Hazardous Substance Migration Path
The definition of the hazardous substance migration path is the first step in the analysis of the surface water pathway. If it is done well, the rest of the surface water pathway is relatively straight forward. Prepare a map showing the run-off patterns from sources to surface water and then for 15 miles in surface water. It may be necessary to use two maps if one map does not have the right scale to show runoff patterns and the entire 15-mile TDL. Later, you will add sampling points to this map and show the locations of the various targets. This map is essential to the understanding of the surface water pathway and is basic to the HRS documentation package.
Define pathway terms by referring to Section 8.1 of the HRS Guidance Manual, page 203.
For the overland flow/flood component:
- Overland segment: just what it sounds like, the route hazardous substances take to get from a source to the nearest surface water body. May or may not be present at each source. We'll review this one in some detail later. (See Highlight 8-1 of the HRS Guidance Manual, page 205)
- PPE: the probable point of entry where hazardous substances reach the surface water body at the end of the overland segment.
- In-water segment: from PPE to end of target distance limit (TDL).
- TDL: 15 miles downstream from the PPE. The TDL may be further than 15 miles if an observed release can be established beyond the 15-mile distance. The TDL may also be shorter than 15 miles if the in-water segment ends before 15 miles is reached.
Mapping out the hazardous substance migration path is crucial to the evaluation of the surface water pathway.
Read the following three steps that are also found on pages 204 through 206 of the HRS Guidance Manual.
Step 1: Identify all sources with a surface water containment factor value greater than 0. Note that contaminated soil is a source.
Step 2: Determine the overland flow paths that runoff would take.
Use a topographic map in a pinch, but site observations are better. Carefully document site conditions during the site visit. These observables will help in determining sampling locations, if needed at a later date. Take photographs!
Look at Highlight 8-3 of the HRS Guidance Manual, page 206. You'll need a diagram of any engineered drains that are part of the overland (or in-water) segment.
Last paragraph: if distance to SW is greater than 2 miles, do not evaluate LR by overland flow!
Step 3: Identify the PPE.
Turn to Highlight 8-5 of the HRS Guidance Manual, Page 208 to determine whether the PPE is into a wetland or an intermittent stream.
Now look at Highlight 8-6 of the HRS Guidance Manual, page 209. Notice that there may be more than one PPE at a single site. Review this highlight. This section also highlights some confusing and complex situations.
The in-water segment defines the maximum distance over which surface water targets are considered in scoring.
This segment starts at the farthest upstream PPE and extends to the end of the TDL.
The TDL is 15 miles downstream from the farthest downstream PPE. Thus, the in-water segment may be longer than 15 miles.
- The TDL may be extended beyond 15 miles to any sampling point that meets the observed release criteria.
- The TDL may be less than 15 miles if surface water bodies end.
Highlight 8-6 of the HRS Guidance Manual, page 209, there are two PPEs on the same stream.
Where does the in-water segment begin?
- Click here for answer.
Where does the in-water segment end?
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Highlight 8-7 of the HRS Guidance Manual, page 205. Discuss multiple PPEs and how to draw the TDL (e.g., line in river, arc in lakes). Then look at Highlight 8-8 of the HRS Guidance Manual, page 210 and discuss extending the TDL with an OR sample.
Watersheds
Like "aquifers" in the ground water pathway, each "watershed" within a site's TDL must be evaluated separately (a copy of the HRS table 4-1 would be completed for each) and the highest-scoring watershed is selected to score the pathway. In most cases, you'll have only one watershed.
If the Red River and the Blue/Yellow River meet further downstream, would it be one watershed?
Highlight 8-16 of the HRS Guidance Manual, page 222, notice that if all hazardous substance migration paths meet within the TDL, it is considered a single watershed.
Highlight 8-17 of the HRS Guidance Manual, page 223, two watersheds are depicted (PPE4< to TDL4 and PPE1, PPE2, and PPE3 to TDL 1).
If the Red River and the Blue/Yellow River meet further downstream, would it be one watershed?
- Click here for answer.
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