Catchment scale tracer testing from karstic features in a porous limestone

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 (10 15 ) are recommended for investigating groundwater pathways where attenuation may be high.

Advances in modelling groundwater behaviour in Chalk catchments

“…On the other hand, tracer tests conducted at a sink hole around about 5 km away gave a breakthrough after about one day (Maurice et al 2006) confirming evidence from previous studies (e.g. Banks et al 1995) of highly connected rapid flow pathways (Maurice et al 2010). Water quality measurements indicate a dual source of Chalk and Palaeogene waters, with an increased proportion of Palaeogene water during high flows ).…”

“…Water quality measurements indicate a dual source of Chalk and Palaeogene waters, with an increased proportion of Palaeogene water during high flows ). The catchment area of the Blue Pool has been estimated to be 58 km 2 (Maurice et al 2010), although there is still some uncertainty as to its actual location. The results from such tracer tests, combined with analyses of groundwater levels, suggest that a network of conduits associated with particular sink holes situated along the edge of the Chalk/Palaeogene boundary act as collectors for diffuse groundwater flow to the NW of the site (Maurice et al 2010).…”

“…The catchment area of the Blue Pool has been estimated to be 58 km 2 (Maurice et al 2010), although there is still some uncertainty as to its actual location. The results from such tracer tests, combined with analyses of groundwater levels, suggest that a network of conduits associated with particular sink holes situated along the edge of the Chalk/Palaeogene boundary act as collectors for diffuse groundwater flow to the NW of the site (Maurice et al 2010). This important feature particularly demonstrates the complexities that often confront groundwater modellers when dealing with the effects of 'focussed' v. diffuse recharge in the vicinity of such geological boundaries in Chalk catchments (MacDonald et al 1998;Soley & Heathcote 1998).…”

“…For the simulation of groundwater flow, characterization of karstic features also remains a challenging problem, particularly for sitespecific studies. For example, after several years of relatively intensive field research, including tracer testing (Maurice et al 2006), there is still uncertainty over recharge area and subsurface mechanisms and pathways that govern flow to the Blue Pool (see above), even though this is a major source of flow, particularly during drought periods (Maurice et al 2010).…”

Comparison of varied complexity models simulating recharge at the field scale

“…It is an important process to consider due to the potential for runoff-recharge which has implications for both recharge quantity and location. Furthermore, if the study catchments were considered as a whole, then quantifying runoff accurately would be invaluable due to the abundance of stream sinks along the Chalk-Palaeogene boundary which are collectively important for recharge (Maurice et al, 2010).…”