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

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

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

Nitrogen sources, transport and processing in peri-urban floodplains

Peri-urban floodplains are an important interface between developed land and the aquatic environment and may act as a source or sink for contaminants moving from urban areas towards surface water courses. With increasing pressure from urban development the functioning of floodplains is coming under greater scrutiny. A number of peri-urban sites have been found to be populated with legacy landfills which could potentially cause pollution of adjacent river bodies. Here, a peri-urban floodplain adjoining the city of Oxford, UK, with the River Thames has been investigated over a period of three years through repeated sampling of groundwaters from existing and specially constructed piezometers. A nearby landfill has been found to have imprinted a strong signal on the groundwater with particularly high concentrations of ammonium and generally low concentrations of nitrate and dissolved oxygen. An intensive study of nitrogen dynamics through the use of N-species chemistry, nitrogen isotopes and dissolved nitrous oxide reveals that there is little or no denitrification in the majority of the main landfill plume, and neither is the ammonium significantly retarded by sorption to the aquifer sediments. A simple model has determined the flux of total nitrogen and ammonium from the landfill, through the floodplain and into the river. Over an 8 km reach of the river, which has a number of other legacy landfills, it is estimated that 27.5 tonnes of ammonium may be delivered to the river annually. Although this is a relatively small contribution to the total river nitrogen, it may represent up to 15% of the ammonium loading at the study site and over the length of the reach could increase in-stream concentrations by nearly 40%. Catchment management plans that encompass floodplains in the peri-urban environment need to take into account the likely risk to groundwater and surface water quality that these environments pose

Method for analysing phosphate 18O/16O ratios for waters with high C:P ratios

In this report we outline a method to isolate phosphate as pure Ag3PO4 for waters with high C:P ratios for 18O analysis. This report details a method that can be used to minimise residual organic contamination of the final Ag3PO4, therefore significantly reducing uncertainties in the final interpretation of 18O results. This includes the use of column resins in series to (i) remove the majority of dissolved organic carbon and (ii) isolate/pre-concentrate PO4. This is followed by the use of a modified McLaughlin et al (2004) method for PO4 precipitation, with an additional final hydrogen peroxide clean-up step to remove residual organic matter following precipitation of Ag3PO4. The reagents required for this method are first listed, and then a step-by-step account of the process is outlined. Importantly, it contains adequate detail to be used by other researchers in this field or modified to suit their particular research objectives

Use and application of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater residence time: a review

Groundwater residence time is a fundamental property of groundwater to understand important hydrogeological issues, such as deriving sustainable abstraction volumes, or, the evolution of groundwater quality. The anthropogenic trace gases chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulphur hexafluoride (SF6) are ideal in this regard because they have been released globally at known rates and become dissolved in groundwater following Henry's Law, integrating over large spatial (global) and temporal (decades) scales. The CFCs and SF6 are able to date groundwater up to ∼100 years old with the caveat of certain simplifying assumptions. However, the inversion of environmental tracer concentrations (CFCs and SF6) to derive groundwater age rests on the accurate determination of groundwater recharge parameters, namely temperature, elevation, salinity and excess air, in addition to resolving the potential for contamination, degradation and unsaturated zone effects. This review explores the fundamentals of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater age and recommends complementary techniques throughout. Once this relatively simple and inexpensive technique has been used to determine initial concentrations at the recharge zone, setting the groundwater dating ‘clock’ to zero, this review then explores the meaning of groundwater ‘age’ in relation to measured environmental tracer concentrations. It is shown that the CFCs and SF6 may be applied to a wide-range of hydrogeological problems and suggests that environmental tracers are particularly powerful tools when integrated with numerical flow and transport models

Impacts of extreme flooding on riverbank filtration water quality

Riverbank filtration schemes form a significant component of public water treatment processes on a global level. Understanding the resilience and water quality recovery of these systems following severe flooding is critical for effective water resources management under potential future climate change. This paper assesses the impact of floodplain inundation on the water quality of a shallow aquifer riverbank filtration system and how water quality recovers following an extreme (1 in 17 year, duration > 70 days, 7 day inundation) flood event. During the inundation event, riverbank filtrate water quality is dominated by rapid direct recharge and floodwater infiltration (high fraction of surface water, dissolved organic carbon (DOC) > 140% baseline values, > 1 log increase in micro-organic contaminants, microbial detects and turbidity, low specific electrical conductivity (SEC) 400% baseline). A rapid recovery is observed in water quality with most floodwater impacts only observed for 2–3 weeks after the flooding event and a return to normal groundwater conditions within 6 weeks (lower fraction of surface water, higher SEC, lower DOC, organic and microbial detects, DO). Recovery rates are constrained by the hydrogeological site setting, the abstraction regime and the water quality trends at site boundary conditions. In this case, increased abstraction rates and a high transmissivity aquifer facilitate rapid water quality recoveries, with longer term trends controlled by background river and groundwater qualities. Temporary reductions in abstraction rates appear to slow water quality recoveries. Flexible operating regimes such as the one implemented at this study site are likely to be required if shallow aquifer riverbank filtration systems are to be resilient to future inundation events. Development of a conceptual understanding of hydrochemical boundaries and site hydrogeology through monitoring is required to assess the suitability of a prospective riverbank filtration site