Boreholes reveal potential for springs to emerge in South D

GNS Science’s principal scientist Dr Simon Cox shows off a core of earth from South Dunedin,...
GNS Science’s principal scientist Dr Simon Cox shows off a core of earth from South Dunedin, revealing mainly thick, clay-rich material. PHOTO: PETER MCINTOSH
Pulling up a core sample of the earth below South Dunedin pavements is like pulling up a piece of spaghetti as long as a football field, said GNS chief scientist Dr Simon Cox.

One core sample, taken from Kettle Park, which sits on top of dunes, was laid out in a GNS workshop by Dr Cox for the ODT to admire. The top layers were predictably sand, but 14m down there was 8,500 year-old silt, and below that there was metre after metre of thick, clay-rich earth — laid down when South Dunedin was an ancient estuary — with only rare layers of sand.

It is stinky stuff when you pull it up — "sulphury, like a swamp", Dr Cox said.

At 50 metres down, bedrock was hit.

It’s important information for planning South Dunedin’s future. The area’s water table rises and falls significantly in response to rain, but can also be affected by the tide, due to seawater pushing in the side.

Rising sea level means more seawater is expected to permeate — get into — South Dunedin’s earth, joining the groundwater and contributing to pushing the water table further up.

However, earth composition determines its permeability. It’s harder for water to get into clay-rich earth than get into sand. Boreholes away from the dunes are typically full of the former.

At 35 bore holes across South Dunedin, automated sensors have been checking water pressure every fifteen minutes. Typically, high tides are raising the water table in the boreholes by between 1 millimetre and 10 centimetres.

Greater upwards movement has rarely been monitored and only at two boreholes. One of these, unsurprisingly close to the dunes, records a much higher 40cm shift in groundwater at every 1.8m high tide. It is a rare exception — while also proving the rule that there is a connection between the ocean and the groundwater.

Significant tidal-driven rises of the water table are not as widespread as originally feared, so unlikely by themselves to cause the arrival of emergent groundwater springs in most places.

However, the other problem remains — the rising sea inhibiting rainwater’s escape route to the sea using gravity.

"Springs will eventually happen across the suburb," Dr Cox said.

The monitoring network and water table modelling enables the forecasting of where and when the water table will be as high as the surface and where and when springs might emerge first. This, in turn, enables a focus on "areas where planning and mitigation may be necessary soonest," Dr Cox said.