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Natural arsenic contamination of groundwater is a severe public health crisis affecting nearly 60 million people in South Asia and Southeast Asia alone. Contamination is spatially heterogeneous and results from the coupled microbial decomposition of organic matter and reductive dissolution of arsenic-bearing iron minerals. Here we demonstrate that elevated arsenic concentrations arise in regions of recent organic matter deposition and thus are controlled by fluvial geomorphic processes. Arsenic contamination is best expressed within recent geomorphic features such as docked islands, scroll bars, and avulsions. Within these features, the deposition of rapidly buried reactive organic matter facilitates microbial iron reduction and arsenic release. Ultimately, the organic matter supply is exhausted and the conditions necessary for soluble arsenic to persist diminish.

We report fallout radionuclide (FRN) and major/trace element (MTE) contributions to bulk atmospheric deposition in Hanover, NH, USA (43.7022°N, 72.2896°W). Deposition of 7Be, 210Pb, and SO4 covary [R2 > 0.4, n = 461] but are discriminated by production sources, depositional mechanisms, meteorological controls, MTE associations, and seasonal biases. 7Be is dominated by rainout (78% of total deposition), recharged by long‐range transport (+23% over mean, o.m.), influenced by stratosphere‐troposphere exchange (+9% o.m.), and solar activity (−2% per doubling of sunspot count). Correlation with particulate nitrogen (+9% per doubling of N) indicates 7Be affinity for biogenic aerosols. 210Pb is dominated by dry + washout deposition (54% of total) and convective storms (+107% o.m), depleted in marine moisture sources (−133% o.m.), correlated with S (+9% per doubling of S), and biased to autumn with Mn, Hg, and V (+7% o.m.). Coincident long‐term declines in S and 210Pb (−14%, −4% per year) suggest co‐scavenging by PbSO4. 7Be:210Pb ratios increase asymptotically with precipitation through the dry‐washout‐rainout transition and recharge of 7Be. At the global scale, 7Be:210Pb increases with precipitation for North American/European sites due to recharge of 7Be in midlatitude storm belts [R2 = 0.64, n = 31]. Conversely, 7Be:210Pb is independent of precipitation for Southeast/East Asian sites where 7Be recharge is low [R2 = 0.01, n = 40]. Globally, 7Be:210Pb ratios in dry deposition reflect resuspended aerosols with mean age of ca. 200 days, contributing <5% of 210Pb deposition. Different aerosol populations contributing to FRN deposition across spatial and temporal scales should emerge as a focus in terrestrial 7Be, 210Pb, and 10Be tracer applications. Plain Language Summary: Beryllium‐7 (7Be) and lead‐210 (210Pb) are natural radioactive elements that are produced in the atmosphere. They are powerful tools in earth and atmospheric sciences because they allow us to trace the movement of other atmospheric elements and particles such as sulphate (SO4), toxins such as mercury (Hg), or particulate matter small enough to damage human lungs (PM2.5). After production, 7Be, 210Pb, and associated particles reach Earth's surface in rainfall, pass through soils and sediments, and may be resuspended as dust that can pose ongoing risks of particulate exposure. To better understand how they trace atmospheric particles, we measured 7Be, 210Pb, SO4, and other elements and particles in rainwater each week for 9 years. By making many measurements we were able to see important differences in how 7Be, 210Pb, and SO4 are delivered to the Earth's surface, and differences among other elements that they associate with. These differences are likely to have implications for how 7Be and 210Pb are used to date geological deposits or to measure erosion or contaminant transport, but more work will be needed to fully understand them. Key Points: 7Be, 210Pb, and SO4 in bulk deposition represent unique mixtures of distinct aerosol populationsPrimary controls on 7Be:210Pb ratios include airmass altitude and origin, meteorology, precipitation depth, and resuspended aerosolsMultiple aerosol populations controlling FRN deposition should be investigated for their implications to terrestrial tracer studies [ABSTRACT FROM AUTHOR]

