DIFFERENCES BETWEEN HYDROLOGICAL AND TOPOGRAPHIC WATERSHED BOUNDARIES IN CARBONATE AQUIFERS

Nahanni National Park Reserve, a World Heritage Site in Canada's Northwest Territories, covers some 14% of the topographic watershed of the South Nahanni River. Expansion of the park to include the whole watershed is currently being considered. Limestone and dolostone occur in the eastern part of the watershed and field work was carried out to investigate the possibility that topographic and hydrogeological watersheds differ substantially due to karstic groundwater flow. Two major springs with discharges each of several cubic meters per second drain the carbonate outcrop: White Spray Spring, which is adjacent to the South Nahanni River, and Bubbling Springs, which are 33 km further north. The north-dipping Devonian strata consist of 1000 m of dolostones overlain by 180 m of limestones, with White Spray Spring being situated close to the base of the dolostones and Bubbling Springs being located in the limestones.

Three simultaneous tracer tests were carried out, using the fluorescent dyes uranine, eosin and phloxine B. All three traces were injected into sinking streams into the limestone. Two tracers flowed updip to White Spray Springs, traveling distances of 12 km and 21 km, respectively, in less than one week. The third trace, injected only 4 km from Bubbling Springs, was not recovered, which is probably due to insufficient sampling in this challenging environment. The tracing showed that the steeper hydraulic gradients south to White Spray Spring had more influence on tracer flow paths than either lithology (limestone versus dolostone) or stratal dip, both of which would be considered to favor northward flow. The tracing showed that the hydrological boundary for the South Nahanni River extends at least 17 km north of the topographic boundary. Such large deviations are not unexpected in carbonate aquifers.