δ18O as a tracer of PO43- losses from agricultural landscapes.

Accurately tracing the sources and fate of excess PO43- in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO43- (δ18OP) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ18OP could distinguish between agricultural PO43- sources by measuring the integrated δ18OP composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in southwestern Australia. δ18OP composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall δ18OP values (21 ± 1‰) than the loams (23 ± 1‰), which corresponded with a smaller, but more readily leachable, PO43- pool. Fertilisers had greater δ18OP variability (∼8‰), with fluctuations due to type and manufacturing year. Consequently, catchment 'agricultural soil leaching' δ18OP signatures could span from 18 to 25‰ depending on both fertiliser type and timing (lag between application and leaching). These findings emphasise the potential of δ18OP to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO43- losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation.

A New Technique to Determine the Phosphate Oxygen Isotope Composition of Freshwater Samples at Low Ambient Phosphate Concentration.

The oxygen isotope composition of dissolved inorganic phosphate (δ18Op) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining δ18Op are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which δ18Op may provide new and important insights into the biogeochemistry of P. In this Article, we report the development, testing and initial application of a new technique that enables δ18Op analysis to be extended into such ecosystems. The twist spinning mode (TSM) protocol described here enables >1000 L of sample with a P concentration <0.016 mg P L-1 to be initially processed within the field in approximately 24 h. Combined with a new freeze-drying method to maximize the yield and minimize the contamination of silver phosphate generated for isotope ratio mass spectrometry, the TSM protocol is able to generate accurate and precise δ18Op data. We evaluated the TSM protocol using synthetic test solutions and subsequently applied the protocol to samples from locations around the Saint Lawrence River in Montreal, Canada. We believe that the novel technique reported here offers the methodological basis for researchers to extend the application of δ18Op into a much wider range of freshwater ecosystems than has been possible to date.

Public Water Supply Is Responsible for Significant Fluxes of Inorganic Nitrogen in the Environment.

Understanding anthropogenic disturbance of macronutrient cycles is essential for assessing the risks facing ecosystems. For the first time, we quantified inorganic nitrogen (N) fluxes associated with abstraction, mains water leakage, and transfers of treated water related to public water supply. In England, the mass of nitrate-N removed from aquatic environments by abstraction (ABS-NO3-N) was estimated to be 24.2 kt N/year. This is equal to six times the estimates of organic N removal by abstraction, 15 times in-channel storage of organic N, and 30 times floodplain storage of organic N. ABS-NO3-N is also between 3 and 39% of N removal by denitrification in the hydrosphere. Mains water leakage of nitrate-N (MWL-NO3-N) returns 3.62 kt N/year to the environment, equating to approximately 15% of ABS-NO3-N. In urban areas, MWL-NO3-N can represent up to 20% of the total N inputs. MWL-NO3-N is predicted to increase by up to 66% by 2020 following implementation of treated water transfers. ABS-NO3-N and MWL-NO3-N should be considered in future assessments of N fluxes, in order to accurately quantify anthropogenic disturbances to N cycles. The methodology we developed is transferable, uses widely available datasets, and could be used to quantify N fluxes associated with public water supply across the world.

Opening Opportunities for High-Resolution Isotope Analysis - Quantification of δ15NNO3 and δ18ONO3 in Diffusive Equilibrium in Thin-Film Passive Samplers.

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3- stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3- stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments.

Role of Humic Acid in the Stability of Ag Nanoparticles in Suboxic Conditions.

Stability and temporal changes in size distributions have been observed for citrate- (cit) and polyvinylpyrrolidone- (PVP) capped silver nanoparticles (AgNPs), in the presence or absence of sulfide and natural organic matter (NOM, as humic acid), while under suboxic conditions. There were substantial differences in the influence of the two capping agents, with PVP-AgNPs showing few or no significant changes in apparent stability or particle size distribution under the conditions examined, while the apparent size distributions of citrate-capped AgNPs changed rapidly. Sulfide and humic acid each individually caused immediate increases in cit-AgNP size distributions, which were then relatively stable over 60-145 days. This may be due to sulfide bridging and cation bridging, respectively. However, in competition, it was the influence of the humic acid that dominated that of the sulfide. These observations have implications for environmental fate and toxicity of AgNP. The increased stability in the presence of even low concentrations of NOM may limit the rapidity of Ag dispersal but may also concentrate the dose received by organisms, which subsequently ingest the stabilized particles.

