Recharge through till : developing a methodology for estimating groundwater recharge with examples from two case studies in East Anglia

Abstract

This report describes the results of a desk study to investigate recharge in an area of discontinuous low permeability till in the Waveney catchment. Within the study area the till is absent in the valley of the River Waveney and also in the lower parts of some of the tributary valleys. The study develops a methodology to identify the main recharge areas and make initial estimates of recharge in such hydrogeologically complex areas. Following earlier work on Chalk recharge through till this combined study area was selected to include both the Chalk and the Crag aquifers. It was thought that the difference between these two aquifers may shed further light on the recharge mechanisms through the overlying till. The main outcomes of the study have been: 1. A recharge estimate methodology is devised based on the effective rainfall, the till thickness, estimates of runoff from the till sheet and delimiting the main recharge areas where the till is thin or absent. 2. The infiltration through thick till (>10 m) is low and as a consequence, runoff from the till sheet is large and is potentially a significant component of recharge at the margins of the till sheet. Estimating the quantity of water that may runoff the till sheet is essential when attempting to assess the amount and distribution of groundwater recharge. 3. An important issue, when considering catchment water balances, is the relative proportion of runoff that infiltrates to groundwater at the margins of the till sheet, compared with that which flows directly into the river. It has not been possible in this study to devise a methodology to split these two components. More catchment scale studies are required to evaluate how catchment characteristics influence the infiltration rates. 4. The time-lag between rain falling at the soil surface and recharge arriving at the water table will be relatively short at the margins of the till sheet where the water table is generally shallow. This has important implications for water quality, as widespread changes in land-use are likely to be observed more rapidly in groundwaters at the edge of the sheet than in areas of extensive Chalk-Crag outcrop