From Ground Water Currents, July 1995, Issue No. 12

Natural Attenuation of Hexavalent Chromium

By Robert W. Puls, EPA National Risk Management Research Laboratory, Ada, Oklahoma

Recent research findings support that there is natural attenuation of hexavalent chromium in ground water and soil. Chromium is an important industrial metal used in diverse products and processes. At many locations chromium has been released to the environment via leakage, poor storage or improper disposal practices. Concerns about the impact of chromium on human health and the environment require an evaluation of the potential risk of chromium entering the ground water flow system and being transported beyond compliance boundaries. At such sites where such potential exists, active remedial measures such as excavation or pump-and-treat have been undertaken. Experience at sites where pump-and-treat remediation of chromium-contaminated ground water is currently under way suggests that, although it is feasible to remove high levels of chromium from the subsurface, as concentrations decrease it becomes more difficult to remove the remaining chromium. While several new remedial technologies are being investigated, there is still concern about the cost of such remediation technology; and, at many sites, there is a debate about the need for expensive remediation.

Researchers have identified natural reductants that can trans-form the more toxic hexavalent form of chromium [Cr(VI)] to the less toxic trivalent form [Cr(III)]. Under alkaline to slightly acidic conditions, this Cr(III) precipitates as a fairly insoluble hydroxide, thereby immobilizing it within the soil. Such natural attenuation of hexavalent chromium is of great interest because it suggests that strict water quality standards do not have to be attained everywhere within and beneath the site. If natural attenuation does occur, pump-and-treat remediation could desist after the most contaminated ground water has been removed, even if the maximum contaminant level (MCL) has not been achieved. Under certain circumstances, expensive remedial measures may not even be necessary. However, before the natural attenuation option is adopted, it should be demonstrated that natural attenuation is likely to occur under the specific conditions at the site being investigated.

If natural attenuation is to be considered a viable option, then ideally it must be demonstrated that: (1) there are natural reductants present within the aquifer; (2) the amount of Cr(VI) and other reactive constituents does not exceed the capacity of the aquifer to reduce them; (3) the time scale required to achieve the reduction of Cr(VI) to the target concentration is less than the time scale for the transport of the aqueous Cr(VI) from source area to the point of compliance; (4) the Cr(III) will remain immobile; and (5) there is no net oxidation of Cr(III) to Cr(VI). The most difficult types of information to obtain are the time scales for the reduction and oxidation of chromium in the soil.

Demonstrating Cr(VI) reduction in aquifer by mass balances that rely primarily on the aqueous concentrations from monitoring well networks are valid only if it is demonstrated that Cr(VI) precipitates are not forming in the aquifer. The monitoring network must be sufficiently dense that estimates of Cr(VI) are accurate.

In the research findings, several soil tests are described that are useful in determining the mass of Cr(VI) and Cr(III) in the source areas and the reduction and oxidation capacities of the aquifer materials. Some simple conceptual models are presented whereby this information, combined with knowledge of the residence time of the chromium between the source and the point of compliance, can be used to determine the feasibility of natural attenuation of Cr(VI). The major limitation to this approach is the lack of information about the rate of oxidation and reduction of chromium under conditions likely to be encountered by plumes emanating from chromium sources. Without better information about these rate processes under a wider range of conditions with respect to pH, the use of the natural attenuation option for contaminated soils will continue to be a highly debated issue.

The findings are presented in GROUND WATER ISSUE; NATURAL ATTENUATION OF HEXAVALENT CHROMIUM IN GROUND WATER AND SOILS (Document No. EPA/540/S-94/505). The document can be ordered from the EPA Center for Environmental Research Information (CERI) by calling 513-569-7562.

For any additional information, call Bob Puls at EPAs National Risk Management Research Laboratory at 405-436-8543.