Demonstration test catchments : the role of hydrogeological conceptual modelling

Agricultural diffuse pollution, particularly from nitrate and phosphate, is a significant problem in the UK and is the focus of the national Demonstration Test Catchments (DTC) study — a UK Government initiative. The DTC programme is providing evidence for investigating how on-farm mitigation measures can reduce the impact of agricultural diffuse water pollution on ecological function. This will involve studying how such measures affect pollutant concentrations in so-called receptors, such as the streams which drain the catchments. It is therefore important to investigate how water moves from the land surface to the receptors and in particular to quantify the amounts and timescales involved in the different water flow routes. The DTC catchments are the Eden, the Avon and the Wensum and groundwater flow is a significant component of main river flow in all of the catchments, ranging in overall terms from around 50% of river flow in the Eden to 90% in the Avon. It is therefore important that robust conceptual models of the groundwater flow systems of the catchments — and in particular of the monitored sub-catchments — are developed. BGS is contributing to the creation of DTC groundwater conceptual models in all three study catchments. The BGS work is funded principally by NERC and by DEFRA

Towards a global interpretation of dual nitrate isotopes in surface waters

Modern anthropogenic activities have significantly increased nitrate (NO3-) concentrations in surface waters. Stable isotopes (delta N-15 and delta O-18) in NO3- offer a tool to deconvolute some of the human-made changes in the nitrogen cycle. They are often graphically illustrated on a template designed to identify different sources of NO3- and denitrification. In the two decades since this template was developed, delta N-1(5)- and delta O-1(8)-NO3- have been measured in a variety of ecosystems and through the nitrogen cycle. However, its interpretation is often fuzzy or complex. This default is no longer helpful because it does not describe surface water ecosystems well and biases researchers towards denitrification as the NO3- removal pathway, even in well oxygenated systems where denitrification is likely to have little to no influence on the nitrogen cycle. We propose a different scheme to encourage a better understanding of the nitrogen cycle and interpretation of NO3- isotopes. We use a mechanistic understanding of NO3- formation to place bounds on the oxygen isotope axis and provide a means to adjust for different environmental water isotope values, so data from multiple sites and times of year can be appropriately compared. We demonstrate that any interpretation of our example datasets (Canada, Kenya, United Kingdom) show clear evidence of denitrification or a mixture of NO3- sources simply because many data points fall outside of arbitrary boxes which cannot be supported once the range of potential delta O-1(8)-NO(3)(- )values has been considered.Modern anthropogenic activities have significantly increased nitrate (NO3-) concentrations in surface waters. Stable isotopes (delta N-15 and delta O-18) in NO3- offer a tool to deconvolute some of the human-made changes in the nitrogen cycle. They are often graphically illustrated on a template designed to identify different sources of NO3- and denitrification. In the two decades since this template was developed, delta N-1(5)- and delta O-1(8)-NO3- have been measured in a variety of ecosystems and through the nitrogen cycle. However, its interpretation is often fuzzy or complex. This default is no longer helpful because it does not describe surface water ecosystems well and biases researchers towards denitrification as the NO3- removal pathway, even in well oxygenated systems where denitrification is likely to have little to no influence on the nitrogen cycle. We propose a different scheme to encourage a better understanding of the nitrogen cycle and interpretation of NO3- isotopes. We use a mechanistic understanding of NO3- formation to place bounds on the oxygen isotope axis and provide a means to adjust for different environmental water isotope values, so data from multiple sites and times of year can be appropriately compared. We demonstrate that any interpretation of our example datasets (Canada, Kenya, United Kingdom) show clear evidence of denitrification or a mixture of NO3- sources simply because many data points fall outside of arbitrary boxes which cannot be supported once the range of potential delta O-1(8)-NO(3)(- )values has been considered.A

A field study to assess the degradation and transport of diuron and its metabolites in a calcareous soil

An experimental plot has been established on a calcareous soil in southern England to investigate the fate and transport of diuron (N'-[3,4-dichlorophenyl]-NN-dimethylurea), a commonly used phenylurea herbicide. An agricultural grade of diuron was applied to the soil surface at a rate of 6.7 kg/ha along with a potassium bromide conservative tracer applied at 200 kg/ha, in early January, 2001. Hand augured samples were taken at regular intervals over the next 50 days, with samples collected down to 54 cm. Porewaters were extracted from the soil cores by using high speed centrifugation and the supernatant fluids were retained for analysis by HPLC, for diuron and three of its metabolites, N'-[3,4-dichlorophenyl]-N,N-methylurea (DCPMU), N'-3,4-dichlorophenylurea (DCPU) and 3,4-dichloroaniline (DCA). The centrifuged soil was retained and then extracted with methanol prior to HPLC analysis for the same suite of phenylureas. A mass balance approach showed large variations in diuron distribution, but on average accounted for 104% of the diuron applied. Concentrations of diuron and its metabolites were roughly five times higher in the soil than in the soil porewaters. After 50 days, metabolites comprised 10% of the total diuron present in the porewater and 20% of the total diuron sorbed to the soil matrix. After 36 days, a large pulse of diuron and DCPMU appeared in the porewaters and soil matrix at a depth of 54 cm, travelling an average of 0.15 cm/day faster than Br. A preferential route for diuron transport is suggested. There is evidence to suggest that degradation occurs at depth as well as at the soil surface. Metabolites generally appear to move more slowly than the parent compound. All metabolites were encountered, but interpreting transport and degradation processes simultaneously proved beyond the scope of the study. Diuron was detected once in a shallow (5 m) observation well, situated on the experimental plot. High concentrations of diuron and metabolites were still present in the soil and soil solutions after 50 days and remain as a source of potential groundwater contaminatio

