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Actual Contamination

The surface water in-water segment is broken into zones of actual and potential contamination based on sampling points.

  • If a sampling point (aqueous, sediment, sessile benthic tissue) meets observed release criteria, including attribution to the site, all drinking water and environmental target between that point and the PPE are considered subject to actual contamination. For the human food chain threat, further constraints are added to this principle.
Persistance Training Slide

Using the graphic, is intake #1 subject to actual or potential contamination? Intake #2? Would the "river" or "lake" value for persistence be used for the drinking water threat? ANSWER

The zone of actual contamination is broken into zones of Level I and Level II concentrations, based on whether the substances that meet observed release criteria also meet or exceed a drinking water benchmarks in SCDM.

  • Go to the most downstream (or farthest away) sample with substances that meet observed release criteria and that also meet or exceed a drinking water benchmark either individually or in the aggregate (the I or the J index).

    • All intakes from this point back to the PPE are considered subject to Level I concentrations.

    • Note that the drinking water benchmarks are in aqueous units (ug/l). Only aqueous samples can establish Level I concentrations. This is considered protective because most people do not drink the sediments.


  • Continue downstream (or farther away) to the farthest sample that meets observed release criteria, but fails to have any substances that meet or exceed a drinking water benchmark either individually or in the aggregate.

    • All intakes that lie between this point back to the zone of Level I are considered subject to Level II concentrations.

    • Any intakes that lies beyond the most downstream (or farthest) observed release sample, but still lie within the TDL are considered subject to potential contamination.

    • Although the 15-mile TDL can be extended by samples that document actual contamination, potential contamination is never extended beyond the 15 miles.

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