5- ua.vo.oi r s r . Natural Environment Research Council British Geological Survey Institute of Hydrology Methods for the prediction of the impact of groundwater abstraction on East Anglian wetlands A Williams1, K Gilman2 & J Barker 1 BGS Hydrogeology Wallingford 2 IH Plynlimon This report is produced as part of NRA Project 01/558 'Protection of East Anglian wetlands' for NBA Anglian Region Bibliographic reference: Authors: A Williams, K Gilman &J Barker Title: Methods for the prediction of the impact of groundwater abstraction on East Anglian wetlands British Geological Survey report WD/95/SR © NERC 1995 Keyworth, Nottinghamshire British Geological Survey 1995 1 CONTENTS Executive summary Introduction 1 1.1 Background 1 1.2 Previous studies 2 1.3 Present study 3 Wetlands and water use 9 2.1 Wetland types 10 2.2 Water requirements of different plant types 11 2.3 The wetland water budget - sources of water 13 2.4 Estimation of present water balance and modelling the wetland water table 14 2.5 Data requirements 20 Impact of groundwater abstraction on wetlands 25 3.1 Aquifer system water balance 25 3.2 Role of wetlands in regional context 25 3.3 The effects of changes in water levels on wetland sites 26 3.4 Effect of groundwater abstraction on groundwater levels 27 3.5 Quantification of the effects of abstraction on wetland flows 30 3.6 The 10% rule 33 3.7 Relating drawdown in the aquifer to impact on the wetland water regime 34 Methodologies 41 4.1 Methodology to apply when considering an abstraction proposal 42 4.2 Impacts of current abstractions 44 4.3 Desk studies - classification of wetlands 45 4.4 Water requirements of wetlands - using a water budget model 46 Monitoring requirements and use of additional data 51 5.1 Wetland water levels 51 5.2 Local water table measurements 52 5.3 Spring flow 53 5.4 Monitoring during pumping tests 54 5.5 Incorporation of new data into existing methodologies 54 Conclusions and Recommendations 55 6.1 Introduction 55 6.2 Protection of wetlands 56 6.3 Methodologies 56 6.4 The way forward 57 Acknowledgements 59 References 61 ENVIRONMENT AGENCY 061437iniiiini Appendix A Background to Suggested Methodologies A1 Wetland water budgets, the MIROS model and the Sum Exceedance Value A2 A new well function for estimating drawdown at a wetland and induced leakage A3 Numerical modelling A4 Dimensionless groups characterising the 'drain' model Appendix B Application of methodologies to specified wetlands Appendix C Terms of Reference Executive Summary This is the final report of the Anglian Regional Operational Investigation 558 : "The Protection of East Anglian Wetlands". The objective of the project was the development of methodologies for evaluation of the likely impact of groundwater abstraction on wetland sites. The methodologies should be suitable for use at all wetland sites in East Anglia, regardless of the amount of data available for the site. A primary requirement of the methodologies is that they should indicate whether a wetland might be at risk from proposed abstractions. In the Terms of Reference, the project objective was defined in relation to the groundwater catchments of wetland sites. The project was run in two phases, and in the course of the first phase the significance of groundwater catchments in the protection of wetlands from the effects of groundwater abstraction was examined. It was concluded that identification of the groundwater catchment was not required when assessing the possibility of the wetland being adversely affected by a specific groundwater abstraction. The emphasis of the project thus changed to a consideration of wetland behaviour in response to variations in water budget, and to the effect of groundwater pumping on that water budget. The maintenance of wetlands in a low-rainfall region depends upon groundwater inputs, but groundwater is not the only important component of the water budget. In this report emphasis is placed on the construction of a simple but comprehensive water budget model, on a monthly timescale, that will predict the impact of changes in the groundwater supply on water levels in the wetland, most importantly during the summer months and in drought years. This MIROS model, which can be used for sites with no local data but can also be refined to take account of available data, has been developed in the form of a spreadsheet. Prohibition of abstractions that would cause any fall whatsoever in water levels at wetland sites is not a practical solution. For a given wetland site, predicted changes in water level and flow, resulting from a proposed groundwater abstraction, should be evaluated by an objective procedure which must recognise the water needs of plant communities. As an improvement to the simple Theis method for predicting the propagation of impacts through the aquifer, a new well function has been developed to quantify the effect of pumping on an idealised wetland. The analytical model characterises a wetland by its size, expressed as an effective radius, and a parameter which is a measure of the