Source Boundaries and Dimensions
Accurate, documented determinations of source boundaries are very important in evaluating many HRS pathway factors due to their dependence on distances from sources to features of concern. Examples of factors whose evaluation depends on measuring distances from sources include:
- Ground water target factors: In the
absence of an observed release, only those targets located within 4 miles of
sources at the site are included in the target factor category evaluation.
- Ground water depth to aquifer and travel time
factors: In the absence of an observed release, determine depth to aquifer and travel time based on locations within 2 miles of sources at the site.
- Surface water pathway overland flow distance to
surface water factor: Evaluated based on distance from nearest source over
the overland flow segment; if distance is greater than 2 miles then overland
flow potential to release is 0.
- Air pathway target factors: In the
absence of an observed release, the nearest individual, potential contamination
populations, and sensitive environments factors are evaluated based on those
targets present within 4 miles of sources at the site.
- Air pathway target factors: Targets are subject to
actual contamination if they are closer to sources at the site than is the
observed release sampling location.
- Air pathway resource target factor: Evaluated based on resources located within 1/2 mile of sources at the site.
Sources and source boundaries are usually identified using:
- Visual observations during the site visit;
- Historical records;
- Sampling results;
- Aerial photographs; and/or
- Interviews with individuals having knowledge of the site.
Site maps showing accurate source boundaries should be included in the HRS documentation record. Site maps can also be used to estimate the area of sources using a planimeter.
Estimates of source volumes can frequently be derived from survey data, visual observation, and sampling data. If the survey data is available.
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