Rob Ward

The natural (baseline) quality of groundwater: a UK pilot study

Knowledge of the natural baseline quality of groundwaters is an essential prerequisite for understanding pollution and for imposing regulatory limits. The natural baseline of groundwaters may show a range of concentrations depending on aquifer mineralogy, facies changes, flow paths and residence time. The geochemical controls on natural concentrations are discussed and an approach to defining baseline concentrations using geochemical and statistical tools is proposed. The approach is illustrated using a flowline from the Chalk aquifer in Berkshire, UK where aerobic and anaerobic sections of the aquifer are separately considered. The baseline concentrations for some elements are close to atmospheric values whereas others evolve through time-dependent water-rock interaction. Certain solutes (K, NH(4)(+)), often considered contaminants, reach naturally high concentrations due to geochemical controls; transition metal concentrations are generally low, although their concentrations may be modified by redox controls. It is recommended that the baseline approach be incorporated into future management strategies, notably monitoring.

Changes in Major and Trace Components of Landfill Gas During Subsurface Migration

A gas plume emanating from the Foxhall Landfill in Suffolk (U.K.) has been defined within unsaturated ferruginous sands on the basis of elevated concentrations of methane, carbon dioxide and volatile organic compounds (VOCs). The plume is relatively narrow, extends more than 100 m from the landfill boundary, and lies mainly between 2 m bgl (below ground level) and the water table at 9.5 m bgl. With increasing distance along the axis of the plume, the ratio of methane to carbon dioxide gradually decreases, while nitrogen increases. Oxygen appears beyond 80 m from the landfill boundary. Stable carbon and hydrogen isotope ratios in methane become heavier with distance, while carbon dioxide becomes isotopically lighter with respect to stable carbon. This provides strong evidence for microbially mediated methane oxidation. Zones of black reduced sediment near the landfill suggest that ferric iron [Fe(III)] may be acting as an electron acceptor for oxidation. No thermal anomaly was observed, thus suggesting that the rate of oxidation/flux of methane is low. Volatile organic compounds in the plume were trapped using a combination of sorbants (Tenax GR, Haysep Q and Carbosieve S-III), and desorbed thermally into a GC/MS for semi-quantitative analysis. The 79 VOCs identified were similar to those found in other landfills, and their concentrations, both in the landfill and in the soil gas, were broadly related to their volatility. Only two compounds (vinyl chloride and dichlorofluoromethane) approached or exceeded the long-term exposure limit (LTEL, as defined by the U.K. Health and Safety Executive, 1992) outside the landfill. Halogenated compounds (dichlorodifluoromethane, dichlorofluoromethane and trichlorofluoromethane) were found to be most mobile but their concentration profiles suggest that they may have been flushed out of the landfill during its early stages. It is suggested that the association of volatile halogenated compounds with methane is good evidence that they are derived from a landfill.

Cost-effective mini drive-point piezometers and multilevel samplers for monitoring the hyporheic zone

The development of cost-effective approaches to monitor groundwater–surface-water exchange processes and contaminant fate within the hyporheic zone fundamentally underpins implementation of legislation such as the European Community Water Framework Directive, which requires integrated management of groundwater and surface water. Cost-effective mini drive-point piezometers (MDPs) and multilevel samplers (MLSs) are presented that use cheap construction materials, involve simple fabrication and installation procedures, and have a proven durability with low vulnerability to flood events and vandalism. They have been used across a range of hydro(geo)logical settings in the UK and proven to be effective in discerning flow exchange, geochemical trends, and contaminant transport and attenuation over monitored depths of 0.25 to 2 m at a resolution as low as 0.05 m. Example depth profiles, cross-river transects and river-reach longitudinal profiles from the River Tame catchment (West Midlands, UK) illustrate the value of MDP–MLS approaches in establishing surface-water–groundwater mixing zone depths, contaminant natural attenuation as a result of biotic activity within the hyporheic zone, and estimates of contaminant flux exchanges between groundwater and surface water. The MDP–MLS approaches allow discernment of contaminated groundwater plume discharges that may go undetected, or at best poorly resolved, if reliance was solely placed on conventional riverside monitoring wells and/or surface-water sampling. The MDP–MLS approaches described also have potential to be used in the investigation of shallow sediment aquifers, lake shorelines and wetland features.

