Report: TR 2007/39

Author: John Hadfield

Abstract

The quality of water in Lake Taupo is under threat, largely as a result of increasing nitrogen from farming in its catchment. Groundwater flow and contaminant transport modelling of the northern catchment using Modflow and MT3D indicate a substantial lag (~250 years) before the full impact of current land use would be realised. Pre-development nitrogen flux from groundwater is estimated at about 65 tonnes annually with about 3,500 tonnes of nitrogen stored in the groundwater system. After 35 years of ‘current’ land use, annual flux from groundwater is estimated to be nearly 175 tonnes at which stage about 10,000 tonnes of nitrogen may be stored in the system. Under simulated equilibrium with current land use, about 300 tonnes of nitrogen flux from the northern groundwater system is predicted annually. Some 18,000 tonnes of nitrogen would be stored in the aquifer. Assumptions include conservative nitrogen transport and porosity reflecting fracture flow in the deeper western model domain. Modelling results are broadly consistent with water age dating and stream water quality information. The extent of groundwater denitrification, currently thought to be minor, is an aspect of ongoing research. The proposed reduction of the manageable load to the lake was found to provide useful mitigation in the northern catchment.

Groundwater Flow and Nitrogen Transport Modelling of the Northern Lake Taupo Catchment [PDF, 826 KB]