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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):

  • Aquifer: 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.

  • Aquifer 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.

  • Aquifer 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.

  • Aquifer 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.

  • Confining Layer: A layer of low hydraulic conductivity (relative to adjacent geologic materials) that is not expected to be used as an aquifer.

  • Hydraulic 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.

  • Single Hydrologic Unit: The combination of geologic materials and aquifers that are determined to be within the same aquifer boundaries, including all interconnected aquifers.

  • Target 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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