Saltwater intrusion from an estuarine river: A field investigation

Abstract

Estuarine rivers provide critical pathways for seawater to travel upstream of the coast and salinize adjacent aquifers. However, this salinization mechanism (forthwith termed riparian saltwater intrusion) has received relatively little attention compared to saltwater intrusion (SI) at the coast. Time series measurements of river and groundwater freshwater head and specific conductance (SC), as well as horizontal river-aquifer hydraulic gradient were collected at transects of piezometers perpendicular to an estuarine river in Ōtautahi Christchurch, Aotearoa New Zealand. The uncertainties of freshwater head and hydraulic gradient were estimated using error propagation methods. Discrete Fourier Transforms were applied to river and groundwater freshwater head and SC time series data, which confirmed the tidal influence in both systems. Cross–correlation analyses of river and groundwater freshwater heads showed very strong relationships with varying time lags. Hydraulic gradients and river SC fluctuated with tides, resulting in the alternation of SI (increase in groundwater SC) and saltwater retreat (decrease in groundwater SC) with various time delays that may be driven by cyclic flow processes. While riparian SI occurred in most piezometers, the hydraulic gradient on the outside of the river meander was steeper than on the inside of the river meander resulting in less SI. Although positive hydraulic gradient (aquifer to river flow direction) occurred most of the time at all sites, land subsidence and climate change conditions of sea–level rise, increased drought, and decreased river flows could increase the occurrence of negative hydraulic gradient, which may result in increased groundwater salinization.