Hydro-chemical responses at different scales in a rural catchment, UK, and implications for managing the unintended consequences of agriculture

Diffuse pollutant transfers from agricultural land often constitute the bulk of annual loads in catchments and storm events dominate these fluxes. There remains a lack of understanding of how pollutants move through catchments at different scales. This is critical if the mismatch between the scales used to implement on-farm management strategies, compared to those used for assessment of environmental quality, is to be addressed. The aim of this study was to understand how the mechanisms of pollutant export may change when assessed at different scales and the corresponding implications for on-farm management strategies. A study was conducted within a 41 km2 catchment containing 3 nested sub-catchments, instrumented to monitor discharge and various water quality parameters. Storm data over a 24-month period were analysed and hysteresis (HI) and flushing (FI) indices calculated for two water quality variables that are typically of environmental significance; NO3–N and suspended sediment (SSC). For SSC, increasing spatial scale had little effect on the mechanistic interpretation of mobilisation and the associated on-farm management strategies. At the three smallest scales NO3–N was chemodynamic with the interpretation of dominant mechanisms changing seasonally. At these scales, the same on-farm management strategies would be recommended. However, at the largest scale, NO3–N appeared unaffected by season and chemostatic. This would lead to a potentially very different interpretation and subsequent on-farm measures. The results presented here underscore the benefits of nested monitoring for extracting mechanistic understanding of agricultural impacts on water quality. The application of HI and FI indicates that monitoring at smaller scales is crucial. At large scales, the complexity of the catchment hydrochemical response means that mechanisms become obscured. Smaller catchments more likely represent critical areas within larger catchments where mechanistic understanding can be extracted from water quality monitoring and used to underpin the selection of on-farm mitigation measures

Dynamics of fluvial hydro-sedimentological, nutrient, particulate organic matter and effective particle size responses during the U.K. extreme wet winter of 2019–2020

The floc size distribution of suspended sediment is a critical driver for in-channel sedimentation and sediment associated contaminant and nutrient transfer and fate in river catchments. Real-time, in situ, floc size characterisation is possible using available technology but, to date, limited high resolution floc data have been published for fluvial systems draining upland extensive grassland catchments. To that end, suspended sediment floc size distribution and turbidity were characterised at 15-minute intervals using Laser In-Situ Scattering and Transmissometry (LISST) diffraction and a YSI turbidity sonde for six storm events in the upper River Taw (15 km2) catchment in SW England. Maximum event discharges (Q) ranged between 4.3 and 20.0 m3 s−1, with clockwise hysteretic responses (HI = 0.18–0.48) of total suspended solid concentrations (TSS) and Q. The sediment flushing index was highest in the early autumn (0.93) and storm event TSS fluxes varied from 0.04 to 2.9 t km−2. This suggests a change in sources or composition of sediment during higher Q and highly variable patterns of sediment flux from event-to-event. The proportion of particulate organic matter (POM) to TSS was highly variable (5–89%) and did not increase with Q, indicating POM source limitation. The fine-grained tail (D10 and D16) of the floc size distributions decreased during hydrograph rising limbs, with the finest floc sizes associated with the highest TN and TP concentrations at peak Q. The results suggest that dynamic interactions between wet ground and extreme rainfall events can flush significant amounts of sediment from the relatively undisturbed extensive grassland upland catchment. We strongly encourage a sensors-based approach to reveal the spatio-temporal complexity of floc size and associated pollutant export during high Q generated by extreme rainfall since this can help to elucidate processes and mechanisms and generate high-resolution data for water quality modelling without significant user intervention

Comparison of sediment biomarker signatures generated using time-integrated and discrete suspended sediment samples

