A global rise in alluvial mining increases sediment load in tropical rivers.

Increasing gold and mineral mining activity in rivers across the global tropics has degraded ecosystems and threatened human health1,2. Such river mineral mining involves intensive excavation and sediment processing in river corridors, altering river form and releasing excess sediment downstream2. Increased suspended sediment loads can reduce water clarity and cause siltation to levels that may result in disease and mortality in fish3,4, poor water quality5 and damage to human infrastructure6. Although river mining has been investigated at local scales, no global synthesis of its physical footprint and impacts on hydrologic systems exists, leaving its full environmental consequences unknown. We assemble and analyse a 37-year satellite database showing pervasive, increasing river mineral mining worldwide. We identify 396 mining districts in 49 countries, concentrated in tropical waterways that are almost universally altered by mining-derived sediment. Of 173 mining-affected rivers, 80% have suspended sediment concentrations (SSCs) more than double pre-mining levels. In 30 countries in which mining affects large (>50 m wide) rivers, 23 ± 19% of large river length is altered by mining-derived sediment, a globe-spanning effect representing 35,000 river kilometres, 6% (±1% s.e.) of all large tropical river reaches. Our findings highlight the ubiquity and intensity of mining-associated degradation in tropical river systems.

Rapid changes to global river suspended sediment flux by humans.

Rivers support indispensable ecological functions and human health and infrastructure. Yet limited river sampling hinders our understanding of consequential changes to river systems. Satellite-based estimates of suspended sediment concentration and flux for 414 major rivers reveal widespread global change that is directly attributable to human activity in the past half-century. Sediment trapping by dams in the global hydrologic north has contributed to global sediment flux declines to 49% of pre-dam conditions. Recently, intensive land-use change in the global hydrologic south has increased erosion, with river suspended sediment concentration on average 41 ± 7% greater than in the 1980s. This north-south divergence has rapidly reconfigured global patterns in sediment flux to the oceans, with the dominant sources of suspended sediment shifting from Asia to South America.

Sorption Behavior and Aerosol-Particulate Transitions of 7Be, 10Be, and 210Pb: A Basis for Fallout Radionuclide Chronometry.

We investigated the partitioning of 7Be, 10Be, and 210Pb aerosols between operationally dissolved and >0.5 µm particulate fractions in wet and dry atmospheric deposition. Bulk deposition in situ-log(KD) averaged 4.27 ± 0.46 for 7Be and 4.79 ± 0.59 for 210Pb (±SD, n = 163), with corresponding activity-fractions particulate (fP) = 24 and 48%. KD was inversely correlated with particulate mass concentration (pC), a particle concentration effect (p.c.e.) that indicates that dissolved 7Be and 210Pb are bound to submicron colloids. Experimental desorption-KD was higher than in situ by a factor of 20 for 7Be and 4 for 210Pb (n = 27), indicating that FRN sorption to particulates was irreversible. 7Be:10Be 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 fBe7, fBe10, and fPb210 each increased with 7Be:10Be bulk age, a particle-age effect (p.a.e). In multiple regression, fBe7 was best predicted by N, Mn, Al, and Ni [R2 = 0.75, p < 0.0001], whereas fPb relied on N, S, Fe, and Mn [R2 = 0.69, p < 0.0001]. Despite differences in magnitude and controls on partitioning, the ratio fBe:fPb converged to 1 with pC in the range of 10-100 mg L-1. Given sufficient solid surfaces, irreversible sorption and p.a.e. form a basis for 7Be:210Pb chronometry of aerosol biogeochemical cycling.

Spatially coherent regional changes in seasonal extreme streamflow events in the United States and Canada since 1950.

Complex hydroclimate in the United States and Canada has limited identification of possible ongoing changes in streamflow. We address this challenge by classifying 541 stations in the United States and Canada into 15 "hydro-regions," each with similar seasonal streamflow characteristics. Analysis of seasonal streamflow records at these stations from 1910 to present indicates regionally coherent changes in the frequency of extreme high- and low-flow events. Where changes are significant, these events have, on average, doubled in frequency relative to 1950 to 1969. In hydro-regions influenced by snowmelt runoff, extreme high-flow event frequency has increased despite snowpack depletion by warming winter temperatures. In drought-prone hydro-regions of the western United States and Southeast, extreme low-flow event frequency has increased, particularly during summer and fall. The magnitude and regional consistency of these hydrologic changes warrant attention by watershed stakeholders. The hydro-region framework facilitates quantification and further analyses of these changes to extreme streamflow.

Relationship between altitude and lithium in groundwater in the United States of America: results of a 1992-2003 study.

Therapeutic dosages of lithium are known to reduce suicide rates, which has led to investigations of confounding environmental risk factors for suicide such as lithium in groundwater. It has been speculated that this might play a role in the potential relationship between suicide and altitude. A recent study in Austria involving geospatial analysis of lithium in groundwater and suicide found lower levels of lithium at higher altitudes. Since there is no reason to suspect this correlation is universal given variation in geology, the current study set out to investigate the relationship between altitude and lithium in groundwater in the United States of America (USA). The study utilised data extracted from the National Water-Quality Assessment programme implemented by the United States Geological Survey that has collected 5,183 samples from 48 study areas in USA for the period of 1992 to 2003. Lithium was the trace-element of interest and 518 samples were used in the current analyses. Due to uneven lithium sampling within the country, only the states (n=15) with the highest number of lithium samples were included. Federal information processing standard codes were used to match data by county with the mean county altitude calculated using altitude data from the Shuttle Radar Topography Mission. The study was controlled for potential confounding factors known to affect levels of lithium in groundwater including aquifer, aquifer type, lithology, water level and the depths of wells. The levels of lithium in groundwater, increased with altitude (R(2) = 0.226, P <0.001) during the study period. These findings differ from the Austrian study and suggest a need for further research accounting also for the impact of geographical variation.

