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Further investigation of direct groundwater seepage to Lake Taupo

TR 2005/34
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Report: TR 2005/34

Author: Max Gibbs, John Clayton, Rohan Wells (NIWA)

Abstract

Environment Waikato commissioned NIWA to undertake further investigations of direct groundwater seepage into Lake Taupo through Whangamata Bay and Whakaipo Bay. This investigation is a follow-up study to a masters degree study by Ross Hector in 2004. The original water balance by Hector indicated that shallow groundwater accounted for only about 5 percent of the residual water input to these catchments after allowing for stream flow and evaporation. The suggestion was that the remaining groundwater was entering the lake as deep seepages or springs.

This study found that there were areas of deep groundwater inflow within the depth range of 2 m to 6.5 m and that these were as diffuse seepages rather than discrete springs. The size of seepage areas varied on a range of scales. Small patches (cm2) of seepage were dotted over larger areas (m2) which were found in zones that covered several hectares of lake bed. Inshore and further offshore from these there were no apparent seepages. Flow estimations indicate deep groundwater inflows totalled about 0.46 m3 s-1 in Whangamata Bay and about 0.24 m3 s-1 in Whakaipo Bay. These inflow estimates have potentially large errors due to the variability of seepage within the groundwater zones and the degree of bay-wide extrapolation, but indicate that almost all of the groundwater unaccounted for in the Hector water balance can be attributed to direct deep groundwater seepage.

It was found that the groundwater inflow was colder than the overlying lake water and thus flowed as a thin layer down the slope of the lake bed, or pooled in depressions. The nutrients in the deep groundwater were being utilised by benthic algae and the lake bed in the areas of inflow was covered with thick algal mats. Nutrient uptake by benthic algae effectively removed a substantial proportion of the dissolved reactive phosphorus and dissolved inorganic nitrogen from the deep groundwater inflows.

Because of the temperature-induced density effect initially holding the deep groundwater seepage close to the lake bed and hence protecting it from wave-induced mixing, a high level of nutrient uptake is considered likely throughout the year. However, as this study was conducted in late summer, it is uncertain what the seasonal effects will be on the level of nutrient uptake by the benthic algal mats, especially in winter.

If seasonal effects are minimal, the deep groundwater inflow is unlikely to constitute a major direct nutrient source to the lake water column. However, while the nutrient load entering the lake via the deep groundwater is not immediately available to support phytoplankton growth in the lake water column, they will eventually be recycled from the sediments after the algae decay. Other in-lake processes will then determine their fate.

Further Investigation of Direct Groundwater Seepage to Lake Taupo [PDF, 574 KB]