Assessing seawater intrusion vulnerability associated with sea level rise in Christchurch, New Zealand using GIS-based methods

Abstract

Seawater intrusion (SWI) is the landward movement of the seawater-freshwater interface in coastal aquifers. Causes of SWI include groundwater pumping, sea level rise, reduced recharge and land drainage. Christchurch aquifers provide one of the highest quality drinking water sources in the world, which local residents completely rely on for critical needs. In this study, a qualitative GIS-based method called GALDIT (Lobo-Ferreira et al., 2007) was used to assess SWI vulnerability in the shallow confined Riccarton Gravel aquifer in Christchurch, under different sea level rise (SLR) scenarios. To overcome limitations of this method, the analytic solutions of Morgan and Werner (2015) were developed and applied within a GIS framework, for the first time. Both methods were applied based on the following scenarios: current sea level, 1 m SLR with fixed head condition, and 2 m SLR with both fixed head and fixed flux conditions. Both methods showed that the Riccarton Gravel aquifer was most vulnerable to SWI in the locations of Brooklands, Woolston and Ferrymead. The differences between the two methods and the implications of the results in the local Christchurch context are discussed. The analytic solution was able to quantify SWI vulnerability in greater detail by determining the change in the theoretical seawater wedge toe position along the coast under SLR scenarios. However, neither the analytic solution nor GALDIT account for the possible offshore extension of the Riccarton Gravel aquifer and this is a limitation of both approaches.