16.3  Vulnerability assessment at a water source

A vulnerability assessment is used to determine the likelihood that potential contaminant sources in the drinking water protection area will degrade the source water quality. A vulnerability determination will include consideration of several factors including hydrological sensitivity, the source of contaminants, how these sources can be managed, and the condition of any water source protection.

Hydrological sensitivity means assessing how sensitive a water source is to contamination. Higher sensitivity ratings apply if geological conditions allow contamination to move quickly from its place of origin through the rocks and soil to water sources. Lower ratings apply when contamination moves more slowly.

Another factor is vegetation and surrounding land use. If the land around a water source has no vegetation it is more susceptible to contamination than a water source surrounded by land with thick vegetation. This is because plants and trees can act as a physical and biological barrier to pollutants.

You also need to look at the condition of source water structures. For example, at a well you need to check the well casing, joints, delivery structures and equipment to move water from the well and assess their integrity. ‘Integrity’ means the quality of design, construction, maintenance and state of repair of the structure.

Factors affecting vulnerability to contamination include:

• Drinking water wells located close to potential sources of contamination have more risk than wells located further away.
• Whether the potential source of contamination is from a single identifiable point which would be easier to manage but could have greater potential for major contamination than diffuse contamination spread over a wider area.