THE SURFACE AND SUBSURFACE CONTROLS ON GROUNDWATER ARSENIC: CAN SURFACE MORPHOLOGY ALWAYS HELP IN DETERMINING VULNERABILITY TO SHALLOW GROUNDWATER ARSENIC?

In recent years, one of the more helpful tools used in identifying Asian areas prone to natural groundwater arsenic contamination has been geomorphology. On both country (regional) and district-wide (local) scales, maps of surface geology (i.e., Ahmed et al., 2004; Yu et al., 2003), satellite imagery, and topography (i.e., Papacostas et al., 2008; Buschmann et al., 2007; Mukhopadyay et al., 2006) have shown correlations between geomorphic features and shallow groundwater arsenic (i.e., areas with recently abandoned channels and uplifted Pleistocene surfaces having more or less shallow groundwater arsenic, respectively). While this makes topography and geographic information particularly valuable in delineating areas susceptible to arsenic, there are still many areas where laterally thick, recent fluvio-deltaic mud sequences obscure geomorphic features, leaving little clue as to the type of arsenic vulnerability in the groundwater just beneath it. Examples of areas where surface signatures of abandoned channels and/or Pleistocene units are either buried or erased include modern floodplains within the Nepalese Terai and the Red River delta. Optical luminescence ages of aquifers in two study sites in Parasi, Nepal and Van Phuc, Vietnam show that active fluvial scouring (laterally ~10m/yr) and rapid sedimentation (0.01-1cm/yr) leave behind mud-filled surfaces with little-to-no trace of the significant aquifer differences lying only meters beneath them (i.e., thousands of years differences in aquifer depositional ages and redox-properties such as sediment reflectance and organic matter). This is important because it means that the source(s) and release mechanism(s) of naturally occurring arsenic into Asia’s groundwater is not necessarily a surface-driven (or top-down) process, and that areas prone to shallow groundwater arsenic cannot always be distinguished via remote mapping. Inasmuch, we focus here on areas where Asian shallow groundwater heterogeneity is not predictable by surface features alone, promoting the need for resolving aquifer facies at a scale comparable to the length over which arsenic often varies (10-100m)--which is necessary for better understanding, modeling, and predicting how Asia’s aquifers will respond to their increased use and other development activity.