Enhancing Microbial Sulfate Reduction of Hydrocarbons in Groundwater Using Permeable Filled Borings

At a service station closed in 1993, groundwater contained benzene that persisted above the cleanup goal of 1 mg/L in zones depleted of background sulfate. The benzene and other petroleum hydrocarbons (PHCs) were present as much as 36 feet (11 m) below the water table and therefore remediation of a thick saturated zone interval was required. Microcosms using site sediments demonstrated that anaerobic benzene biodegradation occurred only if sulfate was added, suggesting sulfate addition as a remediation approach. Twenty-four boreholes (9.1″ diameter and 56′ deep) were drilled around four monitoring wells, in which benzene concentrations exceeded 1 mg/L. The boreholes were backfilled with a mixture of gravel and 15,000 pounds of gypsum (which releases sulfate as it dissolves) to create “Permeable Filled Borings” (PFBs). Concurrently, nine high pressure injections (HPIs) of gypsum slurry were conducted in other site locations (312 pounds of gypsum total). PFBs were expected to release sulfate for up to 20 years, whereas HPIs were expected to produce a short-lived plume of sulfate. Concentrations of benzene and sulfate in groundwater were monitored over a 3-year period in six monitoring wells. In two wells near PFBs, benzene concentrations dropped below the cleanup goal by two to three orders of magnitude; in one well, sulfate concentrations exceeded 500 mg/L for the most recent 18 months. Benzene concentrations in two other PFB monitoring wells declined by a factor of 2 to 4, but remained above 1 mg/L, presumably due to high-dissolved PHC concentrations and possibly greater residual PHC mass in adjacent sediments, and therefore greater sulfate demand. However, hydrogen and sulfur isotopic enrichment in benzene and sulfate, respectively, confirmed biodegradation of benzene and stimulation of sulfate-reducing conditions. Thus, it is hypothesized that the PHC mass in adjacent sediments will decline over time, as will dissolved PHC concentrations, and eventually benzene concentrations will decrease below the cleanup goal. Benzene in two HPI monitoring wells was below the cleanup goal for all but one sampling event before HPIs were conducted and remained below the cleanup goal after HPIs; there was no stimulation of sulfate-reducing conditions. It is concluded that sulfate released from PFBs contributed to declining benzene concentrations.     

Rate control in feldspar dissolution—II. The protective effect of precipitates

The ability of precipitates formed during incongruent dissolution of feldspars in aqueous solutions to act as barriers to mass transfer between the feldspar and the bulk solution is examined using the available textural evidence on such precipitates.

The propensity of precipitates for forming compact coatings on the dissolving grains is measured by a characteristic distance of diffusion of the precipitating component, a function of the diffusion coefficient of the component in solution and of the rate constant for its precipitation. The available evidence on precipitates formed during dissolution of feldspars, complemented by the evidence on similar precipitates produced from comparable solutions in the absence of feldspars, is sufficient for evaluating the protective effect of precipitates under laboratory conditions, and for setting limits to the conditions under which such an effect could be important during weathering and diagenesis.

Feldspar dissolution in the reported kinetic experiments at room temperature and 3 pH 10 could not have been controlled by diffusion through the precipitate, and changes in the rate of release of alkalies after the first couple of days in Lagache's experiments at 200°C also could not have been controlled by diffusion through the precipitate. Other things being equal, characteristic distances of diffusion of components reprecipitated during incongruent dissolution of silicates decrease with increasing temperature, and coating of dissolving grains by compact precipitate layers should be more common at metamorphic temperatures than in the temperature range of weathering and diagenesis.     

