Coupled Reactive Transport Modeling of Redox Processes in a Nitrate-Polluted Sandy Aquifer
22
Citation
17
Reference
10
Related Paper
Citation Trend
Keywords:
Geochemical Modeling
Permeable reactive barrier
Geochemical Modeling
Permeable reactive barrier
Cite
Citations (22)
The Byram and Walls Island members in the lower and upper sections, respectively, of the Lockatong Formation in the Newark basin near the border between Pennsylvania and New Jersey were chosen to assess (i) the role of euxinic/anoxic conditions in sequestering arsenic (As) and other trace elements and (ii) the redistribution of these elements during catagenetic transformations. ἀese members are rich in organic matter and host pyrite which occurs as disseminations, small patches, and subparallel veins. ἀe sulfur isotope values of pyrite samples range between -7.5 and 0.5 ‰CDT (average = -3.5‰CDT). ἀe negative δ34S values are indicative of Bacterial Sulfate Reduction (BSR) under low temperature and euxinic/anoxic conditions. ἀe total organic carbon (TOC) values in this member ᴀuctuate between 0.5 and 2.1%. ἀese euxinic/anoxic conditions enhanced the incorporation of As and other trace elements in both organic matter and pyrite. ἀe As concentrations range from 13 to 800 mg/kg and from 1.4 to 34 mg/kg in pyrite and black shale samples, respectively. Rock Eval analyses reveal that organic matter is over-mature which altered the correlation between TOC and As. ἀe thermal cracking of organic matter resulted in the removal of these elements from organic matter and their subsequent incorporation in pyrite and bitumen. Organic matter- and pyrite-rich anoxic black shale layers and bitumen veins are potential sources of arsenic in groundwater in the Newark basin, with arsenic values that reach up to 215 μg/L.
δ34S
Cite
Citations (3)
The study was conducted in the Northern Atlantic Coastal Plain aquifer system, in the eastern USA. Groundwater pH and redox conditions are fundamental chemical characteristics controlling the distribution of many contaminants of concern for drinking water or the ecological health of receiving waters. In this study, pH and redox conditions were modeled and mapped in a complex, layered aquifer system. Machine-learning methods (boosted regression trees) were applied to data from 3000 to 5000 wells. Predicted pH and the probability of anoxic conditions, defined by three thresholds of dissolved oxygen (0.5, 1, and 2 mg/L), were mapped at the 1-km2 scale for each of 10 regional aquifer layers. Maps depict the extent of acidic groundwater and oxic conditions in the shallow, unconfined surficial aquifer and in unconfined, recharge-proximal areas of underlying aquifers, in contrast to alkaline and anoxic groundwater elsewhere. Geographic patterns and influential predictors–including elevation, overlying confining-units thickness, and simulated groundwater age and flux–are consistent with prior understanding of the processes controlling pH and redox in the aquifer system. The model-based maps support robust estimates of aquifer proportions, either areal or volumetric, likely to contain groundwater of a specified quality or be vulnerable to specific pH- or redox-sensitive contaminants. The machine-learning methods were an effective tool to map groundwater quality at the regional scale.
Cite
Citations (49)
Urban aquifer of Mashhad city, northeastern Iran. Measuring the ratios of the stable nitrate isotopes and hydrogeochemistry of major components in order to specify the origin of nitrate contamination and potential transformation processes in the aquifer. This study represents the first isotopic characterization of groundwater nitrate to identify the sources of excessive nitrate in the urban aquifer and provides a reference for the potential implementation of executive programs on groundwater quality and protection. Nitrate concentration exceeded the limits for nitrate in drinking water in most of the wells (110 out of 261). Isotopic composition of the oxygen and nitrogen of NO3− in groundwater indicated sewage as the primary source of nitrate contamination. Denitrification was identified as a non-significant process in the aquifer. Concentrations of anions (Cl−, SO42− and PO43−) and NO3− showed strong correlations confirming potential wastewater influence. Most of the groundwater was weakly acidic, and the chemistry of the groundwater was rather affected by urbanization and land-use than by aquifer rock interactions. The absence of a reliable sewer collecting system, particularly in the central and southern parts of the study area, directly resulted in the poor water quality. Considering the lack of natural attenuation processes in the groundwater, management options need to be considered for reducing nitrogen input into groundwater.
Cite
Citations (60)
Despite the recognition that both organic sulfur and pyrite form during the very early stages of diagenesis, and that the amount of H{sub 2}S generated in bacterial sulfate reduction primarily limits their formation, the mechanisms and the active species involved still are not clear. In this study, we quantified the major forms of sulfur distributed in sediments to assess the geochemical mechanisms involved in these transformations. XANES spectroscopy, together with elemental analysis, were used to measure sulfur speciation in the organic-rich sediments from the Bay of Concepcion, Chile. Organic polysulfides constituted the major fraction of the organic sulfur, and occurred maximally just below the sediment surface (1--3 cm), where intermediates from H{sub 2}S oxidation were likely to be generated most abundantly. Sulfonates, which could be formed through the reactions of sulfate and thiosulfate, also showed a sub-surface maximum in the vicinity of the ``oxic-anoxic interface``. These results strongly suggest a geochemical origin for organic polysulfides and sulfonates, and illustrate that intermediates from H{sub 2}S oxidation play a dominant role in incorporating sulfur into organic matter. Pyrite was absent in the surficial layer, and first appeared just below the H{sub 2}S maximum, where organic polysulfides began to decrease in abundance. From these results, we argue, that an iron monosulfide precursor formed first from reactions with H{sub 2}S, and then reacts with organic polysulfides, completing the synthesis of pyrite in the sediment column.
Sulfur Cycle
Cite
Citations (23)
In marine sediments, Fe(III) is found predominantly as a solid. Recently, however, soluble species of Fe3+ complexed by natural organic ligands have been detected in coastal marine sediments with voltammetric microelectrodes. The role of soluble Fe3+ complexes in diagenesis is unknown. In anoxic conditions, soluble Fe3+ can effectively oxidize FeS2 and recycle iron and sulfur for use as terminal electron acceptors during natural organic matter (NOM) degradation. Alternatively, soluble Fe3+ complexes can catalyze the formation of FeS and FeS2 through the rapid chemical reduction of Fe3+ by dissolved sulfide. To better understand the role of soluble Fe3+ in the transformation of iron and sulfur in marine sediments, we incubated the first few centimeters of unvegetated salt marsh sediment in plug-flow reactors. Microbial iron reduction seemed to prevail in suboxic conditions, but sulfate reduction outcompeted microbial iron reduction in the presence of reactive organic metabolites. The dominance of sulfate reduction led to the complete removal of reactive iron oxides by precipitation of FeS. Experiments mimicking the enrichment of soluble Fe3+ complexes in reduced sediments show that soluble Fe3+ does not reoxidize FeS and pyrite; rather, it promotes pyrite precipitation by enhancing sulfate reduction via complex bacterial interactions. The rate of pyrite formation in the presence of soluble Fe3+ is much higher than previously reported in the literature, suggesting that soluble Fe3+ might promote alternative pathways for microbial degradation of NOM that ultimately results in the immobilization of Fe and S as reduced iron sulfide minerals.
Iron sulfide
Greigite
Cite
Citations (46)