Applied Geochemistry (v.46, #C)

Understanding fluid sources, water–rock interactions and the biogeochemical processes involved in terrestrial mud volcanoes is necessary in order to predict the chemical processes most responsible for methane emissions to the atmosphere. Mud sediments ejected from the Dushanzi and Sikeshu mud volcanoes, located along the southern margin of the Junggar Basin, northwestern China, were collected by hand core sampling in order to explore whether surface and subsurface geochemical processes occur in their fluids. The ionic compositions of the pore fluids, minerals and major elements of the ejected sediments and surface sediments were analyzed. The pore fluids were mainly derived from ancient deeper sedimentary fluids which had mixed with meteoric surface water, but altered by diagenesis processes. Relative to seawater, the mud volcano pore fluids have higher ratios of Na/Cl and Li/Cl and lower ratios of K/Cl and Mg/Cl. The mud sediments are also enriched in illite, chlorite and calcite, but depleted in smectite. In addition, they are enriched in Ca and Mn, followed by Fe, Mg and P, and depleted in Si relative to the wall rocks. These chemical and mineralogical changes in the mud sediments and pore fluids are related to diagenesis processes. Clay mineral dehydration (mainly involving the conversion of smectite to illite) released large amounts of water. Ion exchange among clay minerals increased Na+ in the pore fluid. Water–rock interaction increased Fe and Mn, but decreased Si in the mud sediments. Carbonate precipitation decreased Ca2+ and Mg2+ concentrations of the pore fluid but increased Fe, Mg and P in the mud sediments. These results indicate that the mud volcanoes system is continuously recharged from deeper sedimentary sources. The difference in fluid and sediment geochemistry of the mud volcanoes can be ascribed to the different depths of the fluid and mud sources and the different diagenesis processes during the rising of fluid and mud.

The dissolved organic carbon (DOC) quantity and quality in streams regulate many ecosystem processes at the watershed scale. There is, however, a dearth of information on the spatial variability of in-stream DOC quality in small catchments. Our study used direct DOC concentration measurements and fluorescence spectroscopy indices to determine how stream DOC quantity and quality changed over space and time in a forested catchment of the US Northeast, and provided insight into how these systems might respond to changes in land use and/or climate in the coming years. Land cover (e.g., wetlands, lakes) exerted a dominant role over changes in flow and/or air temperature at regulating DOC concentrations in the watershed. Wetland areas acted as large sources of humic-rich DOC, while lakes were DOC sinks, especially for humic-rich DOC. DOC quality indices were generally significantly (p  < 0.05) correlated to DOC concentrations regardless of location and time of year, with high DOC concentrations (>7 mg C L−1) primarily tied to the mobilization of terrestrial highly degraded humic rich DOC, likely to be less bioavailable (and less fresh) than the DOC exported at times when DOC concentrations are low (<7 mg C L−1). Overall, results pointed to a two-phase DOC export whereby mobile and bioavailable fractions of DOC in surface water (protein-like DOC) are produced throughout the watershed (including the wetland), while recalcitrant humic-like DOC fractions are predominantly generated by the wetland, before preferentially sedimenting out in the lake. As the climate continues to change, we will likely find not only an increase in the amount of DOC exported at the watershed scale, but also a shift in the quality of this DOC toward less bioavailable DOC fractions rich in humic substances.

Attic dust reflects long-term airborne contamination of an industrial area: A case study from Ajka, Hungary by Péter Völgyesi; Gyozo Jordan; Dóra Zacháry; Csaba Szabó; András Bartha; Jörg Matschullat (19-29).
Recent research suggests that airborne pollutants, deposited in the urban environment, can be efficiently studied by attic dust analysis. Here, an attic dust study was carried out to determine the long-term airborne contamination load in the industrial town of Ajka, Hungary, by attic dust analysis. The spatial distribution of trace element contamination in airborne attic dust samples was to be mapped in order to relate spatial distribution to potential sources. The sampling strategy followed a grid-based stratified random design and samples were collected in 27 houses in the 64 km2 project area. Houses with attics intact for at least 30–40 years were chosen to represent long-term industrial pollution. The concentrations of six trace elements (As, Cd, Cu, Ni, Pb and Zn) were measured with ICP-OES, whereas Hg content was analyzed by CV-AAS. Measured concentrations above the environmental standards were encountered. The univariate distribution analysis shows that Pb, Hg, Zn and Cd distribution is dominated by anthropogenic sources, and characterized by high extreme values, such as elevated Hg-concentrations around the lignite-fired power plant. Arsenic and Cu-distributions seem equally influenced by background and anthropogenic processes. Nickel shows lower concentrations and variability, and its major source is the natural geochemical background. To study the geochemical element behavior, a correlation analysis was performed to estimate the metal sorption to Fe-oxy-hydroxide phases. Results show a good spatial correlation of contamination and sources such as lignite mines, lignite-fired power plant and traffic. Attic dust appears to be an efficient and cheap sampling medium to study and map long-term airborne contamination and possibly associated human health risks in an industrial setting.

