Groundwater Quality in the Cambrian-Ordovician Aquifer System, Midwest

By Paul Stackelberg. USGS Fact Sheet 2017–3056, June 2017, 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 aquifers that matter most, among them the Cambrian-Ordovician (C-O) aquifer system of the Midwest.

The aquifer

• People: about 26 million live above it, in parts of seven states, including the Chicago, Milwaukee and Minneapolis–St. Paul areas.
• Use: it ranks 9th in the nation for public supply and 11th for domestic supply, with about 631 million gallons a day pumped for those uses.
• Land above: mostly farmland (about 73 percent), undeveloped (about 16 percent) and urban (about 3 percent).

The study area and the 60 sampled wells. Map: USGS.

The aquifer is mostly marine sandstone and carbonate rock, at the surface or under glacial deposits in southeastern Minnesota, northeastern Iowa, Wisconsin, northern Illinois and the far northwest of Indiana. The layers dip and thicken south and east into basins in southwestern Iowa, central Illinois and western Michigan. In eastern Wisconsin, most of Iowa, Illinois and Indiana, and Missouri, the aquifer is buried under younger rock.

• Recharge: where it is near the surface, rain soaks in directly or through glacial deposits. Much of the buried part was probably filled by glacial meltwater in the Pleistocene.
• Flow: shallow water travels short paths to local streams; deeper water flows toward major rivers, Lake Michigan or the basins.
• Salt: in far western Iowa, most of Missouri, south-central Illinois and Indiana, the buried water is too salty to drink.

The study

In 2014 the USGS sampled 60 evenly spread public-supply wells. Their solid casing ran from about 40 to 2,250 feet down, and total depths from about 100 to 2,630 feet. 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.

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 an 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.

The overall picture

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

Water from half the wells had no constituent above a human-health benchmark. Inorganic constituents (mostly natural) were high in about 50 percent of the area and moderate in about 22 percent. Organic ones (from human activity) were never high and moderate in about 2 percent.

Health-related results

• 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, animal and human waste.

Radium: the big finding

The EPA limit for radium is 5 picocuries per liter (pCi/L) of radium-226 and radium-228 combined. Combined radium was high in about 42 percent of the study area — and it depended on what lies above the aquifer:

• Not buried by younger rock: moderate or high in about 37 percent.
• Buried: moderate or high in about 91 percent.

Combined radium by well, and the share of each part of the aquifer, low (under 2.5 pCi/L), moderate and high (over 5 pCi/L). Map: USGS.

The difference had been noted before (Wilson, 2012). Chemistry explains it: where the aquifer is exposed, conditions favor radium sticking to the rock; where it is buried, radium is released into the water.

Taste, color and staining

Of 11 constituents with SMCLs, one or more were high in about 63 percent of the area and moderate in about 25 percent.

pH was below 6.5 — acidic and possibly corrosive — in about 2 percent of the area.

Benchmarks used

As listed in the fact sheet. MCL: EPA maximum contaminant level. SMCL: secondary maximum contaminant level. ppb, ppm: parts per billion, per million.

About these studies

NAWQA has assessed groundwater since 1991 with 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.

Sources

Based on Groundwater Quality in the Cambrian-Ordovician Aquifer System, Midwestern United States, by Paul Stackelberg, USGS Fact Sheet 2017–3056, U.S. Geological Survey; a work of the United States government in the public domain. The import had lost the background, the overview and the radium section; they, the map, the charts and the benchmark table are taken from the fact sheet's PDF.