Assessing Aquifer Exploitation Using Observation Boreholes

Observation boreholes are used to assess the degree of exploitation of aquifers, but if the borehole is open over a considerable depth of the aquifer the results may be misleading. The need for individual shallow and deep boreholes is illustrated by case studies of an alluvial aquifer. An examination of groundwater heads in deep and shallow boreholes in the Bromsgrove Sandstone aquifer demonstrates that deep boreholes respond mainly to the abstraction but a shallow borehole reflects the overall response of the aquifer.     

The Influence of Unlined Boreholes on Groundwater Chemistry: A Comparative Study Using Pore-Water Extraction and Packer Sampling

In the consolidated formations which form most of the major aquifers in the UK, many monitoring boreholes are left unlined (open) for their effective length. They therefore provide pathways for groundwater to flow from one part of the aquifer to another.
A borehole was core-drilled through Triassic sandstones into Permian mudstones at a site in North Yorkshire. Pore water was extracted from the core and compared with conventional depth samples and with samples taken from intervals isolated using packer systems: (a) during drilling, (b) about one month after drilling, and (c) two years after drilling.
The study revealed that natural flow down the borehole led to rapid changes in the quality of water in some parts of the formation.
The implication is that open boreholes can act as conduits to allow flow to take place from one part of the aquifer to another. They are therefore not reliable as sampling points, and may actually accelerate the vertical spread of pollutants. However, this last effect should not be over-stated; in this study, the flow down the borehole was equivalent to the natural flow through only a relatively small area of land.     

A NEW CONCEPTUAL GROUNDWATER-FLOW SYSTEM FOR THE CENTRAL SOUTH DOWNS AQUIFER

The central part of the South Downs Chalk aquifer is intensely used for public water supply. Although the aquifer has a long history of careful development and management, the outline conceptual groundwater-flow vision has not been formally investigated for many years. A major programme of work has now been completed which (a) provides a new insight into the hydraulic workings of the aquifer, and (b) highlights a number of hitherto poorly understood concepts controlling groundwater movement in the Chalk. These include the key role of geological structure and the occurrence of hard bands and karst-type groundwater flow as major controls over preferred flow-paths. The new conceptualisation concludes that the main discharge area for the aquifer is laterally to the north-south rivers and not directly southwards to the sea.     

Contamination of Chalk Groundwater by Chlorinated Solvents: A Case Study of Deep Penetration by Non-Aqueous Phase Liquids

The transport behaviour of chlorinated solvents, both in the aqueous phase and as a dense non-aqueous phase liquid (DNAPL), in fissured microporous aquifers is reviewed. The presence of DNAPL in aquifers is especially serious as it is likely to be the main subsurface source of contamination and, given the slow rates of dissolution in groundwater, may persist for decades. However, the identification and quantification of DNAPLs in fractured aquifers present many practical problems and are often not achievable.
A case study of a Chalk site which had been contaminated by chlorinated solvents demonstrated that the use of a range of techniques, including depth profiling of solvent porewater concentrations in cored boreholes, can provide clear evidence for the presence of DNAPL at depth, although DNAPL was not itself observed. Theoretical considerations and field observations confirmed that DNAPL movement is via fractures rather than through the microporous matrix.     

Prioritisation of abstraction boreholes at risk from chlorinated solvent contamination on the UK Permo-Triassic Sandstone aquifer using a GIS

A 250 km2 area of the Permo-Triassic Sandstone aquifer in the West Midlands of England, UK, was selected as a test region for the development of a geographic information system (GIS)-based risk assessment methodology that incorporates contaminant source, groundwater vulnerability and groundwater abstraction catchment elements in order to prioritise areas and boreholes potentially at risk from chlorinated solvent pollution on a regional scale. Factors incorporated in the vulnerability assessment include the nature of soils, presence or absence of superficial or glacial deposits, fault density and depth to water table. ARCVIEW GIS was employed with a simple ranking system from which the derived vulnerability assessment index was combined with current chlorinated solvent user industry data and source protection zone components. Results indicate the presence of high-risk areas in urban locations where locally dense distributions of chlorinated solvent user industries combine with high vulnerability aquifers within the catchment of supply boreholes. Ranking of catchment-specific risk reveals the abstraction points under greatest stress. The proposed methodology has applications as a regional-scale initial screening tool to guide site selection for regulatory inspections and assist in prioritising monitoring strategies for existing boreholes. Future developments will provide guidance for locating new urban boreholes in areas of lowest risk.     

