Natural Attenuation of Chlorinated Solvent Ground-Water Plumes Discharging into Wetlands

WRD PROJECT #: MD194
PROJECT CHIEF: Lorah, Michelle
BEGIN DATE: 01-January- 1999
END DATE: 31-December- 2001

Customers currently supporting the project:

U.S. Department of Defense, Environmental Security Technology Certification Program (ESTCP)

Problem

A significant number of DoD chlorinated solvent groundwater plumes discharge into ecologically sensitive wetland environments. Conventional cleanup of groundwater contaminated with chlorinated solvents (e.g., TCE) can be extremely costly and/or detrimental to wetlands due to dewatering. Chlorinated solvents tend to be relatively resistant to degradation within most natural aquifer systems, however, substantial beneficial transformations can occur within the highly reducing environment of wetland sediments - suggesting that natural attenuation may be a viable, cost-effective option for discharges into wetlands. A number of questions remain unanswered: What processes are relevant and require monitoring to assure adequate attenuation? What methodologies should be used for monitoring? What level of contaminant reduction is acceptable?

In aquifers, TCE and other chlorinated solvents tend to be relatively resistant to transformations, either biotic or abiotic, compared to the biodegradation potential of petroleum hydrocarbons. This has led to the development of long chlorinated solvent plumes at a number of DoD facilities which have reached surface water discharge points such as wetlands. Wetlands are extremely important ecosystems, being habitat for one third of the species listed as threatened or endangered (U.S. Fish and Wildlife Service, 1990) giving rise to heightened regulatory concern. Exposure of the chlorinated solvents to threatened species may be significant once the groundwater-born contaminants cross the sediment/surface water interface. Traditional pump-and-treat remediation could destroy wetland ecosystems by dewatering and could also be prohibitively expensive because of the potential to pump surface water in addition to contaminated groundwater. Costly, and potentially damaging, engineered remedial interventions may be avoided if sufficient natural attenuation of the dissolved chlorinated solvents occurs within the reduced organic carbon-rich wetland sediment zone prior to discharge into the surface water of the wetlands. To illustrate the practical importance of this approach, U.S. Army Aberdeen Proving Grounds (APG), MD is pursuing a Record of Decision (ROD) with natural attenuation as the remediation method for a chlorinated solvent plume discharging into a wetlands (M.M. Lorah, USGS, personal communication).

A draft protocol for natural attenuation of chlorinated solvents has recently been prepared by AFCEE (Wiedemeier, 1996). It was recognized in this protocol that `For sites where contaminated groundwater discharges to surface water, the philosophy of monitoring is not well developed.' To date, the most rigorous study on the natural attenuation of chlorinated solvent plumes entering a freshwater wetlands has been conducted by USGS at APG (Lorah et al., 1997). Our knowledge of the fate of chlorinated organic contaminants in wetlands has been greatly enhanced through the APG study, and much experience has been gained on methods to evaluate natural attenuation processes occurring in wetlands. The present study is an extension of the USGS wetlands research at APG.

Objectives

The central elements involved in the consideration of natural attenuation as a remedial action include: a) determination and documentation of operational natural attenuation processes; b) assessment of the level or extent of natural attenuation taking place, as well as its potential for future occurrence, relative to action levels. The basic approach taken by USGS to address these elements included installation of multilevel piezometer transects; determination of groundwater flow paths; determination of redox zones; and evaluation of transformation by determining levels of parent contaminants, reactive intermediates and non-toxic end products. The present study will include those elements and expands upon them by including:

1. the evaluation of additional sampling and monitoring methodologies for the assessment of natural attenuation in wetlands (under consideration is field analysis of volatile chlorinated organics by ion trap mass spectrometery, a cost-effect analytical methodology recently developed by U.S. Army - WES, in conjunction with direct sampling with a simple in situ purging device which minimizes losses of extremely volatile components such as vinyl chloride, ethene and ethane):
2. examination of the natural attenuation processes at three DoD wetland chlorinated solvent discharge sites exhibiting a wide range of hydrogeologic and subsurface conditions;
3. differentiation of the biotic vs. abiotic transformation processes (based upon the recent research at AFRL on the role of humic complexes in reducing chlorinated solvents); and
4. development of a protocol for the assessment of chlorinated solvent natural attenuation at wetland sites.

Approach

• Select candidate sites and review available site information
• Review available data & literature concerning relevant chlorinated solvent fate processes
• Select crucial indicator parameters for natural attenuation (coordinate with AFCEE and AEC)
• Select three sites for study
• Select additional wetland site monitoring techniques for study
  • Complete initial site reconnaissance (including preliminary sample collection & analysis) and review existing data
  • Select multi-level piezometer transect locations at sites
  • Develop microcosms & other techniques for evaluating natural attenuation potential in the laboratory
  • Instrument sites with multi-level piezometers and monitoring points
• Complete first full sampling round
  • Evaluate ground-water flow paths and velocities
  • Evaluate direct sampling with in situ purging device & on site analysis with ion trap mass spectrometry
• Complete second full sampling round
  • Determine redox zones at sites
  • Complete third full sampling round
  • Assess biotic & abiotic components to observed transformations
  • Evaluate natural attenuation indicator parameters
  • Prepare final technical report
  • Prepare natural attenuation protocol for chlorinated solvent wetland discharge sites (coordinate with AFCEE and AEC)