Groundwater Quality in the Eastern Carbonate-Rock Aquifers

By Bruce Lindsey and Kenneth Belitz. USGS Fact Sheet 2016–3079, September 2016, National Water-Quality Assessment Project.

Groundwater supplies nearly 50 percent of the nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project checks the quality of the aquifers that matter most for drinking water — here, the carbonate-rock aquifers of the Valley and Ridge (V&R) and the Piedmont and Blue Ridge (P&BR).

The aquifers

• People: more than 40 million live above them, in 10 states.
• Domestic supply: together they rank second in the nation for private wells, about 470 million gallons a day.
• Public supply: about 195 million gallons a day.
• Land above: mostly undeveloped (49 percent), farmland (35 percent) and urban (17 percent).

Carbonate rock dissolves more easily than other rock, so the valleys tend to sit on carbonate aquifers while ridges are made of tougher rock such as sandstone. Wells in carbonate rock are usually more productive. But the same dissolution channels make these aquifers vulnerable: where sinkholes open, contaminants can move in fast. In places, thick clay or shale shields them.

The study

Public-supply water usually comes from a deep zone: its top is typically 50 to 250 feet down, and it is typically 50 to 300 feet thick. In 2013 the USGS sampled 60 evenly spread public-supply wells in that zone, for a long list of natural and human-made constituents. The results give the share of the study area with high, moderate or low concentrations; their accuracy depends on the number and spread of wells, not the area's size.

The water was tested before any treatment; what reaches the tap may differ.

Share of the study area with high, moderate and low concentrations. Chart: USGS.

How "high" is defined

Results are compared with drinking-water benchmarks — federal regulatory limits where they exist, otherwise non-regulatory health or aesthetic ones. Of 55 inorganic constituents and properties and 317 organic ones, 24 and 157 have human-health benchmarks.

• High: above a human-health benchmark or SMCL (secondary, aesthetic limit).
• Moderate (inorganic): above half a benchmark.
• Moderate (organic): above one-tenth — a lower bar, because organic compounds are usually rarer and weaker.

Benchmarks for the constituents found at high levels:

MCL: EPA maximum contaminant level. HBSL: health-based screening level. SMCL: secondary maximum contaminant level. ppb, ppm: parts per billion, per million.

Results: health-related

Inorganic constituents (mostly natural) were high in about 15 percent of the study area and moderate in about 17 percent:

• Radioactivity comes mostly from the natural decay of uranium and thorium in the rock.
• Nutrients at high levels usually come from people: fertilizer, septic systems, and human and animal waste.

Organic constituents (from human activity) were never high, and moderate in about 2 percent:

• VOCs (volatile organic compounds): 90 tested, 38 with benchmarks. Only chloroform was moderate, in 2 percent.
• Pesticides: 227 compounds tested, 119 with benchmarks. None were high or moderate.

Results: taste, color and staining

Some constituents affect taste, color or odor, or cause staining and scaling. Of 11 with SMCLs, one or more were high in about 15 percent of the area and moderate in about 18 percent:

• Total dissolved solids (salinity, from rock weathering or from road salt, fertilizer and other chemicals): high in about 5 percent.
• Iron and manganese, released where water is low in oxygen: each high in about 5 percent.
• Sulfate: high in about 2 percent.
• pH: a few samples were below 6.5 — acidic and possibly corrosive.

Nitrate: a north–south split

Nitrate was high or moderate in 13 percent of samples overall, but:

• South (Alabama, Georgia, Tennessee): none.
• North (Virginia, West Virginia, Maryland, Pennsylvania, New Jersey): 23 percent of the area.

The same split in domestic wells has been tied to more farming above the northern aquifers. Arsenic, manganese, gross alpha and strontium showed no clear geographic pattern; their sources are mostly natural.

About these studies

NAWQA has assessed groundwater since 1991 with three kinds of study: Land Use Studies (shallow observation wells), Major Aquifer Studies (domestic wells) and Principal Aquifer Studies (deeper public-supply wells, as here). Its measurements go far below regulatory limits, and include tracers of where groundwater comes from and how it moves. Data: Arnold and others (2016), USGS Data Series 997.

Sources

Based on Groundwater Quality in the Valley and Ridge and Piedmont and Blue Ridge Carbonate-Rock Aquifers, Eastern United States, by Bruce Lindsey and Kenneth Belitz, USGS Fact Sheet 2016–3079, U.S. Geological Survey; a work of the United States government in the public domain. The overview charts and benchmark table are taken from the fact sheet's PDF; its two maps, on an Esri shaded-relief base, are not reproduced.