The potential for methane emissions from groundwaters of the UK

Methane (CH4) is only a trace constituent of the atmosphere but an important greenhouse gas. Although groundwater is unlikely to be a major source of atmospheric CH4, its contribution to the CH4 budget of the UK has up to now been poorly characterised. Groundwater CH4 concentrations have been measured on 85 samples from water-supply boreholes and a further eight from other miscellaneous water sources. Concentrations in abstracted groundwaters ranged from <0.05-42.9 microg/l for Chalk, <0.05-22 microg/l for the Lower Greensand, 0.05-21.2 microg/l for the Lincolnshire Limestone and from <0.05-465 microg/l for the Triassic sandstone. Having the largest abstraction volume, the Chalk is likely to be the main UK groundwater contributor to global CH4 emissions. A calculation to estimate the total emissions of CH4 from water-supply groundwater sources based on the median and the maximum CH4 concentrations gave values of 2.2x10(-6) and 3.3x10(-4) Tg/year. Estimates show groundwater contributes a maximum of 0.05% of all UK CH4 emissions and a further two orders of magnitude less in terms of the global CH4 budget. Other groundwater sources such as inflows to tunnels may have significantly higher CH4 concentrations, but the volume of water discharged is much lower and the overall amount of CH4 outgassed is likely to be of the same order as the aquifer release. The generally low concentrations of CH4 in groundwater supplies suggest no threat of explosion, although groundwater released by excavations remains a hazard.     

Chlorinated Solvents in UK Aquifers

The paper reviews background data on chlorinated solvents and all published data on chlorinated solvents in UK groundwaters. New data from an aquifer-wide survey of the organic water quality of the Birmingham aquifer (carried out by the University of Birmingham and the Water Research Centre (WRc)) show that contamination by chlorinated solvents is extensive and is greater than that observed in previously published UK studies. Trichloroethylene contamination is particularly apparent in the Birmingham aquifer, with 40% of the boreholes sampled containing waters above the new UK limit for this contaminant. Fortunately the Birmingham aquifer is not used for public supply. Land use and hydrogeological factors are shown to influence the contamination observed in particular boreholes. The difficulties associated with locating contaminated zones in aquifers, due to the immiscible flow of chlorinated solvents, are indicated. It is concluded that the occurrence of chlorinated solvents in groundwaters makes a significant contribution to the poor water quality of many UK aquifers that underlie, or are close to, urban and industrial areas.     

The East Kent Chalk Aquifer during the 1988-92 Drought

Groundwater resources of Chalk aquifers may become depleted during drought periods; major causes of this depletion include reductions in the aquifer transmissivity and the interaction between aquifers and rivers. In the East Kent aquifer there are certain catchments where difficulties are encountered in maintaining yields but, in other catchments, drought periods have little effect on the available resources. A mathematical model is developed to help understand the flow processes within the aquifer system, and the model is used to predict the consequences of possible abstraction scenarios.     

Effects on groundwater flow and groundwater quality of a waste disposal site in Noordwijk,The Netherlands

The effects of a waste disposal site in Noordwijk on the groundwater flow and groundwater quality were investigated. Special attention is given to the extension of pollutants in the aquifer. Several boreholes were made and groundwater has been sampled and analysed. Measurement of electrical resistivity and an electromagnetical investigation were carried out. Already beneath the landfill pollution was found on the fresh- salt water boundary at about 40 meters -l.s. A very large vertical flow component due to density flow could be calculated. Local flow patterns indicate an all-sided migration of pollutants. Short-circuit flow caused by the drilling of boreholes can effect the quality of groundwater seriously.     

