Geochemistry Of Molybdenum In The Aquia Aquifer, Maryland, USA
Valle, Teresa Iafelice
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The present study contributes to the understanding of controls on and behavior of molybdenum (Mo) in an aquifer, and its potential as an indicator of changing redox conditions along the groundwater flowpath. The central hypothesis of the present work is that the hydrogeochemical evolution along two separate groundwater flowpaths of the Aquia Aquifer, Maryland, USA, one each on the Eastern and Western Shore of Chesapeake Bay, consists of chemical weathering and oxidation-reduction reactions that dictate the degree of Mo mobilization, as well as mineral co-precipitation/re-adsorption reactions that dictate the degree of Mo immobilization (i.e., scavenging). This hypothesis is evaluated by defining the relationship between the concentration of Mo, the concentration and speciation of dissolved constituents (e.g., Fe, S²⁻, Mn, and N), and the bulk hydrochemical parameters (e.g., alkalinity, pH, DO, and Eh) along the groundwater flowpaths. It is anticipated that the integration of these hydrogeochemical results will demonstrate and further explain 1) the differences in the groundwater evolution between the two Aquia flowpaths, 2) the underlying differences between the Aquia Aquifer and other aquifers, and most importantly, 3) the unique utility of Mo as a groundwater tracer. In order to accomplish these objectives, a series of groundwater samples were collected along the inferred Eastern and Western Shore flowpaths of the Aquia Aquifer. Results of the comparison in groundwater evolution between the two Aquia flowpaths reveal that the redox boundary for both flowpaths correlates with the cation-exchange reaction identified by the change in water chemistry. This is evident near the recharge zone, where waters are identified as Ca-Mg-HCO₃⁻ type and at locations near the redox boundary (~52.5 - 57 km), where water chemistry evolves to Na-HCO₃⁻ type. The present study concludes that Mo can be used as an indicator of changing redox conditions. Molybdenum shows an overall increase in concentration along both flowpaths as a result of reducing conditions. Results from the Eastern Shore flowpath indicate that high concentrations of Mo (1.493 μmol kg⁻¹, at 55 km) are the results of pH-induced desorption/dissolution and microbial dissimilatory processes. At distances greater than 80 km from the recharge zone, Mo decreases in concentration (1.183 - 0.354 μmol kg⁻¹) due to co-precipitation/re-adsorption processes associated with the development of crystallized Fe²⁺ sulfides that form coatings on aquifer mineral surfaces (i.e., pyrite, Chapelle and Knobel, 1983). Results from the Western Shore flowpath reveal an increase in Mo concentration (0.085 - 0.332 μmol kg⁻¹) at a distance of ~44 km from the recharge zone. This indicates that the release of Mo into solution is the direct result of initial Fe³⁺ reduction and respiratory microbial consortia present within the aquifer. At distances greater than ~55 km, Mo increases from 0.186 to 0.409 μmol kg⁻¹, which correlates with the complete reduction of iron. The highest concentration of Mo (0.461 μmol kg⁻¹) along the Western Shore flowpath coincides with strongly reducing waters, identified at distances greater than ~70 km along flowpath (pH > 9.0, Eh < -120).