resistance to flow between the underlying aquifer and the wetland. Quantitative measures of the physical habitat needs of wetland plants are difficult to establish, but one approach based on field evaluation has confirmed the value of the Sum Exceedance Value (SEV) as a measure of the length and intensity of summer drought in wetland soils. The SEV can be calculated from measured or modelled water-level data on an annual or long term average basis. In this report, the combination of the MIROS model with the SEV method has been used as one means of estimating the maximum acceptable drawdown in the wetland. The results lead to a suite of methodologies which allows an assessment of the susceptibility of a wetland to a specific proposed abstraction to be made, regardless of the amount of data available. These allow more data to be used, when available, to further quantify the wetland water budget. I 1. Introduction 1.1 Background There is an abundance of definitions of a wetland, but the for the purposes of this report a wetland is defined as "an area of land whose hydrology and ecology is determined by excess water, usually in the form of a high water table giving rise to saturation or high moisture content in the root zone, and whose natural vegetation communities are dominated by plants that are adapted to these soil conditions”. Wetlands in general constitute a dwindling habitat under considerable developmental pressure. They are often protected from the effects of development by Site of Special Scientific Interest (SSSI) notification, and the presence of SSSIs is taken into account by local authorities and the National Rivers Authority (NRA) as part of the planning and licensing process. For potential threats to the conservation value of a site to outweigh the value of development, it is necessary to prove the extent of the impact of the proposed development. Environmental statements, required for some capital schemes, are often weak in the area of evaluation and prediction of potential impacts on natural systems (Boon, 1991), and it may be difficult to provide necessary scientific evidence to support unequivocal assertions relating to environmental impact on a particular site. In particular, the relationships between surface water, shallow groundwater and deep groundwater are complex and difficult to quantify. Wetlands, by their very nature, require an input of water for their survival. The amount, timing and quality of the water available control the type of wetland and the nature and diversity of the plants and animals which the wetland supports. In many cases, the need for the protection of the wetland is seen, in the first instance, as the need to protect the plant species which thrive in these environments providing a habitat for specialist animal communities, for instance wading birds, which have declined in the wider countryside. In order to adequately protect wetlands it is necessary to understand the role they play in the regional water resource environment. A long term water balance would show groundwater- fed wetlands to be areas of groundwater discharge. However, in the short term wetlands may recharge the underlying aquifer, thus maintaining groundwater levels in times of drought. In the past wetland areas in East Anglia were much more extensive than at present. Thus plant diversity could be maintained even in prolonged periods of low rainfall. Historical development, including land drainage and deepening of river channels, has reduced the wetland area present to an amount which needs careful management to ensure its conservation. The remaining areas of wetland are usually surrounded by land which has been developed, either for agricultural or urban use, and as such the wetlands are threatened by changes in agricultural and urban practices (e.g. deep drainage of fields, deepening of stream channels, diversion of runoff into storm sewers). The occurrence of wetland sites in their present positions is the result of many factors. The most difficult effect to distinguish from visual surveys is the effect of the underlying geology. It is likely that most wetlands occur where they do because of a combination of topographic, hydrological and geological features, which have subsequently been impacted by man-made 1 influences. A knowledge of the combination of features present at each site aids the understanding of the processes which are of importance for that site, and it is important that these features are correctly described. Increasing demand for new groundwater abstraction (for a variety of reasons including, for example, regional increase in demand, nitrate pollution of existing sources, and interference effects from other licensed abstractions) provides additional potential stress on wetland environments. The habitat value of wetlands is almost invariably linked to a pattern of hydrology and hydrogeochemistry that has been stable in the long term, and short term disturbances to this pattern can have important consequences to the composition and diversity of the ecosystem.