Abstract
The nature and distribution of flowing features in boreholes in an area of approximately 400 km2 in a weakly karstic porous limestone aquifer (the Chalk) was investigated using single borehole dilution tests (SBDTs) and borehole imaging. One-hundred and twenty flowing features identified from SBDTs in 24 boreholes have densities which decrease from ∼0.3 m−1 near the water table to ∼0.07 m−1 at depths of more than 40 m below the water table; the average density is 0.20 m−1. There is some evidence of regional lithological control and borehole imaging of three boreholes indicated that most flowing features are associated with marls, hardgrounds and flints that may be developed at a more local scale. Borehole imaging also demonstrated that many flowing features are solutionally enlarged fractures, suggesting that even in carbonate aquifers where surface karst is developed on only a small scale, groundwater flow is still strongly influenced by dissolution. Fully connected solutional pathways can occur over 100s, sometimes 1000s of metres. However, conduits, tubules and fissures may not always be individually persistent along a flowpath, instead being connected together and also connected to unmodified fractures to create a relatively dense network of voids with variable apertures ( 15 cm). Groundwater therefore moves along flowpaths made up of voids with varying shape and character. Local solutional development of fractures at significant depths below the surface suggests that mixing corrosion and in situ sources of acidity may contribute to solutional enhancement of fractures. The study demonstrates that single borehole dilution testing is a useful method of obtaining a large dataset of flowing features at catchment-regional scales. The Chalk is a carbonate aquifer with small-scale surface karst development and this study raises the question of whether other carbonate aquifers with small-scale surface karst have similar characteristics, and what hydrological role small-scale dissolutional features play in highly karstic aquifers.
Highlights
Understanding the distribution of flows in aquifers is important for sustainable use of groundwater resources, and to understand and manage contamination problems
The boreholes are aligned to the Chalk stratigraphy which was interpreted using a combination of geophysical logs, modelled geological surfaces (Aldiss et al, 2002) and published studies of Chalk stratigraphy (Aldiss et al, 2002; Woods and Aldiss, 2004; Mortimore et al, 2001)
The results suggest that lithology is an important control on flowing features, which may be associated with both regionally extensive marker horizons such as the Chalk Rock hardground (e.g. Fig. 1b) and the Glynde Marl, as well as flint layers (e.g. Fig. 1c and e), hardgrounds and marl layers that may be only locally developed
Summary
Understanding the distribution of flows in aquifers is important for sustainable use of groundwater resources, and to understand and manage contamination problems. In fractured aquifers flow distribution is often poorly understood because fracture distributions are spatially highly variable at a range of scales (Neuman, 2005). It is recognised that karstic modification of aquifers can occur on a range of scales (Klimchouk and Ford, 2000; Worthington, 2009; Worthington and Ford, 2009), from highly channelled flows in conduit-dominated aquifers beneath classically karstic landscapes to carbonate aquifers where. Caves are rare but dissolution of both the surface and the subsurface occur on a small-scale. In carbonate aquifers where caves are rare or absent, small-scale subsurface karst can only be inferred from the magnitude and character of springs, tracer testing, or from observation of flowing fractures within outcrops or boreholes
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