DRINKING WATER QUALITY IN THE GLACIAL AQUIFER SYSTEM, NORTHERN USA: GEOCHEMICAL PROCESSES INFLUENCE GROUNDWATER CONTAMINANT PATTERNS

Groundwater supplies 50% of drinking water worldwide, but compromised water quality from geogenic and anthropogenic contaminants can limit use of groundwater as a drinking water source. Geochemical processes often drive small- and large-scale patterns of groundwater contamination. Groundwater quality in the glacial (GLAC) aquifer system, northern USA, is presented in the context of 17 distinct hydrogeologic terranes across the study area. Groundwater withdrawals from the GLAC serve about 30 million people and account for about 5% of water supply in the USA. We present a quantitative assessment of groundwater quality in the GLAC at the aquifer-system scale and regional (terrane) scale using data from numerous sources, compared to health-based and aesthetic (non-health) benchmarks, and evaluated with areal and population metrics.

High concentrations (above a benchmark) of trace elements are widespread, across 28% of the study area. An estimated 5.7 million people rely on groundwater with high concentrations of a trace element, with manganese and arsenic the trace elements most commonly found at high concentrations. Conversely, nitrate concentrations are high in 4.0% and organic compounds (including pesticides and volatile organic compounds) high in 2.0% of the assessed study area. High arsenic and manganese concentrations occur primarily in terranes with thick, stratigraphically complex, fine-grained glacial sediment in the central part of the study area, coincident with groundwater under reducing conditions as indicated by iron concentrations >100 µg/L. In contrast, high nitrate concentrations are uncommon in those same terranes, and when nitrate is high in thick, fine-grained, complex terranes, it is much more commonly associated with groundwater under more oxidizing conditions. Common geogenic trace elements occur at high concentration due to characteristic geologic source materials and geochemical conditions (including redox and pH). Conversely, anthropogenic nitrate and organic compounds are introduced at or near the land surface in developed or agricultural areas. Our results indicate that water resource managers must consider groundwater quality holistically because it can be affected by geogenic contaminants, anthropogenic contaminants, and aquifer geochemical conditions.