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The Midwestern Cambrian-Ordovician aquifer system (MCOAS) covers a large portion of the Midwestern United States and is a major source for public and domestic water supply. Radium (Ra) is a contaminant frequently exceeding the EPA maximum contaminant level of 185 mBq/L (5 pCi/L) for combined [226]Ra and [228]Ra in the MCOAS. While elevated Ra is associated with low dissolved oxygen, old water, and elevated total dissolved solids in groundwater at the aquifer scale, it is difficult to predict elevated Ra occurrence at individual wells. This dissertation uses field and laboratory methods to examine the geochemical influences on Ra mobility within the MCOAS, to gain insight into the influence of local hydrogeology on Ra partitioning to the groundwater. Multiple factors impact Ra sources and mobility in the MCOAS at the local scale; at discrete intervals within the aquifer system, elevated total dissolved solids are related to elevated Ra in an unconfined aquifer, while low dissolved oxygen is related to elevated Ra in a confined aquifer. Local geochemical conditions control Ra sorption to rock, as determined by whole-rock parent-daughter isotope ratios and sequential extractions on representative MCOAS bedrock samples. Potential hydrologic flow from Ra-rich stratigraphy to stratigraphy with lower sorptive capacity may result in increased Ra mobility within the MCOAS. The coupling of field and laboratory work compared the contribution of Ra from aquifer solids across specific stratigraphy. Isotopic evidence ([226]Ra/[238]U) demonstrates radionuclide mobility, where geochemical influences (e.g., ORP, alkalinity) likely influence U partitioning within the system. Additionally, Ra concentrations during a short-term pumping test are shown to be consistent with trends on 1-year timeframe, although the analytical method for Ra is important to verify trend precision. The larger implications of this project are that the presence of parent nuclides, as well as local geochemical and hydrologic conditions are important to consider in terms of predicting where elevated Ra will occur in groundwater samples. This approach provides new insight into how water-rock interactions within specific stratigraphy contribute Ra to the aqueous system within the MCOAS.
The Midwestern Cambrian-Ordovician aquifer system (MCOAS) is a valuable source of drinking water in the North Central United States, but can contain elevated levels of naturally occurring radium (Ra). Long-term consumption of Ra in drinking water is linked to an increased risk of bone cancer, and is therefore federally regulated in the U.S. at 185 millibecquerels per liter (mBq/L) or 5 picocuries per liter (pCi/L) for the total of 226Ra and 228Ra. While the hydrogeochemical conditions associated with elevated activity Ra in the MCOAS are well documented, less is known about how Ra occurrence is affected due to hydrogeochemical changes initiated by natural and anthropogenic aspects. This dissertation uses field and modeling studies to explore Ra occurrence when hydrogeochemical change is instigated by factors including natural hydrogeologic conditions, release of organic contaminants to the subsurface, and well field management. Radium activity increases as groundwater evolves along a regional flow path and approaches the boundary of the Maquoketa shale, a regional confining unit; here, the aquifer becomes locally confined and groundwater is older with geochemical conditions conducive to Ra mobility, including low dissolved oxygen and elevated total dissolved solids (TDS). A complex organic chemical mixture released to the subsurface alters aquifer pH, redox, and TDS conditions, resulting in elevated Ra activity within the dissolved phase plume that highlights the importance of characterizing Ra at hydrocarbon-contaminated sites. As demonstrated by groundwater flow modeling, well construction and pumping rates can alter hydraulic gradients that enhance groundwater exchange between units of different geochemistry and modify the contribution of groundwater from stratigraphic units with different Ra activity to wells. Overall, this dissertation provides insight into how environmental and anthropogenic factors influence hydrogeochemical conditions and Ra occurrence in groundwater, and can help guide long-term groundwater management strategies for mitigating Ra activity in groundwater pumped from the MCOAS.
Groundwater quality monitoring and testing is of paramount importance both in the developed and developing world. This book presents a series of papers illustrating the varied nature of current research into groundwater quality. Urban and rural supplies are covered through a case history approach, and the importance of remedial action to prevent deterioration is emphasized.