Contribution of sedimentary organic matter to arsenic mobilization along a potential natural reactive barrier (NRB) near a river: The Meghna river, Bangladesh.

Elevated dissolved arsenic (As) concentrations in the shallow aquifers of Bangladesh are primarily caused by microbially-mediated reduction of As-bearing iron (Fe) (oxy)hydroxides in organic matter (OM) rich, reducing environments. Along the Meghna River in Bangladesh, interactions between the river and groundwater within the hyporheic zone cause fluctuating redox conditions responsible for the formation of a Fe-rich natural reactive barrier (NRB) capable of sequestering As. To understand the NRB's impact on As mobility, the geochemistry of riverbank sediment (<3 m depth) and the underlying aquifer sediment (up to 37 m depth) was analyzed. A 24-hr sediment-water extraction experiment was performed to simulate interactions of these sediments with oxic river water. The sediment and the sediment-water extracts were analyzed for inorganic and organic chemical parameters. Results revealed no differences between the elemental composition of riverbank and aquifer sediments, which contained 40 ± 12 g/kg of Fe and 7 ± 2 mg/kg of As, respectively. Yet the amounts of inorganic and organic constituents extracted were substantially different between riverbank and aquifer sediments. The water extracted 6.4 ± 16.1 mg/kg of Fe and 0.03 ± 0.02 mg/kg of As from riverbank sediments, compared to 154.0 ± 98.1 mg/kg of Fe and 0.55 ± 0.40 mg/kg of As from aquifer sediments. The riverbank and aquifer sands contained similar amounts of sedimentary organic matter (SOM) (17,705.2 ± 5157.6 mg/kg). However, the water-extractable fraction of SOM varied substantially, i.e., 67.4 ± 72.3 mg/kg in riverbank sands, and 1330.3 ± 226.6 mg/kg in aquifer sands. Detailed characterization showed that the riverbank SOM was protein-like, fresh, low molecular weight, and labile, whereas SOM in aquifer sands was humic-like, older, high molecular weight, and recalcitrant. During the dry season, oxic conditions in the riverbank may promote aerobic metabolisms, limiting As mobility within the NRB.

Origin of tungsten and geochemical controls on its occurrence and mobilization in shallow sediments from Fallon, Nevada, USA.

Tungsten (W) occurrence and speciation was investigated in sediments collected from Fallon, Nevada where previous studies have linked elevated W levels in human body fluids to an unusual cluster of childhood leukemia cases. The speciation of sedimentary W was determined by µ-XRF mapping and µ-XANES. The W content of the analyzed surface sediments ranged between 81 and 25,908 mg/kg, which is significantly higher than the W content in deeper sediments which ranged from 37 to 373 mg/kg at 30 cm depth. The µ-XANES findings reveal that approximately 20-50% of the total W in the shallow sediment occurs in the metallic form (W0); the rest occurs in the oxide form (WVIO3). Because W0 does not occur naturally, its elevated concentrations in surface sediments point toward a possible local anthropogenic origin. The oxidation of metallic W0 with meteoric waters likely leads to the formation of WVIO3. The chief water-soluble W species was identified as WO42- by chromatographic separation and speciation modeling. These results led us to postulate that W0 particles from a currently unknown but local source(s) is (are) deposited onto the soils and/or surface sediments. The W0 in interaction with meteoric water is oxidized to WVIO3, and as these sediment-water interactions progress, WO42- is formed in the water at pH ∼7. Under pH < 7, and sufficient W concentrations, tungstate tends to polymerize, and polymerized species are less likely to adsorb onto sediments. Polymerized species have lower affinity than monomers, which leads to enhanced mobility of W.

Occurrence and distribution of high arsenic in sediments and groundwater of the Claromecó fluvial basin, southern Pampean plain (Argentina).

