Challenging Water Resources Professionals for the 21st Century
By Roy L. Glass
U.S. Geological Survey, Alaska Science Center, Anchorage
Abstract
In 1999, the National Academy of Sciences concluded that the current standard of 50 micrograms per liter (µg/L) does not sufficiently protect public health, and recently, the Federal Government revised the maximum contaminant level to 10 µg/L. This new standard will become effective in January 2006 and applies to public water-supply systems.
Studies of groundwater in the Cook Inlet Basin by the U.S. Geological Survey (USGS), Alaska Department of Environmental Conservation, Alaska Department of Natural Resources, and Municipality of Anchorage indicate that some domestic and public-supply wells yield water containing concentrations of arsenic that exceed the new 10-µg/L standard. High concentrations of arsenic have been observed in groundwater in the Municipality of Anchorage and in the Kenai Peninsula and Matanuska-Susitna Boroughs. Water from 30 percent of the 220 wells sampled for arsenic in the Cook Inlet area by the U.S. Geological Survey from 1969 to 1999 had an arsenic concentration of 10 µg/L or higher. The highest concentration in water sampled by the U.S. Geological Survey was 150 µg/L. Samples of drinking water collected for the Alaska Department of Environmental Conservation indicate that high concentrations of arsenic in some public-supply water; at least one sample from each of more than 60 public-water systems in south-central Alaska reportedly had a sample that had an arsenic concentration greater than 10 µg/L. In contrast, samples of water from streams in the Cook Inlet Basin had very low concentrations of dissolved arsenic. Of 58 surface-water sites sampled by the USGS, 50 had concentrations that were at or less than the 1-µg/L detection level.
Introduction
Long-term exposure to high concentrations of arsenic in drinking water and food has been linked to bladder, lung, and skin cancer in humans (National Research Council, 1999). In 1999, the National Academy of Sciences concluded that the current standard of 50 µg/L does not sufficiently protect public health. In October 2001, the Federal Government revised the maximum contaminant level (MCL) to 10 µg/L (U.S. Environmental Protection Agency, 2001), which will become effective in January 2006. MCLs only apply for public-water supplies but can provide a guide for water users in homes that have private supply wells.
Arsenic is a naturally occurring element in the Earth's crust and is commonly adsorbed to or coprecipitated with iron oxide minerals; adsorbed to clay-mineral surfaces; associated with sulfide minerals or organic carbon; or present in some types of volcanic rocks. Analyses by the U.S. Geological Survey (USGS) of trace elements in streambed sediments sampled for the National Uranium Resource Evaluation and National Water-Quality Assessment programs and studies in cooperation with the National Park Service indicate that concentrations of arsenic in the Cook Inlet Basin appear to be naturally high (Frenzel, 2000). The natural dissolving or desorbing of arsenic from soils and aquifer materials may introduce arsenic into the area's water.
The addition of arsenic to water in the Cook Inlet Basin by humans is minimal. More than 90 percent of the total use of arsenic in the United States is as a preservative in wood products. The total contribution of arsenic from treated wood to water in the Cook Inlet Basin probably is extremely small compared to the large amount of arsenic that occurs naturally in the area's rocks and soils.
Ground water
Many domestic and public-supply wells throughout the Cook Inlet Basin yield water having arsenic concentrations greater than the new MCL of 10 µg/L, but concentrations vary widely, even at a local scale (fig. 1). The only way to be certain of the arsenic concentration in water supplied by a given well is to have the water tested. The health effects of arsenic depend on the concentration of arsenic in drinking water and the amount of water consumed.
Figure 1. Spatial distribution of dissolved arsenic in selected wells and streams sampled by the U.S. Geological Survey
Concentrations of arsenic in water supplied to the public are commonly greater than 10 µg/L. Water sampled from more than 60 public-water systems in south-central Alaska had arsenic concentrations greater than 10 µg/L (Keven Kleweno, Alaska Department of Environmental Conservation, written commun., 2000). Some were from wells supplying water to subdivisions, apartments, trailer courts, restaurants, day-care centers, and schools. High arsenic concentrations were found in water from wells in the Municipality of Anchorage and the Matanuska-Susitna and Kenai Peninsula Boroughs.
