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Water Quality 17 USEPA SDWR for MBAS of 0.5 mg/L (table 2). This exceed- ance may indicate the local contamination of ground water by waste from an ISDS. Detection of anthropogenic surfactants probably relates more to the failure of individual ISDS's rather than being representative of the entire ISDS area. Other detected concentrations of MBAS were too low to discern whether the surfactants were naturally occurring or had an anthropogenic source. Radon is a naturally occurring radioactive gas that is formed from the decay of uranium. Major sources of radon include rocks containing uranium minerals, soils derived from such rocks, and ground water that has been in contact with ura- nium-bearing rocks and soils. Uranium decays to radium (and other products) and then to radon, which is soluble in water and occurs in ground water as a gas. Radon can be present in indoor air as a result of seepage from soils underneath a building (the primary mechanism) or through the use of well water (through aeration) that contains radon. The primary public health risk associated with radon is the breathing of radon from indoor air in homes (U.S. Environmental Protection Agency, 2000b). The National Research Council estimated that the radon in indoor air resulting from the breakdown of uranium in soils has con- tributed to about 20,000 lung cancer deaths each year in the United States (National Research Council, 1999). Only a small portion of radon in indoor air is derived from the aeration of radon-containing well water through household use of the water. This source can, however, increase a person's risk of lung cancer over a lifetime. Drinking water that contains radon presents a risk of internal-organ cancers, particularly stomach cancer (U.S. Environmental Protection Agency, 2000b). Radon can be removed from well water through the use of carbon filters or aeration devices (U.S. Environmental Protection Agency, 2000b). Different isotopes of radon are produced by the decay of radium, but only one, radon-222, has environmental importance (Hem, 1992). Excluding site 1, which was sampled only in 2000 and 2001, three of the water samples collected from each well in 1998 and 1999 were analyzed for radon-222. Concentrations ranged from 365 to 6,380 pCi/L (picocuries per liter), and the median concentration was 760 pCi/L (table 2). All concentra- tions exceeded the USEPA-proposed MCL for radon-222 in drinking water of 300 pCi/L (table 2), which is under review. When variability and uncertainty in radon-222 concentrations were taken into account (table 8, Appendix II), part of the con- centration range for one sample was less than the USEPA- proposed MCL. Among the 10 wells, water samples from site 4, an alluvial aquifer well immediately west of Fraser, had the highest median radon-222 concentration (6,170 pCi/L). This concentration was more than three times higher than the next highest median concentration (fig. 7). All radon-222 concentra- tions for site 4 were greater than the USEPA-proposed alterna- tive MCL for radon-222 in drinking water of 4,000 pCi/L (table 2), which also is under review. Statistically significant differences in radon-222 concentrations were not indicated for samples from wells completed in the alluvial aquifer and wells completed in the Troublesome Formation aquifer (table 3). Radon-222 concentrations in ground-water samples from wells in the Fraser River watershed were similar to concentrations in ground-water samples from wells in other areas of the Upper Colorado River Basin in Colorado, which ranged from 305 to 4,030 pCi/L in 1997 (Apodaca and others, 2000). Changes Over Time For each well except sites 1 and 3, data for field properties and water-quality constituents for ground-water samples were examined qualitatively for changes over time by using LOWESS smooth curves. Insufficient data were available for wells 1 and 3 to be included in the analysis. Properties and con- stituents that were graphed included water temperature, specific conductance, dissolved-oxygen concentration, pH, turbidity, alkalinity, dissolved solids, major ions (calcium, chloride, magnesium, potassium, silica, sodium, and sulfate), manga- nese, nutrients (nitrate and orthophosphate), and dissolved organic carbon. For most constituents from most wells, changes in the data over time were not apparent. Changes were apparent, however, for samples from site 7, an alluvial aquifer well in the Tabernash area. Specific conductance and calcium, magnesium, chloride, and sulfate concentrations appeared to increase over time (fig. 8). These changes are qualitative changes only and are not legitimate statistical trends because of the short time periods of the data collection, the small sample sizes, and data collec- tion only occurring in 2 months of each year. The change in spe- cific conductance probably was a function of higher concentrations of calcium, magnesium, chloride, and sulfate over portions of the sampling period. Calcium and magnesium concentrations generally increased between May 1999 and May 2001, whereas chloride and sulfate concentrations increased after May 2000, respectively (fig. 8). The apparent increases in specific conductance and calcium, magnesium, chloride, and sulfate concentrations for site 7 had little effect on water quality of the well, but the changes indicate that shallow ground water could be susceptible to changes in land use in the watershed. Specific conductance remained moderate, and the water remained suitable for use. Chloride and sulfate concentrations remained well below the USEPA drinking-water standard of 250 mg/L for each constituent (table 2). Surface Water As stated previously, field properties for the surface- water site Fraser River below Crooked Creek at Tabernash (hereinafter Fraser River at Tabernash) were measured bimonthly between August 1998 and September 2001, and water samples were collected for laboratory analyses during each site visit. The resulting data for WY's 1999, 2000, and