Section 8.1 of the HRS Guidance Manual, Page 203
(Excerpt from HRS Guidance Manual pages 203-218)
This section explains how to determine the hazardous substance migration path of the overland/flood component of the surface water pathway, including how to determine the overland and in-water segments, and how to identify the PPE and TDL. The guidance in this section does not apply to the ground water to surface water component of the surface water pathway.
|Section 4.0.2||Surface water categories|
|Section 22.214.171.124||Definition of the hazardous substance migration path for overland flow/flood migration component|
|Section 126.96.36.199||Target distance limit|
|Section 188.8.131.52.2.1.3||Distance to surface water|
Hazardous Substance Migration Path: The path that hazardous substances travel or would travel over land from a source to surface water (overland segment) and within surface water to the TDL (in-water segment). In certain cases (e.g., sites consisting only of contaminated sediments, sites where sources are located in surface water bodies), the hazardous substance migration path consists of only an in-water segment.
Intermittent Water Body: Water bodies that do not contain water during all seasons of the year under normal conditions.
In-water Segment: Portion of the hazardous substance migration path from the PPE to the TDL. For tidally influenced rivers, the in-water segment may include portions of surface water bodies upstream from the PPE to the extent that the in-water migration path is reversed by tides. For contaminated sediments with no identified source, the in-water segment begins at the upstream boundary (for streams and rivers) or center (for water bodies with no direction of flow) of the area of contaminated sediments.
Observed Release: An observed release is established for the ground water, surface water, or air migration pathway either by chemical analysis or by direct observation. Observed release is not relevant to the HRS soil exposure pathway. The minimum requirements for establishing an observed release by chemical analysis are analytical data demonstrating the presence of a hazardous substance in the medium significantly above background level, and information that some portion of that increase is attributable to the site. The minimum criterion for establishing an observed release by direct observation is evidence that the hazardous substance was placed into or has been seen entering the medium.
Overland Segment: Portion of the hazardous substance migration path from a source to a surface water body.
Perennial Water Body: Contains water throughout the year under normal conditions. Under extreme conditions (e.g., severe drought) some water bodies considered perennial may not contain water.
Probable Point of Entry (PPE): Point at which the overland segment of a hazardous substance migration path intersects with surface water. A site may have multiple PPEs. The PPE is assigned as the point at which entry of the hazardous substances to surface water is most likely.
Surface Water: Water present at the earth's surface. Surface water includes rivers, lakes, oceans, ocean-like water bodies, and coastal tidal waters, as defined in HRS section 4.0.2.
Target Distance Limit (TDL) for the Surface Water Migration Pathway: Distance over which the in-water segment of the hazardous substance migration path is evaluated. The TDL extends 15 miles from the PPE in the direction of flow (or radially in lakes, oceans, or coastal tidal waters) or to the most distant sample point establishing an observed release, whichever is greater. In tidally influenced surface water bodies, an upstream TDL is also determined. For some sites (e.g., sites with multiple PPEs), an overall target distance of greater than 15 miles may result.
Watershed: Portion of the watershed downgradient of sources at the site. The watershed includes the surface water bodies between the PPEs and the TDL (i.e., the in-water segment of the hazardous substance migration path). A single watershed includes all in-water segments that intersect within the TDL. A site is in two or more watersheds if two or more hazardous substance migration paths from the sources do not reach a common point within the TDL. In these cases, each distinct watershed is evaluated separately.
The overland segment is used to evaluate potential to release to surface water and establish the PPE. In the simplest case, a site will have one source with a single hazardous substance migration path, with a single overland segment (see Highlight 8-1). Because the overland segment is defined from a source to surface water, a single site with multiple sources may have more than one hazardous substance migration path, and hence more than one overland segment (although they may be very near to one another and/or may converge). The steps below apply to sources in a single watershed. These steps should be repeated for each source within the watershed and for each watershed.
(1) Identify each source at the site with a containment factor value greater than O for the surface water pathway. Do not evaluate sources with a surface water containment factor value of 0.
- In general, each source serves as the beginning of an overland segment.
- Sometimes hazardous substances will have already migrated from the source toward surface water. If evidence of this migration is contaminated soil, the contaminated soil is itself a source. Use the farthest point of documented soil contamination as the beginning of the overland segment.
- If a site consists of contaminated sediments with no identifiable source, then there is no overland segment.
(2) Determine the overland flow paths that surface water would take from a source to a surface water body. Delineate the overland segment by determining the routes that runoff would take from a source to surface water. The routes may be determined solely from topographic maps; however, the overland segment generally should be refined from site observations.
- Storm sewers and other covered drains (or other man-made runoff controls,
such as a wall) along the overland migration path must be considered in
determining the overland flow (see Highlight
- If contaminated soil is directly adjacent to the PPE, the distance to
surface water is 0.
