Global variability and controls on the accumulation of fallout radionuclides in cryoconite.

The accumulation of fallout radionuclides (FRNs) from nuclear weapons testing and nuclear accidents has been evaluated for over half a century in natural environments; however, until recently their distribution and abundance within glaciers have been poorly understood. Following a series of individual studies of FRNs, specifically 137Cs, 241Am and 210Pb, deposited on the surface of glaciers, we now understand that cryoconite, a material commonly found in the supraglacial environment, is a highly efficient accumulator of FRNs, both artificial and natural. However, the variability of FRN activity concentrations in cryoconite across the global cryosphere has never been assessed. This study thus aims to both synthesize current knowledge on FRNs in cryoconite and assess the controls on variability of activity concentrations. We present a global database of new and previously published data based on gamma spectrometry of cryoconite and proglacial sediments, and assess the extent to which a suite of environmental and physical factors can explain spatial variability in FRN activity concentrations in cryoconite. We show that FRNs are not only found in cryoconite on glaciers within close proximity to specific sources of radioactivity, but across the global cryosphere, and at activity concentrations up to three orders of magnitude higher than those found in soils and sediments in the surrounding environment. We also show that the organic content of cryoconite exerts a strong control on accumulation of FRNs, and that activity concentrations in cryoconite are some of the highest ever described in environmental matrices outside of nuclear exclusion zones, occasionally in excess of 10,000 Bq kg-1. These findings highlight a need for significant improvements in the understanding of the fate of legacy contaminants within glaciated catchments. Future interdisciplinary research is required on the mechanisms governing their accumulation, storage, and mobility, and their potential to create time-dependent impacts on downstream water quality and ecosystem sustainability.

Legacy radionuclides in cryoconite and proglacial sediment on Orwell Glacier, Signy Island, Antarctica.

Cryoconite is a specific type of material found on the surface of glaciers and icesheets. Samples of cryoconite were collected from the Orwell Glacier and its moraines, together with suspended sediment from the proglacial stream on Signy Island, part of the South Orkney Islands, Antarctica. The activity concentrations of certain fallout radionuclides were determined in the cryoconite, moraine and suspended sediment, in addition to particle size composition and %C and %N. For cryoconite samples (n = 5), mean activity concentrations (±1SD) of 137Cs, 210Pbun and 241Am were 13.2 ± 20.9, 66.1 ± 94.0 and 0.32 ± 0.64 Bq kg-1, respectively. Equivalent values for the moraine samples (n = 7) were 2.56 ± 2.75, 14.78 ± 12.44 and <1.0 Bq kg-1, respectively. For the composite suspended sediment sample, collected over 3 weeks in the ablation season, the values (± counting uncertainty) for 137Cs, 210Pbun and 241Am were 2.64 ± 0.88, 49.2 ± 11.9 and <1.0 Bq kg-1, respectively. Thus, fallout radionuclide activity concentrations were elevated in cryoconite relative to moraine and suspended sediment. In the case of 40K, the highest value was for the suspended sediment (1423 ± 166 Bq kg-1). The fallout radionuclides in cryoconite were 1-2 orders of magnitude greater than values in soils collected from other locations in Antarctica. This work further demonstrates that cryoconite likely scavenges fallout radionuclides (dissolved and particulate) in glacial meltwater. In the case of 40K, the greater value in suspended sediment implies a subglacial source. These results are amongst the relatively few that demonstrate the presence of fallout radionuclides in cryoconites at remote locations in the Southern Hemisphere. This work adds to the growing contention that elevated activities of fallout radionuclides, and other contaminants, in cryoconites are a global phenomenon and may be a risk to downstream terrestrial and aquatic ecosystems.

Extreme levels of fallout radionuclides and other contaminants in glacial sediment (cryoconite) and implications for downstream aquatic ecosystems.

