Monday, February 13, 2012
The State of the Nation’s Groundwater
The US Geological Survey, USGS, and the US Environmental Protection Agency, US EPA, report that 105 million people, about a third of the population receive their drinking water from one of the 140,000 public water systems across the United States that use groundwater as their source. In addition, 15% of the population obtains their water from groundwater using private drinking water wells. The water quality of the public water supply systems is regulated by the US EPA under the Safe Drinking Water Act (SDWA), but the US EPA only regulates the finished water delivered to consumer and public water is often mixed with supplemental sources and treated so the quality of the underlying groundwater has not been tracked by the US EPA. The USGS monitors the quality and occurrence of contaminants in untreated groundwater. For the past decade and a half, the USGS has been studying groundwater quality in the United States.
Groundwater aquifers are potentially vulnerable to a wide range of man-made and naturally occurring contaminants, including many that are not regulated in drinking water under the SDWA, which defined a contaminant as “any physical, chemical, biological, or radiological substance or matter in water” (U.S. Code, 2002; 40 CFR 141.2). This is a very broad definition of contaminant includes every substance that may be found dissolved or suspended in water, everything but the water molecule itself. However, the SDWA only has MCLs and secondary standards for 91 contaminants. Some substances have non-regulatory human health screening levels and then there are substances where no screening level has been determined and that is of growing concern. The presence of a contaminant in water does not necessarily mean that there is a human-health concern. Whether a particular contaminant in water is potentially harmful to human health depends on the contaminant’s toxicity and concentration in drinking water. Other factors include the susceptibility of individuals, amount of water consumed, and duration of exposure.
Scientists from the U.S. Geological Survey (USGS) tested water-quality conditions in untreated groundwater from 932 public wells, and also tested finished (treated) water from about 10% of the wells. Though the SDWA requires or recommends testing for 91 contaminants this program tested for over 300 contaminants, both naturally occurring and man-made in order to evaluate how widespread contaminants are in groundwater from public wells and their potential significance to human health and whether contaminants that occur in untreated groundwater also occur in finished water after treatment. This study is a great proxy for the state of the nation’s groundwater. I am one of the 45 million Americans who obtain their drinking water directly from groundwater using a private well and care deeply about the quality of the nation’s groundwater.
Though less than 1% of the groundwater public supply wells in the United States were tested, the samples from the systems tested represent source water used by about 26 million people. The USGS was able to test the groundwater that serves such a significant portion of the population by making sure that half of the groundwater wells sampled by the USGS were from very large systems in urban areas that are densely populated. Detection frequencies and the percentages of samples with contaminant concentrations greater than human health screening levels or MCL’s found in this study were similar to those observed in previous USGS studies. This happened because about 30% of the wells sampled in this study were also included in previous USGS studies and groundwater quality changes slowly in deep wells.
The USGS testing found that 10 contaminants were detected at concentrations greater than human-health recommended levels in at least 1% of the groundwater and accounted for most concentrations (74%) greater than MCLs or other human health screening level. Of the ten contaminants, seven were from natural sources and three were man-made. The seven contaminants from natural sources included four geological trace elements (arsenic, manganese, strontium, and boron) and three radionuclides (radon, radium, and gross alpha-particle radioactivity). Radon has been considered several times for regulation in water in the past, but never seems to make the cut. There are; however, two levels of radioactivity that have been proposed as an MCL over the years-4,000 picocuries and 300 picocuries. Radon was found at the higher proposed Alternative MCL of 4,000 picocuries per liter (pCi/L) in less than 1% of samples, but was found above the proposed MCL of 300 pCi/L in 55% of the samples. Each of the remaining six elements and radionuclides was detected at concentrations greater than human-health benchmarks in 3%-19% of samples taken. These contaminants originate from the rocks and sediments that contain the aquifers.
The three contaminants that exceeded MCLs in at least 1% of samples from primarily man-made sources were nitrate (a nutrient), dieldrin (an insecticide that has been banned by the US EPA, but was previously used for termite control and other applications), and perchloroethene (or PCE, a solvent and degreasing agent used for drycleaning). Each of these contaminants was detected at concentrations greater than MCLs or HBSLs in 1% - 3% of the groundwater tested. Nitrate occurs naturally, but most nitrate concentrations greater than 1 milligram per liter (which is one-tenth of the nitrate MCL) originates from man-made sources such as fertilizers, livestock, and human wastewater from septic systems or wastewater treatment plants. Pesticides are released into the environment primarily through their application to agricultural lands, such as croplands, and to non-agricultural lands, such as lawns, golf courses, commercial landscaping and public areas.
There were several chemicals and compounds found at low levels determined to be at least 10% of the MCL or other human health screening levels. Naturally occurring elements, radionuclides and pesticide compounds were extensively found at these low concentrations. Trace levels of pesticide compounds or VOCs were detected in 64% of the groundwater samples from public wells. Three-quarters of the organic-contaminants contained an herbicide (atrazine or simazine) or an herbicide degradate (deethylatrazine), and about 40% contained the solvents perchlorethene or trichloroethene. Pesticides and VOCs were detected in a significantly greater proportion of samples from unconfined aquifers than in samples from confined aquifers. The groundwater with the greatest number of contaminants were consistently from shallower unconfined aquifers demonstrating the natural protection provided by a confining geological layer.
MCLs, HBSLs or other health screening levels were not available for 144 (43%) of the contaminants analyzed for in this study. Most of the contaminants without available health benchmarks were man made organic contaminants. Nine of these unregulated contaminants were detected in 6%-35% of groundwater samples. These contaminants were the gasoline additive MTBE, the solvent 1,1-dichloroethane, and the herbicide breakdown products alachlor ethane sulfonic acid, alachlor oxanilic acid, metolachlor ethane sulfonic acid, metolachlor oxanilic acid, deethylatrazine and deisopropylatrazine are which are break down products of atrazine. The ubiquity of these contaminants is worrisome. Most herbicide degradates found are not currently regulated by the USEPA in drinking water under the SDWA, but may be regulated by USEPA under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). It may be time for the USEPA to develop the toxicology data to evaluate the herbicide degradates for possible human health impacts.
Natural contaminants, the geological trace elements and radionuclides were found at concentrations exceeding human health screening levels or MCLs and at low levels in groundwater samples taken from unconfined and confined aquifers. This was expected since the source of these contaminants is the geological formations of the aquifer. However, most man-made contaminants were found at both trace and concentrations exceeding human health screening levels or MCLs in groundwater samples from unconfined aquifers. These man-made contaminants originate at the surface and the unconsolidated aquifers provided little natural protection from surface infiltration. The degradeates from the newer herbicides alachor and atrazine have penetrated a significant percentage of the nation’s groundwater and needs to be studied further and the widespread use for ornamental use should be reconsidered.