Tracing groundwater flow and sources of organic carbon in sandstone aquifers using fluorescence properties of dissolved organic matter (DOM)

The fluorescence properties of groundwaters from sites in two UK aquifers, the Penrith Sandstone of Cumbria and the Sherwood Sandstone of South Yorkshire, were investigated using excitation-emission matrix (EEM) fluorescence spectroscopy. Both aquifers are regionally important sources of public supply water and are locally impacted by anthropogenic pollution. The Penrith Sandstone site is in a rural setting while the Sherwood Sandstone site is in suburban Doncaster. Fluorescence analysis of samples from discrete sample depths in the Penrith Sandstone shows decreasing fulvic-like intensities with depth and also shows a good correlation with CFC-12, an anthropogenic groundwater tracer. Tryptophan- like fluorescence centres in the depth profile may also provide evidence of rapid routing of relatively recent applications of organic slurry along fractures. Fluorescence analysis of groundwater sampled from multi-level piezometers installed within the Sherwood Sandstone aquifer also shows regions of tryptophan-like and relatively higher fulvic-like signatures. The fluorescence intensity profile in the piezometers shows tryptophan-like peaks at depths in excess of 50 metres and mirrors the pattern exhibited by microbial species and CFCs highlighting the deep and rapid penetration of modern recharge due to rapid fracture flow. Fluorescence analysis has allowed the rapid assessment of different types and relative abundances of dissolved organic matter (DOM), and the fingerprinting of different sources of organic carbon within the groundwater system. The tryptophan:fulvic ratios found in the Penrith Sandstone were found to be between (0.5–3.0) and are characteristic of ratios from sheep waste sources. The Sherwood Sandstone has the lowest ratios (0.2–0.4) indicating a different source of DOM, most likely a mixture of terrestrial and microbial sources, although there is little evidence of pollution from leaking urban sewage systems. Results from these two studies suggest that intrinsic fluorescence may be used as a proxy for, or complimentary tool to, other groundwater investigation methods in helping provide a conceptual model of groundwater flow and identifying different sources of DOM within the groundwater system

Using CFCs and SF6 for groundwater dating : a SWOT analysis

A knowledge of the residence time of groundwater is of importance in understanding key issues in the evolution of water quality. Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) offer a convenient way of dating waters up to ~60 yrs old. In contrast to tritium, these gases are well-mixed in the atmosphere so their input functions are much less problematic. While any one of these gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple endmember mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Mainly for SF6, the phenomenon of ‘excess air’ also requires consideration. Mainly for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. This paper attempts a balanced look at the pros and cons of the trace-gas dating method

Pesticides and metabolites in groundwater: examples from two major UK aquifers

Reducing the impact of anthropogenic pollution on groundwater bodies and ameliorating any deterioration of water quality is central to key legislative drivers such as the EU Water Framework Directive and the proposed daughter Directive relating to the protection of groundwater. Pesticide pollution has a direct impact on groundwater quality and an indirect impact on the associated aquatic ecosystems supported by groundwater. There is currently no legislative requirement to monitor pesticide metabolite concentrations in groundwater. Pesticide and metabolite results from two nationally important aquifers are presented, the Trassic Sandstone and the Chalk of Southern England. Aerobic microbial degradation of diuron in the soil can lead to the formation of three compounds; dichlorophenylmethyl urea (DCPMU), dichlorophenyl urea (DCPU) and dichloroanaline (DCA).Median diuron concentrations were significantly higher than each of the metabolites with outliers exceeding the PVC on at least one occasion. At nine sites in Kent, Southern England, (60%) metabolites were more prevalent than diuron. Both aquifers are an important source of water, locally supplying up to 80% of public drinking water. The sandstone site has a predominantly arable landuse with a potential diffuse source of pesticides although soakaways are possible point sources.The chalk site has a mixture of arable and industrial/urban landuse. A significant source has been from excessive applications of diuron (“over-spray”) on a number of public amenities. Data from both aquifers show that pesticide concentrations have a high degree of temporal variability. Elevated pesticide concentrations are associated with recharge events in both aquifer systems regardless of pesticide source terms. Pesticides from amenity use and diffuse agricultural sources both pose a threat to groundwater quality. Pesticide metabolites are present in significant concentrations in groundwaters. Systematic, long-term monitoring (5-10 years) is required to understand trends in groundwater quality

