Dynamics of the saline interface of the shallow unconfined aquifer at Kapiti

Abstract

This study used electrical resistivity, together with electrical conductivity and water level measurements, to locate and examine the dynamics of the saline interface at Paraparaumu Beach. Various resistivity methods were used to provide 'images' of the geo-electric structure of the saline interface and surrounding area. These included: horizontal traverses, to locate the position of the saline interface; and vertical traverses between bore holes, to determine the vertical nature of the interface. The dynamics of the interface, and changes in its characteristics, were then quantified with respect to tidal fluctuations. Resistivity traverses at different locations along the coast showed that the saline interface is located beneath the beach face at Paraparaumu. The interface is a zone of mixing where the fresh and saline waters meet rather than a discrete boundary. The shallow freshwater aquifer terminates beneath the foredune. The location of the seaward limit of the freshwater in the aquifer was confirmed using both cross-borehole resistivity and conductivity measurements. A significant decrease in salinity occurs between the high tide mark and the base of the foredune opposite Mahana Road. The characteristics and location of the mixing zone, however, vary with distance south of the Waikanae estuary. The 'reservoir' of brackish water in the estuary plays a significant role in determining the site-specific characteristics of the saline interface, particularly the location and shape of the mixing zone. The resolution of the resistivity techniques did not allow the quantification of any landward movement of saline water over a single tide cycle. However, some changes in response to wider tidal fluctuations were observed. Cross-borehole resistivity showed significant changes during spring tides caused by increased wave run-up. No changes in resistivity were observed over neap tides. Significant changes in pore fluid conductivity were observed in all three bores during both neap and spring tides. Water level data from three bores showed the sensitivity of the saline interface to sea level, tidal range, and precipitation. There is an overall draining of the coastal aquifer during neap tides, and recharge during spring tides. Any rise in sea level will therefore affect water levels, and by implication salinity and water quality, further inland than at present. This could cause an overall reduction in freshwater availability along this section of coast of up to 5.2 million m3.