The significance of colloids in the transport of pesticides through Chalk

Agrochemical contamination in groundwater poses a significant long term threat to water quality and is of concern for legislators, water utilities and consumers alike. In the dual porosity, dual permeability aquifers such as the Chalk aquifer, movement of pesticides and their metabolites through the unsaturated zone to groundwater is generally considered to be through one of two pathways; a rapid by-pass flow and a slower 'piston-flow' route via the rock matrix. However, the dissolved form or 'colloidal species' in which pesticides move within the water body is poorly understood. Following heavy rainfall, very high peaks in pesticide concentration have been observed in shallow Chalk aquifers. These concentrations might be well explained by colloidal transport of pesticides. We have sampled a Chalk groundwater beneath a deep (30 m) unsaturated zone known to be contaminated with the pesticide diuron. Using a tangential flow filtration technique we have produced colloidal fractions from 0.45 microm to 1 kDa. In addition, we have applied agricultural grade diuron to a typical Chalk soil and created a soil water suspension which was also subsequently fractionated using the same filtration system. The deep groundwater sample showed no evidence of association between colloidal material and pesticide concentration. In comparison, despite some evidence of particle trapping or sorption to the filters, the soil water clearly showed an association between the <0.45 microm and <0.1 microm colloidal fractions which displayed significantly higher pesticide concentrations than the unfiltered sample. Degradation products were also observed and found to behave in a similar manner to the parent compound. Although relatively large colloids can be generated in the Chalk soil zone, it appears transport to depth in a colloidal-bound form does not occur. Comparison with other field and monitoring studies suggests that rapid by-pass flow is unlikely to occur beneath 4-5 m. Therefore, shallow groundwaters are most at risk from rapid transport of high concentrations of pesticide-colloidal complexes. The presence of a deep unsaturated zone will mean that most of the colloidal-complexes will be filtered by the narrow Chalk pores and the majority of pesticide transport will occur in a 'dissolved' form through the more gradual 'piston-flow' route.     

EXPLORATION OF AQUIFER MANAGEMENT OPTIONS USING A GROUNDWATER MODEL

A methodology is described for harnessing the predictive capacity of a groundwater model to underpin aquifer management decisions for changing abstraction within a licensing regulatory framework. The use of a 'baseline predictive simulation' is advocated from both aquifer management and scientific perspectives, and an example of groundwater modelling predictive work on the Nottingham-Doncaster Sherwood Sandstone aquifer system is used to illustrate the methodology. Differences between the baseline predictive simulation and a predictive simulation are processed to extract quantitative information (spatially and temporally) on the response to changes in aquifer stresses. An explanation is given on the use of this information to aid aquifer management decisions for achieving environmental targets. The importance of the aquifer diffusivity with respect to a predictive simulation exercise is highlighted.     

Management of stream compensation for a large conjunctive use scheme,Shropshire, UK

The Shropshire Groundwater Scheme (SGS) is the largest conjunctive use scheme in the United Kingdom. The SGS operates in drought periods, pumping water from the Permo‐Triassic Sandstone aquifer in Shropshire, England to regulate flows in the River Severn, alongside releases from conventional surface water reservoirs. This regulation protects the environment and increases the security of supply of major public water supply abstractions downstream. An important operational consideration is the impact SGS groundwater abstraction has on small streams that rise on the Shropshire Permo‐Triassic Sandstone. Compensation boreholes have been installed to maintain flows during and after the operation of the SGS. This paper shows how a groundwater model of the East Shropshire Permo‐Triassic Sandstone has been used to estimate the required level of compensation flows for recent climatic conditions to optimise the use of the SGS. The catchment wide conditions are included in this analysis by using the results of a large‐scale surface water model of the River Severn.     