Isotopic Fingerprint for Phosphorus in Drinking Water Supplies.

Phosphate dosing of drinking water supplies, coupled with leakage from distribution networks, represents a significant input of phosphorus to the environment. The oxygen isotope composition of phosphate (δ(18)OPO4), a novel stable isotope tracer for phosphorus, offers new opportunities to understand the importance of phosphorus derived from sources such as drinking water. We report the first assessment of δ(18)OPO4 within drinking water supplies. A total of 40 samples from phosphate-dosed distribution networks were analyzed from across England and Wales. In addition, samples of the source orthophosphoric acid used for dosing were also analyzed. Two distinct isotopic signatures for drinking water were identified (average = +13.2 or +19.7‰), primarily determined by δ(18)OPO4 of the source acid (average = +12.4 or +19.7‰). Dependent upon the source acid used, drinking water δ(18)OPO4 appears isotopically distinct from a number of other phosphorus sources. Isotopic offsets from the source acid ranging from -0.9 to +2.8‰ were observed. There was little evidence that equilibrium isotope fractionation dominated within the networks, with offsets from temperature-dependent equilibrium ranging from -4.8 to +4.2‰. While partial equilibrium fractionation may have occurred, kinetic effects associated with microbial uptake of phosphorus or abiotic sorption and dissolution reactions may also contribute to δ(18)OPO4 within drinking water supplies.

Global scale identification of catchments phosphorus source shifts with urbanization: A phosphate oxygen isotope and Bayesian mixing model approach.

Different scenarios of urban expansion can influence the dynamic characteristics of catchments in terms of phosphorus (P). It is important to identify the changes in P sources that occur during the process of urbanization to develop targeted policies for managing P in catchments. However, there is a knowledge gap in quantifying the variations of potential P sources associated with urbanization. By combining phosphate oxygen isotopes from global catchments with a Bayesian model and the urbanization process, we demonstrate that the characteristics of potential P sources (such as fertilizers, urban wastewater, faeces, and bedrock) change as urban areas expand. Our results indicate that using phosphate oxygen isotopes in conjunction with a Bayesian model provides direct evidence of the proportions of potential P sources. We classify catchment P loadings into three stages based on shifts in potential P sources during urban expansion. During the initial stage of urbanization (urban areas < 1.5 %), urban domestic and industrial wastewater are the main contributors to P loadings in catchments. In the mid-term acceleration stage (1.5 % ≤ urban areas < 3.5 %), efforts to improve wastewater treatment significantly reduce wastewater P input, but the increase in fertilizer P input offsets this reduction in sewage-derived P. In the high-level urbanization stage (urban areas ≥ 3.5 %), the proportions of the four potential P sources tend to stabilize. Remote areas bear the burden of excessive P loadings to meet the growing food demand and improved diets resulting from the increasing urban population. Our findings support the development of strategies for water quality management that better consider the driving forces of urbanization on catchment P loadings.

Discovery of sulfonamide resistance genes in deep groundwater below Patna, India.

Global usage of pharmaceuticals has led to the proliferation of bacteria that are resistant to antimicrobial treatments, creating a substantial public health challenge. Here, we investigate the emergence of sulfonamide resistance genes in groundwater and surface water in Patna, a rapidly developing city in Bihar, India. We report the first quantification of three sulfonamide resistance genes (sulI, sulII and sulIII) in groundwater (12-107 m in depth) in India. The mean relative abundance of gene copies was found to be sulI (2.4 × 10-2 copies/16S rRNA gene) > sulII (5.4 × 10-3 copies/16S rRNA gene) > sulIII (2.4 × 10-3 copies/16S rRNA gene) in groundwater (n = 15) and surface water (n = 3). A comparison between antimicrobial resistance (AMR) genes and wastewater indicators, particularly tryptophan:fulvic-like fluorescence, suggests that wastewater was associated with AMR gene prevalence. Urban drainage channels, containing hospital and domestic wastes, are likely a substantial source of antimicrobial resistance in groundwater and surface water, including the Ganges (Ganga) River. This study is a reference point for decision-makers in the fight against antimicrobial resistance because it quantifies and determines potential sources of AMR genes in Indian groundwater.