Quantitative retention of atmospherically deposited elements by native vegetation is traced by the fallout radionuclides 7Be and 210Pb.

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 (7)Be and (210)Pb, as well as their activities in leaves of colocated trees, for three years in New Hampshire, U.S. The accumulation of both (7)Be and (210)Pb 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 (7)Be and (210)Pb 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 (7)Be and (210)Pb on leaves is thus quantitative and permanent, coupling the fate of (7)Be, (210)Pb 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.

Surficial redistribution of fallout ¹³¹iodine in a small temperate catchment.

Isotopes of iodine play significant environmental roles, including a limiting micronutrient ((127)I), an acute radiotoxin ((131)I), and a geochemical tracer ((129)I). 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 (131)iodine 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) (137)cesium 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 (7)beryllium, 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.

Erosion and physical transport via overland flow of arsenic and lead bound to silt-sized particles.

Understanding of the transport mechanisms of contaminated soils and sediment is essential for the sustainable management of contaminated lands. In New England and elsewhere, vast areas of agricultural lands are contaminated by the historical application of lead-arsenate pesticides. Left undisturbed the physical and chemical mobility of As and Pb in these soils is limited due to their strong affinity for adsorption onto solid phases. However, soil disturbance promotes erosion and overland flow during intense rainstorms. Here we investigate the event-scale transport of disturbed As and Pb contaminated soils through measurement of concentrations of As and Pb in suspended sediment and changes in Pb isotopic ratios in overland flow. Investigation of several rain events shows that where land disturbance has occurred, physical transport of silt-sized particles and aggregates is the primary transport vector of As and Pb derived from pesticide-contaminated soil. Although both As and Pb are associated with similarly-sized particles, we find that solid-phase As is more effectively mobilized and transported than Pb. Our results demonstrate that anthropogenic land disturbance of historical lands contaminated with lead-arsenate pesticides may redistribute, through physical transport, significant amounts of As, and lesser amounts of Pb, to riparian and stream sediments, where they are potentially more bioavailable.

Impact of land disturbance on the fate of arsenical pesticides.

Increasing development of historic farmlands raises questions regarding the fate of pesticides applied when these land were in cultivation. We quantified As and Pb budgets in field soils in two orchards where arsenical pesticides were applied in the early 20th century and a third uncontaminated control field. Sequential extractions and X-ray analyses also were used to determine mineral phases. In addition, we measured metal loads in drainages adjacent to the fields and in two common macroinvertebrate taxa within the wetland at the outlet of the drainages. We find that the applied As and Pb have undergone little vertical redistribution; concentrations of As and Pb in the top 25 cm of contaminated orchard soils are higher than in the uncontaminated control field. However, none of the applied lead arsenate (PbHAsO4) remains in its original mineral phase. Instead, the metals are now primarily adsorbed onto fine silt and clay-sized amorphous oxides and organic matter. Further, physical erosion associated with tilling and replanting appears to have mobilized the fine-particulate-bound As and Pb in one orchard. The remobilized metals are found in sediments in the stream channel draining the tilled orchard. It is unclear if the As and Pb transported sediments are biologically active; average macroinvertebrate metal burdens in the wetland are not elevated above those observed elsewhere in the region. However, little of the mobilized metals may have reached the wetland. These results demonstrate that land use change can significantly impact the retention of arsenical pesticides.

Landfill-stimulated iron reduction and arsenic release at the Coakley Superfund Site (NH).

Arsenic is a contaminant at more than one-third of all Superfund Sites in the United States. Frequently this contamination appearsto resultfrom geochemical processes rather than the presence of a well-defined arsenic source. Here we examine the geochemical processes that regulate arsenic levels at the Coakley Landfill Superfund Site (NH), a site contaminated with As, Cr, Pb, Ni, Zn, and aromatic hydrocarbons. Long-term field observations indicate that the concentrations of most of these contaminants have diminished as a result of treatment by monitored natural attenuation begun in 1998; however, dissolved arsenic levels increased modestly over the same interval. We attribute this increase to the reductive release of arsenic associated with poorly crystalline iron hydroxides within a glaciomarine clay layer within the overburden underlying the former landfill. Anaerobic batch incubations that stimulated iron reduction in the glaciomarine clay released appreciable dissolved arsenic and iron. Field observations also suggest that iron reduction associated with biodegradation of organic waste are partly responsible for arsenic release; over the five-year study period since a cap was emplaced to prevent water flow through the site, decreases in groundwater dissolved benzene concentrations at the landfill are correlated with increases in dissolved arsenic concentrations, consistent with the microbial decomposition of both benzene and other organics, and reduction of arsenic-bearing iron oxides. Treatment of contaminated groundwater increasingly is based on stimulating natural biogeochemical processes to degrade the contaminants. These results indicate that reducing environments created within organic contaminant plumes may release arsenic. In fact, the strong correlation (>80%) between elevated arsenic levels and organic contamination in groundwater systems at Superfund Sites across the United States suggests that arsenic contamination caused by natural degradation of organic contaminants may be widespread.