- At sites with a large source and/or complex topography, there may be
more than one overland segment from a single source in a single watershed.
If all the overland segments are greater than 2 miles, assign a value of 0 for the potential to release by overland flow component. However, potential to release by flood can still be evaluated for that watershed, if applicable.
(3) Identify the PPE. The PPE is the point where the overland segment reaches an eligible surface water body. Eligible surface waters are listed in Highlight 8-4.
- Highlight 8-5 provides additional guidance on determining the PPE for water bodies with
wetlands and for intermittently flowing streams and ditches.
- At sites with a large source and/or complex topography, there may be
more than one PPE to a single surface water body (see Highlight
SCORING THE DISTANCE TO SURFACE WATER FACTOR
Evaluate the distance to surface water factor only for watersheds scored based on potential to release by overland flow. Do not evaluate this factor for watersheds where an observed release to surface water has been documented.
(1) Determine the shortest overland segment from any source with a containment factor value greater than 0 to the surface water body. If this distance is near a breakpoint between distance ranges in HRS Table 4-7, use the mean high water level for tidal waters or the mean water level for other surface waters.
- If there is only one overland segment, the distance to surface water
is the distance from a source to the PPE, measured along the overland segment.
If there are two or more overland segments, the distance to surface
water is the shortest distance from any source to the PPE for the watershed
- If the in-water segments associated with different overland
segments reach a common point within the TDL, select the shortest overland
segment, and use its length as the distance to surface water (see Highlight
- If the in-water segments associated with different overland
segments do not reach a common point within the TDL, the site is in more
than one watershed (see Highlight
8-6). Assign a separate distance to surface water factor and
calculate a separate surface water pathway score for each watershed. Section
8.2 provides guidance on scoring sites with more than one watershed.
- If the in-water segments associated with different overland segments reach a common point within the TDL, select the shortest overland segment, and use its length as the distance to surface water (see Highlight 8-6)
(2) If the shortest overland segment for a watershed exceeds 2 miles, potential to release by overland flow cannot be evaluated for the watershed. Assign potential to release by overland flow a value of 0 for the watershed.
(3) Assign a distance to surface water factor value using HRS Table 4-7. Because this factor is assigned based on distance ranges, precise measurement of the distance of the overland segment is generally not necessary unless the distance is near a breakpoint between two ranges.
SCORING THE FLOOD FREQUENCY FACTOR
(1) Determine the floodplain category (i.e., floods annually, 10-year, 100-year, 500-year) in which the source is partially or wholly located. Potential to release by flood does not consider distance to surface water. Therefore, a source with an overland segment greater than 2 miles can be evaluated if it is located in an appropriate floodplain.
(2) Assign a flood frequency factor value using HRS Table 4-9.
EVALUATING THE TARGET DISTANCE LIMIT FOR NON-TIDALLY INFLUENCED WATER BODIES
This section discusses determining the TDL for sites with and without a PPE, sites with multiple PPEs, and sites where the in-water segment branches.
A. SITES WITH A SINGLE PPE
(A1) Determine the PPE, based on delineation of the overland flow segment.
(A2) Measure the distance along each surface water body. Measure from the PPE to the most distant sampling point that meets the observed release criteria (downstream for rivers and streams; or radially for lakes, oceans, and coastal tidal waters)
- If no sample meets the observed release criteria, or if the
most distant sample that meets this criteria is less than 15 miles from
the PPE (measured along the surface water body), extend the TDL to 15 miles
from the PPE (see Highlight 8-7).
- If the most distant sample that meets the observed release criteria is more than 15 miles from the PPE (measured along the surface water body), extend the TDL to that point (see Highlight 8-8).
(A3) Evaluate those surface water targets that are located partially or wholly within, or contiguous to, the TDL.
B. SITES WITH MORE THAN ONE PPE
For sites with more than one PPE, define an in-water segment for each PPE, and evaluate targets in each in-water segment of a watershed.
(B1) Determine the location of each PPE based on delineation of the overland flow segment.
(B2) Identify the in-water segment from each PPE.
(B3) Determine whether the site is within one watershed.
- If all of the in-water segments from each PPE do not join within the
TDL, consider the site to be in more than one watershed. Evaluate each
watershed separately and use the highest score for any watershed as the
surface water pathway score for the site (see Section 8.2).
If all of 8.2 the in-water segments from each PPE join within the TDL,
consider the site to be in one watershed and evaluate the in-water segment
- If the PPEs for different sources are in the same water body
and are relatively close together, determine a single PPE. Evaluate the
TDL as described for a single PPE.
