Mapping the vulnerability of groundwater to saltwater intrusion from estuarine rivers under sea level rise

Surface water bodies connected to the ocean such as estuarine rivers can act as pathways for saltwater intrusion (SI, i.e., the displacement of fresh groundwater by saltwater in an aquifer) far inland of the coast, presenting one of the earliest risks associated with relative sea level rise (SLR). However, SI vulnerability mapping approaches have largely focused on SI from the coast and do not consider estuarine surface water bodies, except for GALDIT-SUSI (Kazakis et al., 2019); a weighted indexing approach designed for regional-scale mapping applications using a Geographic Information Systems (GIS) framework. However, GALDIT-SUSI is subjective in ranking the importance of factors that can contribute to SI and combining these into a vulnerability index. A less subjective approach to assessing SI vulnerability than weighted indexing methods involves the use of physically-based analytic solutions such as Strack (1976), but these have not been applied in a GIS framework or along estuaries and rivers previously. Here, these analytic solutions, surface water salinity, and surface water-groundwater freshwater head gradients are used in the development of a new SI vulnerability tier system. This new approach was applied in the low-lying coastal city of O over bar tautahi Christchurch, Aotearoa New Zealand, along 70 km of coastal, estuary, and river margins under current sea level and SLR using GIS, then compared to GALDIT-SUSI. The main advantage of the SI vulnerability tiers method is that it considers surface water behaviour, e.g., increased saltwater encroachment up rivers under SLR, which exposed new areas of the aquifer to saltwater and increased SI vulnerability upstream. In contrast, GALDIT-SUSI does not consider surface water conditions and accounts for topography and groundwater level as the main SI vulnerability drivers in this application. The SI vulnerability tiers proposed here are more theoretically robust than GALDIT-SUSI and provide a physicallybased, large-scale, relatively low-budget and rapid screening tool to highlight areas most vulnerable to SI under current and future conditions for further monitoring and management; a gap of national and international relevance.