Tracer Investigations of the Nature and Structure of Subsurface Voids in Mildly Karstic Aquifers: an Example from the English Chalk

Abstract

The Chalk in England is a mildly karstified fractured limestone with high matrix porosity. Chalk landscapes are predominantly a fluviokarst with an extensive dry valley network and with stream sinks and dolines developed in zones close to overlying strata. The nature and extent of subsurface dissolutional voids is poorly understood and field tests using artificial tracers were carried out to investigate their structure. Tracer testing from stream sinks demonstrated rapid groundwater flow (up to 5 km/day) confirming previous Chalk tracer studies. However tracer attenuation was variable with extremely low tracer recoveries at some sites indicating that not all flowpaths fed by stream sinks comprise fully connected conduit systems. A conceptual model of flow along multiple flowpaths comprising a distributary network around stream sinks and a tributary network around springs with variable connectivity between the two is presented. The proposed conceptual model of flowpath structure in the Chalk resembles previous models of the early stages of speleogenesis. It is suggested that groundwater flow in the Chalk and other mildly karstic aquifers occurs within complex small scale dissolutional networks, in which there is a high degree of connectivity between larger conduits, fissures, and a primary fracture network. Advection from karstic channels into smaller voids results in high attenuation of solutes and particulates providing a degree of protection to groundwater outlets that is not seen in more highly karstic aquifers.