A: Dr Helen Rutter, senior hydrogeologist, Aqualinc Research Limited says:

The potential for groundwater contamination is complex and determined by – how much contaminant there is, the rate it’s being leached downwards, how long the contaminants will take to get to the water table, where they enter the system, the aquifer vulnerability, how fast the water is moving and the water system’s removal capacity.

From the land surface, pathogens will be transported through the soils and sediments by recharge.

In many systems, this is relatively slow flow, and pathogens may die off or be reduced in concentration before they can migrate to any significant depth.

However, this is a function of the type of soil and sediments and rate of recharge.

For example, if there is a pathway or crack for rapid flow of water then they can quickly be transported to depth.

These rapid pathways might be formed through natural conditions, such as old root channels, pathways or cracks, or through artificially created pathways such as old wells that do not have a protected well head or are open.

There are also some sources of contaminants that bypass the soils and shallow sub-surface, such as septic tanks or faulty sewers, which, if they leak, have a more rapid pathway to access groundwater.

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.

Source: Science Media Centre