Space and Depth-Resolved Naturally Occurring Toxic Groundwater Species in Bangladesh and Rwanda: Origination and Risk Analysis

Access to safe potable water is a necessity for all. Groundwater is a commonly relied upon drinking water source for many areas around the world. This is especially true for communities in high density, rural settings. Such is the case for populations near Cox’s Bazaar, Bangladesh, and in the Bugesera region of Rwanda. Sediment and groundwater contamination, through toxic dissolved species, represents a significant public health risk to exposed populations. Tropical soils, such as the soil profiles in Bangladesh and Rwanda, often contain higher concentrations of heavy metals (Rieuwerts, 2007). Additionally, nitrate from fertilizers, are widely used on the soils in these regions. While hazardous risk will never be alleviated in its entirety, it is the goal to diminish the threat as much as possible. To accomplish this task, a complete and fundamental understanding of contaminant solid-solution partitioning mechanisms is required. It is also important to note that contaminant distribution over space and depth is a contributing factor to potential exposure. The outcome of this dissertation combines spatial- and depth-resolved sampling to quantify contamination risk posed by heavy metals and other toxic species. In finding and evaluating these hazardous risks, potential solutions to alleviate and manage public health exposure are offered. The first chapter involved groundwater sampling at the Kutupalong settlement camp near Cox’s Bazaar, Bangladesh. Sponsored by the United Nations High Commissioner for Refugees (UNHCR), thorough sampling of the community’s potable groundwater, took place throughout the entire settlement. Sampling uncovered subsurface biogeochemical processes that ultimately govern the release of Pb and NO3-. Pb concentrations were found as elevated as 150µg/L, well above the World Health Organization’s guideline of 10µg/L Pb in drinking water. Further investigation indicated nitrogen dynamics regulate pH in the subsurface. Changes in pH control the solubility of Fe and Mn oxides and, therefore, their associated sorption and desorption of Pb. Contaminant release is spatially heterogeneous within the resettlement’s groundwater. This would make in situ remediation a tedious and possibly ineffective solution. To compensate, geochemical data in combination with GIS spatial analysis generated risk assessment maps. The maps illustrate the heterogeneity of risk associated with distinct contaminants throughout Kutupalong. In doing so, risk maps provide a mitigation strategy of avoidance for contaminated well sites, strategic guidance for currently safe wells, the closure of high-risk wells, and the placement of future ones. The second study in this dissertation looked at naturally occurring metal contamination in groundwater from a tropical region. Water quality mapping in Bugesera, Rwanda, highlighted multiple metal contaminations throughout the region. Multiple in-use boreholes contained Mn exceeding the former WHO guideline of 400µg/L. Several sites also contained U exceeding the WHO guideline of 30µg/L. U was found to be over 50µg/L in some cases and over 400µg/L in one extreme circumstance. Sediment sampling in 2016 and 2017 helped verify that multiple areas within Bugesera contain elevated solid-phase concentrations of various metals with very different mechanisms of potential release into the environment. Three different sites were sampled and assessed for concentrations of multiple heavy and trace metals. Among the metals evaluated, Mn, U, and V presented the highest levels of sediment concentrations. Depth-resolved sampling uncovered subsurface characteristics unique to each locale. Depth to groundwater table and associated redox changes varied by locale. A location with seasonally persistent vadose conditions is contrasted with locations showing redox changes induced by seasonally fluctuation pore saturation via groundwater table oscillation. Sequential extraction experiments were completed to identify the metal speciation. Loss on ignition (LOI), calcination, and soil acidity measurements further characterize the sediment chemical systems and provide depth to interpretation as to the mobility of the metals. This research aims to characterize the elevated metal concentrations in the subsurface as well as determine the mechanisms of release for potential exposure to the environment and the public health. Ultimately, this will permit communities within Bugesera to develop without risk of exposure to toxic metals. The final phase of research looked beyond the soil-water interface and studies contamination exposure through fine sediment particles. Small size airborne particles are well-known pathways for chemical exposure, including exposure to heavy metals. Though metals concentrations are important, they alone do not verify the complete exposure sediments or sediment particles present. Hakanson (1980) and Tomlinson et al. (1980) describe ecological risk indices that give a more accurate assessment of contamination risk. By using contamination factors (Cf), pollution load index (PLI), and potential ecological risk index (RI), an accurate assessment of the toxicity and exposure by fine sediment particles can be calculated. In this study, two different locations in Bugesera are examined. It is found that sub 75µm particles present a far greater level of potential exposure risk than larger particle sizes. The greatest potential hazard comes from a region with constant redox cycling. Metals of most concern, at this location, are Cu and Pb. The other studied site is a region with a yearly constant vadose zone. At this location most concerning metals are Pb, Cd, and Cu. Through understanding the biogeochemical processes, their independent release mechanisms, and potential exposure risk, informed, adequate, and necessary management judgments can be determined. Whether decisions are made for strategic avoidance or direct mitigation, responsible parties cannot do so without pertinent information from an in-depth examination. The complete aim of this dissertation looks to gather all necessary information to make sound decisions in keeping the public health at large protected from toxic metal exposure, specifically metal exposure from tropical soils