The Catchment Runoff Attenuation Flux Tool, a minimum information requirement nutrient pollution model

A model for simulating runoff pathways and water quality fluxes has been developed using the minimum information requirement (MIR) approach. The model, the Catchment Runoff Attenuation Flux Tool (CRAFT), is applicable to mesoscale catchments and focusses primarily on hydrological pathways that mobilise nutrients. Hence CRAFT can be used to investigate the impact of flow pathway management intervention strategies designed to reduce the loads of nutrients into receiving watercourses. The model can help policy makers meet water quality targets and consider methods to obtain “good” ecological status. A case study of the 414 km2 Frome catchment, Dorset, UK, has been described here as an application of CRAFT in order to highlight the above issues at the mesoscale. The model was primarily calibrated on 10-year records of weekly data to reproduce the observed flows and nutrient (nitrate nitrogen – N; phosphorus – P) concentrations. Data from 2 years with sub-daily monitoring at the same site were also analysed. These data highlighted some additional signals in the nutrient flux, particularly of soluble reactive phosphorus, which were not observable in the weekly data. This analysis has prompted the choice of using a daily time step as the minimum information requirement to simulate the processes observed at the mesoscale, including the impact of uncertainty. A management intervention scenario was also run to demonstrate how the model can support catchment managers investigating how reducing the concentrations of N and P in the various flow pathways. This mesoscale modelling tool can help policy makers consider a range of strategies to meet the European Union (EU) water quality targets for this type of catchment

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O-2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-11.08 +/- 0.04) + (1.57 +/- 0.32 kJ mol(-1))/RT ln10 The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H2SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O + SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T -> S process in H2SiOO. This process has a small spin orbit coupling matrix element, consistent with an estimate of its rate constant of 1 x 10(9) s(-1) obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O-2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2 + O-2 and SiCl2 + O-2

Balancing water demand needs with protection of river water quality by minimising stream residence time: an example from the Thames, UK

Freshwater resources in the River Thames basin in southern UK are faced with combined pressures of future population growth and climate change. River basin managers are seeking increasingly innovative methods to meet water demand whilst at the same time maintaining ecological status. Using a river network hydrochemical model modified to account for possible future climate and population, the paper assesses the impact on downstream water quality of changing the location of a major point of abstraction serving the city of Oxford. The rationale behind the hypothetical change, although entailing an increase in energy costs and capital expenditure, was that flows would be maintained along a sensitive stretch of river. Model results at a location a further 23 km downstream suggested that better water quality would arise from this change. The predicted improvements included a decrease in the annual frequency of low DO concentrations (<6 mg L-1) from 8-9 days to 2-3 days and a decrease in 90th percentile (summer) temperatures of 0.6 ⁰C. It is believed these improvements would primarily be attributable to shortening of river residence time which curtails accelerated phytoplankton growth. The overall conclusion, of relevance both for the Thames basin and elsewhere, is that water quality in a river network can be surprisingly sensitive to the location of abstractions. Changing the location of abstractions should be considered as part of a suite of measures available to river basin managers when making plans to meet future water demand

Field-scale evaluation of collection methods for dissolved methane samples in groundwaters

