Characterising groundwater-dominated lowland catchments: the UK Lowland Catchment Research Programme (LOCAR)

Abstract

This paper reports on a major UK initiative to address deficiencies in understanding the hydro-ecological response of groundwater-dominated lowland catchments. The scope and objectives of this national programme are introduced and focus on one of three sets of research basins the Pang/Lambourn Chalk catchments, tributaries of the river Thames in southern England. The motivation for the research is the need to support integrated management of river systems that have high ecological value and are subject to pressures that include groundwater abstraction for water supply, diffuse pollution, and land use and climate change. An overview of the research programme is provided together with highlights of some current research findings concerning the hydrological functioning of these catchments. Despite the importance of the Chalk as a major UK aquifer, knowledge of the subsurface movement of water and solutes is poor. Solute transport in the dual porosity unsaturated zone depends on fracture/matrix interactions that are difficult to observe; current experimental and modelling research supports the predominance of matrix flow and suggests that slow migration of a time-history of decades of nutrient loading is occurring. Groundwater flows are complex; catchments vary seasonally and are ill-defined and karst features are locally important. Groundwater flow pathways are being investigated using natural and artificial geochemical tracers based on experimental borehole arrays; stream-aquifer interaction research is using a combination of geophysics, borehole array geochemistry and longitudinal profiles of stream flow and solutes. A complex picture of localised subsurface inflows, linked to geological controls and karst features, and significant longitudinal groundwater flow below the river channel is emerging. Management implications are discussed. Strategies to control surface application of nutrients are expected to have little effect on groundwater quality for several decades, and new modelling tools for decision support have been developed to represent these effects. Conventional modelling approaches are limited by the complexities of the subsurface system; catchment areas are difficult to define, hence tracking pollutant pathways to stream receptors is also problematic. Conventional distributed groundwater models have difficulty in capturing key aspects of the groundwater system. This raises important questions concerning the confidence that can be placed in models as routinely used for decision support and the level of knowledge required for catchment management to be placed on a secure scientific foundation.

Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford, UK)

“…There was a clear difference between the variograms in the horizontal and vertical planes. Therefore the estimated model was isotropic in the horizontal plane with geometric anisotropy in the vertical plane (Webster and Oliver, 2007). The estimated spherical variograms are shown in Figure 2b …”

“…Studies at a nearby site 100 m upstream of the Observatory, part of the Lowland Catchment Research (LOCAR) programme (Wheater, et al, 2007), have indicated variable hydraulic connection between the Chalk, gravels and surface water (Abesser, et al, 2008;Allen, et al, 2010;Lapworth, et al, 2009). The extent of the interaction remains unclear.…”

“…15, 0.30, 0.45, 0.60, 0.75, and 0.90 (Figure 2b), were interpolated throughout the gridded volume using ordinary kriging (Webster and Oliver, 2007).…”

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

“…The reason for this behaviour might be due to the modelling assumption of a constant vertical porosity, despite the knowledge that there can be a strongly non-linear relation between chalk transmissivity and depth. Several studies acknowledge that hydraulic conductivity in the Chalk follows a non-linear decreasing trend with depth (Allen et al, 1997;Wheater et al, 2007;Butler et al, 2009). This is mainly attributed to the decrease in fractures because of the increasing overburden and absence of water level fluctuations Butler et al, 2012).…”

Sediment storage in the shallow hyporheic of lowland vegetated river reaches

“…As a consequence of their stable flow regimes, intensive abstraction for public and agricultural water supply and extensive growth of aquatic vegetation, chalk rivers are fluvial environments that are particularly vulnerable to problems associated with the storage of fine sediment (Wheater et al, 2007). These rivers support valuable aquatic and riparian ecosystems, so are recognized as a priority habitat for protection under the UK Biodiversity Action Plan (Mainstone and Parr, 2002;Wright et al, 2002).…”