Using fibre-optic distributed temperature sensing and heat modelling to characterize groundwater- surface water interaction in Whakaipo Bay, Lake Taupo, New Zealand

Abstract

The water quality of Lake Taupo is under threat. Agricultural development in the catchment has led to an excess of nutrients in the water systems. Groundwater is the primary link for the transport of nutrients between land and lake. The spatial distribution and groundwater seepages rates in Lake Taupo are still unknown.

This study used horizontal and vertical fibre optic distributed temperature sensing to characterize the groundwater inflow areas in Whakaipo Bay, Lake Taupo. During the summer the temperature of the lake water is warmer than the groundwater, 17 and 12 °C respectively. Horizontal temperature profiles of the lakebed were acquired to study the spatial differences of groundwater inflow zones. Lower lake bottom temperature is an indication for groundwater inflow.

Vertical temperature profiles with a spatial resolution ranging from 1 metre to 1 millimetre where acquired to get a general idea of the vertical temperature differences in the lake. The high resolution temperature profiles where combined with a numerical conduction-convection heat transport model resulting in a temperature based seepage meter. The seepage meter was used in three different temperature areas in the bay, a cold, a medium warm and a warm area in order to verify the differences in groundwater inflow rates.

The horizontal temperature profiles indicated spatial differences in temperature ranging from 13.7 to 18.5 °C. The three high resolution profiles were in agreement with the horizontal profile. The colder the temperature measured in the horizontal profile, the larger the groundwater seepage rate calculated with the numerical heat transport model.