Kevin M. Hiscock

Attenuation of groundwater pollution by bank filtration

Bank filtration, either natural or induced through the river bed by pumping from a system of connected lateral or vertical wells, provides a means of obtaining public water supplies. The success of such schemes is dependent on the microbial activity and chemical transformations that are commonly enhanced in the colmation layer within the river bed compared to those that take place in surface or ground waters. The actual biogeochemical interactions that sustain the quality of the pumped bank filtrate depend on numerous factors including aquifer mineralogy, shape of the aquifer, oxygen and nitrate concentrations in the surface water, types of organic matter in the surface and ground water environments, and land use in the local catchment area. This paper provides an introduction to a series of nine papers contained in this Special Issue that highlight these factors and finishes with a list of recommendations for co-ordinated research into attenuation of groundwater pollution by bank filtration.

Position Paper: The role of expert opinion in environmental modelling

The inevitable though frequently informal use of expert opinion in modelling, the increasing number of models that incorporate formally expert opinion from a diverse range of experience and stakeholders, arguments for participatory modelling and analytic-deliberative-adaptive approaches to managing complex environmental problems, and an expanding but uneven literature prompt this critical review and analysis. Aims are to propose common definitions, identify and categorise existing concepts and practice, and provide a frame of reference and guidance for future environmental modelling. The extensive literature review and classification conducted demonstrate that a broad and inclusive definition of experts and expert opinion is both required and part of current practice. Thus an expert can be anyone with relevant and extensive or in-depth experience in relation to a topic of interest. The literature review also exposes informal model assumptions and modeller subjectivity, examines in detail the formal uses of expert opinion and expert systems, and critically analyses the main concepts of, and issues arising in, expert elicitation and the modelling of associated uncertainty. It is noted that model scrutiny and use of expert opinion in modelling will benefit from formal, systematic and transparent procedures that include as wide a range of stakeholders as possible. Enhanced awareness and utilisation of expert opinion is required for modelling that meets the informational needs of deliberative fora. These conclusions in no way diminish the importance of conventional science and scientific opinion but recognise the need for a paradigmatic shift from traditional ideals of unbiased and impartial experts towards unbiased processes of expert contestation and a plurality of expertise and eventually models. Priority must be given to the quality of the enquiry for those responsible for environmental management and policy formulation, and this review emphasises the role for science to maintain and enhance the rigour and formality of the information that informs decision making.

A dual isotope approach to identify denitrification in groundwater at a river-bank infiltration site

The identification of denitrification in the Torgau sand and gravel aquifer, Germany, was carried out by a dual isotope method of measuring both the delta 15N and delta 18O in NO3-. Samples were prepared by an anion exchange resin method (Silva et al., J. Hydrol. 228 (2000) 22) with a modification to the AgNO3-drying process from a freeze-drying to an oven-drying method. The occurrence of denitrification in the aquifer was confirmed by comparing the reduction of dissolved oxygen, dissolved organic carbon and NO3- concentrations with the dual isotope signatures. High nitrate concentrations were associated with low delta 15N and delta 18O values, and vice versa. The denitrification accords with a Rayleigh equation with calculated enrichment factors of epsilon = -13.62@1000 for delta 15N and epsilon = -9.80@1000 for delta 18O. The slope of the straight-line relationship between the delta 15N and delta 18O data demonstrated that the enrichment of the heavy nitrogen isotope was higher by a factor of 1.3 compared with the heavy oxygen isotope. It is concluded that the identification of this factor is a useful means for confirming denitrification in future groundwater studies.

Marker species for identifying urban groundwater recharge sources: A review and case study in Nottingham, UK

Abstract Urban environments significantly alter the nature of recharge to underlying aquifers. Direct precipitation is reduced, but additional recharge may result from storm water runoff, mains supply leakage and sewer leakage. If urban aquifers are to be effectively and sustainably managed, it is vital that these recharge sources should be identified and quantified. A sound theoretical approach is the use of marker species for identifying the three principal sources of urban recharge (precipitation, mains and sewers). The ideal marker species should be unique to a particular recharge source (irrespective of geographic location), and easily identifiable in the groundwater system, enabling quantification of that source. A review of potential markers and a detailed study of the aquifer beneath the city of Nottingham, UK, was unable to find suitable markers for precipitation and mains leakage. Trihalomethanes, which are chlorination by-products, and so a potential marker of mains water, were hardly detected in either mains or groundwater. More potential markers are available for sewage, including d-limonene, which is a new ingredient in some detergents. For shallow groundwater, the most effective means of identifying sewage recharge was a combination of stable nitrogen isotopes and microbiological indicators; effectively a sewage “fingerprint”. This study confirms the need for a multi-component approach rather than using individual marker species. Additionally it demonstrates that the impact of sewer leakage on groundwater quality beneath Nottingham is generally not high.

Evaluating factors influencing groundwater vulnerability to nitrate pollution: developing the potential of GIS

The 1991 EU Nitrate Directive was designed to reduce water pollution from agriculturally derived nitrates. England and Wales implemented this Directive by controlling agricultural activities within their most vulnerable areas termed Nitrate Vulnerable Zones. These were designated by identifying drinking water catchments (surface and groundwater), at risk from nitrate pollution. However, this method contravened the Nitrate Directive because it only protected drinking water and not all waters. In this paper, a GIS was used to identify all areas of groundwater vulnerable to nitrate pollution. This was achieved by constructing a model containing data on four characteristics: the quality of the water leaving the root zone of a piece of land; soil information; presence of low permeability superficial (drift) material; and aquifer properties. These were combined in a GIS and the various combinations converted into a measure of vulnerability using expert knowledge. Several model variants were produced using different estimates of the quality of the water leaving the root zone and contrasting methods of weighting the input data. When the final models were assessed all produced similar spatial patterns and, when verified by comparison with trend data derived from monitored nitrate concentrations, all the models were statistically significant predictors of groundwater nitrate concentrations. The best predictive model contained a model of nitrate leaching but no land use information, implying that changes in land use will not affect designations based upon this model. The relationship between nitrate levels and borehole intake depths was investigated since there was concern that the observed contrasts in nitrate levels between vulnerability categories might be reflecting differences in borehole intake depths and not actual vulnerability. However, this was not found to be statistically important. Our preferred model provides the basis for developing a new set of groundwater Nitrate Vulnerable Zones that should help England and Wales to comply with the EU Nitrate Directive.

