Phosphate oxygen isotopes within aquatic ecosystems:global data synthesis and future research priorities

The oxygen isotope ratio of dissolved inorganic phosphate (δ18Op) represents a novel and potentially powerful stable isotope tracer for biogeochemical research. Analysis of δ18Op may offer new insights into the relative importance of different sources of phosphorus within natural ecosystems. Due to the isotope fractionations that occur alongside the metabolism of phosphorus, δ18Op could also be used to better understand the intracellular and extracellular reaction mechanisms that control phosphorus cycling. In this review focussed on aquatic ecosystems, we examine the theoretical basis to using stable oxygen isotopes within phosphorus research.We consider the methodological challenges involved in accurately determining δ18Op, given aquatic matrices in which potential sources of contaminant oxygen are ubiquitous. Finally,we synthesise the existing global data regarding δ18Op in aquatic ecosystems, concluding by identifying four key areas for future development of δ18Op research. Through this synthesis, we seek to stimulate broader interest in the use of δ18Op to address the significant research and management challenges that continue to surround the stewardship of phosphorus

Using sorbent waste materials to enhance treatment of micro-point source effluents by constructed wetlands

Sorbent materials are widely used in environmental settings as a means of = enhancing pollution remediation. A key area of environmental concern is that of water pollution, including the need to treat micro-point sources of wastewater pollution, such as from caravan sites or visitor centres. Constructed wetlands (CWs) represent one means for effective treatment of wastewater from small wastewater producers, in part because they are believed to be economically viable and environmentally sustainable. Constructed wetlands have the potential to remove a range of pollutants found in wastewater, including nitrogen (N), phosphorus (P), biochemical oxygen demand (BOD) and carbon (C), whilst also reducing the total suspended solids (TSS) concentration in effluents. However, there remain particular challenges for P and N removal from wastewater in CWs, as well as the sometimes limited BOD removal within these treatment systems, particularly for micro-point sources of wastewater. It has been hypothesised that the amendment of CWs with sorbent materials can enhance their potential to treat wastewater, particularly through enhancing the removal of N and P. This paper focuses on data from batch and mesocosm studies that were conducted to identify and assess sorbent materials suitable for use within CWs. The aim in using sorbent material was to enhance the combined removal of phosphate (PO4-P) and ammonium (NH4-N). The key selection criteria for the sorbent materials were that they possess effective PO4-P, NH4-N or combined pollutant removal, come from low cost and sustainable sources, have potential for reuse, for example as a fertiliser or soil conditioner, and show limited potential for re-release of adsorbed nutrients. The sorbent materials selected for testing were alum sludge from water treatment works, ochre derived from minewater treatment, biochar derived from various feedstocks, plasterboard and zeolite. The performance of the individual sorbents was assessed through preliminary desorption studies, isotherm and kinetic adsorption studies, as well as through final desorption studies. Batch studies demonstrated that alum sludge and ochre effectively removed PO4-P from solution (maximum sorption capacity up to 45 mg/g), whilst biochar from both bamboo and rice feedstocks demonstrated effective removal of NH4-N from solution. The potential benefit of using combined reactive media in conjunction with wastewater recirculation to enhance N, P and C treatment was examined using mesocosm studies, and we report initial data from these mesocosm studies

Pollutant swapping: greenhouse gas emissions from wetland systems constructed to mitigate agricultural pollution

