ARSENIC IN THE “CRITICAL ZONE”: A LOOK AT HOW ARSENIC NATURALLY BEHAVES DURING SOIL FORMATION

In arsenic-prone regions, an important question is the provenance of arsenic in shallow groundwaters (<30m). Some studies suggest that arsenic is naturally sourced from an aquifer’s overlying local soils (i.e., Polizzoto et al., 2008), while others surmise it to be due to weathering of the underlying aquifer matrix (McArthur et al., 2008 and 2011). With new concepts emerging about the “Critical Zone” and the fact that this zone extends from the tops of our trees to the bottommost groundwaters, we show how the natural contamination of arsenic can be better understood within this new context. Building upon early Critical Zone work showing that “younger/fresher” material “weathers” faster than “older/indurated” material (i.e., White and Brantley, 2003), we show how depositional-ages of aquifer material is an important parameter in arsenic groundwater chemistry. Optical ages of aquifer sands from several sites in Bangladesh, Vietnam, and Nepal show that there is an age-difference between aquifer units and that sediments deposited >10,000 years ago are typically low in arsenic (<10 mg/L), while aquifers with recently deposited sediments have increasingly higher arsenic concentrations (>50-500 mg/L). Combining our previous work with more recent studies on soil-formation in the Californian Sierra Nevada, we show that chemical weathering in the overlying soils tends to contribute a negligible amount of weathering constituents, like arsenic--into soil waters and underlying groundwaters. These results suggest that further efforts should be made to understand soil and aquifer processes within context of the Critical Zone, so that we can better understand how elements like arsenic naturally accumulate within our soils and groundwaters.