Waimea Plains aquifer structure as determined by three-dimensional computer modelling

Three-dimensional computer modelling of 274 lithological logs in the Waimea Plains identifies three major aquifer structures. The first is a near-surface structure that covers most of the Waimea Plains and estuary and is consistent with the Appleby Gravel Unconfined Aquifer, the Hope Gravel Unconfined Aquifer and parts of the Upper Confined Aquifer. The second occurs in the vicinity of the Wairoa and Wai-iti rivers and is consistent with part of the Upper Confined Aquifer. The third is a deeper structure that occurs east of the Waimea River and under the estuary and is consistent with the Lower Confined Aquifer. The computer model indicates that the near-surface aquifer is possibly linked to the deeper aquifer to the east of the Waimea River and by two permeable zones to the west and south-west of Richmond Township. Elevated nitrate concentrations in the Lower Confined Aquifer west and south-west of Richmond could be explained by recharge and chemical transport through these zones between the Unconfined Aquifer and the Lower Confined Aquifer.