Factors affecting the occurrence of redox-sensitive elements in groundwater systems

Abstract

Groundwater is an increasingly important source of water for many parts of the world. Redox sensitive elements, like uranium, chromium, arsenic, and vanadium, are known to occur naturally in some groundwater systems, and may pose human health risks in populations that use groundwater as their major drinking water source. This dissertation characterizes factors affecting the occurrence of redox sensitive elements in different aquifer systems from different countries, including hydrogeology, geochemistry, and anthropogenic effects. It also addresses the co-occurrence of these elements in the groundwater resources, which may be associated with greater health risks than exposure to individual elements alone. The first three chapters of this dissertation focus on uranium occurrence in groundwater from different parts of India. Groundwater overexploitation has caused massive groundwater depletion and raised concerns for water and food security in India. Fieldwork in India combined with retrieving available data show that uranium is a widespread problem in groundwater across India, with concentrations greater than the World Health Organizations provisional guideline of 30 µg/L, mostly found in semiarid alluvial aquifers, as well as granitic and metamorphosed granitic crystalline aquifers. High concentrations of uranium in groundwater from the semiarid parts of Northwestern India is also associated with extensive agricultural activities. A detailed analysis, utilizing chemical (major and trace elements) and isotopic (δ18O, δ2H, δ11B, δ13C) data show that redox conditions, bicarbonate concentrations, and salinity have interconnected effects that control groundwater uranium concentrations, and that geochemical processes associated with irrigation under arid and semiarid conditions further intensify the groundwater uranium concentrations. An in-depth investigation of the groundwater chemistry from Rajasthan shows that in addition to uranium, groundwater used for drinking without any treatment contains multiple inorganic contaminants at levels that exceed both Indian and World Health Organization drinking water guidelines. The processes affecting Rajasthan's groundwater quality are common in many other groundwater resources from arid areas worldwide, and the lessons learned from evaluating the mechanisms that affect the groundwater quality in this study are universal and can be applied for other parts of the world. The last two chapters of this thesis address redox sensitive elements in groundwater form North Carolina, USA. Uranium, chromium, vanadium, and arsenic are all known to occur in North Carolina groundwater at health-relevant concentrations, with the highest concentrations of all elements occurring in fractured igneous and metamorphic formations from the Piedmont region. Arsenic occurs most frequently in suboxic to mixed redox character groundwater, whereas uranium, vanadium, and chromium occur mostly in oxic groundwater. Occurrences of arsenic, and to a lesser extent uranium, increase with pH, likely reflecting desorption from clay minerals on the aquifer rocks, while higher chromium and vanadium levels were measured in near-neutral pH and oxic groundwater, reflecting greater sensitivity to redox conditions. Due to similar geochemistry, vanadium and chromium co-occur most frequently. There is not enough health data to quantify the risks associated with such element co-occurrence, and there are large gaps between the current health recommendations and enforceable drinking water standards, especially for chromium given that hexavalent chromium, which is the predominant dissolved species of chromium in groundwater, is not regulated. The large complied dataset of measurments for chromium and hexavalent chromium in drinking water wells across North Carolina I demonstrate that most chromium in North Carolina occurs as the more toxic hexavalent chromium, presenting greater risks to human health. Several approaches to predicting chromium occurrence in NC groundwater are explored to help target future monitoring efforts and highlight areas of higher risks for occurrence and co-occurrence of these contaminants in groundwater resources of the Piedmont region of Eastern U.S.