The changing trend in nitrate concentrations in major aquifers due to historical nitrate loading from agricultural land across England and Wales from 1925 to 2150

Nitrate is necessary for agricultural productivity, but can cause considerable problems if released into aquatic systems. Agricultural land is the major source of nitrates in UK groundwater. Due to the long time-lag in the groundwater system, it could take decades for leached nitrate from the soil to discharge into freshwaters. However, this nitrate time-lag has rarely been considered in environmental water management. Against this background, this paper presents an approach to modelling groundwater nitrate at the national scale, to simulate the impacts of historical nitrate loading from agricultural land on the evolution of groundwater nitrate concentrations. An additional process-based component was constructed for the saturated zone of significant aquifers in England and Wales. This uses a simple flow model which requires modelled recharge values, together with published aquifer properties and thickness data. A spatially distributed and temporally variable nitrate input function was also introduced. The sensitivity of parameters was analysed using Monte Carlo simulations. The model was calibrated using national nitrate monitoring data. Time series of annual average nitrate concentrations along with annual spatially distributed nitrate concentration maps from 1925 to 2150 were generated for 28 selected aquifer zones. The results show that 16 aquifer zones have an increasing trend in nitrate concentration, while average nitrate concentrations in the remaining 12 are declining. The results are also indicative of the trend in the flux of groundwater nitrate entering rivers through baseflow. The model thus enables the magnitude and timescale of groundwater nitrate response to be factored into source apportionment tools and to be taken into account alongside current planning of land-management options for reducing nitrate losses.

Report on EU guidance on groundwater monitoring developed under the common implementation strategy of the water framework directive.

The establishment of high quality long-term monitoring programmes is essential if the implementation of the Water Framework Directive (WFD) is to be effective. It is recognised that monitoring can be very expensive and so guidance is needed to establish cost-effective, risk-based and targeted groundwater monitoring across Europe that enables WFD objectives to be met. In this context, the Groundwater Working Group (WGC) of the Common Implementation Strategy (CIS) of the WFD has developed recommendations aiming to implement consistent groundwater monitoring across Europe. This has been published on the internet in the form of a non-legally binding guidance document, which provides useful elements for the development and maintenance of networks at high standards and thereby provide the necessary information to assess (ground)water status, identify trends in pollutant concentrations, support establishment and assessment of programmes of measures and the effective targeting of economic resources. This paper presents this guidance document.

A framework for monitoring regional groundwater quality

The attention currently being focused on diffuse pollution has led to an increased recognition of the role of groundwater as a pathway for transporting diffuse pollution to surface waters. Understanding and managing this issue requires a good understanding of the spatial and temporal distribution of diffuse pollution in groundwater. This in turn requires a co-ordinated approach to groundwater monitoring, to obtain accurate and representative data. This paper describes a strategic framework for groundwater quality monitoring that has recently been developed by the Environment Agency of England and Wales. The framework is implemented in three main stages: assessment of drivers and objectives, planning and implementation. The national framework is currently being implemented by a series of detailed regional programmes. These are developed on the basis of catchment-scale groundwater quality monitoring units, which will be broadly consistent with groundwater bodies defined under the European Union (EU) Water Framework Directive. The monitoring network that results from the application of the framework is designed to provide background information on groundwater quality at a catchment scale. Separate monitoring programmes will be required for monitoring pollution associated with individual sites. This paper focuses on the strategic aspects of designing a monitoring network, rather than the technical details of obtaining representative and accurate data from individual sample points, and is illustrated by a case study of the process of design of such a catchment-scale monitoring system.