Factors affecting denitrification during infiltration of river water into a sand and gravel aquifer in Saxony, Germany

Abstract River infiltration into a sand and gravel aquifer was investigated to assess the importance of denitrification in maintaining low-NO − 3 groundwater supplies. Samples from the River Elbe and groundwater sampling points along a section of the aquifer were analysed for dissolved organic carbon, major ions and the 15 N 14 N isotopic ratio of dissolved NO − 3 . Input of NO − 3 to the aquifer is influenced by seasonal, temperature-dependent denitrification in the river bed sediments. Along an upper flowpath in the aquifer from the River Elbe to a sampling point at a distance of 55 m, the median NO − 3 concentration decreased by 4.8 mg litre −1 and the δ 15 N composition increased by +9.0‰, consistent with denitrification. Similar isotopic enrichment was demonstrated in a laboratory column experiment with a reduction in NO − 3 of 10.5 mg litre −1 for an increase in δ 15 N of +9.8‰, yielding an isotopic enrichment factor of −14.6‰. A mass balance for denitrification shows that oxidizable organic carbon required for denitrification is derived from both the infiltrating river water and solid organic matter fixed in the river bed sediments and aquifer material.

Sustainable Groundwater Development

Abstract Estimated annual water availability per person in 2025 is likely to result in at least 400f the world’s 7.2 billion people facing serious problems with obtaining freshwater for agriculture, industry or human health (Gleick 2001). To meet present and future needs with the currently available surface and groundwater resources, while at the same time preserving terrestrial and aquatic ecosystems, will require a sustainable approach to managing water. This paper discusses the importance of groundwater resources in industrialized and developing countries, and the associated problems of over-abstraction and groundwater pollution, with the objective of defining sustainable groundwater development. It is concluded that sustainable groundwater development at global and local scales is achieved through the maintenance and protection of groundwater resources balanced against economic, environmental and human (social) benefits. This interpretation of sustainable groundwater development is incorporated into the methodologies currently emerging in Europe (the EU Water Framework Directive) and England and Wales (Catchment Abstraction Management Strategies). However, success in achieving future sustainable groundwater development will require a common understanding at the level of the individual based on information and education within a legislatory framework that promotes co-operation and self-responsibility.

Nitrogen isotope hydrochemistry and denitrification within the Chalk aquifer system of north Norfolk, UK

Although there has been much debate over the potential health risks of nitrate in drinking water, there is a real issue of the costs associated with removing nitrate from drinking water supplies. In the Chalk aquifer system of north-east Norfolk, modern contaminants of a mainly agricultural origin produce high levels of nitrate (>15 mg/l NO3-N) in the unconfined valley areas, whereas in confined regions the levels of nitrate are low and commonly below detection limits (<0.04 mg/l NO3-N). To understand the source and fate of nitrate within this aquifer system, a detailed hydrochemical sampling programme has been completed in the River Bure catchment. Nitrogen isotope values (δ15N) for nitrate within the unconfined and semi-confined zones range between +4‰ and +10‰, characteristic of nitrified soil organic nitrogen. However, many Chalk groundwaters possess high N2/Ar ratios (39–72) indicating a significant contribution to dissolved N2 from denitrification. Denitrification is believed to be occurring within the overlying glacial deposits, providing a mechanism for naturally improving groundwater quality. δ15N values of low-nitrate groundwaters from the confined zone are isotopically light (−3‰ to +4‰), inconsistent with an origin from denitrification: it is suggested that these waters have a pre-anthropogenic nitrate signature.

Simulation of the impacts of climate change on groundwater resources in eastern England

Abstract This study investigated the impacts of climate change on the Chalk aquifer in west Norfolk. A two-layer transient groundwater flow model of the aquifer system was calibrated and validated for the period 1980–1995 and provided the historic flow record for the climate change simulations. Two future scenarios were selected from the Hadley Centre’s climate change experiments using HadCM2: (1) a medium-high (MH) emissions scenario; and (2) a medium-low (ML) emissions scenario of ‘greenhouse’ gases. Two future periods were considered: 2020–2035 and 2050–2065. Future recharge to the aquifer was estimated by adjusting the historic record of monthly precipitation and potential evapotranspiration by factors calculated from comparing control and future HadCM2-generated values. Impacts of climate change were evaluated by incorporating the monthly estimated recharge inputs within the flow model. The most noticeable and consistent result of the climate change impact simulations is the decrease in recharge expected in autumn for all scenarios (decreases ranging from 17 to 35%) as a consequence of the smaller amount of summer precipitation and increased autumn potential evapotranspiration. For the 2050MH scenario, these conditions lead to a 42 0ncrease in autumn soil moisture deficit and a 26% reduction in recharge. Hence, west Norfolk can expect longer and drier summers that are predicted to have relatively little effect on summer groundwater levels (generally a 1 to 2 0ecrease) but will result in a decrease of up to 14 0n autumn river baseflow volumes.

Hydrochemical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK

In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ18O and δ2H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l−1, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ18O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ18O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.