Atmospheric deposition is the primary mechanism by which remote environments are impacted by anthropogenic contaminants. Vegetation plays a critical role in intercepting atmospheric aerosols, thereby regulating the timing and magnitude of both contaminant and nutrient delivery to underlying soils. However, quantitative models describing the fate of atmospherically derived elements on vegetation are limited by a lack of long-term measurements of both atmospheric flux and foliar concentrations. We addressed this gap in understanding by quantifying weekly atmospheric deposition of the naturally occurring radionuclide tracers 7Be and 210Pb, as well as their activities in leaves of colocated trees, for three years in New Hampshire, U.S. The accumulation of both 7Be and 210Pb in deciduous and coniferous vegetation is predicted by a model that is based solely on measured atmospheric fluxes, duration of leaf exposure, and radioactive decay. Any "wash off" processes that remove 7Be and 210Pb from foliage operate with a maximum half-time of greater than 370 days (P > 99%), which is an order of magnitude longer than previously assumed. The retention of both 7Be and 210Pb on leaves is thus quantitative and permanent, coupling the fate of 7Be, 210Pb and similar atmospheric species to that of the leaf matter itself. These findings demonstrate that the long-standing paradigm of a short "environmental half-life" for atmospheric contaminants deposited on natural surfaces must be re-evaluated. [ABSTRACT FROM AUTHOR]

Isotopes of iodine play significant environmental roles, including a limiting micronutrient (127I), an acute radiotoxin (131I), and a geochemical tracer (129I). But the cycling of iodine through terrestrial ecosystems is poorly understood, due to its complex environmental chemistry and low natural abundance. To better understand iodine transport and fate in a terrestrial ecosystem, we traced fallout 131iodine throughout a small temperate catchment following contamination by the 11 March 2011 failure of the Fukushima Daiichi nuclear power facility. We find that radioiodine fallout is actively and efficiently scavenged by the soil system, where it is continuously focused to surface soils over a period of weeks following deposition. Mobilization of historic (pre-Fukushima) 137cesium observed concurrently in these soils suggests that the focusing of iodine to surface soils may be biologically mediated. Atmospherically deposited iodine is subsequently redistributed from the soil system via fluvial processes in a manner analogous to that of the particle-reactive tracer 7beryllium, a consequence of the radionuclides' shared sorption affinity for fine, particulate organic matter. These processes of surficial redistribution create iodine hotspots in the terrestrial environment where fine, particulate organic matter accumulates, and in this manner regulate the delivery of iodine nutrients and toxins alike from small catchments to larger river systems, lakes and estuaries. [ABSTRACT FROM AUTHOR]

Abstract: Measurement of cosmogenic 7Be in soils and sediments by gamma spectroscopy has been routine for three decades. However, the analytical constraints of this method have yet to be adequately described. Natural abundance of 7Be in bulk soils and sediments is typically low (106 to 108 atomskg−1), resulting in very low signal-to-noise ratio in the gamma spectrum. 7Be quantification is further complicated by numerous interferences from ubiquitous 238U- and 232Th-series radionuclides. We investigated the environmental gamma spectrum in the region of 7Be emission using spectra obtained from hundreds of natural stream sediment samples, one well-characterized sediment naturally-enriched in 7Be, and pure standard materials enriched in both 238U-series and 232Th-series decay chains. In typical samples no fewer than three discrete photopeaks directly interfere with 7Be measurement, one of which, the 228Ac 478.3keV photon, yields significant positive bias. Risks of 7Be false-positives due to this overlapping photopeak approach 50% for typical soils or sediments. Detection limits for 7Be in these materials using conventional high-purity germanium instruments are as low as 0.1Bq, but increase by up to a factor of four in the presence of modest activities of 238U- and 232Th-series nuclides. Here we demonstrate a methodology for the measurement of low 7Be abundances in soils and sediments that is unbiased and that optimizes precision. [Copyright &y& Elsevier]

