Preliminary Lithogeochemical Map Showing Near-Surface Rock Types in the Chesapeake Bay Watershed, Virginia and Maryland, USGS OFR 01-187

The purpose of this preliminary digital map is to provide a geographically
referenced data base showing regional patterns in the distribution of rock
types in the Virginia and Maryland parts of the Chesapeake Bay watershed. The
map may be used to evaluate the hypothesis that lithology of rocks and
sediments significantly influences the chemistry and quality of waters
traveling over and through them on a regional scale.

This preliminary experimental lithogeochemical map shows the distribution of
rock types in the Virginia and Maryland parts of the Chesapeake Bay watershed.
The map was produced digitally by classifying geologic-map units according to
composition, mineralogy, and texture; rather than by age and stratigraphic
relationships as shown on traditional geologic maps. This map differs from
most lithologic maps in that the lithogeochemical unit classification
distinguishes those rock units having key water-reactive minerals that may
induce acid neutralization, or reduction, of hosted water at the weathering
interface. The validity of these rock units, however, is independent of water
chemistry, because the rock units are derived from geologic maps and rock
descriptions. Areas of high soil carbon content, and sulfide metal deposits are
also shown.

Water-reactive minerals and their weathering reactions yield five
lithogeochemical unit classes: 1) carbonate rock and calcareous rocks and
sediments, the most acid-neutralizing; 2)carbonaceous-sulfidic rocks and
sediments, oxygen-depleting and reducing; 3) quartzofeldspathic rocks and
siliciclastic sediments, relatively weakly reactive with water; 4) mafic
silicate rocks/sediments, oxygen consuming and high solute-load delivering;
and, 5) the rarer calcareous-sulfidic (carbonaceous) rocks, neutralizing and
reducing. Earlier studies in some parts of the map area have related solute
loads in ground and stream waters to some aspects of bedrock lithology. More
recent preliminary tests of relationships between four of the classes of mapped
lithogeochemical units and ground water chemistry, in the Mid-Atlantic area
using this map, have focused on and verified the nitrate-reducing and
acid-neutralizing properties of some bedrock and unconsolidated aquifer rock
types. Sulfide mineral deposits and their mine-tailings effects on waters are
beginning to be studied by others. Additional testing of relationships among
the lithogeochemical units and aspects of ground and surface water chemistry
could help to refine the lithogeochemical classification, and this map. The
testing could also improve the usefulness of the map for assessing aquifer
reactivity and the transport properties of reactive contaminants such as acid
rain, and nitrate from agricultural sources, in the Chesapeake Bay watershed.

Data were originally collected in UTM coordinates, zone 18, NAD 1927, and
projected to geographic coordinates (Lat/Long), NAD 1983. The data base is
accompanied by a large format color map, a readme.txt file, and a explanatory
PDF pamphlet.