Immediately after the 2010 and 2011 quakes in central Canterbury farmers reported limited damage to wells, except for bent and cracked pipes and pumps near fault lines.
Research led by Aqualinc Research senior hydro-geologist Helen Rutter now shows groundwater levels changed dramatically in parts of the Canterbury Plains, ranging from shortages to surplus. Some farmers find it more difficult to abstract water.
A report Rutter wrote with Tim Ezzy at Environment Canterbury found bores, particularly artesian wells near the coast, showed a reduction in flow with some even ceasing to be artesian.
The changes often resulted in the wells no longer meeting the demand on them, with some bores supplying a quarter of their pre-quake amounts.
In other cases groundwater levels rose and it was difficult to throttle back the pumps sufficiently to remain within consented rates of take.
In a long-running study of the 7.1 Darfield quake in September 2010, Rutter found draw-down, the lowering of the water level in a bore when pumped, increased by up to twice the pre-quake amounts.
“The theory is that compaction or silting of sediments close to the bores made it harder for pumps in some areas to pull water to the surface at a certain flow rate – a relationship known as yield draw-down,” she said.
“When a bore became less efficient, increased wear and tear on pumping equipment and/or increased costs of pumping forced some farmers to redevelop their bores.”
At the same time there was evidence in some cases of greater connection between aquifer layers thought to be caused by vertical fractures in the sediment sequence.
Previous work suggested the fractures healed relatively rapidly. However, data collected and analysed from the Canterbury earthquakes suggested quake fractures had not healed in the next few years.
Rutter asked for observations from industry contacts including McMillan Drilling’s Iain Haycock, whose team has a range of rural work around Canterbury. Rutter said Haycock’s field observations dovetailed with hers.
Changes to groundwater levels were widespread and consistent across the northwestern (upper) side of the Canterbury Plains toward the western end of the Greendale fault, near Darfield. The most noticeable changes were in wells deeper than 40m.
The groundwater level rose in many areas by 1-10 metres after the Darfield quake and by up to 40m in extreme cases.
Immediately post-earthquake some bores began to flow artesian. Of the bores that showed a water level rise, some gradually returned to pre-quake levels while others continued to follow their pre-quake trend, at a higher level, and still remain higher than would be predicted based on climate and abstraction.
Rutter said it is possible water is moving differently through the aquifer network since the quakes.
In some cases springs on farms have emerged or disappeared, including in coastal Kaikoura after the quake in November 2016.
Some of the damage might be the result of damage to a sub-surface confining layer during shaking and/or liquefaction that allowed water to rise to the surface. Other emerging springs might have been old artesian wells that were sealed and in some cases then built on.
Earthquakes might have reactivated the wells though complicating factors were the possibility of a rise in groundwater levels in the area or cracked pipes beneath the surface.
Groundwater levels in some areas of the Canterbury Plains are now relatively high because of recent high rainfall but the underlying changes to the aquifer remain.
“Increased draw-down is likely to become a problem when groundwater levels reduce from their present highs. It is therefore important that any changes in yield draw-down are identified now,” Rutter said.
