Cold Downflows of Groundwater at Ohaaki Geothennal Field: Preliminary Results

Abstract

An exploitation-induced downflow of cold groundwater in the northwestern part of the field is inferred from localised changes in groundwater level. Geophysical techniques (GPR, VES,SP) were tested to evaluate their ability tomap hot and cold groundwater surfaces, their changes with time, and to determine the direction of fluid flow in the groundwater zone. None of the techniques were found to be entirely adequate for these purposes. Data from 17 new, shallow monitor holes suggests that shallow, perched groundwater aquifers, of limited horizontal extent, overlie the regional groundwater near several areas of thermal activity, and that production from the field has resulted them being drained. The data also suggest that the regional groundwater level has been depressed by up to 5 m within about 100 m of BR4. The original concept of a single, extensive cone of groundwater depression centred near BR4, should be replaced by the concept of several, small, localised depressions.