Report: TR 2005/57

Author: Jeremy Piper

Abstract

Over the past five years there has been an expansion of pasture irrigation in the Waikato Region, and in particular within the Reporoa Basin. Rivers and streams in the Reporoa Basin are fully allocated. With maximum allocation limits of 10-15% of the one in five year (Q₅) low flow, water users have started to target groundwater resources as an alternative water source. Through the development of groundwater and the perception that groundwater resources may become over exploited, community interest in current and future applications for pasture irrigation from groundwater is high.

This report describes the geology, groundwater chemistry, groundwater and surface water hydrology, groundwater age, water use and future groundwater allocation of the Reporoa Basin.

The geology comprises predominantly rhyolitic formations. The geology of the Basin edges is relatively simple with Taupo Ignimbrite overlying uplifted Rangitaiki Ignimbrite in the eastern Basin and Paeroa Ignimbrite in the western Basin. Rangitaiki Ignimbrite is welded and fractured. The central Basin is characterised by a complex sequence of pyroclastic Taupo and Oruanui Ignimbrites overlying lacustrine sediments of the Huka Formation. These in turn overly the ignimbritic Waiora Formation.

Groundwater flow in the Basin is consistent with topography although more subdued. There is a correlation between groundwater depth and well depth, indicating vertical groundwater recharge. Groundwater predominantly drains locally toward the streams within the Basin and ultimately the Waikato River. A large proportion of groundwater recharge from rainfall is intercepted by streams such as the Torepatutahi, Kaiwhitiwhiti and Waiotapu streams. Average groundwater flow velocities are estimated to range between 0.02 m d^-1 to 0.16 m d^-1.

Groundwater quality was analysed to investigate land use impacts, particularly elevated nitrogen concentrations. General chemical character was also related to geology in order to improve the hydrogeological understanding of the Reporoa Basin. Groundwater was sampled from 32 sites. Groundwater is largely sodium bicarbonate dominated, which is a reflection of carbonic leaching of the rhyolitic formation. There is some evidence of land use impacts, with elevated nitrate-nitrogen concentrations in groundwater sourced from several bores. Sodium and chloride are generally elevated in groundwater at sites with high nitrate-nitrogen concentrations. They also appear to be higher where it is thought that groundwater is influenced by geothermal processes.

Elevated arsenic concentrations were found to occur in groundwater in 20% of wells and within most of the aquifer formations sampled. Possible factors influencing arsenic concentrations are reducing conditions and the influence of geothermal processes.

Age determination of groundwater at eight bore locations within recharge zones using tritium and CFCs, indicate that mean groundwater residence time in the aquifer is between 11 years to 73 years. A spring of the Torepatutahi Stream within a groundwater discharge zone sampled in 2001, was found to have a mean residence time of 150 years.

Water balance model estimates show that in many of the Reporoa Basin catchments, groundwater flow is predominantly intercepted by large springs. These commonly are the source of streams within the Basin. There is limited groundwater resource available for future abstraction in many catchments, particularly the Kaiwhitiwhiti, Torepatutahi and Waiotapu catchments. The inference of limited groundwater for abstraction is based on aquatic and ecologic requirements as well as water balance model estimates.

The reported investigation provides the basis for seeking future more detailed work not included in this report such as groundwater modelling, and also provides much needed scientific information for the understanding and management of surface water and groundwater resources in the Reporoa Basin.

Water Resources of the Reporoa Basin [PDF, 1.2 MB]