Movement of water within the Waikanae shallow gravel aquifer and its interaction with the Waikanae River

Abstract

The Kapiti Coast is a dynamic, water-stressed environment. The past decade has seen a significant rise in issues relating to shortages in public water supply to the area. As the population continues to grow it puts an even greater pressure on the limited water resources. Until 2003, the Waikanae River supplied most of the potable water for Waikanae, Paraparaumu and Raumati. Since then, there has been the development of the Waikanae Borefield, a project undertaken to provide a supplementary supply when river levels are low. The borefield taps deep aquifers in the underlying gravel units. Private residences have, for some time now, been using shallow groundwater, with currently over 500 existing bores in Waikanae. There is, however, still considerable uncertainty about the dynamics of the shallow groundwater, sources and volumes of recharge, and its link with surface water features in the area. Waikanae has two significant shallow aquifers. One is formed in the coastal and marine sediments deposited as a result of coastal progradation following the most recent post-glacial thermal maximum. The other aquifer, and the focus of this study, has developed within fluvial and deltaic/fluvial gravels deposits from the Waikanae River over the last 6500 years. A third, but less significant shallow aquifer is present in the pre-Holocene deposits at the foot of the Tararua Ranges. The fluvial and deltaic/fluvial gravels are in direct contact with the Waikanae River. This leads to a significant hydraulic connection between the shallow aquifer and river. This interaction is confirmed by concurrent river gaugings, shallow groundwater elevation data, chemical analysis of the river and groundwater, and electrical resistivity measurements. The shallow gravel aquifer has significantly higher hydraulic conductivity than the adjacent shallow aquifers. It also discharges a significant volume of water into the Waimea Stream, maintaining the stream's base flow throughout the year. Groundwater also potentially flows from the aquifer back into the Waikanae River west of Jim Cooke Park. There are indications of a significant hydraulic connection between the aquifer and the underlying pre-Holocene deposits. Groundwater elevation data show that the aquifer is recharged in the east by the Waikanae River and the adjacent aquifer in the pre-Holocene deposits. Chemical analysis shows that the signature of this water persists through the aquifer predominantly unchanged. It is likely that precipitation also constitutes a significant volume of the total recharge to the aquifer but its chemical signature is likely to be very close to that of the other sources of recharge water. Water chemistry indicates that there is a hydraulic connection between this aquifer and the shallow aquifer in the coastal and marine sediments to the west. The thickness and hydraulic conductivity of the aquifer varies significantly across its surface extent. This creates complications when modelling the aquifer. It may therefore be difficult to calculate a sustainable groundwater extraction rate from the aquifer. This difficulty is compounded by uncertainty surrounding the links between the shallow gravel aquifer and adjacent aquifers. The direct connections between the aquifer and the Waikanae River and Waimea Stream mean that over-extraction of groundwater would have adverse effects on these environments. However, since extraction is already taking place, attempts to quantitatively investigate sustainable use of groundwater in the aquifer are essential.