Sediment source fingerprinting using biomarker properties has led to new insights in our understanding of land use contributions to time-integrated suspended sediment samples at catchment scale. A time-integrated mass-flux sampler (TIMS; also known as the ‘Phillips’ sampler), a cost-effective approach for suspended sediment collection in situ. Such samplers are being used to collect sediment samples for source fingerprinting purposes, including by studies using biomarkers as opposed to more conventional tracer properties. Here, we assessed the performance of TIMS for collecting representative sediment samples for biomarkers during high discharge events in a small lowland agricultural catchment. Concentrations of long odd-chain n-alkanes (>C23) and both saturated free and bound fatty acids (C14-C32), as well as compound-specific 13C were compared between sediment collected by both TIMS and auto-samplers (ISCO). The results showed that concentrations of alkanes, free fatty acids and bound fatty acids are consistently comparable between TIMS and ISCO suspended sediment samples. Similarly, compound-specific 13C signals were not found to be significantly different in the suspended sediment samples collected using the different samplers. However, different magnitudes of resemblance in biomarker concentrations and compositions between the samples collected using the two sediment collection methods were confirmed by overlapping index and symmetric coordinates-based correlation analysis. Here, the difference is attributed to the contrasting temporal basis of TIMS (time-integrated) vs ISCO (discrete) samples, as well as potential differences in the particle sizes collected by these different sediment sampling methods. Nevertheless, our findings suggest that TIMS can be used to generate representative biomarker data for suspended sediment samples collected during high discharge events

Occurrence of Unapproved Pesticides and their Ecotoxicological Significance for an Agriculturally Influenced Reservoir and its Tributaries in Nepal

Many catchments in Nepal are affected by intensive agricultural activities, leading to extensive pesticide usages. This study aimed to assess pesticide abundance in concurrently collected water, sediment and fish samples for the first time in intensively cultivated catchment (Indra Sarowar) located in the mid-hill region of Nepal during the rice and vegetables growing season. A total of 75 pesticides were analysed, of which 4 pesticides (alachlor, diuron, metalaxyl and pyrimethanil) were present in water with detection frequency (DF) > 40%, with alachlor (0.62 – 2.68 µg L−1) being ubiquitous. In the sediment of tributaries, the pesticides p,p′-DDT, β-HCH, alachlor and diuron were detected with DF exceeding 40%, where β-HCH was commonly observed (DF = 92%) with concentration ranging from 6.29 – 99.22 µg kg−1. The ecotoxicological risk indicated that herbicides (alachlor and diuron) posed a high risk to aquatic organisms in both tributaries and reservoir water. Such risk in sediment was even more pronounced, with alachlor and diuron showing up to 2.3 and 53.7 times higher risk respectively compared to water samples. However, none of these herbicides were detected in fish muscles. Among the fish species studied, pyrimethanil was the only quantified pesticide in edible tissue of both cage cultured (0.35 – 1.80 µg g−1 ww) and open stock fishes (0.06 – 1.12 µg g−1 ww). The consumer risk assessment showed very low human health risk associated with fish consumption (HQ < 0.2). Nonetheless, long-term consumption of contaminated fish may pose some risk that cannot be ignored. Overall, this study generated the benchmark data highlighting pervasive presence of banned (DDT, endosulfan, HCH) and unapproved (alachlor, diuron, pyrimethanil) pesticides in the environmental compartments in the mid-hill’s streams of Nepal

Prolonged heavy rainfall and land use drive catchment sediment source dynamics: appraisal using multiple biotracers

