What is groundwater?
Groundwater is rainwater that has travelled through the soil to underground aquifers - areas of fractured rock or porous sediments such as sand or gravel.
Wells pump groundwater from aquifers. We use it for drinking and in agriculture, horticulture and industry.
Groundwater in Northland
Groundwater in Northland is highly changeable in both quantity and quality - depending on the geology of the aquifer system.
Our main aquifer systems exist in the:
- Kaikohe basalts;
- Whangārei basalts; and
- Aupōuri sands
There are also many small sand and gravel coastal aquifers in Russell, Matapōuri and Taipā, and less productive greywacke aquifers throughout the region. Rainfall is the main source of replenishment for our aquifers.
Groundwater tends to refill during winter due to heavier and more consistent rainfall. Less rain in summer has little impact on groundwater levels, but a dry winter can be a problem. What mainly affects late summer groundwater levels is the length of time since the last significant rainfall.
On sand country, rainfall filters quickly into the ground. This refills the groundwater rather than contributing to stream flows. For example, many streams on the Aupōuri Peninsula have little or no base flow. In sand aquifers along the coast, salt-water can leak into the groundwater when there is over-use or the resource is low.
Our region’s changeable geology has a major influence on flow systems. Some types of rock allow water to pass through more easily than others. In Northland, the fractured basalt rocks absorb rainfall easily and water is slowly released through springs. This slow-release sustains the flow during dry periods.
Catchments that don’t absorb water easily soak up less rain. This leads to less water available in storage. Flows from these catchments tend to dry up more quickly during dry summers.
The main aquifers we study and monitor are generally either basalts, quaternary sands and gravels, volcanic deposits, geothermal or greywacke.
Regional council Hydrology and Groundwater Monitoring Officer, Sandrine Le Gars sampling groundwater.