Groundwater Quality in the Southeastern Coastal Plain Aquifer System

Groundwater provides nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in the aquifers that matter most for drinking water, among them the Southeastern Coastal Plain aquifer system.

The aquifers

The system underlies more than 120,000 square miles of Kentucky, Tennessee, Mississippi, Alabama, Georgia, Florida and South Carolina, home to about 6 million people. It ranks fifteenth in the nation as a source of public-supply groundwater, with about 340 million gallons a day pumped in 2000. The land above is mostly undeveloped (74 percent) or farmed (19 percent), with little urban land (7 percent).

The aquifers are sand, silt, clay and other sediments of Cretaceous and Tertiary age, which thicken and dip toward the Atlantic and Gulf coasts, where they are several thousand feet thick and buried under other aquifers. Rain soaking in where they reach the surface is the main recharge; most water leaves to streams or by evapotranspiration. Four regional sand aquifers are separated by four confining layers of silt and clay. The two lowest, the Chattahoochee River aquifer and the Black Warrior River aquifer, extend throughout and are the main sources of public supply, so they were sampled: 79 public-supply wells, 45 in the Chattahoochee River aquifer (250 to 1,000 feet deep) and 34 in the Black Warrior (2,000 feet deep), open to the aquifer over long intervals (about 155 feet on average).

Results describe untreated groundwater, compared with drinking-water benchmarks: above a benchmark is high; above half a benchmark (a tenth, for organic constituents) is moderate.

Share of the study area with high, moderate and low concentrations. Inorganic constituents with health benchmarks: 6, 13 and 81 percent. Organic constituents: none high, 3 moderate, 97 low. USGS.

Constituents with health benchmarks

• Trace elements and ions (34 analysed, 19 with benchmarks) were high in about 4 percent of the study area and moderate in about 9 percent; only fluoride, lead and manganese were high.
• Radioactive constituents (8 analysed, 4 with benchmarks) were high in about 2 percent — only gross alpha activity and radium — and moderate in about 7 percent.
• Nutrients (5 analysed; nitrate and nitrite have benchmarks) were not high or moderate anywhere.
• Volatile organic compounds (90 analysed, 38 with benchmarks): none high; one, the fumigant DBCP (1,2-dibromo-3-chloropropane), moderate in 1 percent.
• Pesticides (227 compounds analysed, 119 with benchmarks): none high; one, the herbicide alachlor, moderate in 1 percent.

Taste, staining and corrosion

Of 11 constituents with secondary (aesthetic) limits, one or more were high in about 39 percent of the study area and moderate in about 18 percent.

• Total dissolved solids were high in about 14 percent and moderate in 14; chloride high in about 4 and moderate in 6; fluoride above its secondary limit in about 2 and moderate in 5.
• Manganese exceeded its secondary limit in about 16 percent; iron was high in about 14 percent and moderate in 10.
• pH was outside the 6.5–8.5 range in about 52 percent: below 6.5 (acidic and potentially corrosive) in about 37 percent, and above 8.5 (alkaline) in about 15.

Where the problems are

Health-benchmark inorganic constituents were high or moderate in 19 percent of the study area, more often in Georgia and South Carolina. The culprits varied by state: manganese exceeded health benchmarks in about 5 percent of samples from Kentucky, Tennessee and Mississippi; lead and fluoride in about 7 percent of samples from South Carolina; gross alpha activity and combined radium-226/228 only in Georgia (6 percent). In Alabama, constituents reached only moderate levels.

Sources

• Jeannie Barlow, Bruce Lindsey and Kenneth Belitz, Groundwater Quality in the Southeastern Coastal Plain Aquifer System, Southeastern United States, USGS Fact Sheet 2016–3076, September 2016. https://doi.org/10.3133/fs20163076
• USGS National Water-Quality Assessment Project: http://water.usgs.gov/nawqa/