Groundwater sits in the spaces between soil and sediment particles, and within rock fractures. In many parts of the country, groundwater is used as a key source of water for drinking, industry, and agriculture. Most groundwater extracted in New Zealand is taken from coastal aquifers.
In some coastal areas, the water table is not far below the ground and is connected to the sea. As the level of the sea rises, the water table will rise in these areas, which can lead to surface ponding in some places and more extensive flooding after heavy rain. A higher water table can also damage roads, pipes, cables, and the foundations of buildings. If sea level rise makes groundwater more saline, it can reduce the amount of freshwater that can be extracted from coastal aquifers. Interactions between groundwater and seawater are highly localised and complex, and it is uncertain how groundwater in many places will respond as the sea rises. Predicting impacts on aquifers is made particularly difficult by the ‘invisibility’ of groundwater and a scarcity of information. Those places where groundwater is linked directly to the sea are most likely to be affected.