Geophysical Characteristics of the Conterminous United States: Hydraulic Conductivity (µm/s)

This raster depicts the percentage of lithological the hydraulic conductivity (in micrometers per second) of surface or near surface geology. We derived these rasters by calculating the average conductivity for each map unit in combined surficial-bedrock geologic maps. We used state geologic maps (Preliminary Integrated Geologic Map Databases for the United States, Open File Reports 2004-1355, 2005-1305, 2005-1323, 2005-1324, 2005-1325, 2005-1351, and 2006-1272), which depict surficial geology instead of bedrock when the surficial layers are sufficiently deep. For the state maps that do not incorporate surficial geology (i.e., midwestern states), we overlaid surficial geologic map units with thicknesses greater than 100 feet (from Soller and Reheis [2004]) to produce combined surficial-bedrock geologic maps that were similar to other states. We characterized geology based on the 201 different lithologies that the Geologic Map Database lists as occurring in the conterminous United States. Because some of these lithologies are known to have physical attributes that vary widely, we created an additional 50 lithologic classes based on the common modifiers used in the geologic unit descriptions to better parse physical variability within the lithologies (e.g., tuff and nontuff for volcanic rocks). Modifiers were assigned base on descriptions of geologic formations obtained through either the Lexicon of Geologic Names of the United States or literature searches. Nineteen lithologic classes were not characterized because the class was not a specific rock type (e.g., mélange, water, and landslide) or no data was available to characterize it. These classes were characterized as no data. We translated each state’s combined surficial-bedrock geologic maps into characteristics following the methods in Olson and Hawkins (2012) by assigning an estimate of each map unit’s conductivity to every occurrence of that map unit in the combined surficial-bedrock geologic map. This estimate was calculated as the average of literature or database values of the respective property for each lithological class contained within the map unit weighted by the prevalence of each lithological class within the map unit. The accompanying Excel workbook (Lith-Physical.xls) contains a summary of all of the average geochemical characteristics for each lithology (“Lith Summary” tab) and tabs for each individual lithology that include the source of each record (e.g., originating from the Earth Chem Database or the specific literature reference), as well as the calculations used to determine the measure of central tendency (mean or median depending on the data). The final national raster was created by merging each of the individual state rasters. Users should be cognizant that some differences will exist in chemical and physical characterizations across state lines that are caused by unreconciled differences in lithologic descriptions or mapping scales used among the underlying state source maps.

Tags: bedrock, geology, geophysical, hydraulic-conductivity, usgs-552c4877e4b0b22a157f5061
Organization: Department of the Interior