Formatted Title

Effect on Cleanup Timeframe from Heterogeneity, Back-Diffusion, and Abiotic/Biotic Degradation 

Background/Objectives

Monitoring data collected in over 100 wells over the past 35 years at a CERCLA site in Palm Bay, Florida, have shown tailing with TCE, cis-1,2-DCE, and vinyl chloride concentrations still remaining above cleanup standards in 2023 both near the former source area and within the downgradient plume. For the naturally anaerobic aquifer at this site, both native dehaloccoides (DHC) and plume concentration data collected over time indicated cycling of DHC levels and plume concentrations in some monitor wells.  For plume transport in heterogeneous aquifers in which reactive mixing is limited, the standard ADE can over-estimate degradation rates and fail to capture the frequently observed long tails of contaminant concentration versus time leading to underestimated cleanup timeframes. The objective of this paper is to apply an extended ADE model (HET-TRANS) to examine the sensitivity of key model input parameters (mobile/immobile zone degradation rates, log K variance, immobile zone volume fraction, back-diffusion power law exponent) on the estimated cleanup timeframe at a CERCLA site in Palm Bay, Florida. 

Approach/Activities

Building on the success of the continuous time random walk (CTRW) modeling framework in capturing observed tailing in many field and laboratory studies, this paper applies an extended ADE model (HET-TRANS) to examine the variability of the cleanup time at key monitoring wells within the plume. This extended-ADE model simulates heterogeneous advection, sorption, matrix diffusion, and sequential first-order reactions of both parent and degradation products of chemicals including chlorinated VOCs and PFOA. Sensitivity calculations were performed to examine the relative importance of log hydraulic conductivity variance, mobile zone degradation rate, immobile zone degradation rate, and immobile volume fraction on the cleanup time frame.  Consistent with new TCE, cis-1,2-DCE and VC data collected in 2023, model simulations of both mobile and immobile zone concentration levels were also performed to examine spatial zones where back-diffusion is likely to remain important in the future.   

Results/Lessons Learned

Results of the study provided confirmation that standard ADE models, which assume homogeneous media, can under-predict the cleanup time frame. Sensitivity analyses showed that increased heterogeneity in mobile (higher log hydraulic conductivity variance) and immobile zones (lower power law exponent for back-diffusion) cause increased tailing leading to a longer cleanup timeframe.  Sensitivity analyses also showed that the immobile zone degradation rate has an important effect on plume tailing, which can especially be important for slowly degrading compounds such as PFOA, 1,4-dioxane, and chlorinated solvents with low cleanup standards.   As expected, model simulations showed that a higher immobile zone volume fraction will cause a longer cleanup time frame because of greater storage of contaminants in immobile zones.  For each of these key model input parameters, spatial plots of concentration versus distance from the source indicated spatial zones where back-diffusion may continue to control the cleanup timeframe.  The model simulations were also useful in estimating site-specific, field-scale values for the mobile-immobile mass transfer (trapping) rate coefficient, which is difficult to measure in the field.