This report presents the findings of a jointly funded project by the British Geological Survey (BGS) and Environment Agency (EA project SC210014) that addresses some of the research needs identified in the EA project SC190007 “Methods for sampling and analysing methane in groundwater: a review of current research and best practice”. Primary field sampling allowed comparison of sample collection techniques for dissolved methane in groundwaters, to provide a field evidence base to help establish good practice guidelines (or protocols). This included evaluation of purging protocols, groundwater sampling devices, surface collection protocols, and the influence of methane concentration. The field study used two boreholes previously shown to have groundwater of contrasting low methane concentration (~1mg/l) ‘Site A’, and high methane concentration (~25 mg/l) ‘Site B’ in close proximity in the Vale of Pickering. The boreholes were previously used for hydrochemical baseline monitoring and were similar in construction and hydrogeological setting, each having shallow (~ 1 m depth) water table and a conventional 3-m long well screen sampling a fractured Kimmeridge Clay unit with methane naturally present from elevated organic matter contents. A stage 1 zero-purge passive sampling device was used to obtain initial samples, followed by a low-flow methodology with parallel use of submersible, bladder and peristaltic pumped samples, with stage 2 sampled after purging a single screen volume, and stage 3 sampled after purging to hydrochemical parameter stabilisation over 5.7 – 7.5 pumped screen volumes. Finally, a post-purge stage 4 sample was obtained with the same passive sampling device. Sample collection protocols tested were open (direct fill vial), semi-closed inverted (submerged-inverted vial), semi-closed upright (submerged-upright vial) and closed (double valve cylinder closed to atmosphere). All samples were obtained in triplicate from each pump during stages 2 and 3, but in stages 1 and 4 only open samples were possible from the passive sampling device. Data interpretation was supported by logged hydrochemical borehole groundwater depth profiles before and after the sampling programme, and by the historical methane baseline record. Methane concentrations measured at Site A are challenging to interpret: they are highly sensitive to purging volume, with a decrease in mean concentration of 72% over the purging stages. This, and the time required to obtain multiple samples, obscured specific sensitivity of methane concentration to pump and sample collection protocol variables at Site A. Although the differences in concentrations seen between pumps and between collection protocol are not statistically significant, the high variability in Site A data overall, 52-117% relative standard deviation (RSD), mean these data are generally not useful for developing good practice proposals. Site B, with high methane concentration, provided more consistent data that allowed meaningful comparisons of methane sensitivity between purging volume, pump type and collection methods with findings that are generally consistent with existing literature. Methane concentrations had a lower sensitivity to purging than at Site A (21% mean concentration declines with ~30 % RSD). Most of the conclusions made from Site B data can reasonably be expected to also apply to sites with lower concentrations. In isolation, pump selection - bladder, submersible or peristaltic pump - makes little difference to sampled methane concentrations. The HydasleeveTM passive sampler consistently produced the lowest concentrations, but is very likely to have sampled different water in the borehole to that sampled mid-screen by the active pumps. However, bladder and peristaltic pump closed samples yield higher concentrations, which is attributed to their capacity to provide increased pressure at low flow, thereby reducing degassing potential. The bladder pump is preferred for this use, due to its lower closed sample concentration variability, which is attributed to the controllability of the bladder pump pressure. The peristaltic pump was tested under favourable shallow water table conditions, and a negative concentration bias that is generally expected from suction pressure was not evident, but this is acknowledged as a concern, especially for deeper water tables, where its use may need more caution. Closed sampling at Site B consistently yielded the highest methane concentrations across all pumps with lowest variability, and is the recommended sample collection protocol. Commercial availability of closed sample analysis is, however, limited. The semi-closed (inverted and upright) methods yielded intermediate concentrations between closed and passive samples, with inverted methods generally giving higher concentrations than semi-closed. When using the inverted protocol, exsolving gas headspace accumulation leads to uncertainties in concentration measurements, meaning that the semi-closed upright protocol is often preferred, especially where degassing is evident or expected although results in this study do not directly support this. Open samples consistently produced the lowest concentrations and should only be used where other protocols are impractical, e.g. sampling from a non-pumped collection device. Switching protocols from open sampling to upright sampling requires minimal investment, and is likely to produce more robust concentration data and/or reduced variability, although results from this study do not provide undisputable evidence of this. The observed sensitivities of measured methane concentrations to different parts of the sampling methodology underline the central importance of using an identical protocol with specific pumps, similar deployments, identical purging volumes or stabilisation criteria, and specific sample collection protocol, in order to generate robust temporal records. Reliable cross comparison of data produced by different organisations requires sampling protocols to be as identical as possible. Any protocol change should be done using an overlap period in which both old and new protocols are used simultaneously, to prevent a sampling-related step change in data. This study also indicates that extended purging of any borehole leads to lower concentration samples over time, which critically has the potential to underestimate methane risk. Combining the findings of this study and wider literature reviewed under EA project SC190007, a concise ‘lookup’ sheet is presented as a non-prescriptive aid to assist practitioners in ‘Groundwater methane sampling protocol development’. It covers: site selection, pump/sampler selection/deployment, supporting reconnaissance measurements, sample collection and protocol, data management and wider use. Finally, outstanding field research needs are indicated. The foremost of these is comparative field testing of down-hole devices for obtaining closed system samples at in-situ groundwater pressure

