Contaminant Plume Delineation and Monitoring Well Placement
A dissolved chlorinated solvent plume originates from a former dry cleaner over a shallow sandy aquifer 9 m thick. Regional flow is to the southeast at a gradient of 0.004, with an estimated hydraulic conductivity of 12 m/day and effective porosity of 0.25. Three existing monitoring wells along the centerline show concentrations of 900, 40, and 2 micrograms per litre at 60, 210, and 420 m downgradient. A residential well field lies 600 m downgradient. The regulator requires a defensible plume boundary and a sentinel well network. The consultant proposes three wells on a single transect at 500 m.
1. Estimate the seepage velocity and use the concentration profile to assess plume extent and the adequacy of the proposed transect.
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Seepage velocity v = K i / n = 12 x 0.004 / 0.25 = 0.192 m/day, about 70 m/year. Over the 420 m between the first and last well that is roughly 6 years of travel, consistent with a plume that has moved a few hundred metres.
The concentration drop from 900 to 40 to 2 micrograms per litre shows strong attenuation along the centerline, but a single centerline does not define the lateral boundary. Contaminant plumes spread transversely; a single transect at 500 m can miss a lobe that bypasses it.
The proposed three wells on one transect are inadequate as a sentinel network: they sample one line, not the plume cross-section, and 500 m is close to the residential field, leaving little warning time at 70 m/year.
2. Design a defensible monitoring network and state the decision criteria it must support.
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Use at least two cross-gradient transects: one near the source to define lateral extent and one between the plume and the residential field as a sentinel line. Place wells at the centerline, at estimated plume edges, and beyond them, with a screened interval spanning the aquifer thickness.
Decision criteria: the network must detect the plume before it reaches the residential field with enough lead time to act. At 70 m/year, a sentinel line 150 to 200 m upgrader from the field gives roughly 2 to 3 years of warning.
Define triggers in advance: a detection above the regulatory limit at the sentinel line requires notification, confirmatory sampling, and contingency action; the plume boundary is defined by the outermost detections above the limit, not by interpolation between sparse wells. Document detection limits and sampling frequency so the boundary claim is defensible.