Any target not subjected to actual contamination, but within the TDL is subject to potential contamination.
- In rivers, as the stream flow increases by an
order of magnitude, the value of the targets subject to potential
contamination decreases by an order of magnitude. Intakes that lie on large
rivers can get very low HRS values even though they supply large populations.
- Notice, however, the last entry in
HRS Rule, table 4-13, page 51613. When an intake lies
within 3 miles of the PPE and the river reach between the PPE and the intake
can be described as "quiet flowing" (laminar flow, non-turbulent, not much
mixing, probably silted bottom, etc.) the intake is assigned a dilution
weight of 0.5.
- For lakes or reservoirs, assign a dilution weight as described in HRS Rule, page 51613.
The best data for average annual stream flow is from USGS gauging stations which are frequently located at dams, bridges, and intakes. Where this data are unavailable for an intake that is the target of potential contamination, the HRS Guidance Manual, Highlight 8-21, page 233 suggests a number of sources of information.
Pages 233-239 of the HRS Guidance Manual also suggests a number of methods of estimating average annual stream flow at targets.
- On small streams with relatively small watersheds
the method suggested on page 238 of the HRS Guidance Manual may be particularly
helpful. Be aware that this method assumes a homogeneity of drainage
throughout the watershed represented by the gauging station. Similarly, if
the average annual runoff for an area is mapped and the acreage of the
watershed is measured, acre-inches per year of runoff can easily be converted
into cubic-feet per second of average annual stream flow.
- Do not use instantaneous measures of stream flow; for instance, stream flow observed during the site inspection.
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