Groundwater exploration in the Ida Valley

Abstract

Exploration for groundwater resources was carried out in the Ida Valley during 2011. In the valley, existing water supplies are mostly sourced from the water races, and there is a great deal of pressure placed on surface water resources. In contrast, there is little use of groundwater, mainly because previous drilling has had limited success. The availability of aerial geophysical survey data has given the Otago Regional Council (ORC) the opportunity to locate potential groundwater sources with greater confidence. This study has successfully used the aerial geophysical data to delineate groundwater drilling targets in the Ida Valley. Exploratory drilling into the Tertiary sediments has shown the existence of productive groundwater bearing horizons in the south-western part of the Poolburn catchment. Productive sediments comprise medium to coarse grained sand derived from schist fragments. These sediments represent fan deposits of colluvium and alluvium sourced from the Raggedy Range. The fans of coarser material lie as lenses up to 3m thick within reworked Bannockburn Formation silty sands and are typically confined. Pumping tests were carried out on all of the bores drilled in the Tertiary sediments. These tests varied from simple recovery tests following bore development, to a comprehensive 24-hour pumping test at Meade Road. Transmissivity values range from 10-40 m²/d for the reworked silty sand. Coarser sand horizons in the fan deposits have transmissivity values of up to 80 m²/d. Greywacke gravel, presumably of reworked Hawkdun Formation, intercepted in a bore at Idavale, gave a transmissivity value of 324 m²/d. All the Tertiary sediment exploration boreholes are capable of providing farmers with at least a domestic and stock water supply. A small-scale irrigation supply is possible from the higher yielding boreholes. Specific capacity values measured after 100 minutes pumping ranged from 16 to 41 l/min per meter of drawdown. Water quality results show that recharge and groundwater circulation is occurring in this part of the valley. Discharge for the Ida Valley is limited by the elevation of the Poolburn Gorge, which is around 395m. Water quality is expected to deteriorate rapidly in groundwater below this elevation. Two exploratory bores were also drilled into structural targets in the schist bedrock. The rock was highly fractured, but wireline packer testing showed that the permeability of the schist is low. The upper 20-30m of the schist is likely to provide up to 5 l/min/m, which is sufficient for domestic or stock water supply. The high metamorphic grade of the schist renders it an unlikely source for significant volumes of water. This study has identified areas where there is potential for higher yielding groundwater resources in the Ida and Manuherikia valleys. We believe that groundwater for stock and domestic supply can also be sourced from finer grained areas of reworked Tertiary sediments. However, access to groundwater in these finer gained deposits depends on careful screen selection and the installation of a sand-filter pack.