Glaciers in most parts of the world are retreating, releasing water and sediments to downstream rivers. Studies have found elevated levels of fallout radionuclides (FRNs) and other contaminants in glacial sediments, especially cryoconite, in European glaciers and Greenland. However, there are no equivalent studies for glaciers in North America. We report concentrations of FRNs (i.e. 137Cs, 210Pbun and 241Am) and other contaminants (i.e. metal(loids), phosphorus) in cryoconite and proglacial sediments from a glacier in British Columbia, Canada, and compare values to suspended sediments from the downstream river. The mean concentrations of 137Cs, 210Pbun and 241Am in cryoconite were 2,123 ± 74, 7,535 ± 224 and 11.5 ± 3.0 Bq kg-1, respectively, which are an order of magnitude greater than those for most soils and surficial materials. FRNs were much lower in suspended sediments and decreased with distance away from the glacier. Geochemical elements were enriched in cryoconite relative to local clastic materials and upper continental crust. Concentrations of FRNs in cryoconite were correlated with organic matter, which suggests this is important in controlling the scavenging of hydrophobic contaminants in glacial meltwater. Low concentrations of FRNs and contaminants in suspended sediments suggest that glacial meltwater and the delivery of cryoconite have limited impact on downstream aquatic ecosystems.

Effectiveness of Vegetated Buffer Strips in Controlling Legacy Phosphorus Exports from Agricultural Land.

The continued phosphorus (P) impairment of freshwaters and the associated risk of eutrophication raise questions regarding the efficiency of current beneficial management practices (BMPs) for improving water quality. Vegetated buffer strips (VBSs) are widely encouraged BMPs for reducing P export from agricultural land. However, there is a lack of evidence regarding the long-term efficiency of VBSs for reducing legacy P losses. This research used soil analyses to investigate the P removal efficiency of an unmanaged VBS for controlling P loss from agricultural land in Manitoba, Canada, between 1954 and 2011. The results showed statistically significant retention of total P, Olsen extractable P, and 0.01 M CaCl extractable P by a 5-m wide VBS compared with field soils. We found that surface soils at 5-m into the VBS had a significantly greater P sorption capacity and a smaller degree of P saturation (DPS) than adjacent field soils. The elevated DPS in field soils is likely associated with gradual P enrichment as a result of manure or fertilizer application over time and the strong affinity of P compounds for soil. Although P stratification in the VBS over 57 yr resulted in a significant increase (∼11%) in DPS of VBS topsoil compared with VBS subsoil, our findings do not support the saturation of VBS soils with P. However, cutting and removal of vegetation from VBS could be a useful strategy to remove P from VBS and minimize possible P remobilization associated with vegetation senescence, especially where the climate is cold and runoff is dominated by snowmelt.

A deconvolutional Bayesian mixing model approach for river basin sediment source apportionment.

Increasing complexity in human-environment interactions at multiple watershed scales presents major challenges to sediment source apportionment data acquisition and analysis. Herein, we present a step-change in the application of Bayesian mixing models: Deconvolutional-MixSIAR (D-MIXSIAR) to underpin sustainable management of soil and sediment. This new mixing model approach allows users to directly account for the 'structural hierarchy' of a river basin in terms of sub-watershed distribution. It works by deconvoluting apportionment data derived for multiple nodes along the stream-river network where sources are stratified by sub-watershed. Source and mixture samples were collected from two watersheds that represented (i) a longitudinal mixed agricultural watershed in the south west of England which had a distinct upper and lower zone related to topography and (ii) a distributed mixed agricultural and forested watershed in the mid-hills of Nepal with two distinct sub-watersheds. In the former, geochemical fingerprints were based upon weathering profiles and anthropogenic soil amendments. In the latter compound-specific stable isotope markers based on soil vegetation cover were applied. Mixing model posterior distributions of proportional sediment source contributions differed when sources were pooled across the watersheds (pooled-MixSIAR) compared to those where source terms were stratified by sub-watershed and the outputs deconvoluted (D-MixSIAR). In the first example, the stratified source data and the deconvolutional approach provided greater distinction between pasture and cultivated topsoil source signatures resulting in a different posterior distribution to non-deconvolutional model (conventional approaches over-estimated the contribution of cultivated land to downstream sediment by 2 to 5 times). In the second example, the deconvolutional model elucidated a large input of sediment delivered from a small tributary resulting in differences in the reported contribution of a discrete mixed forest source. Overall D-MixSIAR model posterior distributions had lower (by ca 25-50%) uncertainty and quicker model run times. In both cases, the structured, deconvoluted output cohered more closely with field observations and local knowledge underpinning the need for closer attention to hierarchy in source and mixture terms in river basin source apportionment. Soil erosion and siltation challenge the energy-food-water-environment nexus. This new tool for source apportionment offers wider application across complex environmental systems affected by natural and human-induced change and the lessons learned are relevant to source apportionment applications in other disciplines.

