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BACKGROUND: Oregon has approximately 350,000 active private wells that provide drinking water for an estimated 23% of the population. Private wells in Oregon are unregulated, but testing is required for arsenic, total coliform bacteria and nitrates at the point of a real estate transaction. Sellers are required to submit test results to the Oregon Health Authority. The original real estate transaction law, passed in 1989, required testing for nitrates and total coliform bacteria. Arsenic was added to the law in 2009. Drinking water that has high levels of arsenic can cause many health effects, including skin thickening and discoloration, heart, lung, liver, immune, nervous, or reproductive system disorders, and some cancers. High levels of nitrate are associated with methemoglobinemia, thyroid dysfunction, negative reproductive effects, and cancers of the stomach or bladder.
METHODS: The real estate transaction data were cleaned to remove duplicate entries, standardize test result formats, and eliminate outliers. Data were geocoded and mapped by contaminant (arsenic and nitrates). We calculated descriptive statistics for the entire state and for each county including the percentage of test results that met or exceeded safe drinking water standards, the number of test results submitted by year, the range and the maximum of test result values. We analyzed the data by contaminant for statistically significant clusters using a global measure of spatial association (Getis-Ord General G statistic) and a local measure of spatial association (Getis-Ord Gi* statistic).
RESULTS: The real estate transaction database contains over 9,000 arsenic samples. Of these, 3.4% met or exceeded the safe drinking water standard of 10 ppb. 89.3% of arsenic samples were below detectable levels. The real estate transaction database contains over 24,000 nitrate samples. Of these, 1.6% met or exceeded safe drinking water standards of 10 ppm. 82.5% of samples had a nitrates result below background levels. Visual assessment of mapped arsenic and nitrate test results suggested clustering of high values in areas known to be at higher risk because of the underlying geology or human activity. Local and global measures of spatial association indicated statistically significant clusters for both contaminants.
CONCLUSIONS: Test results from real estate transactions provide a unique source of public health surveillance data for contaminants in Oregon’s unregulated drinking water sources. Analysis of the real estate transaction data can be used to guide future outreach efforts to areas of the state with higher frequency of elevated arsenic and nitrate test results.