Quantifying Groundwater-Surface Water Connectivity During Pumping with Applied Dissolved-Gas Tracers

A novel tracer method was developed to characterize groundwater-surface water connectivity and assess the hydrologic impacts of pumping on flow in Doudles Folly Creek, located in the Southern Highlands of New South Wales, Australia.  Specifically, the purpose of the applied tracer test was to better understand the degree of connection and flow between the creek and the fractured sandstone aquifer in order to extrapolate observed behaviour and trends to other stream-aquifer systems within the Hawkesbury Sandstone in the Sydney Basin.  Dissolved gases (sulfur hexafluoride and helium) were continuously applied to Doudles Folly Creek during a 90-day pumping test and regularly monitored within the stream, at several monitoring wells, and in the produced water.  Tracer data provided a direct verification of stream loss, transport times, and the time-dependant percentage of stream contribution to the produced water.  Based on the relative tracer concentration in the extracted water, approximately  5% of total groundwater extraction flow rate (~30 L/sec)  was produced from the stream near the end of the test.  Pumping resulted in fast transport, with SF₆ being detected in the pumping bore located 260 m from Doudles Folly Creek in less than 5 days.  These results indicate an average transport velocity within the primary pathways on the order of 60 m/day.  Generally, the study results emphasize the significance of fracture flow in the Hawkesbury Sandstone and contributed to wellfield development planning.