Uncertainties in vegetated buffer strip function in controlling phosphorus export from agricultural land in the Canadian prairies.

Vegetated buffer strips (VBSs) are widely encouraged as a cost-effective strategy to address phosphorus (P) pollution associated with agricultural production. However, there is a lack of evidence in the effectiveness of these measures for tackling diffuse P pollution in cold-climate regions under concentrated runoff flow conditions. This research aimed to investigate the effects of VBSs on reducing P concentrations in surface runoff at three different watersheds in Manitoba, Canada. Surface runoff samples were collected in four sub-catchments from each watershed by installing paired weirs at 0.5-m and at 5-m into the VBSs along the expected runoff flow path. In addition, P concentrations were measured in soil samples collected within and outside of the runoff flow path to gain further insight into P dynamics within VBSs at each study site. The results indicate that VBSs had little or no significant effect on reducing the concentration of P forms in surface runoff in the majority of situations, resulting in reduced runoff losses of total, dissolved and particulate P concentrations in only 23, 12 and 12% of the situations, respectively. In addition, Olsen extractable P concentrations in VBS soils were not significantly different from field soils both within and outside of the flow path. The ineffective P retention by VBSs in this region is likely associated with the fact that the majority of the runoff flow is concentrated through small portions of VBSs and occurs during snowmelt when biogeochemical processes responsible for P retention in VBSs are limited. Further research is needed to develop alternative management practices that enhance P retention during concentrated snowmelt runoff events in such cold-climate regions.

Selecting Color-based Tracers and Classifying Sediment Sources in the Assessment of Sediment Dynamics Using Sediment Source Fingerprinting.

The use of sediment color as a fingerprint property to determine sediment sources is an emerging technique that can provide a rapid and inexpensive means of investigating sediment sources. The present study aims to test the feasibility of color fingerprint properties to apportion sediment sources within the South Tobacco Creek Watershed (74 km) in Manitoba, Canada. Suspended sediment from 2009 to 2011 at six monitoring stations and potential source samples along the main stem of the creek were collected. Reflectance spectra of sediments and source materials were quantified using a diffuse reflectance spectrometry, and 16 color coefficients were derived from several color space models. Canonical discriminant analysis was used to reclassify and downsize sediment source groups. After the linear additive test and stepwise discriminant function analysis, four color coefficients were chosen to fit the Stable Isotope Analysis in R model. Consistent with the conventional fingerprinting approach, the color fingerprint results demonstrated a switch in the dominant sediment source between the headwaters and the outlet of the watershed, with the main sources being topsoil in the upper reaches, whereas outcrop shale and stream bank materials dominated in the lower reaches. The color fingerprinting approach can be integrated with conventional fingerprints (e.g., geochemical and fallout radionuclide properties) to improve source discrimination, which is a key component for source ascription modeling. We concluded that the use of color fingerprints is a promising, cost-effective technique for sediment source fingerprinting.

