated ground water. Between 2002 and 2015, the number of NPL Indicator Baseline facilities increased by 18 percent (from 1,494 to 1,768) and the number of RCRA facilities tracked by EPA as the Cleanup Baseline increased by 120 percent (from 1,714 to 3,779). Changes in the RCRA baseline are programmatic determinations and do not necessarily reflect changes in the condition of the environment.

Of the RCRA Cleanup Baseline facilities, the percentage of facilities where contaminated ground water has been determined to be under control increased from 32 percent (554 out of 1,714 facilities) in fiscal year (FY) 2000 to 82 percent (3,099 out of 3,779 facilities) in FY 2015 (Exhibit 1). This increase represents a combination of facilities where mitigation has halted the spread of contaminated ground water and facilities where data are sufficient to show that contaminated ground water migration was not continuing, regardless of mitigation activities. The percentage of facilities where officials have determined that ground water that is contaminated above levels of concern is spreading beyond the existing area of contamination decreased from 18 percent (306 out of 1,714 facilities) in FY 2000 to 1.6 percent (59 out of 3,779 facilities) in FY 2015. These facilities, and the remaining 621 facilities for which data are still insufficient for a determination at the end of FY 2015, tend to be very complex facilities where the appropriate data have yet to be collected due to high costs or technical difficulties.

Ground water has not been an issue at all Superfund NPL sites. Of those NPL Indicator Baseline sites where ground water contamination is present, the percentage where contaminated ground water has been determined to be under control increased from 61 percent (772 of 1,275 sites) in FY 2002 to 70 percent (1,138 of 1,618 sites) in FY 2015 (Exhibit 2). As of the end of FY 2015, ground water that is contaminated above levels of concern is spreading beyond the existing area of contamination at 12 percent (188) of these NPL sites, while the remaining 18 percent (292 sites) had insufficient data to confirm whether contaminated ground water is spreading above levels of concern. These percentages do not include NPL Indicator Baseline sites classified as non-ground water sites.

• The NPL does not represent all of the contaminated or potentially contaminated sites listed in the Comprehensive Environmental Response, Compensation, and Liability Information System (CERCLIS) database, which contains information on thousands of hazardous waste sites, potential hazardous waste sites, and remedial activities across the nation. 

• The indicator results are presented for the number of RCRA Cleanup Baseline facilities tracked each year, not the entire group of about 6,000 hazardous waste management facilities that may be subject to RCRA Corrective Action requirements (e.g., initial assessments and, if needed, more thorough investigations and cleanups). The extent to which people have been affected, or could be affected, by the contaminated ground water at NPL or RCRA Cleanup Baseline facilities is not considered in this indicator, but is addressed in the Current Human Exposures Under Control at High-Priority Cleanup Sites indicator.

• The indicator does not address ground water contaminated at other types of sites, such as sites with leaking underground storage tanks and other sites being addressed solely by state cleanup programs.

• Concentrations of toxic and hazardous contaminants in ground water that must not be exceeded to designate a site as under control vary somewhat from state to state, though they fall within a range determined to be acceptable to EPA (U.S. EPA, 2004).

• This indicator is based on the certification by a responsible official that the criteria necessary to designate whether contaminated ground water is continuing to spread above levels of concern have been met (U.S. EPA, 1999, 2004). Trends in the number of sites where the spread of contaminated ground water has been shown to occur above levels of concern may be underestimated to the extent that certification lags behind the migration of contaminated ground water or certification is delayed due to insufficient or outdated information.

Data for this indicator were provided by EPA's Office of Land and Emergency management (OLEM) (U.S. EPA, 2015a,b). A list showing the current status of every RCRA baseline site is published online (U.S. EPA, 2014). A summary of the status of Superfund NPL sites is available online; information on the current status of any individual NPL site can be queried using EPA's CERCLIS database (U.S. EPA, 2016).