Likelihood of offshore freshened groundwater in New Zealand

Abstract

Offshore aquifer research is an emerging field that is becoming increasingly important as population growth and climate change put pressure on coastal water resources. One of the largest reserves, globally, of offshore freshened groundwater (OFG) was recently identified off the South Island of New Zealand. This has highlighted the potential for OFG elsewhere in New Zealand. This study aims to: (1) screen for New Zealand coastal aquifers most likely to contain OFG and, (2) document evidence for OFG in New Zealand. An OFG-likelihood rating scheme was developed as part of the study. An application of the rating scheme used survey responses from regional councils responsible for groundwater management, in combination with national and regional-scale technical documents. The rating scheme was found to be a simple and transparent first-pass approach for highlighting areas where OFG is more or less likely at the national scale. Results are presented in a map showing the likelihood of OFG around the New Zealand coastline. Regions with aquifers where OFG likelihood is high include Greater Wellington, Canterbury, Tasman, Hawkes Bay and Marlborough. Aquifers in these regions are associated with major fluvial depositional systems, including glacial outwash gravels. Despite high dependence on groundwater in these regions and extensive groundwater extraction near the coast, there are no major reported incidences of seawater intrusion, suggesting offshore groundwater may be augmenting onshore extraction.