Individual and cumulative effects of agriculture, forestry and metal mining activities on the metal and phosphorus content of fluvial fine-grained sediment; Quesnel River Basin, British Columbia, Canada.

The impact of agriculture, forestry and metal mining on the quality of fine-grained sediment (<63 µm) was investigated in the Quesnel River Basin (QRB) (~11,500 km(2)) in British Columbia, Canada. Samples of fine-grained sediment were collected monthly during the snow-free season in 2008 using time-integrated samplers at replicate sites representative of agriculture, forestry and mining activities in the basin (i.e. "impacted" sites). Samples were also collected from replicate reference sites and also from the main stem of the Quesnel River at the downstream confluence with the Fraser River. Generally, metal(loid) and phosphorus (P) concentrations for "impacted" sites were greater than for reference sites. Furthermore, concentrations of copper (forestry and mining sites), manganese (agriculture and forestry sites) and selenium (agriculture, forestry and mining sites) exceeded upper sediment quality guideline (SQG) thresholds. These results suggest that agriculture, forestry and metal mining activities are having an influence on the concentrations of sediment-associated metal(loid)s and P in the Quesnel basin. Metal(loid) and P concentrations of sediment collected from the downstream site were not significantly greater than values for the reference sites, and were typically lower than the values for the impacted sites. This suggests that the cumulative effects of agriculture, forestry and mining activities in the QRB are presently not having a measureable effect at the river basin-scale. The lack of a cumulative effect at the basin-scale is thought to reflect: (i) the relatively recent occurrence of land use disturbances in this basin; (ii) the dominance of sediment contributions from natural forest and agriculture; and (iii) the potential for storage of contaminants on floodplains and other storage elements between the locations of disturbance activities and the downstream sampling site, which may be attenuating the disturbance signal.

Using delta15N and delta18O to evaluate the sources and pathways of NO3- in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions.

The origin of NO(3) (-) yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) was measured from the 'inter-flow' and 'drain-flow' of two 1 ha drained lysimeter plots, one of which had received an application of 21 m(3) of NH(4) (+)-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO(3) (-)-N from the two lysimeters were comparable; an initial flush of NO(3) (-)-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO(3) (-)-N L(-1). The isotopic signature of the delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of delta(15)N-NO(3) (-) during rainfall events ranged between -1.6 to +5.2 per thousand and +0.4 to +11.1 per thousand from the inter-flow and drain-flow, respectively. Variation in the delta(18)O-NO(3) (-) ranged from +2.0 to +7.8 per thousand and from +3.3 to +8.4 per thousand. No significant relationships between the delta(15)N-NO(3) (-) or delta(18)O-NO(3) (-) and flow rate were observed in most cases although delta(18)O-NO(3) (-) values indicated a positive relationship and delta(15)N-NO(3) (-) values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical delta(18)O-NO(3) (-) for soil microbial NO(3) (-) indicated that the contribution of rainfall NO(3) (-) accounted for 8% of the NO(3) (-) in the lysimeter drainage at most. The calculated contribution of rainfall NO(3) (-) was not enough to account for the depletion in delta(15)N-NO(3) (-) values observed during the duration of the rainfall event 2. The relationship between delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) from the drain-flow indicated that denitrification was causing enrichment in the isotopes from this pathway. The presence of slurry seemed to cause a relative depletion in delta(18)O-NO(3) (-) in the inter-flow and delta(15)N-NO(3) (-) in the drain-flow compared with the zero-slurry lysimeter. This may have been caused by increased microbial nitrification stimulated by the presence of increased NH(4) (+)-N.

Controls on catchment-scale patterns of phosphorus in soil, streambed sediment, and stream water.

