MANGANESE SOURCING AND TRANSPORT IN MASSACHUSETTS GROUNDWATER: UNDERSTANDING DISSOLUTION MECHANISMS AND CREATING A TEMPORALLY VARIANT DATABASE

Abstract

Manganese in groundwater is ubiquitous in the U.S., with an estimated 2.6 million residents consuming groundwater with elevated Mn concentrations. All New England states have groundwaters with Mn concentrations over the USEPA secondary maximum contaminant limit of 50 ug/L. Studies often utilize point-sampling and do not capture temporal processes, which are suggested to be important as they relate to redox variability across seasons and may offer more insight to intermittent conditions driving Mn dissolution. We have collected surface, near surface, and groundwater samples, building a temporally variant dataset to determine the source of Mn in Massachusetts groundwater. Field parameters such as dissolved oxygen, pH, and ORP are measured in-situ for each sample, and samples are analyzed in the laboratory for trace metals, anions, and dissolved organic and inorganic carbon. Our preliminary data shows 61% of groundwater samples exceed the USEPA SMCL for Mn, with most of these samples coming from confined aquifer waters. This conflicts with other studies that observe higher Mn concentrations in shallow wells and groundwaters. Mn was not widely abundant in solid phase soils nor drilled rock fragments analyzed from the region, with mean Mn concentrations at 77 mg/kg and 122 mg/kg, respectively. We hypothesize that in geologic settings found in Massachusetts, Mn dissolution and transport behaves as a bottom-up process with transport playing an important role, rather than the localized top-down processes observed in aquifers in other regions. Using these preliminary findings along with stable water isotope data, we hope to further investigate the source of Mn to groundwaters and the role and importance of both transport and temporal redox variation. Private wells are often not extensively treated for Mn contamination, as many public groundwater supplies are, but offer an opportunity to better investigate the mechanisms behind manganese sourcing and transport across a wide range of geologic and soil conditions.