Diffuse (non-point) water pollution from agricultural land continues to challenge water quality management, requiring the adoption of new land management practices. The use of constructed agricultural wetlands is one such practice, designed to trap multiple pollutants mobilised by rainfall prior to them reaching receiving water. Through capturing and storing pollutants in bottom sediments, it could be hypothesised that the abundance of nutrients stored in the anoxic conditions commonly found in these zones may lead to pollutant swapping. Under these circumstances, trapped material may undergo biogeochemical cycling to change chemical or physical form and thereby become more problematic or mobile within the environment. Thus, constructed agricultural wetlands designed to mitigate against one form of pollution may in fact offset the created benefits by ‘swapping’ this pollution into other forms and pathways, such as through release to the atmosphere. Pollutant swapping to the atmosphere has been noted in analogous wetland systems designed to treat municipal and industrial wastewaters, with significant fluxes of CO2, CH4 and N2O being recorded in some cases. However the small size, low level of engineering and variable nutrient/sediment inputs which are features of constructed agricultural wetlands, means that this knowledge is not directly transferable. Therefore, more information is required when assessing whether a wetland’s potential to act as hotspot for pollution swapping outweighs its potential to act as a mitigation tool for surface water pollution. Here we present results from an on-going monitoring study at a trial agricultural wetland located in small a mixed-use catchment in Cumbria, UK. Estimates were made of CH4, CO2 and N2O flux from the wetland surface using adapted floating static chambers, which were then directly compared with fluxes from an undisturbed riparian zone. Results indicate that while greenhouse gas flux from the wetland may be significant, the impacts of this may be greatly diminished when considering wetland size in relation to catchment area. As such, this increased understanding will be valuable when considering the implications of rural land use management for water quality improvement. This knowledge could also be applied to further enhancing our knowledge of gas regional/global gas emissions from freshwater systems, which at the moment are poorly constrained

The effect of water oxygen content on the production of greenhouse gases from shallow pond sediments

Shallow lakes and ponds, including those commonly found in agricultural landscapes are often only a few metres deep, with surface areas <1ha. Despite this, landscapes may contain a high number of these ponds, amounting to a considerable cumulative surface area. Many of these features, both naturally formed and man-made, receive and trap runoff with high nutrient and sediment loadings. As such, the potential for the production of greenhouse gases (GHGs) through biogeochemical cycling in the pond sediments may be significant. Furthermore, the abundance of available nutrients coupled with the shallow physical characteristics of these systems, mean that short, irregular eutrophic episodes during the summer are common, causing large fluctuations in th oxygen content of the overlying water column. The oxygen content of the water column is often cited as key factor in the production of GHGs in large lake and reservoir systems. Given the limited research focusing on shallow ponds/lakes, and potential for these systems to be important sources of GHGs, the impacts of variable water oxygen content should be investigated. Here we present the results from a sediment microcosm experiment utilising sediment cores from an agricultural pond system in Cumbria, UK. Intact sediment cores were incubated in the dark at in-situ temperature and continuously fed with filtered pond water for 2 weeks. During this time the oxygen content of the water was manipulated between fully oxygenated and anaerobic. Measurements of GHG release were based on calculated dissolved gas concentrations present in the water columns of these cores. Results indicated that during times of water column anoxia, production of methane and carbon dioxide increased significantly, despite the presence of substantial quantities of nitrate in the water columns. No change in N2O production was detected. These results indicate that while representing a significant cumulative carbon store in agricultural landscapes, shallow pond and lake systems can contribute to emission of GHGs. Furthermore, the physical and ecological characteristics of these systems have the potential to significantly increase the quantity of gas produced. This understanding will be valuable when constraining both freshwater and agricultural GHG budgets

Development and initial application of δ18Op to understand phosphorus cycling in river, lake and groundwater ecosystems.

Variation in the stable isotope composition of oxygen within dissolved phosphate (δ18Op) represents a novel and potentially powerful environmental tracer. In freshwater, marine and terrestrial ecosystems, δ18Op can act as an inherent label for the sources of phosphorus and the extent to which phosphorus from different sources is metabolised. This paper focuses on the methodological development and initial application of δ18Op across a range of freshwater ecosystems. Initially, we report modifications to the analytical protocol for δ18Op that are designed to minimise incorporation of contaminant oxygen in the final silver phosphate precipitate prior to pyrolysis. This is critical given the range of possible sources of contaminant oxygen within freshwater matrices. Subsequently, we consider the potential utility of δ18Op through application of the technique within a range of freshwater ecosystems in England, UK. Firstly, we characterise δ18Op in river water and effluents from Sewage Treatment Works (STW), and examine the opportunity to use the 18Op of STW effluents to trace the entry and downstream fate of phosphorus from these point sources in rivers. Secondly, we analyse δ18Op to gain insights into variations in the sources and biological cycling of phosphorus in a seasonally stratified lake ecosystem. Thirdly, we characterise δ18Op in shallow and deep groundwater samples, considering whether δ18Op might provide evidence for variation in source and extent of metabolism for phosphorus in groundwater ecosystems. Taken together, these data extend the catalogue of δ18Op in freshwater ecosystems, and further the scope of δ18Op as a tool to better understand phosphorus biogeochemistry

