GROUNDWATER AND NUTRIENT DISCHARGES IN THE GREATER LAKE
TARAWERA CATCHMENT
Toews, M.W.,1 White, P.A.,1 Tschritter, C.,1 Barber, J.2
1
GNS Science, 2 Bay of Plenty Regional Council
Introduction
A series of hydrogeological investigations was developed to assist policies that aim to reduce the
discharge of nutrients (nitrogen and phosphorus) to lakes in the greater Lake Tarawera
catchment, which contains eight lakes (Tarawera, Okaro, Rotomahana, Rerewhakaaitu,
Okataina, Okareka, Tikitapu and Rotokakahi). These investigations have included
hydrogeological characterisation with drilling, pump-testing, groundwater chemistry and
groundwater age dating (e.g., Thorstad et al., 2011). Then followed development of a 3D geologic
model and surface water budgets for catchments and lakes. This information allowed the
construction of 3D steady-state groundwater flow models and nutrient transport models that were
based on land-use scenarios.
Methods
A 3D geologic model was constructed for the region, which was translated into a finite difference
MODFLOW model to simulate groundwater flow. A conceptual flow model was characterised
from surface flows in streams and rivers to inform the groundwater flow model. Boundary
conditions of the model include: general head boundaries for simulating lakes; drain boundaries
for simulating streams; and well boundaries for specified-flux boundaries along the southeastern
edge of the model near Lake Rerewhakaaitu that allows flow from the model area to the east.
Five land use options were developed to consider nitrogen loading to groundwater and surface
water, including: forested, low-intensity agriculture, current land use, moderate expansion of highproducing grassland, and large expansion of high-producing grassland. These scenarios were
applied to a MT3DMS groundwater transport model. Budgets of groundwater flow and nutrient
discharge were assessed for zones around lakes.
Results
Surface geology of the greater Lake Tarawera catchment is dominated by volcanic units,
including ignimbrites and other pyroclastic deposits, as well as rhyolite lava domes and flows
(Figure 1). Most of these deposits were sourced from the Okataina Volcanic Centre which is a
large basin structure that includes most of the greater Lake Tarawera catchment.
Figure 1. Geological model of the greater Lake Tarawera catchment (Tschritter and White, 2014).
The groundwater flow model shows that groundwater provides the majority of inflows to most
lakes; Lake Rerewhakaaitu is an exception, as it is generally perched. Groundwater also flows
between lake zones. For example, groundwater flows into the Lake Tarawera zone from all
surrounding zones, i.e., Rotomahana (the largest source), Okataina, Okareka, Tikitapu and
Rotokakahi. The Tarawera River is the sole surface outflow from Lake Tarawera. Two pieces of
evidence indicate groundwater may flow out of Lake Tarawera down the Tarawera River valley:
Tarawera River gaugings that indicate a significant increase in river flow between Lake Tarawera
and below Tarawera Falls and the discovery of permeable, fractured rhyolite in a drill hole at the
Lake Tarawera outlet (Thorstad et al., 2011).
Intensification of land use will result in increasing nitrogen discharge to lakes, except the perched
parts of Lake Rerewhakaaitu. Most of any increase will come from the land in the west and lesser
amount from the south (i.e., the Lake Rotomahana catchment). Any increase in nitrogen
discharge to lakes will be mostly due to nitrogen flowing in groundwater, rather than streams.
Should intensification happen, then additional work is recommended to improve understanding of
groundwater catchment boundaries and aquifer properties in the western area.
References
Thorstad, J.L.; White, P.A.; Rosenberg, R.; van der Raaij, R. 2011 Lake Tarawera Groundwater
Investigation Phase 1, GNS Science consultancy report 2011/27.
Tschritter, C.; White, P.A. 2014 Three-dimensional geological model of the greater Lake
Tarawera catchment, GNS Science consultancy report 2013/155. 42 p.