Surface-derived groundwater contamination in Gulu District, Uganda: Chemical and microbial tracers.

Groundwater is consumed by over 2 billion people globally, though it can be impacted by microbial and chemical contamination in both rural and (peri-)urban areas. This issue is particularly pertinent in regions like East Africa, where rapid urbanisation has strained local infrastructure, including water and sanitation systems. We use selected tracers of human and animal waste to assess the quality of community drinking sources with regards to surface-derived groundwater inputs and to compare urban versus rural water quality, under the rapidly developing urban area of Gulu, Northern Uganda. Specifically, we examine bulk and fluorescent dissolved organic matter (DOM), microorganisms (total coliforms, E. coli) and inorganic tracers of anthropogenic waste (NO3-, SO42-, Cl/Br) from various sources: boreholes (12-76 m depth; n = 90), protected springs (n = 11) and municipal taps (n = 4). Our results show that NO3- and SO42- were elevated in groundwater sources in the Gulu city urban area and the Cl/Br ratio was elevated in springs, compared to concentrations in the more rural Aswa and Omoro County area (p < 0.05). Interestingly, human and animal waste indicators E. coli and Tryp:FA (the ratio of tryptophan-like to fulvic-like fluorescence) displayed no significant difference between rural and urban settings (p > 0.05), though total coliforms were significantly higher in rural boreholes (p < 0.05). The presence of a pollution source, pollution carrier and a breakdown of a sanitary barrier at the borehole, as spot-checked by a visual sanitary risk assessment, was significantly associated with groundwater E. coli abundances. Evidence suggests monitoring and mitigation should be improved for all water types in Gulu District to meet WHO and Uganda Standard guidelines for potable water. This study offers valuable insights for water management planning and risk assessment of community water sources particularly in the context of East Africa and similar settings.

Characterization of suboxic groundwater colloids using a multi-method approach.

Anoxic groundwater colloid properties were measured using a minimally perturbing procedure for sampling, processing, and analysis. Analytical methods included atomic force microscopy (AFM), flow field flow fractionation (FlFFF), and transmission and scanning electron microscopy (TEM and SEM). Shallow groundwater samples showed abundant iron rich nanoparticles (NP) with diameters of 10-30 nm as well as a smaller heterogeneous polydisperse dissolved organic matter (DOM) fraction. AFM results showed NP with average heights of 10 ± 2 nm, which was corroborated by high-resolution TEM and SEM. FlFFF with UV254 nm detection found particles with number average diffusion coefficients of 2-3 × 10(-10) m(2) s(-1) and hydrodynamic diameters between 1.5 and 2 nm probably representing smaller organic macromolecules. Aeration of the samples resulted in extensive agglomeration of NP to form larger (>50 nm) colloids, and a reduction of UV-absorbing material in the 0.5-4 nm range. The complementary methods described have potential applications for investigating the fate and transport of NP in suboxic hotspots such as leachate plumes, wastewater treatment plants, and within the hyporheic mixing zone.

Source partitioning using phosphate oxygen isotopes and multiple models in a large catchment.