Because of concerns about high levels of arsenic in groundwater near Sand Lake in the western part of Anchorage, the Municipality of Anchorage's Department of Health and Human sampled water from 25 domestic wells. Water samples were collected between December 2000 and March 2001 from faucets inside houses; many were collected after the water passes through water softeners and(or) filters. Concentrations were less than the detection level (1 µg/L) at 3 of the 25 sites. Tap water from 19 of the 25 residences had concentrations of arsenic greater than 10 µg/L, of those, 13 were greater than 24 µg/L, and 3 exceeded the current MCL of 50 µg/L. The highest concentration was 89 µg/L.
Analyses by the USGS and Alaska Department of Natural Resources indicate that concentrations of arsenic in groundwater vary widely, even among wells within a single residential subdivision, possibly owing to local differences in geology, geochemistry, and aquifer conditions. Of the 220 wells sampled by the USGS from 1969 through 1999, about one-third had water containing dissolved-arsenic at concentrations of 1 µg/L or less, about 30 percent had concentrations greater than 10 µg/L, and about 5 percent had concentrations greater than 50 µg/L. The highest dissolved-arsenic concentration from the USGS samples, 150 µg/L, was for water from a well on the west side of Cook Inlet. Most ground-water samples that contained high arsenic concentrations were from chemically reduced environments.
Concentrations of dissolved arsenic greater than 50 µg/L were observed in water from several wells on the Kenai Peninsula (in Nikiski, Kenai, Soldotna, and Sterling) by the USGS (Glass, 1996, 1999, 2001) and the Alaska Department of Natural Resources (Maurer, 1993; Maurer and Ireland, 1994). Concentrations also could also be high elsewhere, but sampling and reporting of water-quality conditions have not been done for many locations where groundwater is used for water supplies. For example, few analyses of arsenic are currently available for the hillside areas of Homer, south Anchorage, and Eagle River; in these upslope neighborhoods, thousands of homes have individual wells for their water supply.
Surface water
Although arsenic in surface water is derived primarily from the natural weathering of soils and rocks and from the discharge of groundwater, detectable arsenic concentrations are uncommon in Cook Inlet Basin streams. Analyses of water samples collected by the USGS from 1969 through 2000 are available from several sites in the basin. These analyses include both dissolved arsenic (for which water samples are passed through a 0.45-micrometer filter) and total arsenic (for which water samples are left unfiltered). Of the 58 surface-water sites having dissolved-arsenic analyses, 50 had concentrations that were at or less than the 1-µg/L detection level. The highest dissolved-arsenic concentrations were observed at Fritz Creek near Homer (3 µg/L) and Swanson River near Kenai (9 µg/L). Concentrations of arsenic in the unfiltered samples generally were higher than in the filtered samples, probably because of the arsenic associated with suspended sediment particles. The three sites having the highest total-arsenic concentrations were Knik River near Palmer (14 µg/L), Maclaren River near Paxson (18 µg/L), and a canal on the east side of Sand Lake in Anchorage (24 µg/L).
References cited
Frenzel, S.A., 2000, Selected organic compounds and trace elements in streambed sediments and fish tissues, Cook Inlet Basin, Alaska: U.S. Geological Survey Water-Resources Investigations Report 00-4004, 39 p.
Glass, R.L., 1996, Ground-water conditions and quality in the western part of the Kenai Peninsula, South-Central Alaska: U.S. Geological Survey Open-File Report 96-466, 66 p.
Glass, R.L., 1999, Water-quality assessment of the Cook Inlet Basin, Alaska-Summary of data through 1997: U.S. Geological Survey Water-Resources Investigations Report 99-4116, 110 p.
Glass, R.L., 2001, Ground-water quality, Cook Inlet Basin, Alaska, 1999: U.S. Geological Survey Water-Resources Investigations Report 01-4208, 58 p.
Maurer, M.A., 1993, Ground-water quality data collected from 30 wells in July 1991, west Nikiski, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 93-2, 59 p.
Maurer, M.A., and Ireland, R.R., 1994, Ground-water data for the Sterling area, Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 94-50, 133 p.
National Research Council, 1999, Arsenic in drinking water: Washington, D.C., National Academy Press, 273 p.
U.S. Environmental Protection Agency, 2001, National Primary Drinking Water Regulations-Arsenic and clarifications to com-pliance and new source contaminants monitoring-Final rule: U.S. Code of Federal Regulations, Title 40, Pts. 9, 141, and 142, p. 6975-7066.
(Revised January 2001.) [URL: <http://www.epa.gov/safewater/ars/arsenic _finalrule.pdf> (accessed January 30, 2002)].
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