- If hazardous substances from different sources enter the same
water body at distant points, the target distance is the distance from
the most upstream PPE to 15 miles downstream from the most downstream PPE
(or the combined overlapping arcs for two or more PPEs into a lake, coastal
tidal water, or ocean). This may result in an overall TDL of greater than
15 miles (see 8-9 and 8-10). The downstream
PPE must be within the TDL of the upstream PPE. If this is not true, each
PPE is considered to be in a separate watershed and each watershed is scored
- If the PPEs for different sources are in two different water
bodies (e.g., two rivers, two lakes) that later merge into one water body,
determine the target distance from each PPE for each source. Total targets
are the sum of each segment for each water body. Count targets common to
more than one source only once (see Highlights
8-11 and 8-12).
The merge point should be within the TDL of each PPE; otherwise the sources
are considered to be in two or more watersheds.
- If the PPEs for different sources are in the same water body and are relatively close together, determine a single PPE. Evaluate the TDL as described for a single PPE.
C. SITES WHERE THE IN-WATER SEGMENT BRANCHES
(C1) Measure the TDL separately for each branch.
(C2) Determine if the branches join within the TDL.
- If the branches do not rejoin within the TDL, determine the TDL in
each branch separately (see Highlight
- If the branches rejoin within the TDL, determine the TDL within each
branch and select the one that is farthest downstream (see Highlight
(C3) Score only those surface water targets that are located partially or wholly within, or contiguous to, the TDL. Targets in all branches (see Highlight 8-13) are considered when evaluating the watershed.
D. SITES CONSISTING SOLELY OF CONTAMINATED SEDIMENTS WITH NO IDENTIFIED SOURCE
(D1) Determine if the surface water body containing the contaminated sediments has a clearly defined direction of flow.
- If there is a clearly defined direction of flow, proceed to Step (D2).
- If there is no clearly defined direction of flow, proceed to Step (D4).
(D2) When there is a clearly defined direction of flow, begin measuring the TDL from the farthest upstream sediment sampling point that meets the criteria for an observed release.
(D3) Use the sediment sampling point identified in Step (D2) in lieu of the PPE.
- Follow Steps (A2) and (A3) for sites with a single PPE to determine
- Skip Steps (D4), (D5), and (D6) below.
(D4) When there is no clearly defined direction of flow, begin measuring the TDL at the center of the area of observed sediment contamination. Determine the center using only those sediment sampling points that meet the criteria for an observed release.
(D5) Extend the TDL as an arc with a radius extending either 15 miles along the surface water, or to the most distant sample point that meets the observed release criteria to surface water, whichever is greater. See Highlight 8-14 for an example of determining the TDL in this case.
(D6) Score only those surface water targets that are located partially or wholly within, or contiguous to, the TDL.
EVALUATING THE TARGET DISTANCE LIMIT FOR TIDALLY INFLUENCED WATER BODIES
In tidally influenced water bodies, the TDL may extend upstream from the PPE of hazardous substances. Evaluate the downstream TDL for such water bodies in the same manner as for other surface water bodies. The following procedures describe how to establish the upstream TDL for tidally influenced water bodies.
- If there are no significant upstream targets, do not evaluate the effect
of tidal influence on the TDL.
- If there are significant upstream targets, proceed to Step (2)
(2) Document how far upstream the tide can carry hazardous substances. Use existing documentation on the upstream point of flow reversal, extent of brackish water, or salt water intrusion data. Such documentation is often available from local water authorities. Often, these data can be obtained from appropriate state agencies such as water resource commissions.
(3) Locate the farthest upstream sample establishing an observed release.
(4) Determine the upstream boundary of the TDL. The upstream boundary of the TDL depends on the extent of tidal carry and the farthest upstream observed release sample. Highlight 8-15 illustrates these considerations.
- If the farthest upstream sampling point establishing an observed release
is located greater than 15 miles from the PPE, use the location of that
sampling point as the upstream boundary of the TDL.
- If the tidal influence is at least 15 miles upstream from the PPE, use
15 miles upstream from the PPE as the upstream boundary of the TDL.
- If the tidal influence is less than 15 miles upstream from the PPE,
use the documented distance to which the tide could carry hazardous substances
as the upstream boundary of the TDL.
TIPS AND REMINDERS
- Consider a perennially flowing irrigation ditch as part of the in-water
segment of the hazardous substance migration path.
- If there is no HRS-defined surface water body within 2 miles of the
site, do not evaluate the surface water pathway, unless there is an observed
release to surface water from the site or the site is in a 500-year (or
less) floodplain of the nearest surface water.
- Intermittent rivers in areas with 20 or more inches mean annual precipitation and intermittent lakes (regardless of annual precipitation) are not evaluated as surface water bodies for HRS purposes.