Oxbow lakes are important stores for fine-grained sediment, which potentially makes them critical sinks for sediment-associated pollutants. We leverage an exhaustive public archive of coring data, supplemented by our data collection, to provide a quantitative assessment of the role of oxbows as off-channel sinks. We investigated loading trends of sediment-sorbed polychlorinated biphenyls (PCBs) within oxbows of the Housatonic River, an actively meandering river in western Massachusetts, USA. Our results reveal the efficiency of oxbows as sinks, with average PCB concentrations (14.8 ppm) that are nearly twice that of the surrounding floodplain (7.56 ppm). Even though the 5.83 km² floodplain is the largest sink of PCB-laden material, storing as much as 14.1 t of PCBs or 2.42 g m-2, oxbows store more than 20% of all PCBs (3.63 t of PCBs or 11.2 g m-2) while making up just over 5% of the floodplain surface area. Nearly 85% of the oxbow storage of PCBs occurs within the first 50 m of floodplain, making clear the significance of regular oxbow production to the off-channel storage of sediment-associated pollutants. [ABSTRACT FROM AUTHOR]

The 228 Th: 228 Ra ratios of foliage and organic soil horizons evolve with time following a predictable radioactive decay law and thus provide a new chronometer for absolute age-dating of plant and soil organic matter. Preferential uptake of 228 Th ( t 0.5 = 1.9 years) and 228 Ra ( t 0.5 = 5.9 years) by canopy tree species, ferns, and mosses, drives disequilibrium in the 232 Th- 228 Ra- 228 Th radioactive decay series within forest vegetation and organic soils. With examples from northeastern USA, we verify a new 228 Th: 228 Ra age model by demonstrating its concordance with the fallout radionuclide chronometer 7 Be: 210 Pb in the 0 to 5-year time frame [ R 2 = 0.87, RMSE = 0.5 years]. At our locality, canopy tree species assimilate 228 Th with a typical initial ratio ( 228 Th: 228 Ra) 0 ∼ 0.3, but in several instances, both deciduous and coniferous tree species show a preference for Th over Ra with ( 228 Th: 228 Ra) 0 exceeding 5. While the 228 Th: 228 Ra system is restricted to organic soil horizons, concordance of 228 Th: 228 Ra with established 7 Be: 210 Pb and 241 Am bomb-pulse chronometers establishes a coherent age-dating system of soil organic matter based on three independent chronometers and five particle reactive metals, and spanning 0-200 years in time scale that encompasses both organic and mineral soils to depths of up to 30 cm. Concordance indicates that these metals all follow common processes of organometallic colloid formation and migration and, in conjunction with 14 C, may open new opportunities to understand soil pedogenic processes that regulate the storage of carbon and atmospheric metals such as Pb and Hg.

We investigated the partitioning of 7 Be, 10 Be, and 210 Pb aerosols between operationally dissolved and >0.5 μm particulate fractions in wet and dry atmospheric deposition. Bulk deposition in situ -log( K D ) averaged 4.27 ± 0.46 for 7 Be and 4.79 ± 0.59 for 210 Pb (±SD, n = 163), with corresponding activity-fractions particulate ( f P ) = 24 and 48%. K D was inversely correlated with particulate mass concentration ( p C ), a particle concentration effect (p.c.e.) that indicates that dissolved 7 Be and 210 Pb are bound to submicron colloids. Experimental desorption- K D was higher than in situ by a factor of 20 for 7 Be and 4 for 210 Pb ( n = 27), indicating that FRN sorption to particulates was irreversible. 7 Be: 10 Be ratios confirmed that colloidal and particulate fractions were geochemically distinct, with corresponding ages of 120 ± 30 and 260 ± 45 days, respectively [mean ± SE, n = 9, p = 0.011]. Fractions particulate f Be 7 , f Be 10 , and f Pb 210 each increased with 7 Be: 10 Be bulk age, a particle-age effect (p.a.e). In multiple regression, f Be 7 was best predicted by N, Mn, Al, and Ni [ R 2 = 0.75, p < 0.0001], whereas f Pb relied on N, S, Fe, and Mn [ R 2 = 0.69, p < 0.0001]. Despite differences in magnitude and controls on partitioning, the ratio f Be : f Pb converged to 1 with p C in the range of 10-100 mg L -1 . Given sufficient solid surfaces, irreversible sorption and p.a.e. form a basis for 7 Be: 210 Pb chronometry of aerosol biogeochemical cycling.