Geology and geochemistry of groundwater within the Taieri Basin, East Otago, New Zealand

The Taieri Basin, in east Otago, New Zealand, is a tectonic depression bounded in part by active faults. The basin is floored and surrounded by Otago Schist that is locally overlain by Cretaceous-Tertiary sediments and volcanic rocks. Basin fill is predominantly Quaternary gravels, sands, and silts derived from the Otago Schist, and these sediments are at least 150 m thick in places. Aquifers are hosted in the Quaternary sediments, principally gravels and sands. A wedge of estuarine silts and sands (c. 8000-5000 yr BP) extends for 18 km to the downstream outlet of the basin, where it is 25 m thick. Irregularly interfingered fluvial gravels and sands, interbedded with silts, make up the principal aquifers at the upstream end of the basin. This complex fluvial sequence dips beneath the estuarine wedge. The estuarine wedge acts as a confining layer for groundwater in the underlying aquifers, and artesian water bores occur in the lower reaches of the basin. Alluvial fans on the western side of the basin are conduits for recharge from adjacent schist ranges. Alluvial fans on the southeastern side of the basin have their toes truncated by active faults, and recharge from these fans is structurally inhibited. Ca, Mg, and bicarbonate contents of Taieri Basin surface waters and groundwaters indicate chemical interaction with calcite and chlorite of Otago Schist, in basement and/or aquifer sediment clasts. All waters have Na and Cl- contents dominated by rainout of NaCl in marine aerosols. Surface waters have lower Na and Cl- than groundwaters. Numerical modelling of groundwater flow at the upstream end of the basin can reproduce field-based inferences of flow patterns and together indicate southwestward flow beneath the estuarine wedge confining layer. This is significant because it implies low-concentration NO3+NO2 contamination of shallow aquifers at the upstream end of the basin will be carried to groundwaters in the southwest.