The requirement for sustainable Monitored Natural Attenuation (MNA) is that energy and material inputs be available for efficient and consistent performance over the life-cycle of a project. Currently, the effectiveness of MNA at chlorinated solvent sites is typically assessed over short time periods (several years). However, when MNA becomes part of a long-term remediation strategy, it is assumed that natural attenuation processes documented during site assessment will continue over the operational life of the site. Regulators, however, are reluctant to approve such MNA-based remediation strategies in the absence of direct evidence that MNA will be sustained over the life-cycle of the site.  This project demonstrated and verified an approach to assessing MNA sustainability as applied to chlorinated solvents in groundwater, particularly at sites where microbially-mediated reductive dechlorination is the primary attenuation mechanism.

Technology Description

The MNA sustainability assessment technology consists of three components designed to answer specific questions (shown in italics):

  1. Determine the amount of potentially bioavailable organic carbon (PBOC) that is present in the aquifer sediment and overlying soils. Is there an adequate source of fuel for reductive dechlorination in the long term?
  2. Quantify the range in estimates for the contaminant source zone to deplete over time to acceptable levels. What is the required life-cycle for MNA at a given site and how long must MNA be effective and self-sustaining?
  3. Estimate the long-term trend of dissolved oxygen (DO) in groundwater using a site model calibrated to current site conditions. Will DO levels trend upward to the point at which microbially-mediated reductive dechlorination will not be sustained?

The technology may be applied at sites in which MNA has been adopted and approved as the primary remediation strategy for plume management or at a site where source zone remediation is planned and MNA is the projected follow-up remediation strategy. At a given site, the assumed starting point is that site characterization efforts and data analysis have proven that the proper environmental conditions for microbially-mediated reductive dechlorination exist in the groundwater and that ample evidence exists to confirm that the microbial community is reducing chlorinated compounds.

Demonstration Results

The quantitative performance objectives of this project were to either validate or demonstrate the three components of the MNA sustainability assessment:

  1. Validate a methodology for quantifying PBOC by establishing correlations with field-measured DO concentrations, concentrations of chlorinated volatile organic compounds (CVOCs), concentrations of natural organic carbon compounds present in aquifer sediment, and the rate and extent of reductive dechlorination.
  2. Verify the upscaled source zone depletion (SZD) function using site CVOC concentration data for a range of source zone geometries.
  3. Validate current site conditions (i.e., short-term sustainability) using PBOC and DO concentration data and demonstrate long-term sustainability using the SEAM3D model at a site where the SZD function was used to estimate source persistence.

PBOC was quantified using 168 samples collected at 17 chlorinated solvent sites representing a range of conditions, including sites where microbially-mediated reductive dechlorination was not active. Mean site values for PBOC in aquifer sediment ranged from 12.5 to 998 mg/kg. For the remaining samples, PBOC ranged from 549 mg/kg (soil) to 14,200 mg/kg (confining units). In addition, PBOC was examined using multiple samples at two of the study sites. Results showed a log-normal distribution to PBOC concentrations and variation with depth in the shallow aquifers. Samples inside chloroethene plumes had lower levels of PBOC than the background samples, indicating that carbon loss is measureable.

PBOC data were compared to redox indicator data, CVOC concentration data in groundwater derived from monitoring reports for the study sites, and other related laboratory data. The success criteria were achieved for the primary metric - demonstrating an inverse correlation between PBOC and DO data and a positive correlation between PBOC data and the natural attenuation capacities of chloroethene plumes. 

The SZD function was applied using historical monitoring data at three sites ranging in complexity and with limited data availability. The analytical model was implemented and verified at the site with the most extensive data. Model verification criteria could not be achieved at the other sites due to noise in the historical data and a lack of data over time.

Assessment of the long-term sustainability of the required level of DO was demonstrated at the study site where the distribution of PBOC was investigated. A site flow and transport model was calibrated to observed DO concentration data and success criteria were achieved (errors between observed and simulated DO < 0.25 mg/L). The long-term sustainability results indicated no significant change in DO levels over time but some increase could be expected over the 100-year evaluation period. For a scenario in which the background flux of oxygen increased by an order of magnitude due to a hypothetical change in land use, DO levels within the CVOC plume increase to levels that would prevent efficient reductive dechlorination. The sustainability assessment was easily implemented using the Groundwater Modeling System (GMS) software platform.

Implementation Issues

Costs for implementing the MNA sustainability assessment technology were estimated for each component. The quality and quantity of available data is an important consideration when implementing this technology at other sites. Verification of the SZD function showed that the nature and extent of long-term historic CVOC data will determine if the probability distributions of predicted outcomes can be quantified. In the event that the SZD analysis is problematic or produces an unacceptable level of uncertainty, the last component of this technology may be implemented using a reasonable life-cycle time estimate. Post-auditing of modeling results is recommended as new data are collected and evaluated, allowing revision to estimates of source depletion and MNA sustainability.