Groundwater Quality in the Southeast's Surficial Aquifer System

The surficial aquifer system lies beneath about 63,600 square miles of Alabama, South Carolina, Georgia and Florida. About 14 million people lived above it in 2010, and in 2015 about 270 million gallons a day were drawn from it for public supply; it had ranked 16th in the nation for public-supply withdrawals. Most of the land over it is wetland, forest and open water (71 percent), with 18 percent agricultural and 11 percent urban. Cities in or near the area studied include Savannah, Georgia; Hilton Head, South Carolina; and Palm Beach and Fort Myers, Florida.

The aquifer is made of loose sand, gravel, shelly sand and shell deposits ranging from Miocene to Holocene in age. In Florida these form nine geologic formations, interbedded in lenses so that not every formation is present in any one place; in Georgia and South Carolina they are undivided Pleistocene terrace deposits and Holocene sands. The system is usually less than 100 feet thick, thickening southward in Florida to as much as 400 feet. Its groundwater is mostly unconfined, though fine-grained layers confine it in places.

The study

The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project, which has studied groundwater since 1991, assessed the aquifer as one of its Principal Aquifer Studies, which look at untreated groundwater from public-supply wells. Between June and November 2019 it sampled 45 randomly distributed public-supply wells in parts of Florida, Georgia and South Carolina, chosen within 10-kilometer buffers around supply wells and divided into equal-area grid cells. A network built this way represents the spatial pattern of water quality across the area, and the accuracy of the estimates depends on the number and spread of wells, not the size of the area. The wells ranged from about 20 to 270 feet deep, averaging about 110 feet.

The study area and sampled wells. U.S. Geological Survey.

How results are judged

Concentrations are compared with drinking-water benchmarks: federal regulatory limits where they exist (EPA maximum contaminant levels, or MCLs), otherwise non-regulatory health-based screening levels (HBSLs), and for taste, color, odor, staining or scaling, EPA's non-regulatory secondary maximum contaminant levels (SMCLs). A concentration is high above the benchmark. For inorganic constituents it is moderate above half the benchmark; for organic constituents, above one-tenth, because organics are generally less common and lower relative to their benchmarks. Water delivered to customers is usually treated first, so its quality is likely different.

What was found

Overview of results for constituents with human-health benchmarks. U.S. Geological Survey.

Inorganic constituents with health benchmarks were high in about 11 percent of the study area and moderate in about 18 percent.

• Trace elements and ions. Of 34 analyzed, 22 have health benchmarks (updated in 2018 to include aluminum, cobalt and iron). Eight were elevated somewhere, and high or moderate levels covered 22 percent of the area. Iron was high in 16 percent and moderate in 2 percent; molybdenum high in 4 percent; arsenic high and moderate in 2 percent.
• Radioactivity. Most radioactivity in groundwater comes from the natural decay of uranium and thorium. Of eight radioactive constituents analyzed, four have health benchmarks; together they were high in about 7 percent of the area and moderate in 6 percent. Gross-alpha radioactivity and radium-226 were each high in 2 percent, and combined radium-226 and -228 in 4 percent.
• Nutrients. High nutrient levels usually come from fertilizer, septic systems and human and animal waste. Of five analyzed, two (nitrate and nitrite) have health benchmarks. Nitrate was not found at moderate or high levels.

Results by group of inorganic constituents. U.S. Geological Survey.

Taste, color and nuisance. Eleven constituents with SMCLs were analyzed; one or more were high in about 67 percent of the area and moderate in about 33 percent. Total dissolved solids, a measure of salinity, were high in 31 percent and moderate in 69 percent. Where groundwater is low in oxygen, iron and manganese can dissolve out of minerals: iron was high relative to its SMCL in about 50 percent of the area and moderate in 24 percent, and manganese high in about 4 percent and moderate in 76 percent. pH fell outside the 6.5–8.5 range in about 2 percent.

Organic constituents. Of the man-made organic compounds detected, 30 had health benchmarks, but levels were typically low and none reached moderate or high.

• Volatile organic compounds (85 analyzed, 51 with health benchmarks) were not found at moderate or high levels.
• Pesticides (225 compounds analyzed, including breakdown products; 119 with health benchmarks) were found at low levels across much of the area, but none at moderate or high levels.

PFAS, found often

The wells were also sampled for 24 per- and polyfluoroalkyl substances (PFAS), used in water-repellent fabrics, nonstick products, food packaging, cleaning products, paints and firefighting foams. Half of them were detected, all in Florida wells. PFOS and PFOA were among the most common. About 50 percent of the study area had moderate concentrations, and about 4 percent exceeded the drinking water advisory level of 70 nanograms per liter for the two combined. How much PFAS a well held was related to how close it was to possible sources such as fire training areas, landfills, chemical plants and airports.

Combined PFAS concentrations across the study area. U.S. Geological Survey.

Why these studies matter

NAWQA's data include measurements far below health benchmarks and tracers of where groundwater comes from and how it moves, which routine compliance monitoring does not usually provide. Principal Aquifer Studies, which typically sample public-supply wells 150 to 750 feet deep, allow the quality of untreated water to be compared with health and aesthetic benchmarks and with other principal aquifers.

Sources

• U.S. Geological Survey, "Groundwater quality in the Surficial Aquifer System, Southeastern United States," Fact Sheet 2022–3035. https://pubs.er.usgs.gov/publication/fs20223035/full
• Data: Gill and others (2021); PFAS: McMahon and others (2022a, 2022b); benchmarks: Norman and others (2018), U.S. Environmental Protection Agency (2016, 2020a, 2020b); geology: Miller (1990). NAWQA: https://water.usgs.gov/nawqa/
• The fact sheet spells Fort Myers "Fort Meyers"; it is corrected here. Its inset gives overall counts of constituents with health benchmarks (24 inorganic, 157 organic) that do not match the counts given group by group; this page gives only the group counts.