Different isotopic evolutionary trends of δ34S and δ18O compositions of dissolved sulfate in an anaerobic deltaic aquifer system by Takahiro Hosono; Oranuj Lorphensriand; Shin-ichi Onodera; Hirokazu Okawa; Takanori Nakano; Tsutomu Yamanaka; Maki Tsujimura; Makoto Taniguchi (30-42).
Concentration and isotope ratios (δ34SSO4 and δ18OSO4) of dissolved sulfate of groundwater were analyzed in a very large anaerobic aquifer system under the Lower Central Plain (LCP) (25,000 km2) in Thailand. Groundwater samples were collected in two different kinds of aquifers; type 1 with a saline water contribution and type 2 lateritic aquifers with no saline water contribution. Two different isotopic compositional trends were observed: in type 1 aquifers sulfate isotope ratios range from low values (+2.2‰ for δ34SSO4 and +8.0‰ for δ18OSO4) to high values (+49.9‰ for δ34SSO4 and +17.9‰ for δ18OSO4); in type 2 aquifers sulfate isotope ratios range from low values (−0.1‰ for δ34SSO4 and +12.2‰ for δ18OSO4) to high δ18OSO4 ratios (+18.4‰) but with low δ34SSO4 ratios (<+12.9‰). Isotopic comparison with possible source materials and theoretical geochemical models suggests that the sulfate isotope variation for type 1 aquifer groundwater can be explained by two main processes. One is the contribution of remnant seawater, which has experienced dissimilatory sulfate reduction in the marine clay, into recharge water of freshwater origin. This process accounts for the high salinity groundwater. The other process, explaining for the modest salinity groundwater, is the bacterial sulfate reduction of the mixture water between high salinity water and fresh groundwater. Isotopic variation of type 2 aquifer groundwater may also be explained by bacterial sulfate reduction, with slower reduction rate than that of the groundwater with saline water effect. The origin of groundwater sulfate with low δ34SSO4 but high δ18OSO4 is recognized as an important topic to be examined in a future investigation.

Precipitation of calcium carbonate and calcium phosphate under diffusion controlled mixing by Tsigabu Gebrehiwet; Luanjing Guo; Don Fox; Hai Huang; Yoshiko Fujita; Robert Smith; James Henriksen; George Redden (43-56).
Multi-component mineral precipitation in porous, subsurface environments is challenging to simulate or engineer when in situ reactant mixing is controlled by diffusion. In contrast to well-mixed systems, the conditions that favor mineral precipitation in porous media are distributed along chemical gradients, which evolve spatially due to concurrent mineral precipitation and modification of solute transport in the media. The resulting physical and chemical characteristics of a mixing/precipitation zone are a consequence of coupling between transport and chemical processes, and the distinctive properties of individual chemical systems. We examined the spatial distribution of precipitates formed in “double diffusion” columns for two chemical systems, calcium carbonate and calcium phosphate. Polyacrylamide hydrogel was used as a low permeability, high porosity medium to maximize diffusive mixing and minimize pressure- and density-driven flow between reactant solutions. In the calcium phosphate system, multiple, visually dense and narrow bands of precipitates were observed that were reminiscent of previously reported Liesegang patterns. In the calcium carbonate system, wider precipitation zones characterized by more sparse distributions of precipitates and a more open channel structure were observed. In both cases, formation of precipitates inhibited, but did not necessarily eliminate, continued transport and mixing of the reactants. A reactive transport model with fully implicit coupling between diffusion, chemical speciation and precipitation kinetics, but where explicit details of nucleation processes were neglected, was able to qualitatively simulate properties of the precipitation zones. The results help to illustrate how changes in the physical properties of a precipitation zone depend on coupling between diffusion-controlled reactant mixing and chemistry-specific details of precipitation kinetics.

