Seasonal variation of arsenic concentration in wells in Nevada.

The issue of seasonal arsenic measurement variability poses consequences regarding the interpretation and frequency of well water measurements for both public health research and surveillance. In this study, we evaluated seasonal variability in arsenic concentration in 356 wells in western Nevada. River flow data obtained from US Geological Survey National Water Information System were used to classify seasons as wet or dry, and mean differences in arsenic well concentrations measured during these seasons were compared. Arsenic concentrations in these wells averaged 72.9 microg/L (range, non-detect to 3000 microg/L). The mean difference in arsenic concentrations between the wet and dry seasons was -3.3 microg/L (p = 0.78; average percent difference = 2.3%). Eighty wells (22%) had higher arsenic concentrations in the wet season, 75 wells (21%) had higher arsenic concentrations in the dry season and no difference was seen in 201 wells (56%). The mean differences in wells with arsenic levels of 0-10, 11-50, 51-200, > 200 microg/L were -1.4 microg/L(p=0.43), 9.2 microg/L(p=0.36), 15.1 microg/L(p=0.30), and -49.9 microg/L(p=0.59). In summary, although changes in arsenic concentrations were seen in some wells, clear trends in arsenic concentration over time were not associated with the wet and dry seasons. These findings provide evidence that, in our study area as a whole, little seasonal variability occurs in arsenic concentrations, and repeated assessments of arsenic concentrations based on season might add little value to the accuracy of health effects research or public health surveillance.