Groundwater Quality in the Glacial Aquifer System

From a U.S. Geological Survey fact sheet by Paul Stackelberg, National Water-Quality Assessment Project, 2017.

Groundwater provides nearly 50 percent of the nation's drinking water. The USGS National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water, and the glacial aquifer system is one of them.

The aquifer

The sand and gravel aquifers of the glacial aquifer system underlie parts of 25 states across the northern contiguous United States, home to more than 98 million people — nearly one-third of the U.S. population. The system ranks first in the nation as a source of groundwater for both public and domestic supply, with about 2.6 billion gallons a day pumped for those uses. The land above it is mostly farmland in the central and west-central regions and mostly undeveloped in the east and west, with large cities throughout.

The aquifers are permeable, loose sediments left by continental glaciers. Glaciers laid them down in many different ways, so the deposits vary a great deal, but each region has a general pattern:

• East and west: glacial sediments fill valleys and cover broad lowlands, typically from less than 50 to 200 feet thick. Patchy layers of sand and gravel give the most productive water supplies.
• Central and west-central: thick sequences of coarse and fine glacial sediment, up to 400 feet thick, fill and hide older bedrock valleys. Unsorted, fine-grained till covers the buried valleys, with patchy layers of more permeable sediment; the northern central region also has thick glacial-lake deposits.

Recharge comes mainly from precipitation or from neighbouring aquifers; the water usually discharges to streams and other surface waters.

The glacial aquifer system's principal aquifer study network and the sampled wells. USGS.

How it was sampled

The USGS sampled 90 public-supply wells spread across the system, in 2013 and 2014. These wells are cased from the surface down, with a perforated (screened) section at the bottom; the screens were about 6 to 80 feet long and the wells about 30 to 420 feet deep. The results were used to estimate what share of the study area — the depth zone used for public supply — has high, moderate or low concentrations of each constituent. The accuracy depends on the number and spread of wells, not on the size of the area.

How the results are judged

The samples are untreated groundwater, compared with drinking-water benchmarks for context; water reaching consumers may be treated first. Of 55 inorganic constituents and properties and 317 organic constituents analysed, 24 and 157 respectively have human-health benchmarks.

• High: above a human-health benchmark or a secondary maximum contaminant level (SMCL).
• Moderate: above half a benchmark for inorganic constituents, or above one-tenth for organic ones.

Results

Water from 74 percent of the wells had no constituent with a human-health benchmark at high concentrations.

Overview of water quality: share of the study area with high, moderate and low concentrations. USGS.

Inorganic constituents with health benchmarks

One or more were high in about 26 percent of the study area and moderate in about 23 percent.

• Trace elements and ions: 34 analysed, 19 with health benchmarks; high in about 24 percent, moderate in about 19 percent. Only manganese, arsenic and strontium were found at high concentrations.
• Radioactive constituents: 8 analysed, 4 with health benchmarks; none high, moderate in about 8 percent.
• Nutrients: 5 analysed, 2 with health benchmarks; high in about 1 percent and moderate in about 3 percent — nitrate in every case.

Organic constituents

None was found at high or moderate concentrations: not the 90 volatile organic compounds analysed (38 with health benchmarks), nor the 227 pesticide compounds (119 with health benchmarks).

Taste, colour, staining and scaling

Of 11 constituents with SMCLs, one or more were high in about 65 percent of the study area and moderate in about 13 percent.

• Manganese was high relative to its SMCL in about 50 percent and moderate in 9 percent; iron was high in about 47 percent and moderate in 7 percent. Groundwater low in oxygen can release both from minerals.
• Total dissolved solids were high in 28 percent and moderate in 40 percent; sulfate was high in about 7 percent and moderate in 14, chloride high in about 1 and moderate in 3.
• pH fell outside the SMCL range of 6.5 to 8.5 in 2 percent of the study area, where it was below 6.5 — acidic and potentially corrosive.

Manganese varies by region

Manganese is a natural trace element long known for spoiling the look and taste of water. More recently it has also been linked with harm to human health, and a health-based screening level of 300 micrograms per litre has been set.

• West-central region: high manganese is especially common. Its thick, mostly fine-grained glacial sediments hold groundwater low in dissolved oxygen, which favours the release of manganese from the aquifer materials.
• East and west regions: high manganese is least common. Their thinner, coarser sediments hold groundwater more likely to contain oxygen, so manganese is less likely to be released.

Manganese by glacial region: low is below 150, moderate above 150, and high above the health-based screening level of 300 micrograms per litre. The map's own legend gives the units as mg/L; the text sets the level in micrograms per litre. USGS.

Benchmarks for the constituents found at high levels

MCL: the Environmental Protection Agency's regulatory maximum contaminant level. HBSL: a non-regulatory health-based screening level. SMCL: a non-regulatory secondary maximum contaminant level.

About the project

NAWQA has studied groundwater quality since 1991, through shallow land-use studies, intermediate-depth studies of domestic wells, and deeper Principal Aquifer Studies of public-supply wells like this one. Its data include measurements far below health benchmarks and tracers of where groundwater comes from and how it moves — information not usually collected for regulatory monitoring.

Sources

• Paul Stackelberg, Groundwater Quality in the Glacial Aquifer System, United States, U.S. Geological Survey Fact Sheet 2017–3055. https://doi.org/10.3133/fs20173055 — the description of the aquifer and the sampling, the organic results, the benchmark table, the manganese section and three figures recovered from the PDF, which the imported page had lost.
• Data: Arnold and others (2017a, 2017b); the fact sheet also cites Belitz and others (2010), Bouchard and others (2011), Burow and Belitz (2014), DeSimone and others (2014), Homoncik and others (2010), Olcott (1992, 1995), Toccalino and others (2004, 2014), Warner and Ayotte (2014) and Whitehead (1994).
• Rewritten in hubnx's own words.