Methods to Estimate Source Zone Depletion of Fuel Releases by Groundwater Flow

We compare two methods for estimating the natural source zone depletion (NSZD) rate at fuel release sites that occurs by groundwater flow through the source zone due to dissolution and transport of biodegradation products. Dissolution is addressed identically in both methods. The “mass budget method”, previously proposed and applied by others, estimates the petroleum hydrocarbon biodegradation rate based on dissolved electron acceptor delivery and dissolved biodegradation product removal by groundwater flow. The mass budget method relies on assumed stoichiometry for the degradation reactions and differences in concentrations of dissolved species (oxygen, nitrate, sulfate, reduced iron, reduced manganese, nonvolatile dissolved organic carbon, methane) at monitoring locations upgradient and downgradient of the source zone. We illustrate a refinement to account for degradation reactions associated with loss of reduced iron from solution. The “carbon budget method,” a simplification of approaches applied by others, addresses carbon‐containing species in solution or lost from solution (precipitated) and does not require assumptions about stoichiometry or information about electron acceptors. We apply both methods to a fuel release site with unusually detailed monitoring data and discuss applicability to more typical and less thoroughly monitored sites. The methods, as would typically be applied, yield similar results but have different constraints and uncertainties. Overall, we conclude that the carbon budget method has greater practical utility as it is simpler, requires fewer assumptions, accounts for most iron‐reducing reactions, and does not include CO₂ that escapes from the saturated to the unsaturated zone.     

A Seti Correspondent Helps us to Discover their Signals, Lost in the Noise of our Receivers (A working hypothesis)

It could be that SETI signals have arrived at the Earth, but are lower than the noise level, and our receivers cannot detect them. We analyze the situation when a SETI correspondent sends signals that can be accumulated upon reception, permitting detection of even very weak signals. It is shown that using this method, we can already transmit SETI signals over our entire Galaxy.     

Consistent patterns of rare earth element distribution in accessory minerals from rocks of mafic-ultramafic complexes

This paper summarizes analytical data accumulated in the world literature and other materials about the regularities of the REE distribution in minerals contained in ultramafic and mafic rocks as accessory phases. These minerals are tentatively divided into two groups. The first includes garnets, zircons, apatites and perovskites, which can accumulate increased amounts of REE in their structure. The second consists of minerals whose structure can accumulate only limited contents of these trace elements. These are chrome-spinels, ilmenites, and micas. These minerals, in respect of REE geochemistry, are studied to a varying degree because of the different levels of accumulations of these elements, different degrees of occurrence in rocks, tiny sizes of their grains and other reasons. The analytical database formed on their basis includes about 600 original analyses. The overwhelming majority of presently available data on REE geochemistry in accessory minerals from ultramafic and mafic rocks have been published only in the recent 15 years. The studies became possible due to the development and introduction of new highly sensible microprobe analyses allowing detection of REE and many other trace elements in minerals grains directly in thin sections. The greatest numbers of these analyses were performed for garnets and zircons, fewer for apatites, and the fewest for chrome-spinels, ilmenites, micas, and perovskites. In general, the regularities of REE distribution in these minerals from ultramafic and mafic rocks are less studied compared to the rock-forming minerals from ultramafic and mafic rocks. Among the analytical methods, which were used to study the REE composition of accessory minerals, the most efficient was the mass-spectrometry with inductively coupled plasma (ICP-MS).     

The effect of coherency stress on the mechanism of the reaction albite+K+⇌K-feldspar+Na+ and on the mechanical state of the resulting feldspar

Coherency stress and coherency strain energy generated by Na⁺?K⁺ ion exchange in alkali feldspars are calculated using an isotropic model, and deformation of single crystals of alkali feldspars exposed to molten alkali chlorides at \(P_{H_2 O} \) < 1 bar is described. Coherency stress in alkali feldspars can reach 10–20 kb. When it is large, partial relaxation by fracture and/or plastic deformation takes place under anhydrous conditions, but temporary build-up of stress is unavoidable even under hydrothermal conditions. Because of coherency strain energy, a thin layer of an end-member alkali feldspar produced by cation exchange on a grain of the other end-member alkali feldspar would be unstable with respect to dissolution. Therefore, under hydrothermal conditions one end-member alkali feldspar replaces the other by dissolution and precipitation. The mechanism of the reaction $$Na_x K_{1 - x} AlSi_3 O_{8_{(feld.)} } + yK^ + \rightleftharpoons Na_{x - y} K_{1 + y - x} AlSi_3 O_{8_{(feld.)} } + yNa^ + $$ is primarily controlled by \(P_{H_2 O} \) and by ΔK/(Na + K), the difference between the equilibrium value and the initial value of the atomic K/(Na + K) ratio of the feldspar. When ¦ΔK/(Na + K)¦ is small, the reaction proceeds by cation exchange. When ¦ΔK/(Na + K)¦ is large, cation exchange still occurs if \(P_{H_2 O} \) is very low, but under hydrothermal conditions replacement by dissolution and precipitation occurs.     