Excessive phosphorus (P) loadings cause major pollution concerns in large catchments. Quantifying the point and nonpoint P sources of large catchments is essential for catchment P management. Although phosphate oxygen isotopes (δ18O(PO4)) can reveal P sources and P cycling in catchments, quantifying multiple P sources in a whole catchment should be a research focus. Therefore, this study aimed to quantitatively identify the proportions of multiple potential end members in a typical large catchment (the Yangtze River Catchment) by combining the phosphate oxygen isotopes, land use type, mixed end-element model, and a Bayesian model. The δ18O(PO4) values of river water varied spatially from 4.9‰ to18.3‰ in the wet season and 6.0‰ to 20.9‰ in the dry season. Minor seasonal differences but obvious spatial changes in δ18O(PO4) values could illustrate how human activity changed the functioning of the system. The results of isotopic mass balance and the Bayesian model confirmed that controlling agricultural P from fertilizers was the key to achieving P emission reduction goals by reducing P inputs. Additionally, the effective rural domestic sewage treatment, development of composting technology, and resource utilization of phosphogypsum waste could also contribute to catchment P control. P sources in catchment ecosystems can be assessed by coupling an isotope approach and multiple-models.

Spatial and seasonal controls on dissolved organic matter composition in shallow aquifers under the rapidly developing city of Patna, India.

The distribution and composition of dissolved organic matter (DOM) affects numerous (bio)geochemical processes in environmental matrices including groundwater. This study reports the spatial and seasonal controls on the distribution of groundwater DOM under the rapidly developing city of Patna, Bihar (India). Major DOM constituents were determined from river and groundwater samples taken in both pre- and post-monsoon seasons in 2019, using excitation-emission matrix (EEM) fluorescence spectroscopy. We compared aqueous fluorescent DOM (fDOM) composition to satellite-derived land use data across the field area, testing the hypothesis that the composition of groundwater DOM, and particularly the components associated with surface-derived ingress, may be controlled, in part, by land use. In the pre-monsoon season, the prominence of tryptophan-like components likely generated from recent biological activity overwhelmed the humic-like and tyrosine-like fluorescence signals. Evidence from fluorescence data suggest groundwater in the post-monsoon season is composed of predominantly i) plant-derived matter and ii) anthropogenically influenced DOM (e.g. tryptophan-like components). Organic tracers, as well as Eh and Cl-, suggest monsoonal events mobilise surface-derived material from the unsaturated zone, causing dissolved organic carbon (DOC) of more microbial nature to infiltrate to >100 m depth. A correlation between higher protein:humic-like fluorescence and lower vegetation index (NDVI), determined from satellite-based land use data, in the post-monsoon season, indicates the ingression of wastewater-derived OM in groundwater under the urban area. Attenuated protein:humic-like fluorescence in groundwater close to the river points towards the mixing of groundwater and river water. This ingress of surface-derived OM is plausibly exacerbated by intensive groundwater pumping under these areas. Our approach to link the composition of aqueous organics with land use could easily be adapted for similar hydrogeochemical settings to determine the factors controlling groundwater DOM composition in various contexts.

Emerging organic contaminants in the River Ganga and key tributaries in the middle Gangetic Plain, India: Characterization, distribution & controls.

The presence and distribution of emerging organic contaminants (EOCs) in freshwater environments is a key issue in India and globally, particularly due to ecotoxicological and potential antimicrobial resistance concerns. Here we have investigated the composition and spatial distribution of EOCs in surface water along a ∼500 km segment of the iconic River Ganges (Ganga) and key tributaries in the middle Gangetic Plain of Northern India. Using a broad screening approach, in 11 surface water samples, we identified 51 EOCs, comprising of pharmaceuticals, agrochemicals, lifestyle and industrial chemicals. Whilst the majority of EOCs detected were a mixture of pharmaceuticals and agrochemicals, lifestyle chemicals (and particularly sucralose) occurred at the highest concentrations. Ten of the EOCs detected are priority compounds (e.g. sulfamethoxazole, diuron, atrazine, chlorpyrifos, perfluorooctane sulfonate (PFOS), perfluorobutane sulfonate, thiamethoxam, imidacloprid, clothianidin and diclofenac). In almost 50% of water samples, sulfamethoxazole concentrations exceeded predicted no-effect concentrations (PNECs) for ecological toxicity. A significant downstream reduction in EOCs was observed along the River Ganga between Varanasi (Uttar Pradesh) and Begusarai (Bihar), likely reflecting dilution effects associated with three major tributaries, all with considerably lower EOC concentrations than the main Ganga channel. Sorption and/or redox controls were observed for some compounds (e.g. clopidol), as well as a relatively high degree of mixing of EOCs within the river. We discuss the environmental relevance of the persistence of several parent compounds (notably atrazine, carbamazepine, metribuzin and fipronil) and associated transformation products. Associations between EOCs and other hydrochemical parameters including excitation emission matrix (EEM) fluorescence indicated positive, significant, and compound-specific correlations between EOCs and tryptophan-, fulvic- and humic-like fluorescence. This study expands the baseline characterization of EOCs in Indian surface water and contributes to an improved understanding of the potential sources and controls on EOC distribution in the River Ganga and other large river systems.

