In light rainfall the water moves relatively slowly through small pores in the soil and in these conditions bacteria, which are particles, are readily filtered out.
As the soil becomes more saturated and rainfall is higher intensity, then the larger pores are operational, transport is faster and the filtration of bacteria is less efficient.
As the soil becomes fully saturated the transport velocity becomes exponentially faster.
Q. How long can campylobacter survive in an aquifer?
A. For a significant contamination event - effluent getting into the aquifer - there could still be quite a lot of campylobacter remaining after a week, depending on groundwater velocities for the aquifer in question.
For example, with an alluvial gravel aquifer it could range from 5 to 60 m/day so the campylobacter could travel 50 to 400m in a week.
Dr Paul White, senior groundwater scientist, GNS Science
Q. How does water get into aquifers and how long does it take?
A. The water that gets into aquifers is termed "recharge".
This recharge comes from rainfall on the land that lies over the aquifer, rivers, and other aquifers.
Rainfall recharge occurs where rainfall on the ground soaks into the soil and then flows to an aquifer.
River recharge occurs where river water flows into the bed of a river and then flows into an aquifer.
Recharge from other aquifers occurs where the other aquifer: is in hydrogeological connection; has larger water pressures; and has a positive water budget, i.e. outflows are larger than inflow.
Q. What effect do extreme events, such as drought or heavy rainfall, have on aquifers? Does it affect how quickly they recharge and the risk of contamination?
A. Drought typically reduces groundwater levels as aquifer storage reduces during the event. Heavy rainfall typically increases groundwater levels and increases groundwater storage.
However, heavy rainfall events in summer may not result in rainfall recharge where the soils are very dry before the event.
Generally, little or no rainfall recharge to groundwater is measured in aquifers located on the east coasts of the North Island and South Island.
Generally, most rainfall recharge to aquifers located on the east coasts of the North Island and South Island occurs in winter.
Therefore, the risk of contamination to groundwater, associated with rainfall recharge, is larger in winter.
Dr Helen Rutter, senior hydrogeologist, Aqualinc Research Limited
Q. How are surface waters connected to groundwater?
A. Connections between groundwater and surface waters can be very direct in some cases.
An example would be a shallow well located on a riverbank: the water taken from the well would be almost entirely river water.
Deeper wells and those located further away from surface waters would be likely to have a less direct connection with the surface water and more of the water is from land surface recharge.
In some cases, we can monitor a stream when pumping groundwater from a well and observe a change in flow in the stream.
This implies a close connection between surface water and groundwater.
Q. What effect does drought have on recharge?
A. Recharge usually occurs during winter months, when the soils are close to being saturated, temperatures are low, and there is little soil evaporation or transpiration from plants.
If there is a lack of winter rainfall, then recharge to groundwater will be limited. We have seen this in Canterbury over the past three years.
