Hydrogeochemical evolution of arsenic along groundwater flow paths: Linking aqueous and solid phase arsenic speciation.

摘要

The central hypothesis of this Doctoral research work is that changes occurring along groundwater flow paths that accompany chemical weathering and oxidation-reduction reactions catalyze both arsenic (As) mobilization from, and capture by, minerals via adsorption/desorption and/or mineral dissolution/precipitation reactions. This central hypothesis is addressed by (i) quantifying concentrations and speciation of As, and ancillary geochemical parameters, including a number of redox sensitive parameters in groundwater samples collected along flow paths, and (ii) determining solid phase speciation of As in aquifer sediment samples, with specific emphasis on the labile (i.e., surface exchangeable) and non-labile As fractions (e.g., precipitated with Fe oxides). In order to accomplish these objectives, a series of groundwater samples were collected along flow paths in three well characterized, drinking water aquifers: Carrizo Sand (Texas), Upper Floridan (Florida), and Aquia (Maryland). Results of the groundwater studies demonstrate that the Carrizo and the Upper Floridan aquifers exhibit low dissolved As concentrations (1.4 ppb or ∼19 nmol kg -1), whereas the Aquia aquifer has As concentrations in many wells that exceed the current US Environmental Protection Agency's maximum contaminant level for As in drinking waters of 10 ppb (133 nmol kg-1). The results further indicate that the type of As [As(III) vs. As(V)] released in solution is largely dependent on respiratory microbial consortia present within the aquifer [i.e., Fe(III) reducing bacteria, dissimilatory As(V) reducing prokaryotes (DARPs), and Fe(III) reducing DARPs]. In the final aspect of the project, to address the source and lability of As in sediment, samples were collected from within the Aquia and Carrizo Sand aquifers. These sediment samples were examined petrographically and by X-Ray diffraction, and subsequently subjected to a sequential extraction scheme. By combining the results of Aquia groundwater and sediment studies, it was established that dissilimilatory Fe(III) reduction driven by suboxic, non-sulfidogenic Aquia groundwaters release a significant amount of As from the non-labile fraction of As, which is associated with amorphous and/or well crystallized Fe(III) oxide/oxyhydroxides that form coatings on aquifer mineral surfaces. Results of sequential extraction for the Carrizo Sand aquifer are presented in the Appendix for future consideration.