A portion of the Gambier Plain underlain by an unconfined aquifer with readily definable hydrologic boundaries has been divided into a number of areas within which soil types have similar hydrologic properties. Mean annual recharge has been estimated for each area using both the tritium concentration and the chloride concentration of water within the soil profile. Good agreement was obtained between the two methods with local recharge varying between 50 and 250 mm year-1. Total mean annual recharge for the area has been estimated to be 2.4 ± 0.3 x 108 m3 year-1, and this compares favourably with an estimated discharge of 2.5 ± 0.3 x 108 m3 year-1.

The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer

The use of natural tracers as indicators of soil-water movement in a temperate semi-arid region

On the theory of tracer experiments in fissured rocks with a porous matrix

Cretaceous chalk is a micritic limestone, mostly debris from planktonic algae, largely in micron-sized plates, but some still in their original circular groupings called coccoliths. Nearly all the material was deposited as low-Mg calcite which is stable at surface temperatures and pressures, which means that most chalk has been spared early lithification. Nevertheless, deep burial, high heat-flow or local tectonic stresses can each harden chalk, reduce the porosity and usually reduce the 18 0: 16 0 ratio. During the Late Cretaceous there was little land in north-west Europe available for erosion because of high eustatic sea-levels, which carried pelagic sedimentation on to the continental shelves. A non-seasonal (probably arid) climate meant little erosion from what land there was. Hence the purity of the Chalk which was deposited in a sea of normal salinity, usually 100 to 600 m deep. Instead of the homogeneous rock that it first appears to be, various chalks show rhythmic bedding, lamination bedding, submarine erosion surfaces and current-piled banks. All these may be cut by burrows. Early post-depositional solution has produced wispy-flasers of marl in the chalk, and has preferentially removed planktonic foraminifera. Early lithification has formed hard-grounds and allowed chalk-pebble conglomerates to be formed.

The petrology of the Chalk

One of the principal uses of environmental tracers is for determining the ages of soil waters and groundwaters. (We may refer to this as ‘hydrochronology’by analogy with the dating of solid materials known as geochronology.) Information on soil water and groundwater age enables timescales for a range of subsurface processes to be determined. For example, ‘groundwater stratigraphy’is used increasingly to decipher past recharge rates and conditions in unconfined aquifers, in much the same way that sedimentary stratigraphy yields information about past depositional environments. The use of environmental tracers to determine water ages allows groundwater recharge rates and flow velocities to be determined independently, and commonly more accurately, than with traditional hydraulic methods where hydraulic properties of aquifers are poorly known or spatially variable. Studies of groundwater residence times in association with groundwater contamination studies can enable historic release rates of contaminants and contaminant transport rates to be determined. Where input rates are known, measurements of groundwater contaminant concentrations, together with groundwater dating, can sometimes be used for estimating chemical reaction rates. The combination of these dating methods with stable isotope measurements has sometimes allowed changes in contaminant sources over time to be determined.

P.L. Wearn

Papers

Water Movement in the Unsaturated Zone of High and Low Permeability Strata by Measuring Natural Tritium