ZINC IN GROUNDWATER: A ROLE IN LEARNING AND MEMORY DEFICIT?

Zinc (Zn) is an essential trace mineral and is a cofactor in more than 300 enzymes. Whereas Zn deficiency is well studied, problems with over-consumption are not well known. Our experiments have shown that excessive Zn concentrations in drinking water may impair learning and memory. Behavioral testing of mice provided with drinking water containing 10 mg/L of dissolved Zn showed the development of increased anxiety during stressful tasks. Testing at 4 months and again at ~10 months of age indicated that mice drinking water with high concentrations of Zn showed hippocampal dependent spatial memory deficits compared to similar animals drinking water containing < 0.5 mg/L Zn. Inductively coupled plasma – optical emission spectrometry analysis of the cortex and hippocampus of these animals showed increased levels of Zn within the hippocampus of mice drinking high Zn water, verifying that dissolved Zn in the drinking water accumulates in specific regions of the brain.

The US EPA secondary drinking water standard for Zn is 5 mg/L and most public drinking water sources in the US are well below this limit. However private wells tapping local groundwater sources in the US are not subject to this standard. The World Health Organization does not have health based guidelines for Zn in drinking water, but the background document for the development of guidelines reports concentrations exceeding 5 mg/L Zn in some drinking waters.

To examine the possible relation of zinc-induced impairment to the environment we have targeted areas of high concentrations in private drinking water sources using maps from the USGS National Water Quality Data Assessment Program that illustrate zinc concentration of well-water samples across the US. Many of the natural occurrences can be anticipated from the general geology in areas related to the weathering of metallic mineral deposits, such as the northern Rocky Mountains (especially SE Idaho), southeast Arizona, or the tri-state area of southern Missouri. Other high-zinc areas include central Florida and south-central Texas. Although there are several natural sources of Zn in drinking water, anthropogenic contributions of Zn to drinking water sources can be statistically significant. Experiments are underway to explore the connection of elevated Zn in drinking water to the geochemical environment in the US.