TCE Pump-and-Treat Rebound and MNA Decision
A leaking underground storage tank released trichloroethylene (TCE) into a sandy aquifer. The dissolved plume is about 1,800 ft long, groundwater velocity is 0.6 ft/day, and the source-area concentration is 500 µg/L; the MCL is 5 µg/L. A pump-and-treat system extracts 25 gpm and treats with granular activated carbon. After 3 years, the extraction well is at 40 µg/L and a downgradient well at 1,200 ft is at 25 µg/L, but a 2-week shutdown caused the extraction well to rebound to 180 µg/L. Annual O&M is $600,000. The nearest private drinking-water well is 2,000 ft downgradient and is not slated for replacement. The plume appears stable based on three annual synoptic gauging events. The responsible party proposes shutting down pump-and-treat and switching to monitored natural attenuation (MNA).
1. Explain what the concentration rebound and long tailing indicate about the TCE distribution and the likely performance of continued pump-and-treat.
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The rebound from 40 µg/L to 180 µg/L after only 2 weeks of shutdown, together with the long tailing at the extraction well, indicates that most of the TCE mass is not in the mobile dissolved phase. TCE has diffused into lower-permeability zones and is sorbed to aquifer solids. Pumping removes dissolved TCE mainly from higher-permeability flow paths, producing cleaner water at the extraction well, but the concentration rebounds as TCE back-diffuses from the low-permeability zones.
Continued pump-and-treat can provide containment, but it will likely take decades to reach the 5 µg/L MCL and may approach an asymptotic concentration above the MCL. The 200 kg source mass is not removed efficiently by water extraction alone, and the $600,000 annual O&M cost is difficult to justify without a clear endpoint. Pump-and-treat should be evaluated as a containment or plume-control measure, not assumed to be a complete aquifer restoration technology at this site.
2. Is MNA appropriate here? Compare MNA with continued P&T and with source-zone treatment, considering the 2,000-ft receptor and plume stability.
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MNA may be considered because the plume appears stable and the nearest private well is 2,000 ft downgradient, but it is not automatically appropriate. At 0.6 ft/day, groundwater travel from the plume to the receptor is roughly 2,000 ÷ 0.6 ≈ 3,300 days, or about 9 years. That is enough time for monitoring to detect a problem, but only if the plume is truly stable and natural attenuation is demonstrated. TCE at 25 µg/L at 1,200 ft is still five times the MCL, so the receptor could be at risk if the plume expands or if degradation is weak.
Continued P&T provides active containment but is costly and shows rebound. Source-zone treatment, such as in-situ chemical oxidation, zero-valent iron, or enhanced bioremediation, could reduce the mass causing the rebound and shorten the cleanup timeframe. A defensible approach is often a hybrid: treat or contain the source, then use MNA with sentinel wells and a contingency to restart active remediation. Shutting down P&T and relying on MNA alone requires strong evidence of plume stability, natural attenuation capacity, and no unacceptable receptor risk.
3. Recommend a monitoring and decision framework for the proposed MNA shutdown, including specific triggers for restarting active remediation.
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Before shutdown, establish a baseline with at least three to four quarterly synoptic sampling events. Monitor TCE and degradation products (cis-1,2-dichloroethene and vinyl chloride), redox indicators, dissolved oxygen, pH, alkalinity, and water levels in wells along the plume axis and at a sentinel location between the plume and the 2,000-ft receptor. Use trend analysis, not a single sample, to evaluate plume stability.
Decision criteria should include: no expanding plume boundary, no increasing TCE trend at the 1,200-ft well, no detection above 5 µg/L at the sentinel well, and no accumulation of vinyl chloride above its risk-based level. Specific restart triggers could be a TCE concentration above 5 µg/L at the sentinel well, a statistically significant increasing trend at any downgradient compliance well, plume length increasing by more than 10%, or detection at the private well. Institutional controls should prevent new drinking-water wells in the plume area. Review the monitoring data annually and require a contingency plan that can restart P&T or implement source treatment if triggers are met.