Groundwater is water under the ground’s surface, in the spaces between rocks and soil. The proportion of groundwater increases the further down we look, until we reach what’s known as the “saturated level”. The top of the saturated level is called the water table.
Groundwater drains from the ground through springs, lakes, rivers, streams, drains, manmade wells, and the sea. It tends to flow under the ground’s surface from higher areas to low-lying ones. The speed it moves at depends on how the ground in the area is made up (whether it’s rock, soil, or sand), and how much water that ground material already holds.
Sometimes our soil becomes saturated and groundwater can’t move through the ground quickly. This causes surface ponding that looks very similar to flooding.
When it rains, water mostly flows into stormwater drains and out to the ocean through our stormwater network, or soaks into the ground, becoming groundwater.
Groundwater levels rise and fall as seasons change, and in response to heavy rainfall.
In low-lying areas, the water table’s usually near surface. When it’s very wet the extra groundwater can mean the water table rises further, causing surface flooding. Groundwater surface flooding is different from stormwater flooding in that it can stick around for some time, and can be more difficult to manage. Flooding from raised groundwater levels can remain on low-lying land for a long time.
In Kāpiti our groundwater levels can change a lot, due to how we sit between the mountains and ocean, the type of natural environments we have, and the natural places where groundwater discharge.
Flooding from raised groundwater (sometimes called groundwater inundation) can have a significant impact on private property, and on Council’s land, infrastructure, and work programmes. It causes:
- damage to road and footpath surfaces
- corrosion of pipes
- decreased ability of stormwater networks to cope
- infiltration to septic tanks
- deaths of trees that aren’t suited to excess water for long periods.
High groundwater levels can also prevent or delay access for maintenance, repairs, renewal work and capital works projects, and impact health where water is sitting under houses for a long time with no ability to drain.
Stormwater devices like soakpits, ponds and wetlands are also less able to deal with excess water, leading to ponding issues.
Groundwater can’t be pumped away in the same way stormwater can be – there’s often nowhere to pump it to, as it will only return to the lowest lying areas.
Groundwater levels will drop given time, though; although this could take some time. If there’s standing water on your property, please be patient while it recedes. You can call a plumber or drainlayer for more advice.
To improve our stormwater network and help the situation, we’re carrying out maintenance of our open channel network, renewal and repairs, investigating pipes, and continuing to work on our stormwater capital works programme.
Surface flooding from groundwater is predicted to become more significant in the future due to both increased rainfall volumes, and sea-level rise associated with climate change.