Carbon, nitrogen, and phosphorus stoichiometry and eutrophication in River Thames tributaries, UK

Primary productivity in aquatic systems relies on carbon (C), nitrogen (N), and phosphorus (P) availability, with a reference stoichiometric ratio of 106 C/16 N/1 P, known as the Redfield ratio. This paper presents a methodology to visualize river water C/N/P stoichiometry and examine phytoplankton response. Redfield total dissolved C/N/P concentration ratios (TDC/TDNR/TDPR) from five River Thames tributaries were plotted in a ternary diagram, allowing relationships between nutrient stoichiometry, total P concentrations, and chlorophyll a, as a surrogate for phytoplankton biomass, to be explored. Chlorophyll a concentrations above 100 μg L−1 were not observed below 14% TDPR, and concentrations above 30 μg L−1 were not observed below 13% TDPR. This indicates a potentially lower TDPR limit for highly eutrophic waters. These rivers are C and N rich, and this methodology should be applied to a wider range of rivers to explore C, N and P thresholds across different river typologies

Intense summer floods may induce prolonged increases in benthic respiration rates of more than one year leading to low river dissolved oxygen

The supply of readily-degradable organic matter to river systems can cause stress to dissolved oxygen (DO) in slow-flowing waterbodies. To explore this threat, a multi-disciplinary study of the River Thames (UK) was undertaken over a six-year period (2009–14). Using a combination of observations at various time resolutions (monthly to hourly), physics-based river network water quality modelling (QUESTOR) and an analytical tool to estimate metabolic regime (Delta method), a decrease in 10th percentile DO concentration (10-DO, indicative of summer low levels) was identified during the study period. The assessment tools suggested this decrease in 10-DO was due to an increase in benthic heterotrophic respiration. Hydrological and dissolved organic carbon (DOC) data showed that the shift in 10-DO could be attributed to summer flooding in 2012 and consequent connection of pathways flushing degradable organic matter into the river. Comparing 2009–10 and 2013–14 periods, 10-DO decreased by 7.0% at the basin outlet (Windsor) whilst median DOC concentrations in a survey of upstream waterbodies increased by 5.5–48.1%. In this context, an anomalous opposing trend in 10-DO at one site on the river was also identified and discussed. Currently, a lack of process understanding of spatio-temporal variability in benthic respiration rates is hampering model predictions of river DO. The results presented here show how climatic-driven variation and urbanisation induce persistent medium-term changes in the vulnerability of water quality to multiple stressors across complex catchment systems

Phosphorus fluxes to the environment from mains water leakage:Seasonality and future scenarios

Accurate quantification of sources of phosphorus (P) entering the environment is essential for the management of aquatic ecosystems. P fluxes from mains water leakage (MWL-P) have recently been identified as a potentially significant source of P in urbanised catchments. However, both the temporal dynamics of this flux and the potential future significance relative to P fluxes from wastewater treatment works (WWT-P) remain poorly constrained. Using the River Thames catchment in England as an exemplar, we present the first quantification of both the seasonal dynamics of current MWL-P fluxes and future flux scenarios to 2040, relative to WWT-P loads and to P loads exported from the catchment. The magnitude of the MWL-P flux shows a strong seasonal signal, with pipe burst and leakage events resulting in peak P fluxes in winter (December, January, February) that are >150% of fluxes in either spring (March, April, May) or autumn (September, October, November). We estimate that MWL-P is equivalent to up to 20% of WWT-P during peak leakage events. Winter rainfall events control temporal variation in both WWT-P and riverine P fluxes which consequently masks any signal in riverine P fluxes associated with MWL-P. The annual average ratio of MWL-P flux to WWT-P flux is predicted to increase from 15 to 38% between 2015 and 2040, associated with large increases in P removal at wastewater treatment works by 2040 relative to modest reductions in mains water leakage. However, further research is required to understand the fate of MWL-P in the environment. Future P research and management programmes should more fully consider MWL-P and its seasonal dynamics, alongside the likely impacts of this source of P on water quality

