Evidence for the onset and persistence with depth of preferential flow in unsaturated fractured porous media

Two distinct types of fracture flow can occur in unsaturated fractured porous media: non-preferential flow, whereby the fractures and matrix wet up in equilibrium, and preferential flow, whereby water in the fractures bypasses the matrix. This has important implications for how infiltration, recharge, groundwater flooding and contaminant transport are modelled. Taking the UK Chalk as a case study, we explore evidence for the occurrence of unsaturated preferential fracture flow, considering separately its initiation in the near surface, and persistence at 20–30 m depth. We postulate a link between the apparent hysteretic response of the Chalk soil moisture characteristic, which was observed and simulated, and the initiation of preferential recharge. Focusing on observed water table responses to an extreme rainfall event on 20th July 2007, we show by inverse modelling of recharge that preferential flow persisted to a depth of 20 m below ground level, but not to 30 m, resulting in markedly different water table responses at two boreholes. These findings both lend support to our conceptualisation. However, since the observations from which preferential flow can be inferred are few and are indirect, quantification of the controls on the onset and depth persistence of preferential flow remains a significant challenge.

[1]  A. Ireson,et al.  Flood risk from groundwater: examples from a Chalk catchment in southern England , 2011 .

[2]  Christopher R. Jackson,et al.  Simulation of the spatio‐temporal extent of groundwater flooding using statistical methods of hydrograph classification and lumped parameter models , 2011 .

[3]  Adrian P. Butler,et al.  Controls on preferential recharge to Chalk aquifers , 2011 .

[4]  M. Cuthbert.  An improved time series approach for estimating groundwater recharge from groundwater level fluctuations , 2010 .

[5]  A. Ireson,et al.  A model for flow in the chalk unsaturated zone incorporating progressive weathering , 2009 .

[6]  A. Gallagher,et al.  Groundwater flooding in fractured permeable aquifers. , 2009 .

[7]  A. Ducharne,et al.  Multi-model comparison of a major flood in the groundwater-fed basin of the Somme River (France) , 2009 .

[8]  A. Ireson.  Quantifying the hydrological processes governing flow in the unsaturated Chalk , 2008 .

[9]  S. Mathias,et al.  The significance of colloids in the transport of pesticides through Chalk. , 2007, The Science of the total environment.

[10]  M. Price,et al.  Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer, SE England , 2006 .

[11]  A. Ireson,et al.  Hydrological processes in the Chalk unsaturated zone - Insights from an intensive field monitoring programme , 2006 .

[12]  H. Wheater,et al.  Transient simulations of flow and transport in the Chalk unsaturated zone , 2006 .

[13]  C. Neal,et al.  Hydro-ecological functioning of the Pang and Lambourn catchments, UK: An introduction to the special issue , 2006 .

[14]  Tempei Hashino,et al.  Evaluation of bias-correction methods for ensemble streamflow volume forecasts , 2006 .

[15]  S. Brouyère.  Modelling the migration of contaminants through variably saturated dual-porosity, dual-permeability chalk. , 2006, Journal of contaminant hydrology.

[16]  Delwyn G. Fredlund,et al.  A study of hysteresis models for soil-water characteristic curves , 2005 .

[17]  K. Rushton.  ESTIMATING RECHARGE FOR BRITISH AQUIFERS , 2005 .

[18]  Nadia Amraoui,et al.  Groundwater‐induced flooding in macropore‐dominated hydrological system in the context of climate changes , 2005 .

[19]  S. Mathias.  Modelling flow and transport in the chalk unsaturated zone , 2005 .

[20]  Peter J. Gregory,et al.  Dynamics of water movement on Chalkland , 2002 .

[21]  R. Low,et al.  Mechanisms of water storage and flow in the unsaturated zone of the Chalk aquifer , 2000 .

[22]  A. Binley,et al.  Geophysical investigation of unsaturated zone transport in the Chalk in Yorkshire , 1999, Quarterly Journal of Engineering Geology.

[23]  Christine Doughty.  Investigation of conceptual and numerical approaches for evaluating moisture, gas, chemical, and heat transport in fractured unsaturated rock , 1999 .

[24]  J. Bloomfield.  Characterisation of hydrogeologically significant fracture distributions in the Chalk: an example from the Upper Chalk of southern England , 1996 .

[25]  K. Kosugi.  Lognormal Distribution Model for Unsaturated Soil Hydraulic Properties , 1996 .

[26]  G. Bodvarsson,et al.  Transient dual-porosity simulations of unsaturated flow in fractured rocks , 1995 .

[27]  Horst H. Gerke,et al.  Evaluation of a first-order water transfer term for variably saturated dual-porosity flow models , 1993 .

[28]  M. V. Genuchten,et al.  A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media , 1993 .

[29]  J. P. Bell,et al.  Processes of water movement through a chalk Coombe deposit in southeast England. , 1990 .

[30]  Jack C. Parker,et al.  Development and evaluation of closed-form expressions for hysteretic soil hydraulic properties , 1987 .

[31]  S. Wellings.  Recharge of the Upper Chalk aquifer at a site in Hampshire, England: 1. Water balance and unsaturated flow , 1984 .

[32]  Van Genuchten,et al.  A closed-form equation for predicting the hydraulic conductivity of unsaturated soils , 1980 .

[33]  S. R. Wellings,et al.  Movement of water and nitrate in the unsaturated zone of Upper Chalk near Winchester, Hants., England , 1980 .

[34]  K. Rushton,et al.  The estimation of groundwater recharge , 1979 .

[35]  Y. Mualem,et al.  A conceptual model of hysteresis , 1974 .

[36]  G. C. Topp,et al.  Soil Water Hysteresis in Silt Loam and Clay Loam Soils , 1971 .