Potential Impact of Regional Geology on Arsenic in Ground Water In New England is Released in USGS Report

Is arsenic in ground water --at levels of concern to public health--tied to regional geology? Analysis of arsenic data for public-supply wells in eastern New England shows a statistical relation between the presence of arsenic in ground-water samples and general bedrock geologic units, according to Joseph Ayotte, Hydrologist, and his colleagues at the U.S. Geological Survey.

The USGS study analyzed data from over 800 public bedrock wells, all of which currently meet the U.S. Environmental Protection Agency drinking-water standard of 0.05 milligrams per liter.

The regional study, part of the USGS National Water Quality Assessment Program, showed a significant correlation between the concentration of arsenic (between 0.005 and 0.05 milligram per liter) and the general rock units into which the wells were drilled.

The source of the arsenic in water is unclear as to whether it is from exposure to rocks or from land-use practices. The analysis, however, points to a strong statistical correlation with rock types. Arsenic concentrations are significantly higher in water from wells drilled in a group of metasedimentary rocks trending from eastern Connecticut through east-central Massachusetts, southeast New Hampshire, and coastal Maine. These rocks were once a package of variably calcareous marine sediments formed between 380 and 440 million years ago.

Additional studies are planned and underway to help determine the causes of arsenic in ground water, whether from geologic or manmade sources. Understanding the occurrence and distribution of arsenic is important in that the USEPA is currently working on guidelines that may lower the allowable concentration of arsenic in order to meet a stricter drinking-water standard.

Wells that currently meet the standard for arsenic might, when new guidelines are released, exceed a new drinking-water standard. The USGS study showed that of the 800 wells analyzed, about 20 percent exceeded 0.005 milligrams per liter and 10 percent exceeded 0.01 milligrams per liter. A better understanding of the effect of the local geology and land use on the siting of wells and subsequent water quality will, therefore, be increasingly important in helping water managers to be in compliance with USEPA standards.

Arsenic was present at or above 0.005 milligrams per liter in almost half of the water samples drawn from the metasedimentary rock group, which were derived from calcareous sediments, and in less than 25 percent of the samples drawn from wells completed in five other general rock groups that were part of the study.

The drinking-water limit, maximum contaminant level, or lifetime health advisory for various constituents or chemicals in water are levels set by State or Federal agencies in order to provide protective measures for human, animal, and aquatic health. The presence of a chemical does not necessarily mean that it is in sufficient quantity to be of health or environmental concern.

Data for the USGS study were drawn from water analyses collected for environmental compliance monitoring by the Maine Department of Human Services, New Hampshire Department of Environmental Services, the Massachusetts Department of Environmental Protection and the Rhode Island Department of Health.

The USGS National Water-Quality Assessment (NAWQA) Program is designed to describe the status and trends in the quality of the nation's ground- and surface-water resources and to provide a sound understanding of the natural and human factors that affect the quality of these resources. As part of the program, investigations will be conducted in 59 areas-- called "study units"-- throughout the nation to provide a framework for national and regional water-quality assessment. Regional and national synthesis of information from study units, such as this snapshot of arsenic in ground water in New England, will consist of comparative studies of specific water-quality issues using nationally consistent information.

As the nation's largest water, earth and biological science, and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. Impartial information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.

The Water-Resources Investigation Report 99-4162 titled "Relation of arsenic, iron, and manganese in ground water to aquifer type, bedrock lithogeochemistry, and land use in the New England Coastal Basins," by Joseph D. Ayotte and others is available in university and state libraries and from USGS, Information Services, Box 25286, Federal Center, Denver, CO 80225 at  1-888-ASK-USGS or from the New Hampshire/Vermont District office, 361 Commerce Way, Pembroke, NH 03275. For additional information, contact the Outreach Coordinator at (603) 226-7837.

* * * USGS * * *

Address:

New Hampshire/Vermont District

361 Commerce Way

Pembroke, NH 03275

 

General Contact: Debra Foster	Phone: (603) 226-7837	Email: dhfoster@usgs.gov
Technical Contact: Joseph Ayotte	Phone: (603) 226-7810	Email: jayotte@usgs.gov

In-depth information about USGS water-quality projects in New England may be found on the New Hampshire/Vermont District web site at:  http://nh.water.usgs.gov.

Highlights of the arsenic report:

• USGS has shown that for over 950 public wells in eastern New England, arsenic is primarily a problem for bedrock wells and much less a problem for wells in sand and gravel

 
• There is a distinct belt of bedrock public-supply wells in which nearly half of the wells contain arsenic above 0.005 milligrams per liter. The belt runs from north-central Massachusetts, through south-eastern New Hampshire, and along near-coastal Maine.  In addition, high concentrations also were found in the Concord and Bow, N.H. areas.
   
• It remains unclear if the source of arsenic is geologic or man made. There is limited evidence that points to man-made sources and there is limited evidence that points to rocks. It may also be related to land-use practices in combination with certain geologic settings.

 
• The USGS and others currently have studies underway to address the occurrence and distribution of arsenic, the sources, and movement of arsenic in well waters in New England.

 
• The USGS also will be studying a test site in the seacoast of New Hampshire area to study local arsenic-related proceses that includes two wells containing arsenic. Sampling begins at those wells in June 2000.