Environmental tracers and groundwater residence time indicators reveal controls of arsenic accumulation rates beneath a rapidly developing urban area in Patna, India.

Groundwater security is a pressing environmental and societal issue, particularly due to significantly increasing stressors on water resources, including rapid urbanization and climate change. Groundwater arsenic is a major water security and public health challenge impacting millions of people in the Gangetic Basin of India and elsewhere globally. In the rapidly developing city of Patna (Bihar) in northern India, we have studied the evolution of groundwater chemistry under the city following a three-dimensional sampling framework of multi-depth wells spanning the central urban zone in close proximity to the River Ganges (Ganga) and transition into peri-urban and rural areas outside city boundaries and further away from the river. Using inorganic geochemical tracers (including arsenic, iron, manganese, nitrate, nitrite, ammonium, sulfate, sulfide and others) and residence time indicators (CFCs and SF6), we have evaluated the dominant hydrogeochemical processes occurring and spatial patterns in redox conditions across the study area. The distribution of arsenic and other redox-sensitive parameters is spatially heterogenous, and elevated arsenic in some locations is consistent with arsenic mobilization via reductive dissolution of iron hydroxides. Residence time indicators evidence modern (<~60-70 years) groundwater and suggest important vertical and lateral flow controls across the study area, including an apparent seasonal reversal in flow regimes near the urban center. An overall arsenic accumulation rate is estimated to be ~0.003 ± 0.003 µM.yr-1 (equivalent to ~0.3 ± 0.2 µg.yr-1), based on an average of CFC-11, CFC-12 and SF6-derived models, with the highest rates of arsenic accumulation observed in shallow, near-river groundwaters also exhibiting elevated concentrations of nutrients including ammonium. Our findings have implications on groundwater management in Patna and other rapidly developing cities, including potential future increased groundwater vulnerability associated with surface-derived ingress from large-scale urban abstraction or in higher permeability zones of river-groundwater connectivity.

A systematic approach to understand hydrogeochemical dynamics in large river systems: Development and application to the River Ganges (Ganga) in India.

Large river systems, such as the River Ganges (Ganga), provide crucial water resources for the environment and society, yet often face significant challenges associated with cumulative impacts arising from upstream environmental and anthropogenic influences. Understanding the complex dynamics of such systems remains a major challenge, especially given accelerating environmental stressors including climate change and urbanization, and due to limitations in data and process understanding across scales. An integrated approach is required which robustly enables the hydrogeochemical dynamics and underpinning processes impacting water quality in large river systems to be explored. Here we develop a systematic approach for improving the understanding of hydrogeochemical dynamics and processes in large river systems, and apply this to a longitudinal survey (> 2500 km) of the River Ganges (Ganga) and key tributaries in the Indo-Gangetic basin. This framework enables us to succinctly interpret downstream water quality trends in response to the underpinning processes controlling major element hydrogeochemistry across the basin, based on conceptual water source signatures and dynamics. Informed by a 2019 post-monsoonal survey of 81 river bank-side sampling locations, the spatial distribution of a suite of selected physico-chemical and inorganic parameters, combined with segmented linear regression, reveals minor and major downstream hydrogeochemical transitions. We use this information to identify five major hydrogeochemical zones, characterized, in part, by the inputs of key tributaries, urban and agricultural areas, and estuarine inputs near the Bay of Bengal. Dominant trends are further explored by investigating geochemical relationships (e.g. Na:Cl, Ca:Na, Mg:Na, Sr:Ca and NO3:Cl), and how water source signatures and dynamics are modified by key processes, to assess the relative importance of controls such as dilution, evaporation, water-rock interactions (including carbonate and silicate weathering) and anthropogenic inputs. Mixing/dilution between sources and water-rock interactions explain most regional trends in major ion chemistry, although localized controls plausibly linked to anthropogenic activities are also evident in some locations. Temporal and spatial representativeness of river bank-side sampling are considered by supplementary sampling across the river at selected locations and via comparison to historical records. Limitations of such large-scale longitudinal sampling programs are discussed, as well as approaches to address some of these inherent challenges. This approach brings new, systematic insight into the basin-wide controls on the dominant geochemistry of the River Ganga, and provides a framework for characterising dominant hydrogeochemical zones, processes and controls, with utility to be transferable to other large river systems.

