CL-36 IN GROUNDWATER AS A PALEOCLIMATIC INDICATOR - THE EAST MIDLANDSTRIASSIC SANDSTONE AQUIFER (UK)

The Cl-36 contents of groundwaters from the East Midlands Triassic san dstone have been used to deduce rainfall, its chloride content and eva potranspiration during the Late Pleistocene. At this time, the regiona l precipitation and evapotranspiration would have been reduced relativ e to the modern and Holocene climate. It is estimated that the infiltr ation was reduced by about 40 mm . a-1. The method for evaluation of t hese palaeoclimatic parameters is dependent upon the major Cl- source being a marine aerosol fall-out. This is the general case, but groundw ater east of the exploited zone of the aquifer contains chloride which is derived from saline formation water, with a marine Cl-/Br- ratio, and in equilibrium with the in situ neutron flux. The fall-out of cosm ogenic Cl-36 at the geomagnetic latitude 56.2-degrees-N is evaluated a s 30.6 atoms m-2 s-1 and is significantly greater than that estimated from the theoretical fall-out curve of Bentley et al. [1]. The 'bomb p eak' of Cl-36 remains entirely within the unconfined zone of the aquif er. The assessment of groundwater residence time from the Cl-36 chrono meter is shown to be impossible for this aquifer.