Donald I. Siegel

Crude oil in a shallow sand and gravel aquifer-III. Biogeochemical reactions and mass balance modeling in anoxic groundwater

Abstract Crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near Bemidji, Minnesota. The degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body. The concentrations of Fe2+, Mn2+ and CH4, Eh measurements, and the δ13C ratios of the total inorganic C indicate that the plume became more reducing ver a 5-a period. However, the size of the contaminant plume remained stable during this time. Field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment. In anaerobic microcosm experiments conducted under field conditions, almost complete degradation (98%) was observed for benzene in 125 d and for toluene in 45 d. Concentrations of aqueous Fe2+ and Mn2+ increased in these experiments, indicating that the primary reactions were hydrocarbon degradation coupled with Fe and Mn reduction. Mass transfer calculations on a 40-m flowpath in the anoxic zone, downgradient from the oil body, indicated that the primary reactions in the anoxic zone are oxidation of organic compounds, precipitation of siderite and a ferroan calcite, dissolution of iron oxide and outgassing of CH4 and CO2. The major difference in the two models presented is the ratio of CO2 and CH4 that outgasses. Both models indicate quantitatively that large amounts of Fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume.

The dissolution of quartz in dilute aqueous solutions of organic acids at 25°C

Abstract The dissolution of quartz in dilute aqueous solutions of organic acids at 25° and standard pressure was investigated by the batch dissolution method. The bulk dissolution rate of quartz in 20 mmole/Kg citrate solutions at pH 7 was 8 to 10 times faster than that in pure water. After 1750 hours the concentration of dissolved silica in the citrate solution was 167 μmole/Kg compared to 50 μmole/Kg in water and a 20 mmole/Kg solution of acetate at pH 7. Solutions of salicylic, oxalic, and humic acids also accelerated the dissolution of quartz in aqueous solution at pH 7. The rate of dissolution in organic acids decreased sharply with decreasing pH. The possibility of a silica-organic acid complex was investigated using UV-difference spectroscopy. Results suggest that dissolved silica is complexed by citrate, oxalate and pyruvate at pH 7 by an electron-donor acceptor complex, whereas no complexation occurs between silica and acetate, lactate, malonate, or succinate. Three models are proposed for the solution and surface complexation of silica by organic acid anions which result in the accelerated dissolution and increased solubility of quartz in organic rich water.

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland.

To elucidate the roles of hydrology and vegetation in below ground carbon cycling within peatlands, radiocarbon values were obtained for pore water dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), CH4, and peat from the Glacial Lake Agassiz peatland. The major implication of this work is that the rate of microbial respiration within a peat column is greater than the peat decomposition rate. The radiocarbon content of DOC at both bog and fen was enriched relative to solid-phase peat by ∼150-300‰ consistent with the advection of recently photosynthesized DOC downward into the peat column. Fen Δ14C values for DIC and CH4 closely track the Δ14C of pore water DOC at depth, indicating that this recent plant production was the predominant substrate for microbial respiration. Aceticlastic methanogenesis apparently dominated the upper third of the peat column (α = 1.05), shifting toward CO2 reduction with depth (1.05 < α < 1.08). Upwelling groundwater contributed as much as 15% of the DIC to the bulk DIC pool at depth in the fen. The similarity of Δ14C values for DIC and CH4 suggests that methanogens utilized DIC from this source as well as DIC produced in situ. Bog Δ14C values for pore water DIC and CH4 differ by ≤ 15‰ at all depths and are depleted in 14C relative to DOC by ∼100‰, suggesting microbial utilization of a mixture of older and modern substrates. CO2 reduction was the primary pathway for methanogenesis at all depths in the bog (α = 1.08), and groundwater influence on bulk DIC was negligible. For both sites, Δ14C−DIC and Δ14C−CH4 are approximately equal at depths where stable isotope data indicate a predominance of CO2 reduction and dissimilar when acetate fermentation is indicated.

Simulating vertical flow in large peatlands.

Ground-water flow simulations are used to evaluate the importance of three parameters on vertical flow in peatlands: regional slope, permeability of the mineral soil underlying the peat, and peatland topography. Our results indicate that the extent of vertical ground-water flow in peatlands is primarily controlled by mineral soil permeability. Local ground-water flow cells that form under small water-table mounds at bog domes can drive peat pore water into permeable mineral soil. However, when peat forms over low permeability mineral soil, vertical movement of peat pore water becomes negligible and lateral flow of water in the upper portion of the peat column dominates the peatland hydrology. This effect is not due to the low permeability of humified peat layers, as is commonly assumed in many peatland studies. Field data from the Hudson Bay Lowland (Canada) and the Glacial Lake Agassiz peatlands of northern Minnesota confirm the validity of these models. These results are relevant in settings other than peatlands where relief is low and small topographic mounds exist.

Temporal variations in dissolved methane deep in the Lake Agassiz Peatlands, Minnesota

A study (August 1990 to July 1991) of profiles of dissolved CH4 concentrations, diffusive flux of CH4, and CH4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH4 deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH4 concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH4. At one site (2.5 m depth) the concentration of dissolved CH4 in the peat pore-water was 140 mg L−1. In July 1991, at no site did the concentration of dissolved CH4 in the peat pore water exceed 40 mg L−1 in the peat profile. The average calculated diffusive flux of CH4 decreased from 95 to 45 mg m−2 d−1 between 1990 and 1991. Gaseous CH4 was more in evidence in 1990 than in 1991. In 1990, CH4 at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H2CO3, CO2, HCO3− and CO32− decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L−1 to 38 mg C L−1 in 1991. A dynamic mechanism must exist which traps CH4 deep in the peat column allowing gaseous CH4 to build up, increasing dissolved CH4. Other times, CH4 passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH4 bubbles forms at depth in the peat, trapping gaseous CH4. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the water table elevation.

