Teacher background information
What is a catchment?
A catchment area is a surface area from which runoff flows. A catchment area might include a lake, a reservoir (or dam), a stream or any other water body and areas where water soaks into the ground and recharges as groundwater.
Pollution of catchments
There are many ways in which human activity can impact on the water draining from a catchment. This includes homes and gardens, agriculture, industry, nature reserves and transport.
Many have a negative impact on water quality, such as nutrients from agricultural areas, waste from industrial areas, oil and fuel from roads and wastewater from homes. Everyday activities such as driving in a car or fertilising our gardens can also lead to pollution of water entering dams and waterways.
Much of the Perth area is covered with sandy, porous soils. Most private wells (bores) are located within this shallow aquifer system, as are some Water Corporation bores that supply the Integrated Water Supply Scheme with drinking water. Perth wetlands are often an expression of the shallow aquifer, where the water table is visible in low-lying areas.
Rainfall seeps through these sandy soils into the water table where it collects in groundwater aquifers. Pollutants spilled on the ground, or leaking from storage vessels, have the potential to contaminate groundwater aquifers through seepage. Groundwater contamination in the shallow aquifer system can endanger community health and threaten the environment; especially the fauna and flora of wetlands.
Controlling groundwater contamination
Water Corporation regularly monitors the quality of groundwater used for public water supply, to ensure that scheme water meets the requirements of the Department of Health, which are based on the Australian Drinking Water Guidelines (2011).
Fortunately, the Perth region has not suffered widespread groundwater contamination; there have been only localised areas of contamination at some sites in industrial areas and at petrol stations (where tanks have rusted and leaked fuel).
It is said that groundwater contamination has a long memory: it can take decades to hundreds of years for contamination to disperse naturally. For this reason, ‘prevention is better than cure’ is the prevailing policy.
Potential sources of groundwater contamination
- Septic tanks – high levels of micro-organisms from septic tanks pose a health risk if they enter groundwater used as drinking water. Contaminated groundwater can also contribute excessive nutrients to wetlands and watercourses, resulting in algal blooms that kill fish and aquatic animals and may be harmful to human health. Septic tanks must be set back a safe distance away from bores or reservoirs used to supply drinking water. The Water Corporation is actively reducing the number of septic tanks in the Perth region by installing deep sewerage (through the Infill Sewerage Program) in non-sewered urban areas.
- Fertilisers and pesticides – excessive or incorrect use of fertilisers and pesticides from horticultural industries, market gardens and domestic gardens can be a source of groundwater contamination. Slow-release fertilisers, which gradually break down over time, are recommended where practical and new horticultural developments are discouraged in environmentally sensitive areas (e.g. the Peel-Harvey catchment).
- Solid and liquid waste disposal – in the past, community and industrial wastes in the Perth region were occasionally disposed of in sandy depressions, which in some cases resulted in groundwater contamination. Waste disposal is now allowed only at properly located, appropriately constructed (e.g. impervious-lined), operated and supervised waste disposal sites. There are heavy penalties for the illegal dumping of waste materials.
- Accidental leakage – leakage of industrial chemicals and petrol from surface and subsurface tanks can pollute groundwater and surface water. Chemical transport vehicle accidents can also cause contamination when runoff enters drainage systems that discharge into streams, wetlands or rivers. There are now strict controls on the construction of underground petroleum storage tanks in drinking water catchments and other sensitive areas, and groundwater quality at the sites must be monitored. Above-ground petroleum storage tanks with proper spill protection (e.g. double containment around pipe work) are now used in environmentally sensitive areas.
- Saltwater intrusion – groundwater quality can be seriously affected by the intrusion of salt water from the ocean or an estuary. Saltwater intrusion occurs when fresh groundwater supplies are over pumped, causing the underlying salt water to be drawn upwards and inland. Bores sunk in coastal and riverfront areas should just penetrate the water table and be pumped at low rates so as to avoid drawing up the deeper saline water. Developments (e.g. horticulture) requiring large quantities of groundwater from wells in drinking water catchments are discouraged from using the shallow aquifer.