Many models of phosphorus (P) transfer at the catchment scale rely on input from generic databases including, amongst others, soil and land use maps. Spatially detailed geochemical data sets have the potential to improve the accuracy of the input parameters of catchment-scale nutrient transfer models. Furthermore, they enable the assessment of the utility of available, generic spatial data sets for the modeling and prediction of soil nutrient status and nutrient transfer at the catchment scale. This study aims to quantify the unique and joint contribution of soil and sediment properties, land cover, and point-source emissions to the spatial variation of P concentrations in soil, streambed sediments, and stream water at the scale of a medium-sized catchment. Soil parent material and soil chemical properties were identified as major factors controlling the catchment-scale spatial variation in soil total P and Olsen P concentrations. Soil type and land cover as derived from the generic spatial database explain 33.7% of the variation in soil total P concentrations and 17.4% of the variation in Olsen P concentrations. Streambed P concentrations are principally related to the major element concentrations in streambed sediment and P delivery from the hillslopes due to sediment erosion. During base flow conditions, the total phosphorus (<0.45 microm) concentrations in stream water are mainly controlled by the concentrations of P and the major elements in the streambed sediment.

Spatial variability of soil phosphorus in relation to the topographic index and critical source areas: sampling for assessing risk to water quality.

A measure of soil P status in agricultural soils is generally required for assisting with prediction of potential P loss from agricultural catchments and assessing risk for water quality. The objectives of this paper are twofold: (i) investigating the soil P status, distribution, and variability, both spatially and with soil depth, of two different first-order catchments; and (ii) determining variation in soil P concentration in relation to catchment topography (quantified as the "topographic index") and critical source areas (CSAs). The soil P measurements showed large spatial variability, not only between fields and land uses, but also within individual fields and in part was thought to be strongly influenced by areas where cattle tended to congregate and areas where manure was most commonly spread. Topographic index alone was not related to the distribution of soil P, and does not seem to provide an adequate indicator for CSAs in the study catchments. However, CSAs may be used in conjunction with soil P data for help in determining a more "effective" catchment soil P status. The difficulties in defining CSAs a priori, particularly for modeling and prediction purposes, however, suggest that other more "integrated" measures of catchment soil P status, such as baseflow P concentrations or streambed sediment P concentrations, might be more useful. Since observed soil P distribution is variable and is also difficult to relate to nationally available soil P data, any assessment of soil P status for determining risk of P loss is uncertain and problematic, given other catchment physicochemical characteristics and the sampling strategy employed.

PCB congener dynamics in a heavily industrialized river catchment.

PCB congener concentrations in the water column of a highly industrialized river catchment, the Aire/Calder, in N.E. England were determined weekly on a routine basis, and 2 hourly through selected high flow (flood) events. Bed, suspended and floodplain sediment PCB congener concentrations were also determined along transects of the rivers investigated. Weekly monitoring revealed that the sum of 11 quantified (Sigma11) PCBs rose in concentration by two orders of magnitude during late summer compared to their winter minimum values. This rise was concurrent with sustained periods of low flow. SigmaPCB concentrations were rapidly diluted during high flow (flood) events. Suspended sediment was, on average, 13 times more contaminated with PCBs than bed sediment, with means of 4.0 and 53.8 ng/g, respectively, while floodplain samples had an intermediate concentration of 29.8 ng/g. Principle components analysis (PCA) of congener profiles showed that all three sediment types were similar, but that congener profiles differed considerably between sediment and whole-water samples. There was no change in the percentage contribution of individual PCB congeners apparent from weekly whole-water monitoring. However, the congener pattern in whole-waters changed systematically during high flow events. PCA showed that whole-water samples collected during high flow events had progressively more sediment characteristics, and then returned to whole-water characteristics on cessation of the event. The PCA evidence, dilution of PCB concentrations during events, and suspended sediments more contaminated than bed sediments, indicate that the major sources of PCBs in this catchment are current inputs from sewage treatment works, rather than remobilization of bed sediments.

Fingerprinting suspended sediment sources in a large urban river system.

