The term "aquifer" is not defined in the HRS, nor are several other key terms whose understanding is required in order to properly evaluate the ground water pathway. The following consolidated set of definitions is drawn from the HRS and the HRS Guidance Manual (Section 7.1):
One or more strata of rock or sediment that is saturated and sufficiently
permeable to yield economically significant quantities of water to wells or
springs. An aquifer includes any geologic material that is currently used
or could be used as a source of water (for drinking or other purposes) within
the target distance limit. Note, this definition differs from many common
definitions because it is based on the current or potential future use of the
geologic material for drinking water or other purposes.
Boundary: A physical barrier to ground water flow identified as the
contact between geologic materials defined as an aquifer and materials
defined as non-aquifer (or as an aquifer but with a significantly lower hydraulic conductivity). Where aquifer interconnections are documented, aquifer boundaries are expanded to encompass the interconnected aquifers.
Discontinuity: An aquifer discontinuity occurs only when a geologic,
topographic, or other structure or feature entirely transects an aquifer
(or a single hydrologic unit) within the 4-mile target distance limit,
thereby creating a continuous boundary to ground water flow within this
limit. Aquifer discontinuities are a type of aquifer boundary. This concept
will be discussed further below.
Interconnections: Subsurface conditions that allow two or more aquifers
separated by aquifer boundaries to be combined into a single aquifer
(i.e., a single hydrologic unit). Subsurface conditions must demonstrate that
the aquifer boundaries separating the aquifers do not or would not impede
the flow of ground water and hazardous substances between the aquifers.
Aquifer interconnections are evaluated within two miles of sources at the
site and in areas underlying contamination attributable to the site. This
concept will be discussed below.
Layer: A layer of low hydraulic conductivity (relative to adjacent
geologic materials) that is not expected to be used as an aquifer.
Conductivity: The overall ability of water to flow through a geologic
material, accounting for all openings in the material (e.g., between grains,
through fractures, along lava tubes). For HRS purposes, the terms hydraulic
conductivity and permeability are used interchangeably.
- Layer of
Lower Relative Hydraulic Conductivity: A geologic material with lower
hydraulic conductivity than adjacent geologic materials. If used to establish
aquifer boundaries, the difference in hydraulic conductivity should be at
least two orders of magnitude.
Hydrologic Unit: The combination of geologic materials and aquifers that
are determined to be within the same aquifer boundaries, including all
Distance Limit (TDL): Maximum distance over which targets for the site are
evaluated. The target distance limit varies by HRS pathway.
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