Geology topics

A. Amirbahman

Publications and source records attributed to A. Amirbahman.

4 recordsLinked to original sources

USGS2006

Controls on mercury and methylmercury deposition for two watersheds in Acadia National Park, Maine

Throughfall and bulk precipitation samples were collected for two watersheds at Acadia National Park, Maine, from 3 May to 16 November 2000, to determine which landscape factors affected mercury (Hg) deposition. One of these watersheds, Cadillac Brook, burned in 1947, providing a natural experimental design to study the effects of forest type on deposition to forested watersheds. Sites that face southwest received the highest Hg deposition, which may be due to the interception of cross-continental movement of contaminated air masses. Sites covered with softwood vegetation also received higher Hg deposition than other vegetation types because of the higher scavenging efficiency of the canopy structure. Methyl mercury (MeHg) deposition was not affected by these factors. Hg deposition, as bulk precipitation and throughfall was lower in Cadillac Brook watershed (burned) than in Hadlock Brook watershed (unburned) because of vegetation type and watershed aspect. Hg and MeHg inputs were weighted by season and vegetation type because these two factors had the most influence on deposition. Hg volatilization was not determined. The total Hg deposition via throughfall and bulk precipitation was 9.4 ??g/m2/year in Cadillac Brook watershed and 10.2 ??g/m2/year in Hadlock Brook watershed. The total MeHg deposition via throughfall and bulk precipitation was 0.05 ??g/m2/year in Cadillac Brook watershed and 0.10 ??g/m2/year in Hadlock Brook watershed. ?? Springer Science + Business Media B.V. 2006.

Environmental Monitoring and Assessment↗

USGS2006

Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials

The fate of arsenic in groundwater depends largely on its interaction with mineral surfaces. We investigated the kinetics of As(III) oxidation by aquifer materials collected from the USGS research site at Cape Cod, MA, USA, by conducting laboratory experiments. Five different solid samples with similar specific surface areas (0.6–0.9 m 2 g −1 ) and reductively extractable iron contents (18–26 μmol m −2 ), but with varying total manganese contents (0.5–3.5 μmol m −2 ) were used. Both dissolved and adsorbed As(III) and As(V) concentrations were measured with time up to 250 h. The As(III) removal rate from solution increased with increasing solid manganese content, suggesting that manganese oxide is responsible for the oxidation of As(III). Under all conditions, dissolved As(V) concentrations were very low. A quantitative model was developed to simulate the extent and kinetics of arsenic transformation by aquifer materials. The model included: (1) reversible rate-limited adsorption of As(III) onto both oxidative and non-oxidative (adsorptive) sites, (2) irreversible rate-limited oxidation of As(III), and (3) equilibrium adsorption of As(V) onto adsorptive sites. Rate constants for these processes, as well as the total oxidative site densities were used as the fitting parameters. The total adsorptive site densities were estimated based on the measured specific surface area of each material. The best fit was provided by considering one fast and one slow site for each adsorptive and oxidative site. The fitting parameters were obtained using the kinetic data for the most reactive aquifer material at different initial As(III) concentrations. Using the same parameters to simulate As(III) and As(V) surface reactions, the model predictions were compared to observations for aquifer materials with different manganese contents. The model simulated the experimental data very well for all materials at all initial As(III) concentrations. The As(V) production rate was related to the concentrations of the free oxidative surface sites and dissolved As(III), as r As ( V ) = k ox ′ [ Mn ( IV ) OH ] [ H 3 AsO 3 ] "> rAs(V)=kox′[Mn(IV)OH][H3AsO3] with apparent second-order rate constants of k ox f ′ = 6.28 × 10 - 1 "> koxf′=6.28×10-1 and k ox s ′ = 1.25 × 10 - 2 M - 1 s - 1 "> koxs′=1.25×10-2M-1s-1 for the fast and the slow oxidative sites, respectively. The As(III) removal rate decreased approximately by half for a pH increase from 4 to 7. The pH dependence was explained using the acid–base behavior of the surface oxidative sites by considering a surface p K a = 6.2 ( I = 0). In the presence of excess surface adsorptive and oxidative sites, phosphate diminished the rate of As(III) removal and As(V) production only slightly due to its interaction with the oxidative sites. The observed As(III) oxidation rate here is consistent with previous observations of As(III) oxidation over short transport distances during field-scale transport experiments. The model developed here may be incorporated into groundwater transport models to predict arsenic speciation and transport in chemically heterogeneous systems.

