Characterising groundwater-surface water interaction using fibre-optic distributed temperature sensing and validating techniques in Whakaipo Bay, Lake Taupo, New Zealand

Abstract

Lake Taupo, the largest lake of New Zealand, is blessed with good water quality. However, over the last decades the natural system is under pressure from increased dairy-farming. These agricultural activities primarily export nutrients to the groundwater system, which transports it to the lake. This can possibly have detrimental effects on lake ecology, tourism and animal and human health. Since the sources of nutrients around the lake are numerous and diffuse, some of the methods that have been developed under the SMART Aquifer Characterisation Programme are possibly suitable for developing rapid and cost-effective characterization of the groundwater systems around the lake.

This thesis is a follow-up study of Meijer (2014) and will characterize groundwater-surface water interaction using fibre-optic distributed temperature sensing and validating techniques in Whakaipo Bay, Lake Taupo. To map and quantify the seepage flows into the bay, high-resolution spatial and temporal temperature measurements were conducted and linked to direct seepage measurements and a heat transport model. The seepage areas detected by horizontal distributed temperature sensing were validated using seepage flow calculation by vertical temperature sensing. These flows were extrapolated across the shallow part of the bay and the total seepage flow was compared to water balance studies. Ultimately, a direct way was investigated to detect seepage areas and calculate seepage flows by horizontal distributed temperature sensing.

Temperatures on the sediment-water interface near the shore of Whakaipo Bay vary between 15 and 22 °C. The cold spots are characterised by high seepage flows and rates differ notably throughout the bay. The seepage rates measured by vertical DTS range vary between 0.253 cm³/m²/s and 0.736 cm³/m²/s, whereas the rates by the seepage meter are factor 4 smaller at high-flow seepage locations and almost similar at low-flow locations. Although the seepage meter might underestimate the high-seepage flows, the heat flux modelling results approximate the vertical DTS flow rates. The average flow for the area shallower than 6.5 meter depth is 0.141 m³/s, which is 41% of the seepage component in the water balance. However, it is unlikely that the seepage areas examined in this research are the only major source of seepage. Seasonal variability might affect the seepage as well.