Groundwater Quality in the Northern Atlantic 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 Northern Atlantic Coastal Plain aquifer system.

The aquifers

The system underlies six states with more than 21 million people. It is the nation's seventh-largest source of groundwater for public supply and fourth for domestic supply, with about 930 million gallons a day pumped for those uses. The land above is mostly undeveloped (62 percent), farmed (24 percent) or urban (14 percent).

The aquifers are loose to partly consolidated sediments that thicken and deepen seaward from the Fall Line to the Atlantic coast, reaching about 10,000 feet thick in North Carolina. Clay and silt confining layers separate 10 regional aquifers used for water supply. Recharge enters mostly where the aquifers reach the surface or lie just below it, inland, with some leaking down through the confining layers.

The study focused on two groups important for public supply: the Piney Point–Castle Hayne (PP) aquifers and the Potomac–Magothy (PM) aquifers (the Magothy, Potomac–Patapsco and Potomac–Patuxent). Public-supply water usually comes from a fairly deep zone: in the PP aquifers its top is typically 200 to 350 feet down and it is 50 to 250 feet thick; in the PM aquifers its top is typically 350 to 500 feet down and it is 300 to 1,000 feet thick. 119 public-supply wells were sampled.

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. Of 55 inorganic constituents and properties and 317 organic constituents analysed, 24 and 157 have health benchmarks.

Share of the study area with high, moderate and low concentrations. Inorganic constituents: 4, 18 and 78 percent. Organic constituents: none high, 2 moderate, 98 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 6 percent; only fluoride, arsenic and manganese were high.
• Radioactive constituents (8 analysed, 4 with benchmarks) were high in about 1 percent — gross alpha activity only — and moderate in about 12 percent.
• Nutrients (5 analysed, 2 with benchmarks) were never high; nitrate was moderate in about 1 percent.
• Volatile organic compounds (90 analysed, 38 with benchmarks) were never high and were moderate in about 2 percent: the disinfection by-products dibromochloromethane and bromoform and the solvents trichloroethylene and 1,1-dichloroethylene.
• Pesticides (227 compounds analysed, 119 with benchmarks) were not found at high or moderate levels.

Taste, staining and corrosion

Of 11 constituents with secondary (aesthetic) limits, one or more were high in 58 percent of the study area and moderate in about 24 percent.

• Iron was high in about 32 percent and manganese in about 16 percent, released from minerals where the water holds little oxygen.
• Total dissolved solids, a measure of salinity, were high in about 12 percent; chloride in 2 and fluoride in 5 percent.
• pH: most samples outside the 6.5–8.5 range were below 6.5 — acidic and potentially corrosive — especially in the northern Potomac–Magothy aquifers, where more than half the wells had such water.

Differences by aquifer and region

In the north, arsenic was high in the PP aquifers, and manganese and gross alpha activity in the PM aquifers. In the south, fluoride was high in the PM aquifers. Organic constituents were not high or moderate anywhere, while dissolved solids, iron and chloride were high or moderate in all four aquifer-and-region groups.

Sources

• Bruce Lindsey and Kenneth Belitz, Groundwater Quality in the Northern Atlantic Coastal Plain Aquifer System, Eastern United States, USGS Fact Sheet 2016–3078, September 2016. https://doi.org/10.3133/fs20163078
• USGS National Water-Quality Assessment Project: http://water.usgs.gov/nawqa/