EVIDENCE OF MERCURY BIOGEOCHEMICAL CYCLING ALONG A GROUNDWATER FLOW PATH

Groundwater samples were collected along a groundwater flow paths within the Carrizo Sand aquifer in Texas for analysis of dissolved mercury (Hg) concentrations. In addition, at each sampling site we measured the pH, specific conductance, temperature, alkalinity, dissolved oxygen (DO), oxidation-reduction potential (Eh), iron speciation ([Fe (II); Fe(III)]), and dissolved sulfide [S(-II)] concentrations for these groundwaters. Mercury concentrations in Carrizo Sand aquifer groundwaters range from 1.25 to 7.62 pmol/kg with the lowest concentrations reported from the recharge area. Mercury concentrations subsequently increase 6-fold along the first 13 km of the flow path, coincident with the region of the aquifer where dissimilatory Fe(III) reduction buffers the redox conditions. With groundwater flow beyond the Fe(III) reducing zone and into the region of the aquifer dominated by sulfate reduction, the dissolved Hg concentrations decrease by close to a factor of 3. Dissolved Hg is moderately correlated with dissolved Fe(II) concentrations (r = 0.6), and this relationship is significant at greater than the 90% confidence interval. Mercury concentrations in Carrizo Sand aquifer groundwaters are inversely related to dissolved sulfide concentrations (r = -0.31), although this relationship is not statistically significant. Dissolved Hg concentration show no correlations with pH or colloidal Fe(III) within these groundwaters. Our data indicate that Hg is released to Carrizo Sand aquifer groundwaters by dissimilatory ferric iron reduction of the Fe(III)-coated quartz sands (goethite, hematite) that characterize the aquifer in the recharge zone and the first ~20 km of the confined portion of the aquifer. Our data suggest that in addition to containing sorbed and/or co-precipitated arsenic, these ferric oxide/oxyhydroxide coatings also contain some Hg. Once Hg is released to Carrizo Sand groundwaters, its concentrations dramatically decrease with flow down gradient along the flow path owing to re-adsorption and removal as, or by, precipitating metal sulfides.