Geochemical investigation of tungsten in two groundwater flow systems: The Carrizo Sand Aquifer, Texas, USA, and the Aquia Aquifer, Maryland, USA

Groundwater sampled were collected from well sites located within the Carrizo Sand aquifer in Atascosa and McMullen counties in Texas, and also within the Aquia Aquifer, Maryland in the counties of Queen Anne's, Kent, and Ann Arundel in Maryland. During the field sampling trips, on site measurements were conducted for pH, specific conductance, temperature, alkalinity, dissolved oxygen (DO), oxidation-reduction potential (Eh), iron speciation ([Fe (II)/Fe(III)]), and dissolved S (-II) concentrations. In order to examine and study solid phase tungsten concentrations in the Aquia (archived at the Maryland Geological Survey) and Carrizo Aquifer sediments, we collected a number of aliquots, including duplicates, from pre-existing drill cores (Evergreen Underground Water Conservation District, Pleasenton, TX; Maryland Geological Survey, Baltimore, MD). Tungsten concentrations in the Carrizo Sand Aquifer range from 3.64 to 1297 pmol kg-1 with the lowest concentrations reported from the recharge area followed by progressively increasing dissolved W concentrations with flow downgradient along the flow path. Total tungsten in the Aquia aquifer varies along the flow path ranging from 14.31 to 184.41 pmol kg-1, with an average concentration of 80.41 pmol kg-1. We suggest that increases in W concentrations in our groundwaters are reflective of pH-related desorption. Given that the chemistry of tungsten is very similar to that of Molybdenum, and also that the Carrizo groundwaters contain high levels of S(-II), we suggest that the possible formation of thiotungstates could most likely be responsible for the high tungsten levels in these groundwaters. Again, as was the case with our Aquia sediment samples, the high concentrations of tungsten in the last two fractions of our experiment are in accordance with these studies, and suggest that tungsten is being released into solution under oxidizing conditions. Evolution of W in the Aquia aquifer suggests the element's conservative behavior in these generally oxic to suboxic groundwaters. The observed changes of W concentrations along the studied flow path is also consistent with some direct control due the moderately changing redox conditions in the aquifer, i.e from oxic, to suboxic conditions. We suggest therefore that for W dissolution in groundwaters, pH related adsorption desorption reactions are the key controlling factors in oxic, and sub-oxic waters. Our results also suggest that except for the first few sediment samples, most of the W in the Aquia sediments are bound up in Al-silicate minerals, and are therefore not available for recycling in the groundwaters.