Portraying chemical properties of bedrock for water quality and ecosystem analysis; an approach for the New England region

A classification scheme has been developed to describe bedrock geologic rock units in terms of lithologic units with similar chemical features (termed lithochemical units) that portray the relative reactivity of their constituent minerals to dissolution and other weathering reactions. The lithochemical classification, as developed for the New England Region, consists of 26 lithologic types, which are defined on mineralogy, weathering characteristics, and structural setting. The 26 units can be summarized into 6 major categories, arranged in order of decreasing weatherability or attack by natural waters: (1) carbonate-rich rocks, (2) clastic sedimentary rocks restricted to rift basins, (3) mafic igneous rocks and their metamorphic equivalents, (4) ultramafic rocks, (5) metamorphosed, noncalcareous clastic sedimentary rocks, and (6) felsic igneous and plutonic rocks and their metamorphic equivalents. This classification was developed to provide the USGS National Water Quality Assessment (NAWQA) Program with a scheme to use bedrock geologic data for analysis of the water-quality characteristics of surface water and shallow ground water.