Hydraulic reversals and episodic methane emissions during drought cycles in mires

Reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic-head and dissolved-methane profiles in peat, Glacial Lake Agassiz Peatlands, northern Minnesota, United States. During the summer 1990 drought, regional groundwater discharged from the mineral soil underlying the peat to raised bogs. Concentrations of dissolved CH[sub 4] in pore water showed that much of the peat column was supersaturated with respect to a reference standard of 1 atm partial pressure CH[sub 4]. By the summer of 1991, the severity of the drought lessened, and the upper peat column was resaturated with precipitation-derived water. Groundwater flow was then controlled by local, precipitation-driven, recharge flow systems, and the regional groundwater that was discharged affected only the peat immediately above the mineral soil-peat interface. In contrast, during the summer of 1991, concentrations of dissolved CH[sub 4] in pore water were all undersaturated with respect to 1 atm partial pressure CH[sub 4]. 23 refs., 5 figs.

Silicate mineral dissolution at pH 4 and near standard temperature and pressure

The dissolution of labradorite, microcline, enstatite, augite and forsterite in acidified deionized water was investigated at near standard temperature and pressure and constant pH of 4.00 to determine the kinetics of the release of silica, and cations. Saturation indices and mass balance calculations suggest that after 700 hours, the release of silica from forsterite and augite was controlled by the precipitation of a solid silica phase, whereas silica mass transfer from the feldspars and enstatite was essentially as silicic acid. Iron release from the pyroxenes and olivine was probably controlled by the precipitation of iron oxyhydroxide phases. Linear-rate constants calculated after 700 hours for release of magnesium ranged from 10−15.2 to 10−14.4 M · cm−2 s−1 for augite and forsterite respectively. Linear-rate constants for the release of cations from feldspars ranged from 10−15.8 to 10−15.3 M · cm−2 s−1.

Geochemical controls on peatland pore water from the Hudson Bay Lowland: a multivariate statistical approach

Abstract Pore-water samples were collected in the Albany River drainage basin of the Hudson Bay Lowland. The chemistry of these samples was evaluated using bivariate plots, cluster analysis and principal component analysis to determine the importance of groundwater and to evaluate geochemical processes within the peat. The transport of dissolved constituents from the mineral soil into the peat column is a dominant control on peat pore-water chemistry. Peatland landforms have different signatures for pore-water chemistry; bogs are characterized by elevated concentrations of dissolved organic carbon, CH 4 , SiO 2 , K + and larger mineral ion errors, whereas fens are characterized by their higher pH and alkalinity. Large mineral ion balance errors (up to 99%), the inverse relationship between pH and dissolved organic carbon and the positive correlation between mineral ion balance error and dissolved organic carbon show that organic acids are important anions in bog pore waters. Methane concentrations and SO 4 2− concentration are inversely related, suggesting that SO 4 2− inhibits CH 4 production. Peat pore water at several locations contains high concentrations of marine salts (SO 4 2− , Cl − , and Na + ) in the lower half of the peat column, indicating that the marine sediments contain sea salts. We suggest that SO 4 2− from these marine sediments may reduce methane production in portions of the Hudson Bay Lowland.

Methane concentrations in water wells unrelated to proximity to existing oil and gas wells in northeastern Pennsylvania.

Recent studies in northeastern Pennsylvania report higher concentrations of dissolved methane in domestic water wells associated with proximity to nearby gas-producing wells [ Osborn et al. Proc. Natl. Acad. Sci. U. S. A. 2011 , 108 , 8172 ] and [ Jackson et al. Proc. Natl. Acad. Sci. U. S. A. , 2013 , 110 , 11250 ]. We test this possible association by using Chesapeake Energys baseline data set of over 11,300 dissolved methane analyses from domestic water wells, densely arrayed in Bradford and nearby counties (Pennsylvania), and near 661 pre-existing oil and gas wells. The majority of these, 92%, were unconventional wells, drilled with horizontal legs and hydraulically fractured. Our data set is hundreds of times larger than data sets used in prior studies. In contrast to prior findings, we found no statistically significant relationship between dissolved methane concentrations in groundwater from domestic water wells and proximity to pre-existing oil or gas wells. Previous analyses used small sample sets compared to the population of domestic wells available, which may explain the difference in prior findings compared to ours.

Simulating dispersive mixing in large peatlands

Numerical simulations indicate that mechanical dispersive mixing can be the dominant mass transport mechanism in large peatlands. Dispersive mixing driven by lateral flow can drive solute fluxes from the mineral soil upward to the peat surface and thereby explain observed patterns of bog and fen in large peatlands. Longitudinal and transverse dispersivities of only 0.5 and 0.05 m, respectively, were sufficient to supply solutes to the peat surface in the absence of upward ground-water flow. Incorporation of hydrodynamic dispersion in peatland systems explains apparent contradictions in solute migration in peatlands, allowing the simultaneous downward flux of labile carbon (i.e. root exudates) produced at the peat surface and upward migration of inorganic solutes from the underlying mineral soil. Previous models of peatland hydrogeochemistry that rely on advection alone as the dominant process for solute transport may therefore be inadequate to explain fully the hydrology, geochemistry, and evolution of large peatlands.