Catchment scale tracer testing from karstic features in a porous limestone

Maurice, L., Atkinson, T. C., Williams, A. T., Barker, John A. and Farrant, A. R. (2010) Catchment scale tracer testing from karstic features in a porous limestone. Journal of Hydrology, 389 (1-2), 31-41. (doi:10.1016/j.jhydrol.2010.05.019).

Abstract

Tracer testing was undertaken from sinking streams feeding the Chalk, a porous limestone aquifer characterised
by frequent small-scale surface karst features. The objective was to investigate the nature and
extent of sub-surface karstic development in the aquifer. Previous tracer testing has demonstrated rapid
flow combined with low attenuation of tracer. In this study, at two sites rapid groundwater flow was
combined with very high attenuation and at two other sites no tracer was detected at springs within
the likely catchment area of the stream sinks tested, suggesting that tracer was totally attenuated along
the flowpath. It is proposed that the networks beneath stream sinks in the Chalk and other mildly karstic
aquifers distribute recharge into multiple enlarged fractures that divide and become smaller at each division
whereas the networks around springs have a predominantly tributary topology that concentrates
flow into a few relatively large cavities, a morphology with similarities to that of the early stages of karstification. Tracer attenuation is controlled by the degree to which the two networks are directly connected.
In the first state, there is no direct linkage and flow between the two networks is via primary
fractures in which tracer attenuation is extreme. The second state is at a percolation threshold in which
a single direct link joins the two networks. A very small proportion of tracer reaches the spring rapidly
but overall attenuation is very high. In the third state, the recharge and discharge networks are integrated
therefore a large fraction of tracer reaches the spring and peak concentrations are relatively high. Despite
the large number of stream sinks that recharge the Chalk aquifer, these results suggest that sub-surface
conduit development may not always be continuous, with flow down smaller fissures and fractures causing
high attenuation of solutes and particulates providing a degree of protection to groundwater outlets
that is not seen in more highly karstic aquifers. Bacteriophage tracers that can be detected at very large
dilutions (1015) are recommended for investigating groundwater pathways where attenuation may be
high.

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