USGS Releases Report on Water Quality in Principal United States Aquifers

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On Wednesday, the United States Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program released a summary of findings from its ongoing evaluation of water quality in the principal aquifers of the United States.  Entitled "Water Quality in Principal Aquifers of the United States," the compendium summarizes groundwater quality trends based upon regional- and national-scale assessments conducted between 1991 and 2010, and includes several notable findings for aquifers supplying irrigation water and public drinking water within the Mississippi River Basin.

Background

The USGS Office of Groundwater has identified 62 principal aquifers in the U.S. (see here for background). About one-third of those principal aquifers have been the historical focus of water-quality assessments at a regional scale by NAWQA. Of the principal aquifer systems1 historically evaluated and presented in the compendium, several lie completely or partially within the Mississippi River Basin, including the High Plains aquifer in Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming; the sands and gravels of the glacial aquifer system, in the northern U.S.; the sandy aquifers in the southeastern and southcentral U.S. (including the Mississippi Embayment-Texas Coastal Uplands aquifer, and Mississippi River Valley alluvial aquifer); the Piedmont, Blue Ridge, and Valley and Ridge aquifers; and the Denver Basin deep sandstone aquifer system.

Exceedances of human-health benchmarks (click to enlarge)

Nationally, NAWQA found that groundwater from 22 percent of sampled wells contained at least one chemical constituent at a concentration greater than a U.S. Environmental Protection Agency Maximum Contaminant Level or other human-health benchmark (see figure to the left). Nitrate was the only human-sourced constituent from that exceeded its human-health benchmark in more than one percent of wells.

Mississippi River Basin Regional Findings

High Plains Aquifer

The thick, extensive sediments of this aquifer supply one-third of the Nation's groundwater pumped for irrigation.  Recently-recharged shallow groundwater, especially beneath irrigated cropland, exhibits higher dissolved solids and nitrate concentrations than deep groundwater, and the aquifer has little natural capacity to attenuate those nitrates. High-capacity pumping can cause shallow and deep groundwater to mix, drawing contaminants from human sources into deeper parts of the aquifer and altering groundwater geochemistry.

Glacial Aquifer System

Contaminants from geologic sources (including arsenic and manganese, in more than 10 percent of sampled wells) were present in some areas at concentrations of potential concern for human health. This is especially so in oxygen-depleted groundwater, which is more common in this aquifer than in many others.  Agriculture, especially in the upper Midwest, is a source of nitrate and pesticides to groundwater, although low-permeability soils and artificial drainage reduce the aquifer’s vulnerability to contamination in some areas.

Mississippi Embayment-Texas Coastal Uplands Aquifer; Mississippi River Valley Alluvial Aquifer

These sandy aquifers are pumped heavily for irrigation and public supply, but have few anthropomorphic contaminants.  Concentrations of nitrate are low, despite large surface fertilizer applications because of low recharge rates and a capacity for fairly rapid natural attenuation. 

Piedmont, Blue Ridge, and Valley and Ridge Aquifers

Geology largely determines where contaminants occur in these aquifers, which supply a large suburban and rural population.  Fecal-indicator bacteria were detected in half of the drinking-water sources sampled.  Carbonate-rock aquifers are particularly susceptible to contamination from human activities on the land surface because of karst (solution) features. Concentrations of nitrate in the carbonate-rock aquifers were among the highest in the Nation.

Denver Basin Aquifer System

Groundwater in the deep sandstone layers of this aquifer system provides high-quality drinking water to the Front Range urban corridor of Colorado, but shallow groundwater quality has been degraded by irrigation and other human activities.

NAWQA Principal Aquifer Coordinator, Barb Mahler (512-927-3566, bjmahler@usgs.gov) can be contacted with questions regarding the report.

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1 The USGS defines a principal aquifer as "a regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable water. An aquifer is a geologic formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs. Aquifers are often combined into aquifer systems."