Executive Summary

For some parts of the Nation, large-scale development of groundwater has caused decreases in the amount of groundwater that is present in aquifer storage and that discharges to surface-water bodies. Water supply in some areas, particularly in arid and semiarid regions, is not adequate to meet demand, and severe drought is affecting large parts of the United States. Future water demand is projected to heighten the current stress on groundwater resources. This combination of factors has led to concerns about the availability of freshwater to meet domestic, agricultural, industrial, mining, and environmental needs. To ensure the water security of the Nation, currently [2016] untapped water sources may need to be developed. Brackish groundwater (abbreviated as “BGW” for the purposes of this report), which is defined for this assessment as groundwater having a dissolved-solids concentration ranging from 1,000 to 10,000 milligrams per liter (mg/L), is an unconventional water source that may offer a partial solution to current [2016] and future water demands. In support of the national census of water resources, the U.S. Geological Survey completed the national brackish groundwater assessment to better understand the occurrence and characteristics of BGW in the United States as a potential water resource. This BGW assessment was authorized by section 9507(c) of the Omnibus Public Land Management Act of 2009 (42 U.S.C. 10367), passed by Congress in March 2009. Section 9507(c) states that the Secretary of the Interior, in consultation with State and local water resource agencies, shall conduct a study of available data and other relevant information (1) to identify significant BGW resources in the United States; (2) to consolidate available data related to those groundwater resources; and (3) to submit a report that will describe significant brackish aquifers, data gaps, and current use and summarize information available at the time of passage of the act. Analyses completed as part of this assessment relied on previously collected data from multiple sources; no new data were collected. Compiled data included readily available information about groundwater chemistry, horizontal and vertical extents and hydrogeologic characteristics of principal aquifers (regionally extensive aquifers or aquifer systems that have the potential to be used as a source of potable water), and groundwater use. Although these data were obtained from a wide variety of sources, the compiled data are biased toward shallow and fresh groundwater resources; data representing groundwater that is at great depths and is saline were not as readily available. One of the most important contributions of this assessment is the creation of a database containing chemical characteristics and aquifer information for the known areas with BGW in the United States. Previously published digital data relating to BGW resources were limited to a small number of State- and regional-scale studies. Data sources for this assessment ranged from single publications to large datasets and from local studies to national assessments. Geochemical data included concentrations of dissolved solids, major ions, trace elements, nutrients, and radionuclides as well as physical properties of the water (pH, temperature, and specific conductance). Additionally, the database provides selected well information (location, yield, depth, and contributing aquifer) necessary for evaluating the water resource. The assessment was divided into national-, regional-, and aquifer-scale analyses. National-scale analyses included evaluation of the three-dimensional distribution of observed dissolvedsolids concentrations in groundwater, the three-dimensional probability of BGW occurrence, and the geochemical characteristics of saline (greater than or equal to 1,000 mg/L of dissolved solids) groundwater resources. To describe BGW resources in the context of generalized hydrogeologic characteristics, the United States was divided into 10 BGW regions: Coastal Plains, Eastern Midcontinent, Southwestern Basins, Western Midcontinent, Eastern Mountains and Uplands, Northwestern Volcanics, Western Mountain Ranges, Alaska, Hawaii, and U.S. Territories. Each regional-scale analysis included a summary of the percentage of observed volume in the region that was occupied by BGW within the mixture of air, water, and rock for multiple depth intervals. Aquifer-scale analyses focused primarily on the four regions (Coastal Plains, Eastern Midcontinent, Southwestern Basins, and Western Midcontinent) that contained the largest amounts of observed BGW. Aquifer-scale analyses included a generalized description of hydrogeologic characteristics from previously published work; the distribution of dissolved-solids concentrations; considerations for developing BGW resources, including a summary of other chemical characteristics that may limit the use of BGW and the ability of sampled wells producing BGW to yield useful amounts of water; and the amount of saline groundwater being used in 2010.