Groundwater Quality in the Piedmont and Blue Ridge Aquifers: Radon Stands Out

By Bruce Lindsey, U.S. Geological Survey

Groundwater supplies nearly 50 percent of the Nation's drinking water. To help protect it, the USGS National Water-Quality Assessment (NAWQA) Project checks groundwater quality in aquifers that are important sources of drinking water (Burow and Belitz, 2014). The Piedmont and Blue Ridge crystalline-rock aquifers are one of them.

The aquifers

• People above them: more than 25 million, in 11 states — including Atlanta and Charlotte, and the suburbs of Richmond, Washington, D.C., Baltimore and Philadelphia.
• Land above them: mostly undeveloped (71 percent) and agricultural (19 percent).
• Private wells: with the region's other rock types, they rank second in the Nation as a source of groundwater for private household supply — about 360 million gallons per day (Arnold and others, 2017a).
• Public supply: about 92 million gallons per day.

The rock: gneiss, schist, phyllite, granite and basalt (Chapman and others, 2013), under a layer of loose, weathered material called regolith or saprolite. The saprolite is more permeable than the bedrock and matters for storing and passing water. Most recharge comes from local rain and snow. In the bedrock, water moves only through fractures, so wells yield less than in the region's other bedrock.

The study area and the 60 public-supply wells sampled. USGS.

How the study worked

In 2014, the USGS sampled 60 public-supply wells, spread across the aquifers (data in Arnold and others, 2017b). The study area is the depth zone used for public supply — typically 150 to 700 feet below the surface.

• The water was tested before any treatment, for a large number of constituents from natural and human sources.
• Results show the share of the study area where each is high, moderate or low. Accuracy depends on how many wells there are and how they are spread — not on the size of the area (Belitz and others, 2010).

What "high" and "moderate" mean

The results are compared with drinking-water benchmarks — federal regulatory ones for protecting health where they exist, otherwise non-regulatory health or aesthetic ones. Water that reaches consumers may differ, since groundwater may be treated before delivery. Of 55 inorganic constituents and properties and 317 organic ones analyzed, 24 and 157 have human-health benchmarks.

Overview

Share of the study area with high, moderate and low concentrations. Percentages might not add to 100 because of rounding. USGS.

• Inorganic constituents — many occur naturally, and both natural processes and human activities affect them: high in about 33 percent of the study area, moderate in about 23 percent.
• Organic constituents — from household, business, industrial and farm products, entering the environment through normal use, spills or improper disposal: never high; moderate in about 5 percent.

Inorganic constituents with health benchmarks

• Trace elements and ions come naturally from the minerals of rocks, soils and sediments, and the water touching them.
• Radioactivity is energy or particles released as unstable atoms decay. People are always exposed to a little natural radioactivity. Most of it in groundwater comes from the decay of uranium and thorium in the aquifer's minerals.
• Nutrients occur naturally at low levels; high or moderate levels generally come from human activity — fertilizer on crops and landscaping, septic systems, and human and animal waste.

Radon: among the highest in the Nation

Radon in the sampled wells: high (yellow) above the proposed benchmark of 4,000 picocuries per liter (pCi/L), moderate (green) above 2,000, low (blue) at or below 2,000. USGS.

• Radon was above 4,000 pCi/L in 30 percent of the wells sampled.
• That level is the proposed alternative maximum contaminant level; it has not been adopted. But radon is considered the second leading cause of lung cancer (U.S. Environmental Protection Agency, 2016).
• In household wells, radon in these aquifers is among the highest in the Nation (DeSimone and others, 2014).

Why here? Crystalline rocks share similar origins — igneous and metamorphic — but are made of a wide variety of minerals. They tend to be high in radon, but levels range from high to quite low, depending on things like how much uranium is in the rock. Wells in granite had higher radon than wells in basalt and diabase (Chapman and others, 2013).

Organic constituents

• Volatile organic compounds (VOCs) — in many household, commercial, industrial and farm products, and quick to evaporate: 90 analyzed (38 with health benchmarks). Never high; moderate in 5 percent of the study area — chloroform, a byproduct of disinfection, and the solvent trichloroethylene (TCE).
• Pesticides — herbicides, insecticides and fumigants, used on crops, gardens and lawns, around buildings and along roads: 227 compounds and breakdown products analyzed (119 with health benchmarks). None high or moderate.

Taste, color and scaling

Some constituents affect taste, color or odor, or cause staining and scaling. Their benchmarks are non-regulatory secondary maximum contaminant levels (SMCLs) for public drinking water. (Some, such as fluoride and manganese, have health benchmarks too.) These results are not in the overview charts.

• Of 11 constituents with SMCLs, one or more were high in about 52 percent of the study area and moderate in about 12 percent.
• Acidity: in 35 percent of the study area, the pH fell outside the SMCL range of 6.5 to 8.5 — usually below 6.5, which is acidic and potentially corrosive.
• Manganese and iron: groundwater with little dissolved oxygen can release them from minerals. Manganese was high relative to its SMCL in about 15 percent, iron in about 12 percent.
• Total dissolved solids (TDS), a measure of salinity: all water carries some, from weathering and dissolving minerals; levels can also rise from road salt, fertilizer and other chemicals on urban or farm land. High in about 3 percent. Chloride, fluoride and sulfate, which add to it, were moderate.

Benchmarks for what was found high:

MCL: regulatory maximum contaminant level of the U.S. Environmental Protection Agency. HBSL and SMCL: non-regulatory. ppb: parts per billion.

The bigger program

NAWQA has studied groundwater since 1991, with three kinds of study, each a network of wells across an area:

Shallow and intermediate groundwater has been summarized nationally (DeSimone and others, 2014) and for these aquifers regionally (Lindsey and others, 2014). Principal Aquifer Studies compare untreated water with health and aesthetic benchmarks, and let aquifers be compared with one another. They include analyses not usually part of regulatory monitoring — at levels far below health benchmarks, and of constituents that trace where groundwater comes from and how it moves.

More: NAWQA, or nawqapublicinfo@usgs.gov.

Sources

Based on Groundwater Quality in the Piedmont and Blue Ridge Crystalline-Rock Aquifers, Eastern United States, by Bruce Lindsey, USGS Fact Sheet 2017–3040 (version 1.1), U.S. Geological Survey; a work of the United States government in the public domain. The background, the organic-compound and radon sections, the benchmark table and the three figures are taken from the fact sheet's PDF, which the import had left out.