Geochimica et Cosmochimica Acta↗

USGS2004

The effect of fire on mercury cycling in the soils of forested watersheds: Acadia National Park, Maine, U.S.A

This study compares mercury (Hg) and methylmercury (MeHg) distribution in the soils of two forested stream watersheds at Acadia National Park, Maine, U.S.A. Cadillac Brook watershed, which burned in 1947, has thin soils and predominantly deciduous vegetation. It was compared to the unburned Hadlock Brook watershed, with thicker soil and predominantly coniferous vegetation. Soils in both watersheds were primarily well drained. The fire had a significant impact on the Cadillac watershed, by raising the soil pH, altering the vegetation, and reducing carbon and Hg pools. Total Hg content was significantly higher (P < 0.05) in Hadlock soils (0.18 kg Hg ha-1) compared to Cadillac soils (0. 13 kg Hg ha-1). Hadlock O horizon had an average Hg concentration of 134??48 ng Hg g-1 dry weight, compared to 103??23 ng Hg g-1 dry weight in Cadillac O horizon. Soil pH was significantly higher in all soil horizons at Cadillac compared to Hadlock soils. This difference was especially significant in the O horizon, where Cadillac soils had an average pH of 3.41??0.22 compared to Hadlock soils with an average pH of 2.99??0.13. To study the mobilization potential of Hg in the O horizons of the two watersheds, batch adsorption experiments were conducted, and the results were modeled using surface complexation modeling. The results of Hg adsorption experiments indicated that the dissolved Hg concentration was controlled by the dissolved organic carbon (DOC) concentration. The adsorption isotherms suggest that Hg is more mobile in the O horizon of the unburned Hadlock watershed because of higher solubility of organic carbon resulting in higher DOC concentrations in that watershed. Methylmercury concentrations, however, were consistently higher in the burned Cadillac O horizon (0.20??0.13 ng Hg g-1 dry weight) than in the unburned Hadlock O horizon (0.07??0.07 ng Hg g-1 dry weight). Similarly, Cadillac soils possessed a higher MeHg content (0.30 g MeHg ha-1) than Hadlock soils (0.16 g MeHg ha-1). The higher MeHg concentrations in Cadillac soils may reflect generally faster rates of microbial metabolism due to more rapid nutrient cycling and higher soil pH in the deciduous forest. In this research, we have shown that the amount of MeHg is not a function of the total pool of Hg in the watershed. Indeed, MeHg was inversely proportional to total Hg, suggesting that landscape factors such as soil pH, vegetation type, or land use history (e.g., fire) may be the determining factors for susceptibility to high Hg in biota. ?? 2004 Kluwer Academic Publisher. Printed in the Netherlands.

Water, Air, & Soil Pollution↗

USGS2002

Association of methylmercury with dissolved humic acids

Sorption of methylmercury (MeHg) to three different humic acids was investigated as a function of pH and humic concentration. The extent of sorption did not show a strong pH dependence within the pH range of 5−9. Below pH 5, a decrease in adsorption for all humic samples was observed. The experimental data for equilibrium sorption of MeHg were modeled using a discrete log K spectrum approach with three weakly acidic functional groups. The modeling parameters, which were the equilibrium binding constants and the total binding capacities, represented the data well at all MeHg and humic concentra tions and pH values for a given humic sample. The estimated binding constants for complexes of MeHg with humic acids were similar in magnitude to those of MeHg with thiol-containing compounds, suggesting that binding of MeHg involves the thiol groups of humic acids. The results show that only a small fraction of the reduced sulfur species in humic substances may take part in binding MeHg, but in most natural systems, this subfraction is considerably higher in concentration than ambient MeHg. The model developed here can be incorporated into speciation models to assess the bioavailability of MeHg in the presence of dissolved organic matter and competing ligands such as chloride and sulfide.

Environmental Science & Technology↗