This paper presents the results of an investigation carried out on a geomembrane lined pond in Australia which was temporarily decommissioned due to elevated leakage rates and observed development of geomembrane ‘whales’, after it was in service for only 4 years. The investigation identified a number of geomembrane cracks (or splits) in the vicinity of the whales. The cracks were orientated parallel to the outside edges of fusion wedge weld tracks where adjoining geomembrane panels were seamed. A 400 mm long crack was exhumed and analysed in the laboratory. The testing confirmed the heat affected zone adjacent to the crack was more susceptible to brittle stress cracking compared a control sample, and exhibited raised crystallinity which is indicative of higher than industry standard welding temperatures and slower cooling. It is hypothesized that failure of the geomembrane occurred at the edges of fusion wedge weld tracks due to the combined effects of 1) raised crystallinity from unacceptably high welding temperatures; 2) geometric stress concentrating features along the weld profile, and 3) variations in ambient temperatures causing whales to ‘breath’, inducing a flexing action on the edge of welds. The geomembrane cracks were repaired in situ and the impact on leakage rates was assessed after the pond was recommissioned. The lessons learnt from the investigation are presented in this paper.