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Pages 116-177 of the HRS Guidance Manual (Section 7.1) provide the definitions to some key terms. Of particular importance are:

  • Aquifer: Notice that the phrase "currently used or could be used" implies that the aquifer does not necessarily have to be used as a source of water to be evaluated. This definition differs from many common definitions because it is use-based.
  • Aquifer boundary: Aquifer boundaries often depend on interconnections and relative hydraulic conductivity between geologic materials. These dependencies are themselves important definitions.
  • Aquifer discontinuity
  • Aquifer interconnection
  • Hydraulic conductivity (K)
  • Confining layer: Defined by a 100 times or more difference in K between layers.
  • Layer of lower hydraulic conductivity: Defined by at least 100 times difference in K between layers. Note that Section 7.1 of the HRSGM defines a layer of lower hydraulic conductivity as equal to or greater than 100 times difference in some places, and greater than 100 times in others. The correct definition is equal to or greater than 100 times difference in K.
  • Single hydrologic unit
  • Target Distance Limit (TDL)

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