Sampling and characterising groundwater nanoparticles in sub-oxic environments

Characterising nanoparticles is important for understanding physiochemical and biogeochemical processes occurring within groundwater bodies e.g. those impacted by the migration of leachates from waste storage sites as well as monitoring the use of engineered nanotechnology for pollution attenuation. While characterising nano-scale particles (both natural and engineered) within sub-oxic environments is a challenging task, it is critical for understanding pollution attenuation and migration within a number of different environments. The overall aim of this study was to develop a robust sampling and analytical methodology for characterising nanoparticles in sub-oxic environments using a range of complementary methods. This study has successfully sampled and characterised nano-scale particulate material in sub-oxic groundwaters within an alluvial floodplain aquifer impacted by a landfull plume. The integrity of the sample was maintained throughout the field and laboratory work to ensure that only nanoparticles representative of the sub-oxic environment were characterised. Nanoparticles from two pairs of nested boreholes were characterised by a number of state-of-the-art methods; atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning transmisson electron microscopy (TEM) and field flow fractionation (FFF), to explore particle size distributions, morphology and surface chemistry. It is important to characterise nanoparticles in environmental contexts using multiple techniques as each method has its own benefits and limitations (Lead and Wilkinson 2006). As far as the authors are aware this is the first such study in the UK to isolate and characterise sub-oxic groundwater nanoparticles using these complimentary techniques. Groundwaters were found to have abundant iron and organic nanoparticles with diameters <30 nm. AFM results showed spherical nanoparticles with average diameters of ca 10 nm, while FFF with UV absorbance (254 nm) results indicated that smaller fulvic-like nanoparticles were present with average hydrodynamic diameters of ca. 1.5 nm. FFF with UV absorbance detection at 575 nm showed that another population of organic rich nanoparticles was present with larger hydrodynamic diameters (ca. 3 nm) in the groundwater at nest 26, but were not present in nest 28. These larger organic nanoparticles perhaps represent co-aggregated humic-like particles or another distinct type of organic matter. Scanning TEM analysis with energy-dispersive X-ray diffraction showed that Ca rich nanoparticles were present within the groundwater at a number of sites, and that P was associated with the surface of Fe rich particles in nest 28. Aeration of sub-oxic samples resulted in a dramatic shift in the nanoparticle size distribution. This was a result of the aggregation of smaller nanoparticles to form larger agglomerations with diameters typically >50-100 nm. This is analogous to processes that occur during groundwater aeration for water treatment, and mixing of anaerobic and aerobic environmental waters, e.g. during rapid recharge events, flooding, hyporheic zone mixing, waste water treatment and waste water inputs to surface waters. The techniques developed in this study have potential wider applications for understanding the occurrence and fate of natural and anthropogenic (engineered) nanoparticles in sub-oxic conditions, such as the fate of nanoparticles injected for pollution attenuation, those found below landfill sites, within waste water treatment works and the hyporheic zone which are all important redox hot-spots for pollution attenuation and biological activity

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

Estimating the leakage contribution of phosphate dosed drinking water to environmental phosphorus pollution at the national‐scale

Understanding sources of phosphorus (P) to the environment is critical for the management of freshwater and marine ecosystems. Phosphate is added at water treatment works for a variety of reasons: to reduce pipe corrosion, to lower dissolved lead and copper concentrations at customer’s taps and to reduce the formation of iron and manganese precipitates which can lead to deterioration in the aesthetic quality of water. However, the spatial distribution of leakage into the environment of phosphate added to mains water for plumbosolvency control has not been quantified to date. Using water company leakage rates, leak susceptibility and road network mapping, we quantify the total flux of P from leaking water mains in England and Wales at a 1 km grid scale. This is validated against reported leaks for the UKs largest water utility. For 2014, we estimate the total flux of P from leaking mains to the environment to be c. 1.2 kt P/yr. Spatially, P flux is concentrated in urban areas where pipe density is highest, with major cities acting as a significant source of P (e.g. London into the Thames, with potentially 30% of total flux). The model suggests the majority (69%) of the P flux is likely to be to surface water. This is due to leakage susceptibility being a function of soil corrosivity and shrink‐swell behaviour which are both controlled by presence of low‐permeability clays. The location of major cities such as London close to the coast results in a potentially significant flux of P from mains leakage to estuarine environments. The contribution of leakage of phosphate dosed mains water should be considered in future source apportionment and ecosystem management. The methodology presented is generic and can be applied in other countries where phosphate dosing is undertaken or used prior to dosing during investment planning

Phosphate dosing of mains water : novel approaches to water loss reduction through leakage detection and policy [abstract]

Detection and t racing of leakage in the environment are essential component s of water loss reduction strategies. Industry standard techniques for tracing leaks include analysis of chlorine and trihalomethane concentrations, but levels of these determinands can fall belo w detection limits due to their volatile nature 1 . Consequently additional tools to trace leakage in the environment are a useful step to move towards minimum losses