Mercury speciation and mobilization in a wastewater-contaminated groundwater plume.

We measured the concentration and speciation of mercury (Hg) in groundwater down-gradient from the site of wastewater infiltration beds operated by the Massachusetts Military Reservation, western Cape Cod, Massachusetts. Total mercury concentrations in oxic, mildly acidic, uncontaminated groundwater are 0.5-1 pM, and aquifer sediments have 0.5-1 ppb mercury. The plume of impacted groundwater created by the wastewater disposal is still evident, although inputs ceased in 1995, as indicated by anoxia extending at least 3 km down-gradient from the disposal site. Solutes indicative of a progression of anaerobic metabolisms are observed vertically and horizontally within the plume, with elevated nitrate concentrations and nitrate reduction surrounding a region with elevated iron concentrations indicating iron reduction. Mercury concentrations up to 800 pM were observed in shallow groundwater directly under the former infiltration beds, but concentrations decreased with depth and with distance down-gradient. Mercury speciation showed significant connections to the redox and metabolic state of the groundwater, with relatively little methylated Hg within the iron reducing sector of the plume, and dominance of this form within the higher nitrate/ammonium zone. Furthermore, substantial reduction of Hg(II) to Hg(0) within the core of the anoxic zone was observed when iron reduction was evident. These trends not only provide insight into the biogeochemical factors controlling the interplay of Hg species in natural waters, but also support hypotheses that anoxia and eutrophication in groundwater facilitate the mobilization of natural and anthropogenic Hg from watersheds/aquifers, which can be transported down-gradient to freshwaters and the coastal zone.

Formation and stability of atmospherically relevant isoprene-derived organosulfates and organonitrates.

Isoprene is the precursor for number of alcohol, organosulfate, and organonitrate species observed in ambient secondary organic aerosol (SOA). Recent laboratory and field work has suggested that isoprene-derived epoxides may be crucial intermediates that can explain the existence of these compounds in SOA. To confirm this hypothesis, the specific hydroxy epoxides observed in gas phase isoprene photooxidation experiments (as well as several other related species) were synthesized and the bulk phase aqueous reactions of these species in the presence of sulfate and nitrate were studied via nuclear magnetic resonance (NMR) techniques. The results indicate that both primary and tertiary organosulfates and organonitrates are efficiently formed from the potential SOA reactions of isoprene-derived epoxides. However, the tertiary organonitrates are shown to undergo rapid nucleophilic substitution reactions (in which nitrate is substituted for by water or sulfate) over the whole range of SOA pH, while the tertiary organosulfates are found to undergo a much slower acid-dependent hydrolysis reaction. The primary organonitrates and organosulfates under study were found to be stable against nucleophilic substitution reactions, even at low pH. This finding provides a potential explanation for the fact that organosulfates are more commonly detected in ambient SOA than are organonitrates.

Kinetics of the hydrolysis of atmospherically relevant isoprene-derived hydroxy epoxides.

Isoprene (the most abundant nonmethane hydrocarbon emitted into the atmosphere) is known to undergo oxidation to 2-methyl-1,2,3,4-butanetetraol, a hydrophilic compound present in secondary organic aerosol (SOA) in the atmosphere. Recent laboratory work has shown that gas phase hydroxy epoxides are produced in the low NOx photooxidation of isoprene and that these epoxides are likely to undergo efficient acid-catalyzed hydrolysis on SOA to 2-methyl-1,2,3,4-butanetetraol at typical SOA acidities. In order to confirm this hypothesis, the specific hydroxy epoxides observed in the isoprene photooxidation experiment (as well as several other related species) were synthesized, and the hydrolysis kinetics of all species were studied via nuclear magnetic resonance (NMR) techniques. It was determined that the isoprene-derived hydroxy epoxides should undergo efficient hydrolysis under atmospheric conditions, particular on lower pH SOA. An empirical structure-reactivity model was constructed that parametrized the hydrolysis rate constants according to the carbon substitution pattern on the epoxide ring and number of neighboring hydroxy functional groups. Compared to the previously studied similar nonfunctionalized epoxides, the presence of a hydroxy group at the alpha position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 20, and the presence of a hydroxy group at the beta position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 6.