ELEVATED LEAD IN GROUNDWATER: NATURAL OR ANTHROPOGENIC?

Lead contamination in the environment is a threat to human health, particularly in young children, who absorb four to five times as much lead as adults. Determining the source of elevated lead concentrations in groundwater in southern Missouri is confounded by multiple possible natural and anthropogenic contributors. It is particularly difficult to determine if elevated lead concentrations are (a) the result of contamination from plumbing or well construction materials, (b) the result of groundwater contamination from former mining activities, or (c) the result of groundwater in contact with natural, but unmined, mineralization within the aquifer.

To determine if mining activities have resulted in contamination of domestic wells outside of the Jasper County Superfund site, data from over 1,900 domestic wells in Jasper County collected by the U.S. Environmental Protection Agency were analyzed to determine probable sources of lead in drinking water. All 1,900 wells had total and dissolved lead, zinc, copper, and barium data; 173 wells had total lead isotope data (²⁰⁸Pb, ²⁰⁶Pb, ²⁰⁷Pb); 52 wells had data for dissolved lead isotopes; and less than 100 samples had data for calcium, magnesium, bicarbonate, and sulfate.

A strong relation between the presence of lead isotopes outside the Tristate Mining District (TSMD) range and Pb_t/(Pb_t+Cu_t) ratios less than 0.35 was found, and this ratio was used as a screening tool to indicate possible anthropogenic contamination from plumbing materials. About 92 percent of the samples having Pb_t/(Pb_t+Cu_t) ratios less than 0.35 had lead isotopes outside the TSMD range indicative of anthropogenic contamination. A decision tool, constructed to classify the domestic wells based on geochemical data, indicated that 156 of the 240 wells (65 percent) with total lead concentrations in samples above the maximum contaminant level (MCL) were most likely the result of groundwater interactions with natural, but unmined, lead mineralization within the bedrock aquifer or possibly contamination from former mining operations. Sixty-two (62) wells (about 26 percent) had conflicting data indicating probable anthropogenic plumbing contamination, and 22 wells had insufficient data to make any possible interpretation about the origin of total or dissolved lead detected above the MCL.