Very few studies have attempted to quantify the sources of suspended sediment transported in urban river systems. In this study, statistically verified composite fingerprints and a multivariate mixing model have been used to identify the main sources of the suspended sediment transported by the River Aire and its main tributary, the River Calder. Because of the polluted nature of the Aire/Calder catchment and its effect on fingerprint property concentrations, source tracing was undertaken separately for the upper and lower reaches. The mean contributions from individual source types (i.e. surface materials from woodland, uncultivated and cultivated areas, channel bank material, road dust and solids from sewage treatment works) varied between the upper and lower reaches of the rivers, reflecting the change in land use from primarily pasture and moorland in the upper reaches to mainly urban areas (with some cultivated land) in the lower reaches. The suspended sediment in the upper reaches of the River Aire originates largely from channel bank sources (43-84%) and from uncultivated topsoil (16-57%). In the lower reaches of the Aire/Calder system, local sources of cultivated topsoil contribute 20-45% of the suspended sediment load and there is a significant contribution from urban sources, such as road dust (19-22%) and solids from sewage treatment works (14-18%). In the upper reaches, the proportion of sediment derived from each of the two main geological areas corresponds broadly to the proportion of the catchment occupied by each geological area. The relative contribution from the Rivers Aire and Calder to the suspended sediment load transported below the confluence demonstrates that most of the sediment is derived from the River Calder.

The phosphorus content of fluvial sediment in rural and industrialized river basins.

The phosphorus content of fluvial sediment (suspended sediment and the < 63 microm fraction of floodplain and channel bed sediment) has been examined in contrasting rural (moorland and agricultural) and industrialized catchments in Yorkshire, UK. The River Swale drains a rural catchment with no major urban and industrial areas, and the total phosphorus (TP) content of fluvial sediment is generally within the range 500-1,500 microg g(-1). There is little evidence of any major downstream increase in TP content. In contrast, fluvial sediment from the industrialized catchments of the Rivers Aire and Calder exhibits both higher levels of TP content and marked downstream increases, with values of TP content ranging from < 2,000 microg g(-1) in headwater areas upstream of the main urban and industrial areas, to values > 7,000 microg g(-1) at downstream sites. These elevated levels reflect P inputs from point sources, such as sewage treatment works (STWs) and combined sewer overflows. The influence of STWs is further demonstrated by the downstream increase in the inorganic P/organic P ratio from < 2 in the headwaters to > 4 in the lower reaches. Comparison of the P content of suspended sediment with that of the <63 microm fraction of potential source materials suggests that topsoil from upland moorland/pasture and from cultivated areas, and channel bank material are likely to be the main sources of particulate P (PP) in the River Swale and in the headwaters of the Rivers Aire and Calder. In the middle and lower reaches of the Rivers Aire and Calder, inputs associated with urban and industrial land uses, such as STWs, industrial effluents and street dust, are likely to represent the dominant sources of PP. During high flow events, such urban inputs may be diluted by inputs from moorland and agricultural land in the headwaters. Consequently, for all three rivers, there are inverse relationships between the TP content of suspended sediment and both discharge and suspended sediment concentration, reflecting changes in sediment and P sources during high flow events. Spatial variations in the P contents of the < 63 microm fraction of overbank floodplain deposits and channel bed sediment evidence a similar pattern as those for suspended sediment, with relatively low levels of TP in the River Swale and elevated levels in the middle and downstream reaches of the Rivers Aire and Calder. The PP concentrations associated with floodplain and channel bed sediment are, however, lower than equivalent values for suspended sediment, and this primarily reflects the differences in the particle size composition between the three types of sediments. Rates of floodplain deposition and the amounts of fine-grained sediment stored in the river channels are relatively high, and suggest that such environments may represent important sinks for PP. Based on the sediment samples collected from the study basins, a simple four-fold classification which relates the TP content of suspended sediment to upstream land use has been established. Both the range and the absolute values of TP content tend to increase with an increase in the level of urbanization and industrialization.