Water transport through the unsaturated zone of the Middle Chalk: a case study from Fleam Dyke lysimeter

Abstract

The role of fractures in transporting water in the unsaturated zone of the Chalk has been a source of controversy for many years. There is clearly a potential for fractures to transport water and contaminants rapidly to the saturated zone, although direct observations are rare. The evidence from well hydrographs, solute profiles and soil water measurements is ambiguous concerning the importance of by-pass flow. This paper examines drainage data from several years’ operation of a 5 m cube undisturbed monolith lysimeter at Fleam Dyke, Cambridgeshire, for evidence of by-pass flow and investigates the factors responsible for its onset. Evidence from measurements of unsaturated hydraulic conductivity of the Chalk at the site and in situ measurements of soil water are also presented. Both the lysimeter and soil water observations agree that flow through fractures accounts for approximately 30% of the total drainage from this site, significantly higher than has been inferred at other Chalk sites. By-pass flow was observed to occur above a threshold of approximately −50 cm H 2 O matric potential, or above a flow rate of approximately 1 mm day −1 . During periods of no soil water deficit, about 50% of drainage occurred via fractures. In general, there was a delay of three or four days between rainfally and drainage at the base of the lysimeter. On once occasion, intense summer rainfall satisfied the existing soil moisture deficit and initiated fracture flow, the peak in drainage through the lysimeter occurring within 24 hours. Drainage from the lysimeter was consistently higher than MORECS estimates of hydrologically effective precipitation during the period after the onset of soil water deficits.

Process-based modelling to evaluate simulated groundwater levels and frequencies in a Chalk catchment in south-western England

“…A rather low value that is sensitive to K C was found when calibrating only by discharge operations, indicating some interactions of K C with other model parameters (Saltelli et al, 2008). This is in accordance with the findings by Jones and Cooper (1998) as well as by Reeves (1979), who reported 30 and 10-20 % of the recharge occurring through (macro-) fissures in Chalk catchments. Although groundwater flow in the chalk is dominated by the matrix, given antecedent wet conditions, fracture flow can increase significantly (Lee et al, 2006;Ireson and Butler, 2011;Butler et al, 2012).…”

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer, SE England

“…At Fleam Dyke on the Cambridgeshire Middle Chalk, fissure flow was observed to occur at pore pressures greater than À5 kPa in the winter months (Jones and Cooper, 1998). Lysimeter and soil water observations indicate that approximately 30% of flow occurred through fissures throughout the year, and that during the winter months (when no soil moisture deficit was present) 50% of flow occurred through fissures.…”

Transient simulations of flow and transport in the Chalk unsaturated zone

“…Any remaining recharge and its associated nitrate was bypassed directly to the water table. This idea is based on the hydraulic conductivity relationships observed by Wellings (1984), Hodnett and Bell (1990), Cooper et al (1990), Gardner et al (1990) and Jones and Cooper (1998). Andrews et al (1997) describe their model output as comparing well with observed solute profiles.…”