More than 20 percent of private domestic water wells sampled nationwide contain at least one contaminant at levels of potential health concern, according to a study by the U.S. Geological Survey (USGS).

     About 43 million people - or 15 percent of the Nation's population - use drinking water from private wells, which are not regulated by the Federal Safe Drinking Water Act.  Test your well water.

    USGS scientists sampled about 2,100 private wells in 48 states and found that the contaminants most frequently measured at concentrations of potential health concern were inorganic contaminants, including radon and arsenic.  These contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is drawn.  Nitrate was the most common inorganic contaminant derived from man made sources such as from fertilizer applications and septic tanks that was found at concentrations greater than the Federal drinking water standard for public water supplies (10 parts per million).  Nitrate was greater than the standard in about 4 percent of sampled wells.  The study shows that the occurrence of selected contaminants varies across the country, often following distinct geographic patterns related to geology, geochemical conditions, and land use.  For example, elevated concentrations of nitrate were largely associated with intensively farmed land, such as in parts of the Midwest Corn Belt and the Central Valley of California.  Radon was found at relatively high concentrations in crystalline rock aquifers in the Northeast, in the central and southern Appalachians, and in central Colorado. 

     "The results of this study are important because they show that a large number of people may be unknowingly affected," said Matt Larsen, USGS Associate Director for Water.  "Greater attention to the quality of drinking water from private wells and continued public education are important steps toward the goal of protecting public health."

     The USGS sampled private wells from 1991 to 2004 in 30 of the Nation's principal aquifers used for water supply.  As many as 219 properties and contaminants, including pH, major ions, nutrients, radionuclides, trace elements, pesticides, arsenic, volatile organic compounds, and microbial contaminants, were measured.  Sampled water was taken from private water wells before any whole house filtration home treatment.  Other contaminants found in the private water wells were man made organics, including herbicides, insecticides, solvents, disinfection by-products, and gasoline chemicals.  Few organic contaminants (7 out of 168) exceeded health benchmarks, and were found above health benchmarks in less than 1 percent of sampled wells.  Organic contaminants were detected at lower concentrations in more than half (60 percent) of sampled wells, indicating that a variety of contaminant sources including agricultural, residential, and industrial that can affect the quality of water from private water wells.

     Contaminants found in private water wells usually co-occurred with other contaminants as mixtures rather than alone, which can be a concern because the total combined toxicity of contaminant mixtures can be greater than that of any single contaminant.  Mixtures of contaminants at relatively low concentrations were found in the majority of well water.

     The USGS report identifies the need for continued research because relatively little is known about the potential health effects of most mixtures of contaminants, and the additive or synergistic effects on human health of mixtures of man made chemicals even at low levels are not well understood.

    Bacteria, including total coliform bacteria and Escherichia coli (e coli), were found in as many as one third of a subset of 400 water wells.  These bacteria are typically not harmful but can be an indicator of fecal contamination.

     About half of the 2,100 sampled water wells had at least one property or contaminant outside recommended ranges for cosmetic or aesthetic purposes, such as total dissolved solids, pH, iron, sulfur, and manganese.  Human health benchmarks used in the study included drinking water standards for contaminants regulated under the Federal Safe Drinking Water Act and non-enforceable USGS Health Based Screening Levels (HBSLs) for unregulated contaminants, developed by USGS in collaboration with the U.S. Environmental Protection Agency.

     About half of the wells deemed to have potential health concerns had concentrations greater than Maximum Contaminant Levels specified by the Safe Drinking Water Act for public water supplies.  In relating measured concentrations to health benchmarks, this study offers a preliminary assessment of potential health concerns that identifies conditions that may require further investigation.  The research is not a substitute for comprehensive risk and toxicity assessments.  Private water well owners, who generally are responsible for testing the quality of their well water and treating, if necessary, can find more information about well maintenance, testing and water treatment options including in-home whole house water filter treatment devices at WaterFiltersOnline.com.

Ground Well water quality

Just because you have a well that yields plenty of water doesn't mean you can go ahead and just take a drink.  Because water is such an excellent solvent it can contain lots of dissolved chemicals.  And since ground water moves through rocks and subsurface soil, it has a lot of opportunity to dissolve substances as it moves.  For that reason, ground water will often have more dissolved substances than surface water will.

Even though the ground is an excellent mechanism for filtering out particulate matter, such as leaves, soil, and bugs, dissolved chemicals and gases can still occur in large enough concentrations in ground water to cause problems.  Underground water can get contaminated from industrial, domestic, and agricultural chemicals from the surface.  This includes chemicals such as pesticides and herbicides that many homeowners apply to their lawns, or farmers apply to their fields.

Contamination of ground water by road salt is of major concern in northern areas of the United States.  Salt is spread on roads to melt ice, and, with salt being so soluble in water, excess sodium and chloride is easily transported into the subsurface ground water.  The most common water quality problem in rural water supplies is bacterial contamination from septic tanks, which are often used in rural areas that don't have a sewage treatment system.  Effluent (overflow and leakage) from a septic tank can percolate (seep) down to the water table and maybe into a homeowner's own well.  Just as with urban water supplies, chlorination may be necessary to kill the dangerous bacteria.

The U.S. Geological Survey is involved in monitoring the Nation's ground water supplies. A national network of observation wells exists to measure regularly the water levels in wells and to investigate water quality.

Contaminants can be natural or human induced

Naturally occurring contaminants are present in the rocks and sediments.  As ground water flows through sediments, metals such as iron and manganese are dissolved and may later be found in high concentrations in the water.  Industrial discharges, urban activities, agriculture, ground water pumpage, and disposal of waste all can affect ground water and well water quality.  Contaminants from leaking fuel tanks or fuel or toxic chemical spills may enter the ground water and contaminate the aquifer.  Pesticides and fertilizers applied to lawns and crops can accumulate and migrate to the water table.