A hydrostratigraphic unit can be defined as a formation, part of a formation, or groups of formations in which there are similar hydraulic characteristics allowing for grouping into aquifers or confining layers (aquitards). Consistent with this definition, an aquifer or hydrostratigraphic unit in regions underlain by complex igneous and metamorphic rocks may simply be the interconnected, near surface, water-bearing fracture rind. Fractured crystalline rocks tend to produce relatively low water yields, and as such may not be identified as a significant aquifers. For example, the USGS map of Principal Aquifers of the United States does not identify the crystalline rocks of New England as being a principal aquifer, although 94.6 percent of all new wells in New Hampshire are drilled into the crystalline bedrock, and only 4.6 percent of wells are installed in the overlying stratified drift deposits, which are designated as the only principal aquifers in the state, based on well yield estimates. However, the glacial polish, and lack of saprolite development on the bedrock surface often allow for direct hydraulic connection with overlying unconsolidated drift deposits. Thus, one might posit that both the shallow, fractured crystalline bedrock and the stratified drift may be considered one hydrostratigraphic unit, based on the definition above.

Although the near surface fractured rock may be classified as an aquifer, individual formations, by merit of their structural fabric and mineralogy, may have distinct water producing characteristics that deserve identification as individual hydrostratigraphic units. In addition, chemical characteristics and distinct water types can be associated with a specific stratigraphic unit. However, the lack of deep subsurface data in northern crystalline terranes makes it difficult to identify the extent and continuity of irregular shaped rock bodies that may be identified as specific hydrostratigraphic units. These and other data gaps and anomalies will be presented so that they may be considered in the development of unifying nomenclature for hydrostratigraphy.