An integrated study of controls on solute transport in the Lincolnshire limestone

Abstract A study site located in a quarry on the Lincolnshire Limestone has been selected to examine the validity of the ‘dual porosity’ hypothesis and its application in models for the prediction of solute migration. An integrated approach has been adopted for the experimental programme, combining data obtained from both field and laboratory-scale investigations with mathematical modelling. The results of the first to stages of the experimental developments at the site are reported and the results are discussed in relation to he development of a full tracer test programme. Fracture mapping has been conducted on the face and the floor of the quarry where the vertical fractures are seen to be discontinuous over depth. However, extensive horizontal bedding plane fissures are readily identifiable in the boreholes and represent the main paths for lateral flows. An extensive data collection programme has been conducted to characterize the hydraulic properties of these fissures, as well as the adjacent altered and unaltered matrix. Important results for future tracer testing were obtained from the hydraulic tests which suggest the strong possibility of non-Darcian flow in the fissures under pumping and tracer test conditions and the probability of strong spreading of solutes at injection wells under forced inflows. Preliminary tracer tests confirm the difficulty of employing laboratory defined diffusion coefficients as input data to ‘dual porosity’ models of local-scale transport.

Global patterns of nitrate storage in the vadose zone

Global-scale nitrogen budgets developed to quantify anthropogenic impacts on the nitrogen cycle do not explicitly consider nitrate stored in the vadose zone. Here we show that the vadose zone is an important store of nitrate that should be considered in future budgets for effective policymaking. Using estimates of groundwater depth and nitrate leaching for 1900–2000, we quantify the peak global storage of nitrate in the vadose zone as 605–1814 Teragrams (Tg). Estimates of nitrate storage are validated using basin-scale and national-scale estimates and observed groundwater nitrate data. Nitrate storage per unit area is greatest in North America, China and Europe where there are thick vadose zones and extensive historical agriculture. In these areas, long travel times in the vadose zone may delay the impact of changes in agricultural practices on groundwater quality. We argue that in these areas use of conventional nitrogen budget approaches is inappropriate.Current global-scale nitrogen (N) budgets quantifying anthropogenic impacts on the N cycle do not explicitly consider nitrate storage in the vadose zone. Here, using estimates of depth to groundwater and nitrate leaching between 1900–2000, the authors show that the vadose zone is an important store of nitrate.

Converging flow tracer tests in fissured limestone

As part of an investigation into the dual porosity behaviour of fractured limestone aquifers, a sequence of converging flow tracer tests was devised and conducted at an extensively investigated experimental site in a major aquifer in the UK. The tests were designed specifically to produce detailed, high-resolution information about the tails of the breakthrough curves typically observed in this kind of aquifer and test. A set of mutually compatible, low detection limit tracers was identified through laboratory investigations. Two tests were carried out over distances of 20 m and 40 m along each of the two radii towards an abstraction borehole where tracer concentration was monitored. Simple dual porosity models were calibrated using the data from one test on each radius. Blind validations of these models were undertaken by attempting to predict the outcome of the second test on each radius, producing one success and one significant failure.

A parameter sensitivity analysis of two Chalk tracer tests

Abstract As with most fractured rock formations, Chalk is highly heterogeneous. Therefore, meaningful estimates of model parameters must be obtained at a scale comparable with the process of concern. These are frequently obtained by calibrating an appropriate model to observed concentration–time data from radially convergent tracer tests. Arguably, an appropriate model should consider radially convergent dispersion and Fickian matrix diffusion. Such a model requires the estimation of at least four parameters. A question arises as to whether this level of model complexity is supported by the information contained within the calibration data. A four-parameter model was developed for the analysis of two Chalk aquifer radially convergent tracer tests. The parameters included an advective travel time, ta, a characteristic fracture diffusion time, tcf, a characteristic matrix block diffusion time, tcb and a Peclet number, P. Because the tracer test duration was less than 500 h, tcb was impossible to identify. Further analysis showed that a large set of correlated values of P, ta and tcf would lead to equally good model fits. To resolve this ambiguity, more and better quality data are needed at the very start of the breakthrough curve, to constrain the mechanical dispersion parameter, P.