Comparing Natural Source Zone Depletion Pathways at a Fuel Release Site

Natural source zone depletion (NSZD) refers to processes within chemically impacted vadose and saturated zones that reduce the mass of contaminants remaining in a defined source control volume. Studies of large petroleum hydrocarbon release sites have shown that the depletion rate by vapor phase migration of degradation products from the source control volume through the vadose zone (V‐NSZD) is often considerably higher than the rate of depletion from the source control volume by groundwater flow carrying dissolved petroleum hydrocarbons arising from dissolution, desorption, or back diffusion, and degradation products arising from biodegradation (GW‐NSZD). In this study, we quantified vadose zone and GW‐NSZD at a small unpaved fuel release site in California typical of those in settings with predominantly low permeability media. We estimated vadose zone using a dense network of efflux monitoring locations at four sampling events over 2 years, and GW‐NSZD using groundwater monitoring data downgradient of the source control volume in three depth intervals spanning up to 9 years. On average, vadose zone was 17 times greater than GW‐NSZD during the time interval of comparison, and vadose zone was in the range of rates quantified at other sites with petroleum hydrocarbon releases. Estimating vadose zone and GW‐NSZD rates is challenging but the vadose zone rate is the best indicator of overall source mass depletion, whereas GW‐NSZD rates may be useful as baselines to quantify progress of natural or engineered remediation in portions of the saturated zone in which there are impediments to loss of methane and other gases to the vadose zone.     

Rate control in dissolution of alkali feldspars—I. Study of residual feldspar grains by X-ray photoelectron spectroscopy

Sanidine grains (100–600 μm in diameter) were subjected to dissolution at 82°C in aqueous electrolyte solutions of pH ranging from 4 to 8 for 293 or 377 hr. Dissolution equivalent to the removal of silica from the outer 300–900 A of these grains was accomplished. The shallow subsurfaces of feldspar grains were then analyzed for K, Al, and Si by X-ray photoelectron spectroscopy. The results rule out any continuous precipitate layer; if an alkali-depleted subsurface zone (leached layer) was present in the feldspar, the thickness of such a zone approximated by linear increase of alkali concentration with depth was not more than about 17 Å.It is concluded that in the absence of a compact precipitate layer, dissolution of feldspars in the temperature range corresponding to deep diagenesis is controlled by the processes at the feldspar-solution interface and a leached layer more than one feldspar unit cell thick does not form. Whether the same applies at the temperatures of shallow diagenesis and weathering cannot be judged with certainty, but parallels with leached layers on alkali silicate glasses suggest that it does.     

Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates—I. Deformation and dissolution of quartz under laboratory conditions

The extensive evidence on the properties of ground quartz such as grain morphology and adherence and subsurface structural damage shows a consistent pattern, interpretable in terms of the competing processes of fracture and of local plastic deformation.The well-documented pattern of dissolution of ground quartz in aqueous solutions is also consistent. At large undersaturations and constant other environmental variables, the apparent dissolution rate of ground quartz decreases exponentially with the increasing thickness of the equivalent dissolved disturbed layer, and the rate of release of adherent tiny fragments shows roughly the same dependence on the latter. At small undersaturations distribution of solubilities also needs to be considered.     

“Laser Discoverers” – Web‐based User‐generated Content in Heritage Detection in Poland

This article presents the results and potential of using volunteered geographic information (VGI) in heritage detection. Research was completed under the project entitled “Laser Discoverers – non‐invasive examination and documentation of archeological and historical objects in the ?wi?tokrzyskie Voivodeship”, carried out as a part of the Ministry of Science and Higher Education program entitled “The Paths of Copernicus”. Within the project, strong emphasis was placed on promotional and awareness‐raising activities, to involve as many voluntary users as possible. Project participants had at their disposal a web application, which provided access to a digital terrain model (DTM) where they identified possible heritage objects. All samples of data were additionally available in eight variants of sunshine, based on the simulation of sunlight from eight directions and at a constant angle. In total, 5,989 elementary areas with dimensions of 100 × 100 m were used for the project. After conducting a field inventory, Internet users together with specialists were able to recognize several thousands of potential archaeological and historic objects. During the project, approximately 10% of those features were verified through non‐invasive (field survey) work, with 75% success.