Identifying and Evaluating Aquifers
The first step in scoring the ground water pathway is to identify the geological materials being used as aquifers within the target distance limit. An understanding of these materials can be gained from a geological cross-section of the site.
Karst Terrain and Aquifers
If a geologic cross-section is not already available, one should be created. Such a diagram is an invaluable tool in understanding the ground water pathway at a site. It can be useful in evaluating factors such as observed releases, actual contamination, depth to aquifer, travel time, hazardous substance mobility, and assigning target populations to aquifers. The diagram should distinguish among the various geological materials by identifying:
- materials that
meet the definition of aquifer;
boundaries, interconnections, and discontinuities; and
- aquifers combined
for scoring purposes into single hydrologic units.
The basic geologic cross-section should be included as part of the site inspection report. The cross-section, modified for scoring purposes as indicated above, should be included in the HRS documentation record. The Regional QC Guidance Manual provides details on the recommended content and format of geologic cross-sections.
Once the geologic units are defined, a determination of which ones are aquifers must be made. As indicated in the HRS Guidance Manual definition (see above), the unit must be "saturated and sufficiently permeable to yield economically significant quantities of water to wells or springs." The principal information sources for making such determinations include:
- visual surveys
during site visits;
- USGS topographic
maps (verified by site visits);
- well logs from
current and past well installations; and
- county records of
well installation and ground water use.
Because the definition includes units that are "currently used or could be used" for any economic purpose, State ground water classifications are also useful, particularly in determining whether a unit might be used in the future. Comprehensive documentation of the rationale for determining that a unit is an aquifer is essential and must be included in the HRS documentation record.
The above illustration shows four hydrologic units underlying the source: alluvium, limestone, shale, and the basalt intrusion. The hydraulic conductivity (in centimeters per second or cm/s) is provided for each unit. Assume for purposes of this example, that data are available documenting that the first three of these units are aquifers.
The uppermost aquifer consists of the alluvium (or the "alluvial aquifer"). Since there is no layer between the surface and the alluvium, the alluvial aquifer is not bounded above. Based on the 100-fold difference in hydraulic conductivity, the alluvial aquifer is bounded below by the limestone unit. Similarly, the alluvial aquifer is bounded on the right by the basalt intrusion (due to its complete transection of the unit and its 1,000-fold difference in hydraulic conductivity). Since the HRS Guidance Manual indicates that quarries and quarry water are "non-aquifer materials," and the quarry completely transects this unit, the alluvial aquifer is bounded on the right by the quarry.
For the same reasons, the limestone aquifer is bounded:
- above by the alluvial aquifer;
- below by the shale;
- to the right by the basalt intrusion; and
- to the left by the quarry.
The boundaries of the shale aquifer are not the same. Specifically, this unit is not bounded on the right because the quarry does not completely transect the shale unit. In principal, it is bounded below only by its lower confining layer (e.g., bedrock; not illustrated). It is bounded above by the limestone aquifer and to the right by the basalt intrusion.
The key concepts illustrated here are that (1) aquifer boundaries are defined first in terms of relative hydraulic conductivity and (2) potential aquifer discontinuities must completely transect an aquifer in order to form a boundary.
As illustrated above, aquifer discontinuities are frequently essential in determining aquifer boundaries. Thus, the presence or absence of discontinuities may have a significant affect on an HRS ground water pathway score.
The criteria for determining that a structure or feature constitutes an aquifer discontinuity is specified in the HRS rule (Section 184.108.40.206.2). To paraphrase, the structure or feature must completely transect the aquifer so as to form a continuous barrier to ground water flow. Consistent with other HRS Guidance definitions, changes in hydraulic conductivity of 100-fold or greater and areas of contact between aquifer and non-Aquifer materials are considered as barriers to ground water flow.
Examples of discontinuities include:
- Major offset
formations (dikes and sills);
- Erosion channels
(e.g., rivers and streams); and
- Salt water
See Highlight 7-14 of the HRS Guidance Manual illustrates a salt water aquifer discontinuity.
In this illustration of a potential river discontinuity, aquifer A is entirely transected across the 4-mile radius, while aquifer B is not. Is the river a discontinuity for aquifer A? For aquifer B? ANSWER
Even if aquifers are apparently separated by distinct boundaries or discontinuities,they can sometimes be combined into a single hydrological unit if an interconnection exists. An interconnection occurs between aquifers whenever the nature of the intervening materials allows for transfer of ground water and hazardous substances with minimal or no disruption in flow path or velocity. Interconnections must be within two miles of the sources or within areas of observed ground water contamination that extend beyond two miles from the sources.
Just as in evaluating aquifer discontinuities, if evidence is not available to establish aquifer interconnections, then it cannot be assumed that such an interconnection exists. If such is the case, each aquifer is evaluated as separate hydrologic units.
Karst Terrain and Aquifers
The presence of karst terrain and its implications for aquifer transport characteristics warrant special consideration in HRS evaluations. The HRS Rule (p. 51586) defines karst terrain as: "Terrain with characteristics of relief and drainage arising from a high degree of rock solubility in natural waters." "Karst" refers to a characteristic of a geologic material or formation resulting from the dissolution of the formation by natural waters over time. The majority of karst occurs in limestones, but karst may also form in dolomite, gypsum, and salt deposits. Features associated with karst terrains typically include irregular topography, sinkholes, vertical shafts, abrupt ridges, caverns, abundant springs, and/or disappearing streams. Karst aquifers are aquifers associated with karst terrain and consist of dissolution cavities and the channels. As such, the movement of hazardous substances released into karst aquifers is highly unpredictable, and transport over relatively long distances can occur very rapidly.
The HRS includes special provisions for evaluating several factors for karst aquifers:
Potential to Release
- Depth of aquifer
- Travel time
- Nearest well
- Population/potential contamination
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