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Sewage effluent discharged from wastewater treatment plants (WWTPs) is a major driver of nitrogen (N) and phosphorus (P) enrichment, but tertiary treatment methods such as P-stripping have previously been shown to mitigate eutrophication risk. The aim of this study was to evaluate the impacts of sewage effluent discharged from WWTPs with contrasting classifications of tertiary treatment on nutrient dynamics across the River Wensum catchment, UK. River water samples were collected from 20 locations across the catchment at monthly intervals between October 2010 and September 2013, with 677 samples collected in total and analysed for a suite of hydrochemical parameters. The 20 sampling locations were divided into four classifications based on the type of upstream WWTP: (1) no WWTP; (2) WWTPs without P-stripping; (3) WWTPs with and without P-stripping; (4) WWTPs with P-stripping. Results revealed substantial overlaps in riverine nutrient composition making differentiation between classifications difficult. The majority of N (>97%) and P (~75%) was present in dissolved bioavailable forms across all sites and there was no significant difference in total N speciation between classifications. Total P (TP) speciation did, however, reveal higher proportions of particulate P at sites with no WWTP, indicating a greater P contribution of agricultural origin. Ratios of total dissolved to particulate P (TDP:TPP) and chloride concentrations proved effective discriminators of agricultural and sewage P, respectively, but phosphate‑boron ratios (PO 4 :B) were ineffective discriminators in this catchment. Most importantly, there was no evidence that P-stripping reduced overall TP concentrations downstream of WWTPs, despite evidence of a per capita reduction, nor reduced the proportion of dissolved P released. These findings were attributed to P-stripping facilities serving larger populations and thus releasing greater effluent P load, thereby demonstrating that the presence of tertiary P-stripping alone is insufficient to overcome population pressures and ensure that rivers achieve good hydrochemical status., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Metaldehyde, a widely used molluscicide, is one of the most commonly detected pesticides in aquatic environments in the UK. In this study, metaldehyde concentrations and fluxes in stream water over a ten-year period (2008-2018) are reported for the River Colne catchment (Essex, southeast England), and the influence of hydrological conditions and application regimes are assessed. In general, peaks in metaldehyde concentration in river water occasionally exceeded 0.25 μg L -1 , and concentrations did not typically exceed the European Union Drinking Water Directive (EU DWD) regulatory limit of 0.1 μg L -1 . Metaldehyde concentration peaks displayed a seasonal pattern. Metaldehyde concentrations during periods when the molluscicide was not applied to agricultural land (January, July) and during the spring-summer application period (February to June) were generally low (0.01-0.03 μg L -1 ). Peaks in metaldehyde concentration mainly occurred during the autumn-winter application season (August to December), and were typically associated with high intensity hydrological regimes (daily rainfall ≥10 mm; stream flow up to 18 m 3 s -1 ). Where metaldehyde concentrations exceeded the EU DWD regulatory limit, this was short-lived. The annual flux at the top of the Colne catchment (0.2-0.6 kg a -1 ) tended to be lower than in the middle of the catchment (0.3-1.4 kg a -1 ), with maximum flux values observed at the bottom of the catchment (0.5-25.8 kg a -1 ). Metaldehyde losses from point of application to surface water varied between 0.01 and 0.25%, with a maximum of 1.18% (2012). Annual flux was primarily controlled by the annual precipitation and stream flow (R 2 = 0.9) rather than annual metaldehyde use (kg active applied). Precipitation explained 37% and 81% of variability in metaldehyde concentration and flux, respectively. Annual ranges in metaldehyde concentration were greater in the years 2012 and 2014 with an overall reduction in the range of metaldehyde concentrations evident over the period 2015-2018. It is the expectation that metaldehyde concentrations in stream water will continue to decrease following the withdrawal of metaldehyde for outdoor use in the UK from March 2022., Competing Interests: Declaration of competing interest The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript., (Crown Copyright © 2021. Published by Elsevier B.V. All rights reserved.)