Occurrences of high arsenic (As) in sediments and groundwaters were investigated in the Claromecó fluvial basin, southern Pampean plain, Argentina. This investigation includes sedimentology, mineralogy, and hydrogeochemistry of the Neogene and Quaternary aquifers to determine possible sources and transport mechanisms for As in the Claromecó basin. Characterization of the sediments revealed homogeneous mineralogy in both Neogene highlands and Quaternary floodplains with abundant plagioclase, volcanic glass shards (VGS), K-feldspar, quartz, clay minerals and minor concentrations of clinopyroxenes, orthopyroxenes, hornblende, epidote, Fe-(oxy)hydroxides and fluorapatite. The sedimentary As concentrations ranged between 2.8 and 31 mg kg-1 in both aquifers. The average total dissolved As (dissolved AsT) concentrations was 47.2 ±â€¯30.8 µg L-1 (15.3-110 µg L-1) in groundwater in Neogene aquifer (GW1), while it was 97.1 ±â€¯30.6 µg L-1 (45-144 µg L-1) in Quaternary floodplain aquifer (GW2), with all samples exceeding WHO's guideline for dissolved AsT in safe drinking water of 10 µg L-1. Some GW1 (33%) and all GW2 samples contained high levels of fluoride (F-) ranging from 0.6 to 2.6 mg L-1 (1.37 ±â€¯0.59 mg L-1) in GW1 and 2 to 5 mg L-1 (3.2 ±â€¯0.9 mg L-1) in GW2 which also exceeded WHO's guideline for F- in safe drinking water of 1.5 mg L-1. Elevated concentrations of Na+, Cl- and SO42- in the Quaternary flood plain groundwater (GW2) could indicated some degree of sea water mixing as well as some contribution from inland processes (e.g. high evapotranspiration rates, long residence time and soil-water interactions). Dissolution of As bearing VGS or Fe-(oxy)hydroxides, alkaline desorption or competitive desorption with HCO3- from Fe-(oxy)hydroxides appear to be dominating processes of As mobilization, while desorption from fluorapatite elevate dissolved F- levels. This study provides valuable insights on As mobilization processes in Neogene and near coast Quaternary floodplain aquifer.

Investigating Fluorescent Organic-Matter Composition as a Key Predictor for Arsenic Mobility in Groundwater Aquifers.

Dissolved organic matter (DOM) is linked to the heterogeneous distribution of elevated arsenic (As) in groundwater used for drinking and irrigation purposes, but the relationship between DOM characteristics and arsenic mobility has yet to be fully understood. Here, DOM from groundwater sampled in the Bengal Basin region was characterized using both conventional bulk emission-excitation (EEM) spectroscopy and high-performance size-exclusion chromatography coupled to spectroscopy (HPSEC-EEM). Notably, application of the novel HPSEC-EEM approach permitted the total fluorescence of individual samples to be independently resolved into its underlying components. This allowed the external validation of the bulk-sample fluorescence decomposition and offered insight into the molecular size distribution of fluorescent DOM. Molecular size distributions were similar for the UVA fluorescent (C310 and C340) as well as the three visible fluorescent (C390, C440, and C500) components. There was a greater visible fluorescence in shallow aquifer samples (10-33 m) with high As (SH, up to 418 µg/L) than in samples from the same depth with lower As (up to 40 µg/L). This indicated a link between DOM quality and As mobility within the shallow aquifer. The deep aquifer samples (170-200 m) revealed DOM characteristics similar to SH samples but had low As concentrations (<4 µg/L), signifying that the deep aquifer is potentially vulnerable to As contamination. These findings pave the way for a more comprehensive assessment of the susceptibility of drinking water aquifers, thereby supporting the management of groundwater resources.

Water Recovery from Advanced Water Purification Facility Reverse Osmosis Concentrate by Photobiological Treatment Followed by Secondary Reverse Osmosis.