Excessive sediment loss degrades freshwater quality and is prone to further elevation and variable source contributions due to the combined effect of extreme rainfall and differing land uses. To quantify erosion and sediment source responses across scales, this study integrated work at both field and catchment scale for two hydrologically contrasting winters (2018-19 and 2019-20). Sediment load was estimated at the field scale (grassland-arable conversion system). Sediment source apportionment work was undertaken at the catchment scale (4.5 km2) and used alkanes, and both free and bound fatty acid carbon isotope signatures as diagnostic fingerprints to distinguish sediment sources: arable, pasture, woodland and stream banks. Sediment source apportionment based on bound fatty acids revealed a substantial shift in contributions, from stream banks dominating (70 ± 5%) in winter 2018-19, to arable land dominating (52 ± 7%) in the extreme wet winter 2019-20. Increases in sediment contributions from arable (~3.9 times) and pasture (~2.4 times) land at the catchment outlet during the winter 2019-20 were consistent with elevated sediment losses monitored at the field scale which indicated that low-magnitude high frequency rainfall alone increased sediment loss even from pasture by 350%. In contrast, carbon isotope signatures of alkanes and free fatty acids consistently estimated stream banks as a dominant source (i.e., ~36 % and ~70 % respectively) for both winters regardless of prolonged rainfall in winter 2019-20. Beyond quantifying the shifts in field scale sediment load and catchment scale sediment sources due to the changes in rainfall patterns, our results demonstrate valuable insight into how the fate of biotracers in soil and sediment manifests in the δ13C values of homologues and, in turn, their role in information gain for estimating sediment source contributions. Discrepancies in the estimated sediment source contributions using different biotracers indicate that without a careful appreciation of their biogeochemical limitations, erroneous interpretation of sediment source contributions can undermine management strategies for delivering more sustainable and resilient agriculture

Diffuse water pollution during recent extreme wet-weather in the UK: Environmental damage costs and insight into the future?

Periods of extreme wet-weather elevate agricultural diffuse water pollutant loads and climate projections for the UK suggest wetter winters. Within this context, we monitored nitrate and suspended sediment loss using a field and landscape scale platform in SW England during the recent extreme wet-weather of 2019–2020. We compared the recent extreme wet-weather period to both the climatic baseline (1981–2010) and projected near- (2041–2060) and far- (2071–2090) future climates, using the 95th percentiles of conventional rainfall indices generated for climate scenarios downscaled by the LARS-WG weather generator from the 19 global climate models in the CMIP5 ensemble for the RCP8.5 emission scenario. Finally, we explored relationships between pollutant loss and the rainfall indices. Grassland field-scale monthly average nitrate losses increased from 0.39- to 1.07 kg ha−1 (2016–2019) to 0.70–1.35 kg ha−1 (2019–2020), whereas losses from grassland ploughed up for cereals, increased from 0.63- to 0.83 kg ha−1 to 2.34–4.09 kg ha−1 . Nitrate losses at landscape scale increased during the 2019–2020 extreme wet-weather period to 2.04–4.54 kg ha−1 . Field-scale grassland monthly average sediment losses increased from 92- to 116 kg ha−1 (2016–2019) to 281–333 kg ha−1 (2019–2020), whereas corresponding losses from grassland converted to cereal production increased from 63- to 80 kg ha−1 to 2124–2146 kg ha−1. Landscape scale monthly sediment losses increased from 8- to 37 kg ha−1 in 2018 to between 15 and 173 kg ha−1 during the 2019–2020 wet-weather period. 2019–2020 was most representative of the forecast 95th percentiles of >1 mm rainfall for near- and far-future climates and this rainfall index was related to monitored sediment, but not nitrate, loss. The elevated suspended sediment loads generated by the extreme wet-weather of 2019–2020 therefore potentially provide some insight into the responses to the projected >1 mm rainfall extremes under future climates at the study location

Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers

In recent decades, the diatom Didymosphenia geminata has emerged as nuisance species in river systems around the world. This periphytic alga forms large “blooms” in temperate streams, presenting a counterintuitive result: the blooms occur primarily in oligotrophic streams and rivers, where phosphorus (P) availability typically limits primary production. The goal of this study is to examine how high algal biomass is formed under low P conditions. We reveal a biogeochemical process by which D. geminata mats concentrate P from flowing waters. First, the mucopolysaccaride stalks of D. geminata adsorb both iron (Fe) and P. Second, enzymatic and bacterial processes interact with Fe to increase the biological availability of P. We propose that a positive feedback between total stalk biomass and high growth rate is created, which results in abundant P for cell division. The affinity of stalks for Fe in association with iron-phosphorus biogeochemistry suggest a resolution to the paradox of algal blooms in oliogotrophic streams and rivers

Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers

In recent decades, the diatom Didymosphenia geminata has emerged as nuisance species in river systems around the world. This periphytic alga forms large “blooms” in temperate streams, presenting a counterintuitive result: the blooms occur primarily in oligotrophic streams and rivers, where phosphorus (P) availability typically limits primary production. The goal of this study is to examine how high algal biomass is formed under low P conditions. We reveal a biogeochemical process by which D. geminata mats concentrate P from flowing waters. First, the mucopolysaccaride stalks of D. geminata adsorb both iron (Fe) and P. Second, enzymatic and bacterial processes interact with Fe to increase the biological availability of P. We propose that a positive feedback between total stalk biomass and high growth rate is created, which results in abundant P for cell division. The affinity of stalks for Fe in association with iron-phosphorus biogeochemistry suggest a resolution to the paradox of algal blooms in oliogotrophic streams and rivers

ROAD NETWORK MAPPING USING AIRBORNE LiDAR DATA

The accurate, detailed and up-to-date road information is highly essential geo-spatial databases for transportation, smart city and other related applications. Thus, the main objective of this research is to develop an efficient algorithm for road network extraction from airborne LiDAR data using supervised classification approach. The proposed algorithm first classifies the input data into the road and non-road features using modified maximum likelihood classification approach. Then Digital Terrain Model (DTM) mask is generated by removing non-ground features from Digital Surface Model using hierarchical morphology and road candidate image if obtained. The parking lots are removed and road network is extracted successfully

Current advisory interventions for grazing ruminant farming cannot close exceedance of modern background sediment loss – Assessment using an instrumented farm platform and modelled scaling out

Water quality impairment by elevated sediment loss is a pervasive problem for global water resources. Sediment management targets identify exceedance or the sediment loss ‘gap’ requiring mitigation. In the UK, palaeo-limnological reconstruction of sediment loss during the 100–150 years pre-dating the post-World War II intensification of agriculture, has identified management targets (0.20−0.35 t ha−1 yr−1) representing ‘modern background sediment delivery to rivers’. To assess exceedance on land for grazing ruminant farming, an integrated approach combined new mechanistic evidence from a heavily-instrumented experimental farm platform and a scaling out framework of modelled commercial grazing ruminant farms in similar environmental settings. Monitoring (2012–2016) on the instrumented farm platform returned sediment loss ranges of 0.11−0.14 t ha−1 yr−1 and 0.21−0.25 t ha−1 yr-1 on permanent pasture, compared with between 0.19−0.23 t ha−1 yr-1 and 0.43−0.50 t ha−1 yr−1and 0.10−0.13 t ha−1 yr−1and 0.25−0.30 t ha−1 yr-1 on pasture with scheduled plough and reseeds. Excess sediment loss existed on all three farm platform treatments but was more extensive on the two treatments with scheduled plough and reseeds. Excessive sediment loss from land used by grazing ruminant farming more strategically across England, was estimated to be up to >0.2 t ha−1 yr−1. Modelled scenarios of alternative farming futures, based on either increased uptake of interventions typically recommended by visual farm audits, or interventions selected using new mechanistic understanding for sediment loss from the instrumented farm platform, returned minimum sediment loss reductions. On the farm platform these were 2.1 % (up to 0.007 t ha−1 yr−1) and 5.1 % (up to 0.018 t ha−1 yr-1). More strategically, these were up to 2.8 % (0.014 t ha−1 yr−1) and 4.1 % (0.023 t ha−1 yr−1). Conventional on-farm measures will therefore not fully mitigate the sediment loss gap, meaning that more severe land cover change is required