Errors in geophysical parameters incorporated simultaneously in in-situ SWCC inversion: Improved slope stability analyses

The Cocoa Grove Group is planning a housing development on approximately 5.5 acres in the area of Saphire, Diamond, St Lucia. Directly downhill of the property is a WASCO (Water and Sewage Company) Spring that has supplied drinking water to the town of Soufriere, going back to historic times. This potable water is of high purity. Concern was therefore expressed by WASCO as to the advisability of implementing any development directly above the emanation of the spring. The obvious concerns are the possible change in flow volumes and chemical and biological contamination if a housing development was completed. F.D. Morgan and the St Lucia Water Resources and personnel representing the Cocoa Grove Group discussed whether or not geophysical methods could be used to map the direction of groundwater flow into the spring. The group agreed that geophysical methods could be used in an attempt to delineate the underground flow paths into the spring. But it was made clear that the investigations could produce results that could be either favorable or unfavorable to the proposed development. Consequently, geophysical surveys were planned and executed in the area of concern, to measure resistivity and self potentials (SP). The geophysics did not indicate evidence, as expected, of substantial fluid flow into the spring. However, we were able to locate the main water supply from the top of Terre Blanche down towards the spring. The flow comes downhill via a significant geological fracture that is hidden from sight in the dense tree cover. Recommendations were made that it was relatively safe to develop the site as intended, with respect to possible water contamination. In conclusion, we also recommend that WASCO could take water from another point of a topographic step on Terre Blanche. Doing so, would reduce the contamination risk essentially to zero from the planned housing development area.