National Brackish Groundwater Assessment: Pilot Saline Studies

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Three pilot studies were conducted (2010-2012) to assess saline groundwater resources at regional scales. The goals of the pilot studies were to determine the availability of data for assessing the distribution and character of saline groundwater and test and develop methodologies for assessing the resource.

Southern Midcontinent Pilot Study

Map showing six aquifer systems in the midcontinental U.S.

Regional aquifer systems in the southern midcontinental U.S.

Principal Investigator: Noel Osborn

The hydrogeology, distribution, and volume of saline water in 22 aquifers in the southern midcontinent of the United States were evaluated to provide information about saline groundwater resources that may be used to reduce dependency on freshwater resources. Those aquifers underlie six States in the southern midcontinent-Arkansas, Kansas, Louisiana, Missouri, Oklahoma, and Texas-and adjacent areas including all or parts of Alabama, Colorado, Florida, Illinois, Kentucky, Mississippi, Nebraska, New Mexico, South Dakota, Tennessee, and Wyoming and some offshore areas of the Gulf of Mexico. Saline waters of the aquifers were evaluated by defining salinity zones; digitizing data, primarily from the Regional Aquifer-System Analysis Program of the U.S. Geological Survey; and computing the volume of saline water in storage.

The distribution of saline groundwater in the southern midcontinent is substantially affected by the hydrogeology and groundwater-flow systems of the aquifers. Many of the aquifers in the southern midcontinent are underlain by one or more aquifers, resulting in vertically stacked aquifers containing groundwaters of varying salinity. Saline groundwater is affected by past and present hydrogeologic conditions. Spatial variation of groundwater salinity in the southern midcontinent is controlled primarily by locations of recharge and discharge areas, groundwater-flow paths and residence time, mixing of freshwater and saline water, and interactions with aquifer rocks and sediments.

 

Southeastern United States Pilot Study

Principal Investigator: Lester Williams

Map showing the location and overlap of the Southeastern Coastal Plain and Floridan aquifer systems.

Location of study area showing the extent of the Southeastern Coastal Plain and Floridan aquifer systems in the southeastern United States.

Brackish and saline groundwater resources were assessed in two regional aquifer systems in the southeastern United States to determine their potential for supplementing existing freshwater supplies in the area. These resources were mapped in terms of their dissolved-solids concentrations, estimated mostly from borehole geophysical logs, and supplemented with water-quality data collected from wells with known open intervals. Maps and cross sections were constructed for each of the major hydrogeologic units that depict salinity variations in four zones:

  1. the freshwater zone, containing dissolved-solids of less than 1,000 milligrams per liter (mg/L);
  2. the brackish-water zone, containing between 1,000 and 10,000 mg/L of dissolved solids;
  3. the salinity transition zone, containing between 10,000 and 35,000 mg/L of dissolved solids; and
  4. the saline-water zone, containing between 35,000 and 100,000 mg/L of dissolved solids.

The brine zone, containing greater than 100,000 mg/L of dissolved solids, was not differentiated from the saline-water zone.

The results of the study indicate brackish groundwater resources are present in a variety of clastic and carbonate rock aquifers ranging in depth from a few hundred feet to several thousand feet or more. Brackish zones were mapped in two aquifers of the Southeastern Coastal Plain aquifer system (the Chattahoochee River aquifer and Black Warrior River aquifer) and two aquifers of the Floridan aquifer system (the Upper and Lower Floridan aquifers). Because of differences in permeability, proximity to recharge areas, and the local presence of relict seawater or brine that has not been fully flushed out of the freshwater flow system, vertical and lateral salinity variations in successively deeper aquifers can be complex. A cross section through southeastern Georgia demonstrates the varying extents of brackish water zones in the Southeastern Coastal Plain and Floridan aquifer systems. Regionally, the shallowest and most accessible brackish groundwater resources are present in the Upper Floridan aquifer, mostly in coastal areas of Georgia, Alabama, South Carolina, and throughout most of the coastal areas in Florida. Deeper, less accessible aquifers containing brackish groundwater resources are present throughout much of the Southeastern Coastal Plain aquifer system in Georgia, Alabama, and South Carolina.

Additional study will be needed to assess the extent, water quality, and potential of using brackish- and saline-water resources to supplement existing freshwater supplies. Major considerations for development include the depth and cost of drilling into these deeper zones, alternatives for brine disposal that may be associated with treatment of brackish water, and the potential yield of these aquifers.

 

Cretaceous Aquifer System of the Mississippi Embayment Pilot Study

Map of the Mississippi Embayment pilot study area covering the eastern and southern parts of Arkansas.

Map of the West Gulf Coastal Plain, Mississippi embayment, and underlying Cretaceous aquifer study area, including the Nacatoch Sand and Tokio Formation, Arkansas.

Principal Investigator: Jonathan Gillip

The Mississippi Embayment and underlying Cretaceous groundwater systems are rich in water resources. However, large areas of aquifers have elevated total dissolved-solids concentrations. The Nacatoch Sand and the Tokio Formation of the Cretaceous aquifer system were chosen for investigation since the aquifers are known to have increased salinity as the distance from the outcrop increases. Limited water-quality data are available for deeper aquifers, as other shallower aquifers are used for water supply. Historical geophysical logs were used to determine the properties of the deeper aquifers and the quality of the water contained within. Based on the results of this study, in Arkansas, the Nacatoch Sand contains over 80 million acre feet of water with a TDS (total dissolved solids) concentration between 1,000 and 10,000 mg/L, over 60 million acre feet of water with a TDS concentration between 10,000 and 35,000 mg/L, and nearly 180 million acre feet of water with a TDS concentration over 35,000 mg/L. The Tokio formation, in Arkansas contains over 18 million acre feet of water with a TDS concentration between 1,000 and 10,000 mg/L, over 40 million acre feet of water with a TDS concentration between 10,000 and 35,000 mg/L, and nearly 145 million acre feet of water with a TDS concentration over 35,000 mg/L.