Hydrological analysis as a technical tool to support strategic and economic development: A case study of Lake Navaisha,Kenya

Effective integrated water resources management requires reliable estimation of an overall basin water budget and of hydrologic fluctuations between groundwater and surface‐water resources. Seasonal variability of groundwater‐surface water exchange fluxes impacts on the water balance. The long term lake water balance was calculated by Modflow using the stage‐volume rating curve of Lake Package LAK3. The long term average storage volume change is 8.4 × 10⁸ m³/month. The lake water balances suggests that the lake is not in equilibrium with the inflow and outflow terms. Using field abstraction data analysis and model simulation, the combined volume of lake‐groundwater used for industrial abstraction since the last three decades was estimated. This requires an average abstraction amount of 7.0 × 10⁶ m³/month with a long term trend of abstraction ratio 30% (groundwater) and 70% (lake water) since 1980. The amount resulted in a lake which might have been 4.8 m higher than was observed in the last stress period (2010). A long term regional groundwater budget is calculated reflecting all water flow in to and out of the regional aquifer. The model water balance suggests that lake Navaisha basin is in equilibrium with a net outflow about 1% greater than the inflow over the calibrated period of time (1932–2010). The regional model is best used for broad‐scale predictions and can be used to provide a general sense of groundwater to surface water and groundwater to groundwater impacts in the basin. A basin wide water resource management strategy can be designed by integrating the lake/wetland within the regional groundwater model to increase the level of sustainable production and good stewardship in Lake Navaisha. Such hydrological analysis is crucial in making the model serve as simulator of the response of lake stage to hydraulic stresses applied to the aquifer and variation in climatic condition.     

Development of a Groundwater Resource Model for the Yorkshire Chalk

This paper (a) aims to demonstrate the basic principles of groundwater resource model development by means of an example, and (b) describes the development of the Yorkshire Chalk groundwater model (YORKMOD), undertaken on behalf of the National Rivers Authority † and Yorkshire Water Services. The model incorporates a permeability-depth function to represent fissuring in the Chalk, and has been successfully calibrated against observed groundwater heads and springflows for the period 1975–1992 inclusive.     

A METHOD FOR INVESTIGATING THE POTENTIAL IMPACTS OF CLIMATE-CHANGE SCENARIOS ON ANNUAL MINIMUM GROUNDWATER LEVELS

Previous approaches to assessing the impact of climate-change scenarios on groundwater levels and groundwater droughts have focused on modelling specific recharge processes or phenomena. However, statistical methods, based on correlations between historic groundwater level and rainfall time-series, provide an alternative and robust approach to predicting minimum groundwater levels and droughts. For the purposes of this study, groundwater droughts are defined in terms of the return period of a given groundwater level. A multiple linear regression model (regression of monthly rainfall totals for a given period against values of minimum annual groundwater levels for the same period), when used with synthetic rainfall data based on climate-change scenarios, enables changes in future annual groundwater-level minima to be modelled. The method is illustrated at three sites on the Chalk, Permo-Triassic sandstone and Jurassic limestone aquifers.     

A GIS-based DRASTIC model for assessing aquifer vulnerability in Kakamigahara Heights, Gifu Prefecture, central Japan