Soils derived from black shale can accumulate high concentrations of elements of environmental concern, especially in regions with semiarid to arid climates. One such region is the Colorado River basin in the southwestern United States where contaminants pose a threat to agriculture, municipal water supplies, endangered aquatic species, and water-quality commitments to Mexico. Exposures of Cretaceous Mancos Shale (MS) in the upper basin are a major contributor of salinity and selenium in the Colorado River. Here, we examine the roles of geology, climate, and alluviation on contaminant cycling (emphasis on salinity and Se) during weathering of MS in a Colorado River tributary watershed. Stage I (incipient weathering) began perhaps as long ago as 20 ka when lowering of groundwater resulted in oxidation of pyrite and organic matter. This process formed gypsum and soluble organic matter that persist in the unsaturated, weathered shale today. Enrichment of Se observed in laterally persistent ferric oxide layers likely is due to selenite adsorption onto the oxides that formed during fluctuating redox conditions at the water table. Stage II weathering (pedogenesis) is marked by a significant decrease in bulk density and increase in porosity as shale disaggregates to soil. Rainfall dissolves calcite and thenardite (Na2SO4) at the surface, infiltrates to about 1 m, and precipitates gypsum during evaporation. Gypsum formation (estimated 390 kg m−2) enriches soil moisture in Na and residual SO4. Transpiration of this moisture to the surface or exposure of subsurface soil (slumping) produces more thenardite. Most Se remains in the soil as selenite adsorbed to ferric oxides, however, some oxidizes to selenate and, during wetter conditions is transported with soil moisture to depths below 3 m. Coupled with little rainfall, relatively insoluble gypsum, and the translocation of soluble Se downward, MS landscapes will be a significant nonpoint source of salinity and Se to the Colorado River well into the future. Other trace elements weathering from MS that are often of environmental concern include U and Mo, which mimic Se in their behavior; As, Co, Cr, Cu, Ni, and Pb, which show little redistribution; and Cd, Sb, V, and Zn, which accumulate in Stage I shale, but are lost to varying degrees from upper soil intervals. None of these trace elements have been reported previously as contaminants in the study area.

The Cretaceous Mancos Shale (MS) is a known nonpoint source for a significant portion of the salinity and selenium (Se) loads in the Colorado River in the southwestern United States and northwestern corner of Mexico. These two contaminants pose a serious threat to rivers in these arid regions where water supplies are especially critical. Tuttle et al. (companion paper) investigates the cycling of contaminants in a Colorado River tributary watershed (Uncompahgre River, southwestern Colorado) where the MS weathers under natural conditions. This paper builds on those results and uses regional soil data in the same watershed to investigate the impact of MS geology, weathering intensity, land use, and climate on salt and Se storage in and flux from soils on the natural landscape, irrigated agriculture fields, areas undergoing urban development, and wetlands. The size of salinity and Se reservoirs in the MS soils is quantified. Flux calculations show that during modern weathering, natural landscapes cycle salt and Se; however, little of it is released for transport to the Uncompahgre River (10% of the annual salinity and 6% of the annual Se river loads). When irrigated, salinity and Se loads from the MS soil increase (26% and 57% of the river load, respectively), causing the river to be out of compliance with Federal and State Se standards. During 100 years of irrigation, seven times more Se has been removed from agricultural soil than what was lost from natural landscapes during the entire period of pedogenesis. Under more arid conditions, even less salt and Se are expected to be transported from the natural landscape. However, if wetter climates prevail, transport could increase dramatically due to storage of soluble phases in the non-irrigated soil. These results are critical input for water-resource and land-use managers who must decide whether or not the salinity and Se in a watershed can be managed, what sustainable mitigation strategies are possible, and what landscapes should be targeted. The broader implications include providing a reliable approach for quantifying nonpoint-source contamination from MS and other rock units elsewhere that weather under similar conditions and, together with results from our companion paper, address the complex interplay of geology, weathering, climate, and land use on contaminant cycling in the arid Southwest.

By use of a multi-site surface complexation model, Duputel et al. (2013) showed that citrate can decrease the solubility of phosphorus in soils, in contrast to what is commonly expected. We have identified several major errors in their model, which put the conclusions in doubt. We argue that major re-evaluation of their modelling approach is needed.

This review of applied geochemical methods for monitoring CO2 leakage focuses on shallow freshwater aquifers overlying CO2 storage areas. Geochemical tracer tools form a set of geochemical techniques, each of which is examined in this review and classified according to its sensitivity in CO2 detection. The purpose of any monitoring programme is to be able to provide sufficient information to enable site remediation in the case of unforeseen events and also to enable a satisfactory site closure strategy. Therefore, CO2 monitoring tools must be able to detect a precursor signal or an early warning signal of leakage associated with potentially minute geochemical modifications (i.e. associated with small amounts of CO2).We have classified the monitoring/tracing tools into two groups: (A) direct indicators of the CO2 itself, and (B) indirect indicators (i.e. reaction products) of the presence of CO2 that take into account displacement of the chemical equilibria under the conditions imposed by CO2 dissolution. Included in these tools are isotopic monitoring tools that are very sensitive to physico-chemical changes and can therefore provide early CO2 detection. These tools include carbon and oxygen isotope systematics that are conventionally used with respect to CO2 in the Carbon Capture and Storage (CCS) context. Finally, the review offers new perspectives on sensitive indirect detection methods using isotopes that are ‘non-traditional’ in the sense that they have not yet been applied to the field of CO2 geological storage. The complementarity of these geochemical methods provides a powerful monitoring strategy.