Chlorinated Solvent Plume: Is MNA Defensible?
Groundwater beneath a former dry cleaner contains PCE at 120 µg/L at the source well MW-1 and 35 µg/L at MW-3, 150 ft downgradient. Sentinel well MW-5, 400 ft downgradient, has shown 8-10 µg/L for three years. The state groundwater standard is 5 µg/L. The aquifer is silty sand with hydraulic conductivity 5 ft/day, gradient 0.004, porosity 0.30, bulk density 1.6 g/cm3, and a measured PCE retardation factor of 7.4. First-order attenuation estimated from three years of data gives a half-life of 2.5 years. A municipal well field lies 1,200 ft downgradient, but the city reports it is not currently pumping that well. The responsible party proposes monitored natural attenuation with semiannual sampling and no source treatment. The regulator asks for a technical justification.
1. Calculate the retarded groundwater velocity and use it with the half-life to evaluate whether the plume can reach the municipal well field above 5 µg/L. State your assumptions.
Read the worked answer
Seepage velocity is v = K i / porosity = (5 ft/day)(0.004)/0.30 = 0.067 ft/day, or about 24 ft/year. Retarded velocity is 24/7.4 = 3.3 ft/year. Travel 1,200 ft would take about 360 years. With a 2.5-year half-life, the decay factor over 360 years is exp[-(0.693/2.5)(360)], which is effectively zero. A simple calculation therefore suggests no exceedance at the well field. But the same model predicts near-zero PCE at MW-5 after about 120 years of travel, yet MW-5 has remained at 8-10 µg/L for three years. That inconsistency suggests a continuing source, preferential flow, or an overestimated attenuation rate. Assumptions include steady uniform flow, no dispersion, constant source strength, and first-order decay. The calculation supports long-term attenuation but does not prove current or future protection, especially while MW-5 exceeds the standard.
2. What lines of evidence would you require before accepting monitored natural attenuation, and what would make you reject it?
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I would require source control evidence, a statistically significant downward concentration trend at MW-5 and other sentinel wells, and geochemical evidence of biodegradation: PCE daughter products such as TCE, DCE, and vinyl chloride, plus dissolved organic carbon, redox indicators, alkalinity, ethene, and methane. I would also require a site-specific mass balance and decay rate from multiple wells, not a single estimate, and a receptor analysis that includes the municipal well and vapor intrusion. I would reject MNA if the plume is expanding, if MW-5 is stable or rising above 5 µg/L, if vinyl chloride is accumulating, if the municipal well could be used without institutional controls, or if the source is still discharging. Because MW-5 has been above the standard and not declining for three years, I would not accept MNA alone; I would require source treatment or an active barrier plus monitoring and institutional controls.
3. Recommend a monitoring plan and performance criteria for the site.
Read the worked answer
Sample MW-1, MW-3, and MW-5 quarterly for the first two years, then semiannually if stable, plus at least two new sentinel wells between MW-5 and the municipal well field. Analyze PCE, TCE, DCE, vinyl chloride, ethene, methane, dissolved organic carbon, redox parameters, nitrate, sulfate, iron, and manganese. Use low-flow sampling with consistent purge and analytical methods. Establish trends with at least eight to ten sampling rounds. Set performance criteria: MW-5 must remain below 5 µg/L or show a statistically significant downward trend, vinyl chloride must not accumulate, and the municipal well must be sampled annually with pumping records reviewed. If MW-5 rises above 5 µg/L with an upward trend, or if the plume reaches a new sentinel well, trigger contingency active remediation. Report semiannually to the regulator and document all deviations from the plan.