Emerging organic contaminants in groundwater under a rapidly developing city (Patna) in northern India dominated by high concentrations of lifestyle chemicals.

Aquatic pollution from emerging organic contaminants (EOCs) is of key environmental importance in India and globally, particularly due to concerns of antimicrobial resistance, ecotoxicity and drinking water supply vulnerability. Here, using a broad screening approach, we characterize the composition and distribution of EOCs in groundwater in the Gangetic Plain around Patna (Bihar), as an exemplar of a rapidly developing urban area in northern India. A total of 73 EOCs were detected in 51 samples, typically at ng.L-1 to low µg.L-1 concentrations, relating to medical and veterinary, agrochemical, industrial and lifestyle usage. Concentrations were often dominated by the lifestyle chemical and artificial sweetener sucralose. Seventeen identified EOCs are flagged as priority compounds by the European Commission, World Health Organisation and/or World Organisation for Animal Health: namely, herbicides diuron and atrazine; insecticides imidacloprid, thiamethoxam, clothianidin and acetamiprid; the surfactant perfluorooctane sulfonate (and related perfluorobutane sulfonate, perfluorohexane sulfonate, perfluorooctanoic acid and perfluoropentane sulfonate); and medical/veterinary compounds sulfamethoxazole, sulfanilamide, dapson, sulfathiazole, sulfamethazine and diclofenac. The spatial distribution of EOCs varies widely, with concentrations declining with depth, consistent with a strong dominant vertical flow control. Groundwater EOC concentrations in Patna were found to peak within ∼10 km distance from the River Ganges, indicating mainly urban inputs with some local pollution hotspots. A heterogeneous relationship between EOCs and population density likely reflects confounding factors including varying input types and controls (e.g. spatial, temporal), wastewater treatment infrastructure and groundwater abstraction. Strong seasonal agreement in EOC concentrations was observed. Co-existence of limited transformation products with associated parent compounds indicate active microbial degradation processes. This study characterizes key controls on the distribution of groundwater EOCs across the urban to rural transition near Patna, as a rapidly developing Indian city, and contributes to the wider understanding of the vulnerability of shallow groundwater to surface-derived contamination in similar environments.

The need to integrate legacy nitrogen storage dynamics and time lags into policy and practice.

Increased fluxes of reactive nitrogen (Nr), often associated with N fertilizer use in agriculture, have resulted in negative environmental consequences, including eutrophication, which cost billions of dollars per year globally. To address this, best management practices (BMPs) to reduce Nr loading to the environment have been introduced in many locations. However, improvements in water quality associated with BMP implementation have not always been realised over expected timescales. There is a now a significant body of scientific evidence showing that the dynamics of legacy Nr storage and associated time lags invalidate the assumptions of many models used by policymakers for decision making regarding Nr BMPs. Building on this evidence, we believe that the concepts of legacy Nr storage dynamics and time lags need to be included in these models. We believe the biogeochemical research community could play a more proactive role in advocating for this change through both awareness raising and direct collaboration with policymakers to develop improved datasets and models. We anticipate that this will result in more realistic expectations of timescales for water quality improvements associated with BMPs. Given the need for multi-nutrient policy responses to tackle challenges such as eutrophication, integration of N stores will have the further benefit of aligning both researchers and policymakers in the N community with the phosphorus and carbon communities, where estimation of stores is more widespread. Ultimately, we anticipate that integrating legacy Nr storage dynamics and time lags into policy frameworks will better meet the needs of human and environmental health.

