Riparian saltwater intrusion: implications, dynamics, and vulnerability under sea level rise : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Surface water bodies connected to the ocean, such as estuarine rivers, are crucial pathways for saltwater intrusion (SI, i.e., the displacement of freshwater by saltwater in an aquifer) far inland of the coast, presenting one of the most immediate challenges posed by relative sea level rise (SLR). However, SI from estuarine rivers (forthwith riparian SI) has received little attention compared to SI at the coast. Shallow groundwater salinisation can prematurely deteriorate co-existing municipal assets such as subsurface infrastructure and roads, salinize deeper wells under downward hydraulic gradients, impact wastewater treatment plant operations, and create unfavourable living environments for fresh groundwater-dependent species. This thesis aims to improve the understanding of riparian SI implications, dynamics, and vulnerability under SLR in an urban setting. This research was undertaken in the low-lying coastal city of Ōtautahi Christchurch, Aotearoa New Zealand, mostly built on drained swampland. In the second chapter, shallow groundwater salinity was measured at high resolution, mapped, and interpolated across the city. Areas with the highest groundwater salinity were centred on a spit of dunes between an estuary and the ocean and inside the meander of an estuarine river. The municipal assets located within brackish groundwater areas were highlighted, including: (i) stormwater and wastewater pipes made of steel-reinforced concrete that are vulnerable to premature damage when exposed to chloride, particularly under wetting-drying cycles and (ii) 41 parks and reserves totalling 236 ha, where salt intolerant species that are exposed to groundwater can be at risk. Additionally, the salinity and chloride concentration of wastewater have not been regularly monitored in the city’s wastewater treatment plant, thus presenting a vulnerability. The vertical hydraulic gradient between the shallow aquifer and the gravel aquifer underneath was upward at two locations near brackish groundwater zones. Therefore, the deeper aquifer used for water supply was not at risk of downward salinisation during the period assessed. This chapter contributes to the literature by linking shallow groundwater salinisation to its implications on municipal assets, which can become increasingly vulnerable under climate change and intensifying anthropogenic pressure. The third chapter investigates riparian SI dynamics in the field via time series measurements of river and groundwater freshwater head and specific conductance (SC) at two transects perpendicular to an estuarine river, focused on a dry summer period. Discrete Fourier transform confirmed the tidal influence in the river and groundwater freshwater head and SC, with the most saline bore showing the strongest tidal signal. Cross-correlation analysis of river and groundwater freshwater heads showed very strong relationships with varying time lags across sites. River-aquifer freshwater head gradients fluctuated with tides, resulting in a cycle of SI (increase in groundwater SC) and saltwater retreat (decrease in groundwater SC) with various time delays, which can be driven by cyclic flow processes. Steeper river-aquifer freshwater head gradients and fresher groundwater were found on the outside of the river meander compared to the inside. Future climatic and anthropogenic stresses can increase the occurrence of negative freshwater head gradients, which can result in more groundwater salinisation. The fourth chapter mapped riparian SI vulnerability by creating a new SI vulnerability tier system based on surface water salinity, surface water-groundwater freshwater head gradients, and the physically-based analytic solutions of Strack (1976). This new approach was applied in a Geographic Information System framework along 70 km of ocean, estuary, and river margins under current sea level and SLR scenarios. Then, the method was compared to GALDIT-SUSI; a weighted indexing approach that considers surface water bodies as salinity sources but is highly subjective. The main benefit of the SI vulnerability tiers is that it accounts for surface water conditions, e.g., increased river salinity further upstream under SLR, exposing new stretches of the aquifer to saltwater, and increasing SI vulnerability upstream. In comparison, GALDIT-SUSI does not consider surface water behaviour and regarded elevation and groundwater level as the main factors contributing to SI vulnerability. The SI vulnerability tiers approach is more theoretically robust than GALDIT-SUSI. It provides a physically-based, large-scale, relatively low-budget, and rapid screening tool to categorize SI vulnerability from brackish and saline surface water bodies under current and future conditions for further monitoring and management—a gap of national and international relevance.