GEOCHEMICAL AND HYDROCHEMICAL ANALYSIS OF A QUARTZITE-DOLOSTONE BEDROCK AQUIFER IN THE CENTRAL CHAMPLAIN VALLEY, VERMONT

Previous studies have identified groundwater contamination from naturally occurring inorganic constituents in fractured bedrock aquifers in some areas of Vermont. For example, quartzites and phyllites of the hanging wall of the Hinesburg thrust fault are sources of elevated radionuclides (uranium and alpha radiation). This study aims to assess a fractured bedrock aquifer where lithology is comprised mainly of Cambrian Monkton Quartzite and Dunham Dolostone. The area is situated in the footwall of the Paleozoic Hinesburg thrust and is bisected by the Mesozoic St George normal fault. To date, no systematic information on groundwater quality, aquifer potential or bedrock composition has been collected from this part of the Champlain Valley. Groundwater from 28 wells and from representative rock outcrops have been sampled and are undergoing analysis for major and trace element composition. Water samples were collected from purged wells unaffected by water softeners.

Data from 20 wells has been obtained thus far. Gross alpha radiation is above the Vermont Department of Health action level of 5 pCi/L in 10 % of wells tested to date – this is the level at which further testing for radium is recommended. Groundwater sampled from wells completed in the Dunham Dolostone tends to have higher alpha radiation levels than other formations. Hydrochemical data indicate that 50 % of wells contain > 2 µg/L uranium, 10 % contain > 5 µg/L U, and 5% contain > 10 µg/L U; notably, none exceed the Vermont U MCL of 20 µg/L. Alpha radiation and U are positively correlated in groundwater, suggesting that U is the likely source of alpha radiation; calculations of residual gross alpha content suggest that radium is < 5 pCi/L in this aquifer system. Variation in reduction-oxidation potential of the aquifer system is indicated by varied abundances of manganese and iron; for example, 20 % of wells contain Mn concentrations >100 µg/L and Fe exceeds 100 µg/L in 35 % of wells; in other cases, both are below detection limit. Spatial and chemical associations of these redox-sensitive elements are being explored. Arsenic in all wells was less than the EPA maximum contaminant level of 10 µg/L. Results thus far imply that the Monkton Quartzite contributes lower amounts of radionuclides to groundwater than do the older quartzites of the Cheshire and Pinnacle formations.