ABSTRACT

Bruce D.R. Misstear1 and Vincent P. Fitzsimons2 1 Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Ireland 2 Scottish Environmental Protection Agency, Clearwater House, Heriot Watt Research Park, Edinburgh, EH14 4AP, UK (formerly of the Geological Survey of Ireland, Beggar’s Bush, Hadington Road, Dublin 4, Ireland)

ABSTRACT: Recharge assessment has become increasingly important in Ireland in response to recent European legislation. The reliability of groundwater recharge estimates is influenced by the complexity of the geology – fractured bedrock aquifers with a thick covering of superficial deposits, generally glacial tills, often of low permeability – and by the relative scarcity of hydrometric and hydrogeological data. The two main approaches applied to recharge estimation are the soil budgeting method and river baseflow analysis. A recent study of the soil moisture budgeting approach showed that the recharge estimates are more dependent on the characteristics of the till cover (especially hydraulic conductivity) than on the particular soil budgeting method applied or the assumptions concerning land use and crop root constant (readily available water). A sensitivity analysis of baseflow separation parameters highlighted the importance of having a good conceptual model of the various subsurface pathways contributing flow to a river in order to achieve realistic baseflow results. The baseflow separation can be improved by comparing the flow data against borehole water level hydrographs, and by also calibrating the output against both major summer recessions and short term winter recessions

1 INTRODUCTION

The main impetus for the current study of recharge in the fractured bedrock aquifers of Ireland was the introduction of the European Union Water Framework Directive (European Commission, 2000). Prior to the directive, recharge received little attention in Ireland, primarily because the total groundwater usage was small (in national terms), the resource was not regarded as being under pressure and because there was no permitting system for well abstractions. Due to the implementation of the directive, with its emphasis on an integrated approach to managing and protecting water resources and dependent ecosystems, there is now a greater emphasis on acquiring estimates of recharge. Reliable estimates of recharge are needed for many purposes, including: quantifying groundwater resources within river basins; issuing abstraction licences; determining groundwater vulnerability; assessing the groundwater contributions to rivers (baseflow) and to sensitive wetland habitats, and hence protecting these resources. The paper considers the two main approaches used for estimating

groundwater recharge in Ireland: soil moisture budgets and river baseflow analysis. The results obtained are examined both in relation to the assumptions made in the analysis, and also with regard to the influence of the geology.