Agricultural point source pesticide pollution arising from contaminated machinery washings and accidental spillages pose a significant threat to river water and groundwater quality. In this study, we assess the effectiveness of a three-stage on-farm biobed for treating pesticide contaminated wastewater from a large (20 km(2)) commercial arable estate. The facility consisted of an enclosed machinery wash-down unit (stage 1), a 49 m(2) lined compost-straw-topsoil biobed (stage 2), and a 200 m(2) drainage field with a trickle irrigation system (stage 3). Pesticide concentrations were analysed in water samples collected fortnightly between November 2013 and November 2015 from the biobed input and output sumps and from 20 porous pots buried at 45 cm and 90 cm depth within the drainage field. The results revealed that the biobed removed 68-98% of individual pesticides within the contaminated washings, with mean total pesticide concentrations reducing by 91.6% between the biobed input and output sumps. Drainage field irrigation removed a further 68-99% of individual pesticides, with total mean pesticide concentrations reducing by 98.4% and 97.2% in the 45 cm and 90 cm depth porous pots, respectively. The average total pesticide concentration at 45 cm depth in the drainage field (57 μg L(-1)) was 760 times lower than the mean concentration recorded in the input sump (43,334 μg L(-1)). There was no evidence of seasonality in the efficiency of biobed pesticide removal, nor was there evidence of a decline in removal efficiency over the two-year monitoring period. However, higher mean total pesticide concentrations at 90 cm (102 μg L(-1)) relative to 45 cm (57 μg L(-1)) depth indicated an accumulation of pesticide residues deeper within the soil profile. Overall, the results presented here demonstrate that a three-stage biobed can successfully reduce pesticide pollution risk from contaminated machinery washings on a commercial farm., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Agricultural diffuse water pollution remains a notable global pressure on water quality, posing risks to aquatic ecosystems, human health and water resources and as a result legislation has been introduced in many parts of the world to protect water bodies. Due to their efficiency and cost-effectiveness, water quality models have been increasingly applied to catchments as Decision Support Tools (DSTs) to identify mitigation options that can be introduced to reduce agricultural diffuse water pollution and improve water quality. In this study, the Soil and Water Assessment Tool (SWAT) was applied to the River Wensum catchment in eastern England with the aim of quantifying the long-term impacts of potential changes to agricultural management practices on river water quality. Calibration and validation were successfully performed at a daily time-step against observations of discharge, nitrate and total phosphorus obtained from high-frequency water quality monitoring within the Blackwater sub-catchment, covering an area of 19.6 km(2). A variety of mitigation options were identified and modelled, both singly and in combination, and their long-term effects on nitrate and total phosphorus losses were quantified together with the 95% uncertainty range of model predictions. Results showed that introducing a red clover cover crop to the crop rotation scheme applied within the catchment reduced nitrate losses by 19.6%. Buffer strips of 2 m and 6 m width represented the most effective options to reduce total phosphorus losses, achieving reductions of 12.2% and 16.9%, respectively. This is one of the first studies to quantify the impacts of agricultural mitigation options on long-term water quality for nitrate and total phosphorus at a daily resolution, in addition to providing an estimate of the uncertainties of those impacts. The results highlighted the need to consider multiple pollutants, the degree of uncertainty associated with model predictions and the risk of unintended pollutant impacts when evaluating the effectiveness of mitigation options, and showed that high-frequency water quality datasets can be applied to robustly calibrate water quality models, creating DSTs that are more effective and reliable., (Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Mechanistic insights into the relative contribution of sorption and biodegradation on the removal of the herbicide isoproturon (IPU) are reported. (14)C-radiorespirometry indicated very low levels of catabolic activity in IPU-undosed and IPU-dosed (0.1, 1, 100 μg L(-1)) river water (RW) and groundwater (GW) (mineralisation: <2%). In contrast, levels of catabolic activity in IPU-undosed and IPU-dosed river sediment (RS) were significantly higher (mineralisation: 14.5-36.9%). Levels of IPU catabolic competence showed a positive log-linear relationship (r(2) = 0.768) with IPU concentration present. A threshold IPU concentration of between 0.1 μg L(-1) and 1 μg L(-1) was required to significantly (p < 0.05) increase levels of catabolic activity. Given the EU Drinking Water Directive limit for a single pesticide in drinking water of <0.1 μg L(-1) this result suggests that riverbed sediment infiltration is potentially an appropriate 'natural' means of improving water quality in terms of pesticide levels at concentrations that are in keeping with regulatory limits., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

In the UK agriculture is by far the largest source of nitrous oxide (N(2)O) emissions. Direct N(2)O emissions as a result of nitrogen (N) application to soils have been well documented in the UK, whereas indirect emissions produced in surface waters and groundwaters from leached N are much less understood with limited data to support IPCC emission factors. Indirect emissions were studied in surface waters in the Upper Thurne, a lowland drained arable catchment in eastern England. All surface waters were found to have dissolved N(2)O concentrations above that expected if in equilibrium with ambient concentrations, demonstrating all surface waters were acting as a source of N(2)O. The drainage channels represented 86% of the total indirect N(2)O flux, followed by wetland areas, 11%, and the river, 3%. The dense drainage network was found to have the highest dissolved N(2)O concentrations of all the water bodies studied with a combined N(2)O flux of 16 kg N(2)O-N per day in March 2007. Such indirect fluxes are comparable to direct fluxes per hectare and represent a significant proportion of the total N(2)O flux for this catchment. Separate emission factors were established for the three different surface water types within the same catchment, suggesting that the one emission factor used in the Intergovernmental Panel on Climate Change (IPCC) methodology for predicting all indirect N(2)O emissions is inappropriate.

The 1991 EU Nitrate Directive was designed to reduce water pollution from agriculturally derived nitrates. England and Wales implemented this Directive by controlling agricultural activities within their most vulnerable areas termed Nitrate Vulnerable Zones. These were designated by identifying drinking water catchments (surface and groundwater), at risk from nitrate pollution. However, this method contravened the Nitrate Directive because it only protected drinking water and not all waters. In this paper, a GIS was used to identify all areas of groundwater vulnerable to nitrate pollution. This was achieved by constructing a model containing data on four characteristics: the quality of the water leaving the root zone of a piece of land; soil information; presence of low permeability superficial (drift) material; and aquifer properties. These were combined in a GIS and the various combinations converted into a measure of vulnerability using expert knowledge. Several model variants were produced using different estimates of the quality of the water leaving the root zone and contrasting methods of weighting the input data. When the final models were assessed all produced similar spatial patterns and, when verified by comparison with trend data derived from monitored nitrate concentrations, all the models were statistically significant predictors of groundwater nitrate concentrations. The best predictive model contained a model of nitrate leaching but no land use information, implying that changes in land use will not affect designations based upon this model. The relationship between nitrate levels and borehole intake depths was investigated since there was concern that the observed contrasts in nitrate levels between vulnerability categories might be reflecting differences in borehole intake depths and not actual vulnerability. However, this was not found to be statistically important. Our preferred model provides the basis for developing a new set of groundwater Nitrate Vulnerable Zones that should help England and Wales to comply with the EU Nitrate Directive.