Water Resources Research Act Program

Details for Project ID 2016MN368B

Development of a reactive-transport model for arsenic mobility in glacial aquifers using arsenic and iron X-ray absorption spectroscopy data

Institute: Minnesota
Year Established: 2016 Start Date: 2016-03-01 End Date: 2017-02-28
Total Federal Funds: $24,767 Total Non-Federal Funds: $48,135

Principal Investigators: Brandy Toner, GeneHua Ng

Abstract: Western Minnesota is a geographic nexus of drinking water wells with arsenic (As) concentrations above 10g/L, the current drinking water standard. The complex distribution of elevated-As wells in Minnesota glacial aquifers is a long standing public health problem. The strong geographic heterogeneity of elevated-As wells, the relatively low As concentration in aquifer sediments, and the lack of correlation between solid-phase and aqueous-phase As concentrations have contributed to the intractability of this human health problem. Previous work identified a correlation between well construction and As concentration that suggested that the interface between an aquifer and aquitard is a geochemically active zone releasing As to waters. Recently, we have described and quantified As speciation in aquifer (sand and gravel), aquitard (till), and aquifer-aquitard interface sediments obtained by rotosonic drilling using X-ray absorption spectroscopy (XAS). These novel data were then used, along with existing well water chemistry data and new bulk sediment chemistry, to identify several specific chemical mechanisms leading to the release of As to groundwater. Three major As release mechanisms were identified using samples from rotosonic drill cores: (1) As desorption from aquifer sediments; (2) reductive dissolution of iron (Fe) oxyhydroxide minerals to which As is sorbed; and (3) oxidative dissolution of As-bearing sulfide minerals. The diversity of As release mechanisms is consistent with the geographic heterogeneity observed in the distribution of elevated-As wells. Our results confirm that in two of the three locations studied, the till forming the aquitard is the source of As to aquifer sediments. Further, we have confirmed that the interface between an aquitard and aquifer is a geochemically active zone for As release to water (Nicholas et al. in prep). In the research proposed here, we will use our new XAS data to develop a reactive-transport model to extrapolate our core-specific observations to larger geographic areas. The modeling effort will be supported with an existing database of well water chemistry for As-affected areas of Minnesota (Toner et al. 2011) and published stratigraphy (Berg 2006; 2008). The model development will focus on incorporating the As release mechanisms identified by XAS into a geochemical reaction path framework for an aquifer-aquitard contact zone. These modeling efforts will be used to generate seed data for a larger proposal to the National Science Foundation that will include new sediment and groundwater sampling to expand and test the modeling activities. Our modeling study scope will encapsulate detailed understanding at three intensive study sites, with the greater perspective toward regional trends. We will: (1) compile and synthesize physical and geochemical data on As-release processes at example study sites; (2) develop and calibrate a reactive-transport model of As dynamics using the compiled data, including XAS speciation; and (3) with the calibrated reactive-transport model, assess controls on As release at the example study sites and evaluate the potential for regional trends. The proposed work will advance As modeling capabilities by incorporating detailed speciation data from our newly developed XAS methods. The proposed research aims to facilitate strategic well placement to improve drinking water quality.