Reverse osmosis (RO)-based desalination and advanced water purification facilities have inherent challenges associated with concentrate management and disposal. Although enhanced permeate recovery and concentrate minimization are desired, membrane scaling due to inorganic constituents, such as silica, calcium, phosphate, and iron, hinders the process. To solve this problem, a new diatom-based photobiological process has been developed to remove these scaling constituents by biological uptake and precipitation. In this study, RO concentrate samples were collected from a full-scale advanced water reclamation facility in California and were treated in 3.8 and 57 L photobioreactors inoculated with a brackish water diatom  Pseudostaurosira trainorii PEWL001 using light-emitting diode bulbs or natural sunlight as a light source. The photobiological treatment removed 95% of reactive silica and 64% of calcium and enabled additional water recovery using a secondary RO at a recovery rate up to 66%. This represents 95% overall recovery, including 85% recovery in the primary RO unit. In addition to the scaling constituents, the photobiological treatment removed 12 pharmaceuticals and personal care products, as well as N-nitrosodimethylamine, from RO concentrate samples primarily via photolysis. This novel approach has a strong potential for application to brackish water desalination and advanced water purification in arid and semiarid areas.

Influence of monsoonal recharge on arsenic and dissolved organic matter in the Holocene and Pleistocene aquifers of the Bengal Basin.

Arsenic (As) mobilization in the Bengal Basin aquifers has been studied for several decades due to the complex redox bio-geochemistry, dynamic hydrogeology and complex nature of dissolved organic matter (DOM). Earlier studies have examined the changes in groundwater As in the dry season before monsoon and during the wet season after monsoonal recharge. To investigate the more immediate influence of recharge during the active monsoon period on As mobilization and DOM character, groundwater samples were analyzed in the pre-monsoon and during the active monsoon period. Groundwater samples were collected from shallow (<40 m) and deep (>40 m) tube-wells in West Bengal, India. Dissolved AsT in shallow groundwater ranged from 50 to 315 µg/L exceeding the WHO guideline of 10 µg/L. Shallow groundwater also showed high total dissolved nitrogen, carbon to nitrogen (C:N) <1, and humic-like DOM with a humic:protein ratio >1. By contrast, deep groundwaters contained AsT between 0.5 and 11 µg/L with carbonaceous and protein-like DOM, C:N >1, and humic:protein <1. Stable isotopes of δ18O and δ2H and Cl/Br results indicated three recharge scenarios in the shallow aquifer including direct recharge of dilute rainwater, evaporated surface water, and anthropogenically impacted surface water. Monsoonal recharge did not cause notable changes in AsT in deep or shallow groundwater, including two As hotspots in the Pleistocene aquifer. However, the monsoon did result in a two-fold decrease in SUVA254, increase in nitrite and nitrate in the shallow groundwater. The DOM in the deep groundwater at the two As hotspots (with AsT 132 and 715 µg/L) had optical properties with much greater humic-like DOM than the surrounding groundwater, which had low AsT and highly protein-like DOM. Overall, these results support that protein-like DOM associated with low groundwater As concentrations and suggest that the monsoonal influence on nitrate and nitrite is limited to shallow aquifers.

Dissolved fulvic acids from a high arsenic aquifer shuttle electrons to enhance microbial iron reduction.

It was demonstrated more than two decades ago that microorganisms use humic substances, including fulvic acid (FA), as electron shuttles during iron (Fe) reduction in anaerobic soils and sediments. The relevance of this mechanism for the acceleration of Fe(III) reduction in arsenic-laden groundwater environments is gaining wider attention. Here we provide new evidence that dissolved FAs isolated from sediment-influenced surface water and groundwater in the Bengal Basin were capable of electron shuttling between Geobacter metallireducens and Fe(III). Moreover, all four Bangladesh sediment-derived dissolved FAs investigated in this study had higher electron accepting capacity (176 to 245µmol/g) compared to aquatic FAs, such as Suwanee River Fulvic Acid (67µmol/g). Our direct evidence that Bangladesh FAs are capable of intermediate electron transfer to Fe(III) supports other studies that implicate electron shuttling by sediment-derived aqueous humics to enhance Fe reduction and, in turn, As mobility. Overall, the finding of greater electron accepting capacity by dissolved FAs from groundwater and other sediment-influenced environments advances our understanding of mechanisms that control Fe reduction under conditions where electron transfer is the rate limiting step.