Identifying multiple stressor controls on phytoplankton dynamics in the River Thames (UK) using high-frequency water quality data

River phytoplankton blooms can pose a serious risk to water quality and the structure and function of aquatic ecosystems. Developing a greater understanding of the physical and chemical controls on the timing, magnitude and duration of blooms is essential for the effective management of phytoplankton development. Five years of weekly water quality monitoring data along the River Thames, southern England were combined with hourly chlorophyll concentration (a proxy for phytoplankton biomass), flow, temperature and daily sunlight data from the mid-Thames. Weekly chlorophyll data was of insufficient temporal resolution to identify the causes of short term variations in phytoplankton biomass. However, hourly chlorophyll data enabled identification of thresholds in water temperature (between 9 and 19 °C) and flow (<30 m3 s−1) that explained the development of phytoplankton populations. Analysis showed that periods of high phytoplankton biomass and growth rate only occurred when these flow and temperature conditions were within these thresholds, and coincided with periods of long sunshine duration, indicating multiple stressor controls. Nutrient concentrations appeared to have no impact on the timing or magnitude of phytoplankton bloom development, but severe depletion of dissolved phosphorus and silicon during periods of high phytoplankton biomass may have contributed to some bloom collapses through nutrient limitation. This study indicates that for nutrient enriched rivers such as the Thames,manipulating residence time (through removing impoundments) and light/temperature (by increasing riparian tree shading) may offer more realistic solutions than reducing phosphorus concentrations for controlling excessive phytoplankton biomass

A novel application of remote sensing for modelling impacts of tree shading on water quality

Uncertainty in capturing the effects of riparian tree shade for assessment of algal growth rates and water temperature hinders the predictive capability of models applied for river basin management. Using photogrammetry-derived tree canopy data, we quantified hourly shade along the River Thames (UK) and used it to estimate the reduction in the amount of direct radiation reaching the water surface. In addition we tested the suitability of freely-available LIDAR data to map ground elevation. Following removal of buildings and objects other than trees from the LIDAR dataset, results revealed considerable differences between photogrammetry- and LIDAR-derived methods in variables including mean canopy height (10.5 m and 4.0 m respectively), percentage occupancy of riparian zones by trees (45% and 16% respectively) and mid-summer fractional penetration of direct radiation (65% and 76% respectively). The generated data on daily direct radiation for 2010 were used as input to a river network water quality model (QUESTOR). Impacts of tree shading were assessed in terms of upper quartile levels, revealing substantial differences in indicators such as biochemical oxygen demand (BOD) (1.58–2.19 mg L−1 respectively) and water temperature (20.1 and 21.2 °C respectively) between ‘shaded’ and ‘non-shaded’ radiation inputs. Whilst the differences in canopy height and extent derived by the two methods are appreciable they only make small differences to water quality in the Thames. However such differences may prove more critical in smaller rivers. We highlight the importance of accurate estimation of shading in water quality modelling and recommend use of high resolution remotely sensed spatial data to characterise riparian canopies. Our paper illustrates how it is now possible to make better reach scale estimates of shade and make aggregations of these for use at river basin scale. This will allow provision of more effective guidance for riparian management programmes than currently possible. This is important to support adaptation to future warming and maintenance of water quality standards