Low-Flow Purging and VOC Sampling in a Monitoring Well
A 2-inch Schedule 40 PVC monitoring well is installed to 40 ft with a 10-ft screen from 30 to 40 ft. The water column is 25 ft, and the casing ID is 1.5 inches. The site has chlorinated solvents in the smear zone. A low-flow pump is set at 35 ft and purged at 0.5 L/min. After 30 minutes, drawdown is 0.05 ft, turbidity has fallen from 80 to 15 NTU, pH is 6.8, specific conductance is 450 µS/cm, DO is 2.0 mg/L, and ORP is −50 mV. The state’s low-flow guidance requires stable parameters for three consecutive readings and turbidity below 10 NTU or stable. The sampler wants to collect VOC samples after one well volume because “the water is clear enough.” The calculated well volume is 2.3 gallons.
1. Calculate the purge volume for one well volume and the time to purge it at 0.5 L/min. Does the sampler’s plan meet the low-flow criteria? Show your work.
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One well volume is given as 2.3 gal. At 0.5 L/min, convert: 0.5 L/min × 0.2642 gal/L = 0.132 gpm. Time = 2.3 gal / 0.132 gpm = 17.4 min. So one well volume is about 17 minutes, not 30. But low-flow purging is not volume-based; it is stabilization-based. At 30 minutes the turbidity is 15 NTU, above the 10-NTU target, and there is only one reading shown, not three consecutive stable readings. Sampling after one volume would be even earlier and would likely bias VOC results with turbidity and fine sediment. The sampler’s “clear enough” judgment does not meet the written criteria. Continue purging and record pH, conductance, DO, ORP, and turbidity until three consecutive readings are stable and turbidity is below 10 NTU or documented as stable.
2. What could cause turbidity to remain at 15 NTU, and what would you check before sampling?
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Possible causes: the pump intake is too close to the screen or sediment sump; the purge rate is still too high for the formation; the filter pack or formation contains fines; bentonite or drilling mud was not fully developed; the well was recently surged or disturbed; or the pump is entraining air. Check pump placement, reduce the rate to 0.25 L/min if drawdown allows, inspect the pump and tubing for air leaks, and measure turbidity at a consistent point. Do not surge or bail a monitoring well just before VOC sampling. If turbidity stabilizes at 15 NTU after a reasonable period, document it, but do not call it stable until three readings confirm it.
3. Describe the VOC sample collection procedure, including containers, preservation, and field QA.
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Use 40-mL amber VOC vials with HCl preservative, filled slowly from the pump discharge with no headspace. Fill to a meniscus, cap immediately, invert, and check for bubbles; if a bubble larger than a pea is present, refill. Chill to 4°C, protect from light, and ship with chain-of-custody. Collect a field duplicate and a trip blank, and use a dedicated or properly decontaminated pump. Do not use a bailer that disturbs the water column unless it is the only option. Record purge parameters, sample time, and any turbidity anomaly. Because chlorinated solvents are the target, avoid air contact and do not sample before the low-flow criteria are met.