64 research outputs found

    The Removal Kinetics of Dissolved Organic Matter and the Optical Clarity of Groundwater

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    Concentrations of dissolved organic matter (DOM) and ultraviolet/visible light absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and along flowpaths within aquifers. These changes occur over distances of tens of meters (m) implying rapid removal kinetics of the chromophoric DOM that imparts color to groundwater. A one-compartment input-output model was used to derive a differential equation describing the removal of DOM from the dissolved phase due to the combined effects of biodegradation and sorption. The general solution to the equation was parameterized using a 2-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOC loss were rapid and ranged from 0.093 to 0.21 micromoles per liter per day (μM d−1), and rate constants for DOC removal ranged from 0.0021 to 0.011 per day (d−1). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of DOC over flow-path distances of 200 m or less and in timeframes of 2 years or less. These results explain the low to moderate DOC concentrations (20–75 μM; 0.26–1 mg L−1) and ultraviolet absorption coefficient values (a 254 \u3c 5 m−1) observed in groundwater produced from 59 wells tapping eight different aquifer systems of the United States. The nearly uniform optical clarity of groundwater, therefore, results from similarly rapid DOM-removal kinetics exhibited by geologically and hydrologically dissimilar aquifers

    Rates of Microbial Metabolism in Deep Coastal Plain Aquifers

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    Rates of microbial metabolism in deep anaerobic aquifers of the Atlantic coastal plain of South Carolina were investigated by both microbiological and geochemical techniques. Rates of [2-(14)C]acetate and [U-(14)C]glucose oxidation as well as geochemical evidence indicated that metabolic rates were faster in the sandy sediments composing the aquifers than in the clayey sediments of the confining layers. In the sandy aquifer sediments, estimates of the rates of CO(2) production (millimoles of CO(2) per liter per year) based on the oxidation of [2-(14)C] acetate were 9.4 × 10(−3) to 2.4 × 10(−1) for the Black Creek aquifer, 1.1 × 10(−2) for the Middendorf aquifer, and <7 × 10(−5) for the Cape Fear aquifer. These estimates were at least 2 orders of magnitude lower than previously published estimates that were based on the accumulation of CO(2) in laboratory incubations of similar deep subsurface sediments. In contrast, geochemical modeling of groundwater chemistry changes along aquifer flowpaths gave rate estimates that ranged from 10(−4) to 10(−6) mmol of CO(2) per liter per year. The age of these sediments (ca. 80 million years) and their organic carbon content suggest that average rates of CO(2) production could have been no more than 10(−4) mmol per liter per year. Thus, laboratory incubations may greatly overestimate the in situ rates of microbial metabolism in deep subsurface environments. This has important implications for the use of laboratory incubations in attempts to estimate biorestoration capacities of deep aquifers. The rate estimates from geochemical modeling indicate that deep aquifers are among the most oligotrophic aquatic environments in which there is ongoing microbial metabolism

    Acetogenic Microbial Degradation of Vinyl Chloride

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