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 (δ18OPO4), 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 δ18OPO4 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 δ18OPO4 of the source acid (average = +12.4 or +19.7‰). Dependent upon the source acid used, drinking water δ18OPO4 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 δ18OPO4 within drinking water supplies

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

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 (d18Op) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining d18Op are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which d18Op may provide new and important insights into the biogeochemistry of P. In this paper, we report the development, testing and initial application of a new technique that enables d18Op 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 hours. Combined with a new freeze-drying method to maximise the yield and minimise the contamination of silver phosphate generated for isotope ratio mass spectrometry, the TSM protocol is able to generate accurate and precise d18Op 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 d18Op into a much wider range of freshwater ecosystems than has been possible to date

Mitigation of diffuse water pollution from agriculture in England and China, and the scope for policy transfer

• To mitigate diffuse water pollution from agriculture (DWPA) in China, the right mix of complementary policy approaches is needed. • The public agricultural extension service is relatively well resourced and is the primary means available to mitigate DWPA. The extension service needs re-orientation and re-skilling to help farmers maintain and increase agricultural productivity whilst balancing this with environmental protection. A new ethos of input use efficiency and environmental stewardship of natural resources is needed, based on 2-way knowledge exchange with farmers. Four policies to achieve this are: 1. A ‘reference level’ of enforceable regulation for all large commercial farms is needed. This can be transposed from existing laws with appropriate variation by farming system and region. Intensive livestock units have the greatest potential to cause significant pollution and take first priority. Resources for monitoring and enforcement of regulation are limited, but as land transfer and farm consolidation continue in accord with local needs, regulations for use of manure and chemical fertiliser in arable systems can be developed for large farms. 2. For small farms monitoring and enforcement of regulations is difficult. Simple, locally well-adapted guidelines are needed. Adoption by farmers must be achieved through an accredited advisory and voluntary approach developed by the public agricultural extension service and its wider agricultural knowledge and innovation systems partners. 3. Targeted incentive payment schemes should be used strategically to protect water resources from DWPA in key locations. For example, payments for retirement, or low intensity use, of vulnerable land adjacent to watercourses or in aquifer recharge zones used for water supply. 4. To support these approaches more applied research is needed to build an accessible and comprehensive knowledgebase. This should span, for example, from methods for public participation, through design of regulation and incentive payments, to design and costing of farm best management practices and estimation of modelling coefficients empirically derived for conditions in China. • None of these approaches are completely absent from China and attempts at international policy transfer or ‘lesson-drawing’ must consider what can be better developed rather than what could commence. Innovation in farmer participation, advice provision, design of incentive schemes, data sharing and applied research are leading examples

Watermains leakage and outdoor water use are responsible for significant phosphorus fluxes to the environment across the United States

Human activity has led to excess phosphorus (P) concentrations and the continued eutrophication of coastal and freshwaters across the United States (US). Developing more effective P management policy requires a comprehensive understanding of P sources in the environment. Public water systems across the United States widely dose water with phosphate (PO4) in order to control the corrosion of lead and copper within their distribution networks. Using public water system PO4 dosing facility data and target PO4-P dosing concentrations, we estimate that PO4 dosing added 4–14.9 kt PO4-P yr−1 into the US water distribution network in 2015. Using estimates of public water supply inputs and domestic water deliveries, we estimate that 0.7–2.6, and 0.8–3.1 kt PO4-P yr−1 were then lost from the network due to watermains leakage and outdoor water use, respectively. After accounting for these fluxes, we estimate that 9.3 kt PO4-P yr−1 was then returned to wastewater treatment plants (WWTPs) and accounted for up to 2.7% of the national WWTP influent P load. As sources of P to the environment, lower and upper estimates of combined watermains leakage and outdoor water use PO4-P fluxes exceeded P loads to surface waterbodies from documented point sources across 461–541 counties. The exceedance of these fluxes above other major components of the US P-budget emphasizes the need to include them in P source apportionment studies, both across the US and in other countries where public water supplies are dosed with PO4