Predictive geospatial model for arsenic accumulation in Holocene aquifers based on interactions of oxbow-lake biogeochemistry and alluvial geomorphology

Sedimentary controls on arsenic distribution in meander-belt deposits of the Po Valley, Italy

Geogenic arsenic in soils and aquifers is a threat to public health, which can be mitigated by improving our understanding of arsenic distribution in natural environments. In alluvial plains traversed by meandering rivers, solid- and aqueous-phase arsenic concentrations tend to vary across sedimentary deposits accumulated by different processes linked to river morphodynamics: mud-prone and commonly organic-rich abandoned-channel fills arising from meander cut-off act as local sources of arsenic, which can be transferred to adjacent point-bar deposits related to meander growth. Meanwhile, spatial variability in arsenic contamination may also arise from variations in sediment provenance across a fluvial landscape, and from inherent downstream changes in a fluvial system. Yet, the relative importance of facies and provenance as controls on arsenic concentrations in fluvial sediments still needs to be assessed. Through integrated analyses of geomorphological, sedimentological and geochemical data, this study examines the spatial variability in arsenic distribution from deposits of the late Holocene channel belt of the Po River, Italy. Three study areas were investigated along a >100 km stretch of channel belt to evaluate the possible roles of downstream changes in river behaviour and sediment supply from variably arsenic-rich catchments. Sedimentological controls on solid-phase arsenic concentrations are recognized at the scales of both elementary lithologies (facies) and depositional sub-environments, highlighting the roles of morphologically recognizable abandoned-channel fills as sources of arsenic that can be mobilized via groundwater flow and become trapped in point-bar elements. Although solid-phase arsenic concentrations are dominantly related to the presence of organic matter and clay, local arsenic enrichment may be related to arsenic advection across meander-belt sediments, a process that is itself controlled by petrophysical heterogeneity. No evident relationship is seen between arsenic concentrations and point-bar facies distributions related to styles of meander morphodynamic evolution. Yet, limited variability across the study areas suggests that the facies control remains dominant over a potential provenance control related to catchment integration. The results help elucidate the role of sedimentary heterogeneity in the distribution of arsenic in sediments, soils and aquifers, in the Po Valley and other analogous fluvial environments.

Mobilization and enrichment of geogenic iodine in a floodplain groundwater system: New insights from sources and characterization of dissolved organic matter

High iodine groundwater occurs widely in the lower reaches of Yellow River floodplain, which has aroused public concern. The biogeochemical behavior of dissolved organic matter (DOM) plays a crucial role in the mobilizing iodine from aquifer media. In this study, the molecular composition of DOM in groundwater characterized by FT-ICR-MS, and the optical properties of organic matter obtained by combining three-dimensional fluorescence spectroscopy and parallel factor analysis (EEM ⁃ PARAFAC), were used to elucidate the effect of DOM on the migration and enrichment of iodine in groundwater in the eastern Henan Plain, which is located in the lower reaches of Yellow River floodplain, Northern China. The results show that，the total iodine concentration in groundwater in the study area is ranged from 4.68 to 1598 μg/L, and the average value was 216.4 μg/L. High iodine groundwater shows a distribution pattern along the Paleochannels of Yellow River, which is closely related to the richness of organic matter in the buried sediments of the Paleochannels of Yellow River. Organic matter in the sedimentary aquifers plays an important role in regulating the mobilization and enrichment of iodine, and its degradation process is conducive to the release of iodine. DOM components in high iodine groundwater are more homogeneous, more unsaturated, and has more aromatic molecules than those in low iodine groundwater. The activation of organic iodine in groundwater system may be accompanied by the degradation of N+ aliphatic compounds (CHON, CHONSP and CHON) and the formation of oxygen-poor highly unsaturated phenols (CHOSP, CHOP and CHOS) organic compounds. In addition to biodegradation, the adsorption of iron oxide rich in sedimentary aquifers can partially remove the high AI and O/C components of DOM in groundwater and enrich the remaining OPHUP components. The findings provide new insights into the coupling mechanism between iodine release and DOM in aquifers.

Predicting natural arsenic enrichment in peat-bearing, alluvial and coastal depositional systems: A generalized model based on sequence stratigraphy

Hazardously high concentrations of arsenic exceeding the threshold limits for soils and drinking waters have been widely reported from Quaternary sedimentary successions and shallow aquifers of alluvial and coastal lowlands worldwide, raising public health concerns due to potential human exposure to arsenic. A combined sedimentological and geochemical analysis of subsurface deposits, 2.5-50 m deep, from the SE Po Plain (Italy) documents a systematic tendency for naturally-occurring arsenic to accumulate in peat-rich layers, with concentrations invariably greater than maximum permissible levels. A total of 366 bulk sediment samples from 40 cores that penetrated peat-bearing deposits were analysed by X-ray fluorescence. Arsenic concentrations associated with 7 peat-free lithofacies associations (fluvial-channel, levee/crevasse, floodplain, swamp, lagoon/bay, beach-barrier, and offshore/prodelta) exhibit background values invariably below threshold levels (<20 mg/kg). In contrast, total arsenic contents from peaty clay and peat showed 2-6 times larger As accumulation. A total of 204 near-surface (0-2.5 m) samples from modern alluvial and coastal depositional environments exhibit the same trends as their deeper counterparts, total arsenic peaking at peat horizons above the threshold values for contaminated soils. The arsenic-bearing, peat-rich Quaternary successions of the Po Plain accumulated under persisting reducing conditions in wetlands of backstepping estuarine and prograding deltaic depositional environments during the Early-Middle Holocene sea-level rise and subsequent stillstand. Contamination of the Holocene and underlying Pleistocene aquifer systems likely occurred through the release of As by microbially-mediated reductive dissolution. Using high-resolution sequence-stratigraphic concepts, we document that the Late Pleistocene-Holocene lithofacies architecture dictates the subsurface distribution of As. The "wetland trajectory", i.e. the path taken by the landward/seaward shift of peat-rich depositional environments during the Holocene, may help predict spatial patterns of natural As distribution, delineating the highest As-hazard zones and providing a realistic view of aquifer contamination even in unknown areas.

Fundamentals and application in phytoremediation of an efficient arsenate reducing bacterium Pseudomonas putida ARS1

Arsenic is a ubiquitous environmental pollutant. Microbe-mediated arsenic bio-transformations significantly influence arsenic mobility and toxicity. Arsenic transformations by soil and aquatic organisms have been well documented, while little is known regarding effects due to endophytic bacteria. An endophyte Pseudomonas putida ARS1 was isolated from rice grown in arsenic contaminated soil. P. putida ARS1 shows high tolerance to arsenite (As(III)) and arsenate (As(V)), and exhibits efficient As(V) reduction and As(III) efflux activities. When exposed to 0.6 mg/L As(V), As(V) in the medium was completely converted to As(III) by P. putida ARS1 within 4 hr. Genome sequencing showed that P. putida ARS1 has two chromosomal arsenic resistance gene clusters (arsRCBH) that contribute to efficient As(V) reduction and As(III) efflux, and result in high resistance to arsenicals. Wolffia globosa is a strong arsenic accumulator with high potential for arsenic phytoremediation, which takes up As(III) more efficiently than As(V). Co-culture of P. putida ARS1 and W. globosa enhanced arsenic accumulation in W. globosa by 69%, and resulted in 91% removal of arsenic (at initial concentration of 0.6 mg/L As(V)) from water within 3 days. This study provides a promising strategy for in situ arsenic phytoremediation through the cooperation of plant and endophytic bacterium.