Changes in global groundwater organic carbon driven by climate change and urbanization.

Climate change and urbanization can increase pressures on groundwater resources, but little is known about how groundwater quality will change. Here, we use a global synthesis (n = 9,404) to reveal the drivers of dissolved organic carbon (DOC), which is an important component of water chemistry and substrate for microorganisms that control biogeochemical reactions. Dissolved inorganic chemistry, local climate and land use explained ~ 31% of observed variability in groundwater DOC, whilst aquifer age explained an additional 16%. We identify a 19% increase in DOC associated with urban land cover. We predict major groundwater DOC increases following changes in precipitation and temperature in key areas relying on groundwater. Climate change and conversion of natural or agricultural areas to urban areas will decrease groundwater quality and increase water treatment costs, compounding existing constraints on groundwater resources.

Seasonal variability of sediment controls of carbon cycling in an agricultural stream.

Streams and rivers are 'active pipelines' where high rates of carbon (C) turnover can lead to globally important emissions of carbon dioxide (CO2) and methane (CH4) from surface waters to the atmosphere. Streambed sediments are particularly important in affecting stream chemistry, with rates of biogeochemical activity, and CO2 and CH4 concentrations far exceeding those in surface waters. Despite an increase in research on CO2 and CH4 in streambed sediments there is a lack of knowledge and insight on seasonal dynamics. In this study the seasonally variable effect of sediment type (sand-dominated versus gravel-dominated) on porewater C cycling, including CO2 and CH4 concentrations, was investigated. We found high concentrations of CO2 and CH4 in the streambed of a small agricultural stream. Sand-dominated sediments were characterised by higher microbial activity and CO2 and CH4 concentrations than gravel-dominated sediments, with CH4:CO2 ratios higher in sand-dominated sediments but rates of recalcitrant C uptake highest in gravel-dominated sediments. CO2 and CH4 concentrations were unexpectedly high year-round, with little variation in concentrations among seasons. Our results indicate that small, agricultural streams, which generally receive large amounts of fine sediment and organic matter (OM), may contribute greatly to annual C cycling in freshwater systems. These results should be considered in future stream management plans where the removal of sandy sediments may perform valuable ecosystem services, reducing C turnover, CO2 and CH4 concentrations, and mitigating greenhouse gas (GHG) production.

Provenance of drinking water revealed through compliance sampling.

Understanding drinking water hydrochemistry is essential for maintaining safe drinking water supplies. Whilst targeted research surveys have characterised drinking water hydrochemistry, vast compliance datasets are routinely collected but are not interrogated amidst concerns regarding the impact of mixed water sources, treatment, the distribution network and customer pipework. In this paper, we examine whether compliance samples retain hydrochemical signatures of their provenance. We first created and subsequently undertook the first hydrochemical analysis of a novel national database of publically available drinking water compliance analyses (n = 3 873 941) reported for 2015 across England and Wales. k-means cluster analysis revealed three spatially coherent clusters. Cluster 1 is dominated by groundwater sources, with high nitrate concentrations and mineralisation, and lower organic carbon, residual chlorine and THM formation. Cluster 2 was dominated by surface water sources and characterised by low mineralisation (low conductivity and major ion concentrations), low nitrate and high organic carbon concentrations (and hence residual chlorine and THM formation). Cluster 3 shows a mixture of groundwater overlain by confining layers and superficial deposits (resulting in higher trace metal concentrations and mineralisation) and surface water sources. These analyses demonstrate that, despite extensive processing of drinking water, at the national scale signatures of the provenance of drinking water remain. Analysis of compliance samples is therefore likely to be a helpful tool in the characterisation of processes that may affect drinking water chemistry. The methodology used is generic and can be applied in any area where drinking water chemistry samples are taken.