Selecting a TCE Remedy at a Low-Permeability Site with a Municipal Well Downgradient
A former aerospace parts-cleaning facility has trichloroethylene in groundwater. The source area is a 30 m by 40 m former vapor degreaser pit; the plume is 220 m long and 60 m wide, with a maximum detected concentration of 850 µg/L. The water-table aquifer is a silty clay with hydraulic conductivity of 3 × 10⁻⁷ cm/s, a hydraulic gradient of 0.002, and an effective porosity of 0.35. The nearest downgradient receptor is municipal Well #4, 700 m away, which has never detected TCE; the state MCL is 5 µg/L. Four quarters of monitoring show the plume front stable or retreating slightly, and a compliance well 40 m downgradient of the source has averaged 12 µg/L. The state allows monitored natural attenuation with a long-term monitoring plan and a contingent remedy. Pump-and-treat is estimated at $700,000 capital and $450,000 per year for at least 30 years. An SVE pilot removed about 40% of the vadose-zone mass in 12 weeks. An ISCO bench test showed 60% reduction in 8 weeks, but a nearby site rebounded within six months. The board has asked for "a remedy with a finish line."
1. Recommend a remedy and defend it quantitatively against the board's request for a definitive cleanup.
Read the worked answer
Run the transport numbers first, because they decide the remedy.
- Darcy flux: q = Ki = (3 × 10⁻⁷ cm/s)(0.002) = 6 × 10⁻¹⁰ cm/s, which is about 0.019 cm/yr.
- Seepage velocity: v = q/n = 6 × 10⁻¹⁰ / 0.35 ≈ 1.7 × 10⁻⁹ cm/s, about 0.054 cm/yr.
- Time to reach Well #4: 70,000 cm at 0.054 cm/yr is on the order of a million years.
Advective transport to the receptor is effectively not occurring. That single fact drives everything else.
Against pump-and-treat: over 30 years the present-value cost is well over $13 million, and at this conductivity the system would capture very little contaminant mass - it would pump water, not clean up the site. It is the least defensible option on both cost and effectiveness.
Recommended remedy: source-zone mass removal in the vadose zone and smear zone, where SVE already demonstrated 40% removal, combined with monitored natural attenuation and institutional controls for the dissolved plume. ISCO is not recommended as the primary remedy: the bench result is encouraging but delivery in a 10⁻⁷ cm/s unit is poor and rebound within six months is the expected outcome, not a surprise.
On the finish line: the honest answer is that the endpoint is risk-based closure with an environmental covenant and long-term monitoring, not a return to background. Presenting a million-year transport calculation to the board is more persuasive than any adjective.
2. What must the monitored natural attenuation plan contain, and what performance triggers convert it to active remediation?
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Monitoring network:
- A sentinel well between the plume front and Well #4, plus the existing compliance well near the source.
- Well #4 itself sampled on the utility's schedule, coordinated with the water system so results feed the operator's reporting.
Analytes and parameters: TCE and its degradation products - cis-1,2-dichloroethene, trans-1,2-DCE, 1,1-DCE, and vinyl chloride - plus ethene and ethane to show complete reductive dechlorination; dissolved oxygen, ORP, nitrate, sulfate, sulfide, ferrous iron, alkalinity, TDS, and chloride to document the redox regime. Quarterly for two years, then annually with a trend analysis.
Why a trend analysis and not a single number: in a low-permeability unit, back-diffusion from the clay matrix will sustain concentrations at the compliance well long after the source is removed. Expect a long tail, state it explicitly, and set the schedule accordingly so the monitoring is not declared a failure at year three.
Performance triggers, quantified:
- Plume front advances more than 15 m over two consecutive monitoring events.
- A concentration at the sentinel well above 5 µg/L.
- Two consecutive quarters above 5 µg/L at the compliance well with an upward trend.
- Any detection at Well #4.
- Any new pathway - utility trench, sand seam, or new pumping well - that changes the transport assumption.
Contingency: re-treatment of the source zone, a permeable reactive barrier sized to the actual flux, or wellhead treatment for Well #4 if a detection occurs.
3. A neighborhood group and the water utility ask what you will tell residents. Draft the core of that communication.
Read the worked answer
Lead with the receptor, not with the chemistry, and give the limits plainly.
What to say:
- Well #4 has never detected TCE. It is sampled on a schedule, and the results are shared with the utility and the state.
- The geology between the site and the well is a tight silty clay. Groundwater there moves on the order of a centimeter per year, so contaminated water is not traveling to the well in any timeframe residents need to worry about.
- The contamination is not in the air or in the municipal supply; it is in groundwater at the site, and the pathway to a home does not exist.
- The remedy at the source is active - soil vapor extraction removed about 40% of the mass in the vadose zone during the pilot - and the rest of the plume is being monitored, with specific numeric triggers that force more aggressive action.
What not to say: "the site is clean," "there is no risk," or any prediction of a completion date the data do not support. Residents have heard those statements before and they undermine the next message.
Anticipate the hard question: "if it isn't moving, why monitor for 30 years?" The honest answer is that contamination in low-permeability clay slowly diffuses back out, so concentrations can persist even with no new release, and the monitoring exists to prove the plume is behaving as modeled and to catch any pathway we did not anticipate. Pair the message with a named contact and a commitment to publish each monitoring round.