Vulnerability assessment to delineate areas that are more susceptible to contamination from anthropogenic sources has become an important element for sensible resource management and land use planning. This contribution aims at estimating aquifer vulnerability by applying the DRASTIC model as well as utilizing sensitivity analyses to evaluate the relative importance of the model parameters for aquifer vulnerability in Kakamigahara Heights, Gifu Prefecture central Japan. An additional objective is to demonstrate the combined use of the DRASTIC and geographical information system (GIS) as an effective method for groundwater pollution risk assessment. The DRASTIC model uses seven environmental parameters (Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity) to characterize the hydrogeological setting and evaluate aquifer vulnerability. The western part of the Kakamigahara aquifer was dominated by "High" vulnerability classes while the eastern part was characterized by "Moderate" vulnerability classes. The elevated north-eastern part of the study area displayed "Low" aquifer vulnerability. The integrated vulnerability map shows the high risk imposed on the eastern part of the Kakamigahara aquifer due to the high pollution potential of intensive vegetable cultivation. The more vulnerable western part of the aquifer is, however, under a lower contamination risk. In Kakamigahara Heights, land use seems to be a better predictor of groundwater contamination by nitrate. Net recharge parameter inflicted the largest impact on the intrinsic vulnerability of the aquifer followed by soil media, topography, vadose zone media, and hydraulic conductivity. Sensitivity analyses indicated that the removal of net recharge, soil media and topography causes large variation in vulnerability index. Moreover, net recharge and hydraulic conductivity were found to be more effective in assessing aquifer vulnerability than assumed by the DRASTIC model. The GIS technique has provided efficient environment for analyses and high capabilities of handling large spatial data.     

Groundwater Resources in the Lagan Valley Sandstone Aquifer, Northern Ireland

Beneath Belfast and the Lagan Valley lies Northern Ireland's most important aquifer, the Triassic Sherwood Sandstone. Up to 300 m in thickness and with a total abstraction of around 31 000 m³/d, it is a modest aquifer by UK and world standards; nonetheless, it is an important local water source. The use of this aquifer system as a water supply will undoubtedly increase as growth of industry and population continues in the Belfast metropolitan area. Even with the mesic climate of Ireland, groundwater mining of this aquifer system is already occurring, and thus there is a need for detailed aquifer planning and protection to be implemented in order to preserve this resource for the future.     

Controls on the discharge of Chalk streams of the Berkshire Downs,UK

The water quality and biological functioning of major UK rivers draining permeable basins cannot be fully understood without knowledge of the spatial and temporal controls on the aquifer-river system. In this paper, hydrogeological features of Chalk streams feeding the Thames, the focus of this special issue, are described. Large-scale features, such as line sinks along the Thames and major tributaries such as the Kennet, tend to determine the regional groundwater flow patterns in the Chalk aquifer, whereas the complex characteristics of the Chalk dominate the local spatial and temporal patterns of groundwater discharge to the tributaries draining the dip slopes. These different controls are described and illustrated by particular reference to the Lambourn (a sub-catchment of the Kennet) and Pang catchments. Various issues requiring further investigation are also highlighted.     

Identifying Land Contaminated by Chlorinated Hydrocarbon Solvents

This paper provides a method of identifying land contaminated with chlorinated hydrocarbon solvents by sampling and analysing soil gas. Relationships are presented which enable levels of soil gas contamination to be related to levels of soil and groundwater pollution. Two case studies are presented. Soil gas surveying of an industrial site with severe groundwater pollution demonstrated that standards for contaminated land are insufficient to protect groundwater. Solvent levels in groundwater were one to two orders of magnitude higher than UK permitted concentrations in drinking water, whereas soil contamination values did not reach polluted levels. Surveying of a site occupied by a disused fire station showed significant differences in patterns of soil gas and groundwater pollution. This is explained by direct pollution of groundwater though the well at the site and/or possible flow of polluted groundwater to the site from adjacent areas.     

地下水开采引起地面沉降的机理研究

过量开采地下水会造成地面沉降已经是不争的事实,但其发生发展的机理目前还缺乏深入研究,学术界的观点也不统一。本文基于多层地下水水头分布的理论,应用有效应力原理对此进行了详细的分析和研究,从理论层面揭示了地下水开采引起地面沉降的机理,并给出了预测地面沉降的计算方法。研究显示:潜水水位下降在引起有效应力增加的同时也会产生总应力的减小,因此在部分土层中发生固结沉降而另一部分地层中发生卸荷回弹;承压水水头降低只会在其含水层和上下两个隔水层中引起有效应力增加,并产生土层的固结沉降。