The saltwater-freshwater interface, Te Horo Beach, Kapiti Coast

Abstract

Urban and rural development on the Kapiti Coast is putting increased pressure on the region's groundwater resources. Coastal communities such as Te Horo Beach, which rely on bores for a potable water supply, are at risk of seawater intrusion if groundwater abstraction exceeds recharge. The principle aims of this study are to define the boundaries of the unconfined aquifer at Te Horo, and to determine the characteristics of the saltwater interface. In addition, the study evaluates the effectiveness of earth resistivity methods for saltwater interface studies. The unconfined aquifer at Te Horo is composed of postglacial Holocene sediments that reach a maximum thickness of 30m near the coast. Sedimentary units consist of marine and dune sands, fluvial gravels, and peat beds. Marine sands lie beneath the coast, and form the medium for the saline interface. The hydraulic conductivity of these sands has been determined as 2.5m/day, and the mean hydraulic gradient has been estimated as 0.006. Together these give a discharge of 0.375 m2/day, and a throughflow of~3350 m3/day, for the unconfined aquifer. The nature of the saline interface has been characterised using earth resistivity profiling. The results show that the interface in the vicinity of the Te Horo Beach settlement has intruded more than 10m further than in undeveloped areas. Resistivity profiling has been successful in defining subsurface areas of higher salinity, although resolution is lost for earth resistivities below 32Ωm. This is because the groundwater in this area has high concentrations of dissolved iron and manganese (i.e., 4.6 mg/L and 0.12 mg/L). A resistivity formation factor has been derived to allow the pore fluid resistivity to be estimated for future coastal surveys. The results of the resistivity surveys are supported by water quality data, which show evidence of saltwater mixing. Bores on beachfront properties at Te Horo Beach settlement have concentrations of up to 1% seawater. The Glover analytical solution was used in this study to predict the theoretical location of the saline interface, and to understand its behaviour. The predicted interface curve, however, does not match that observed on the resistivity profiles. Despite this, the model is a useful reference point for understanding the dynamics of the interface. Analysis of the model assumptions indicates that this is because dispersion, rather than advection, is the dominant salt transport process at Te Horo. The effect of dispersion is to steepen the interface shape, and to extend the area affected by salinity inland. Resistivity profiles indicate that the shape of the interface is also controlled strongly by tidal motion, particularly within the top half of the aquifer. The effect of tidal motion is to increase salt dispersion, and to "push" the shallow upper half of the interface inland. This has resulted in an "inverted interface" at Te Horo, consisting of a perched lens of saline water situated within the upper 10-15m of the aquifer. Analysis of the extent that salt water will disperse in response to tidal motion supports the observation that the interface at Te Horo beach settlement has intruded at least 10m beyond its natural location. This suggests that saline intrusion is occurring at Te Horo beach settlement in response to excessive groundwater pumping.