DIRECT MEASUREMENT OF GROUNDWATER DISCHARGE TO A LARGE LAKE

Long term groundwater pumping comes from either the permanent depletion of storage during the creation of the cone of depression, or from streamflow depletion. Direct measurement of groundwater - surface water interaction can help quantify net streamflow depletion. This study examined the aquifer system in Kelowna British Columbia, Canada. Prior modelling studies had derived varying estimates of groundwater discharge to Okanagan Lake. Stakeholders were not convincingly engaged with groundwater resource issues, as the modelling efforts varied in magnitude and were complex. The study initiated direct monitoring of surface water groundwater interactions to provide evidence of the existing throughflow in the system, and therefore give direction to the state of groundwater resources. A 2 year groundwater discharge monitoring project was conducted. Seepage meters were deployed along the 15 kilometres of shoreline. Individual location measurements were scaled temporally using long term station data over 2 years. Spatial corrections were applied from detailed profiles perpendicular to shore. Field data were then compared to 2-D Modflow profiles to estimate total discharge.

The data determined annual flow between 2.9 and 4.1 x 10⁵ m3/year. Long‐term station seepage meter measurements showed a large range of annual variability with flux measurements which ranged from 10‐11 to 10‐9/10‐8 m3/m2/s. The annual discharge is less that 1% of some modelled results from other studies, and is an order of magnitude lower than existing known extraction from the aquifer for water supply and irrigation. The study results have been used to provide clear, direct evidence into discussions with stakeholders as the technology and findings are simple to place in context, and easy to interpret. The results have prompted investigations into the surface water-groundwater interactions along main regional rivers and streams to derive similar hard estimates of total discharge, and therefore better understand the state of the aquifer system. The direct discharge measurements were time consuming, but had great value in stakeholder engagement.