The pressures on our estuaries are mostly due to human activities in their catchments. Therefore, to manage estuaries you actually need to manage their catchments. Unless land use and development is carefully planned and managed, it can harm our waterways. The Department of Water supports land planning authorities to integrate water and land use planning in ways that support population growth and state development, meet environmental needs and balance economic, community and cultural benefits.

Land clearance and the resulting agricultural, urban or industrial land uses, as well as the drying climate of southwest Western Australia, have placed our estuaries under significant stress. Additionally, many catchments have been artificially drained to lower the water table to increase the area of land suitable for agriculture and/or urban development. Drains typically have low ecological value and increase the delivery rate of nutrients and other contaminants to our estuaries, meaning less assimilation can occur on-route. If left unchecked, these modifications in our estuary catchments can lead to the severe deterioration of our estuaries.

Catchment management is guided by the Leschenault Estuary water quality improvement plan. Water Quality Improvement Plans (WQIPs) provide a consolidated understanding of water quality issues in the catchment and estuary.

Water quality monitoring and modelling identifies diffuse agricultural sources – primarily beef and dairy farming – as the main sources of nutrient load to the estuary. Septic tank contributions are also a concern, particularly those located adjacent to the estuary foreshore. The WQIP has shown that nutrient (nitrogen and phosphorus) load reductions of around 35% are required to alleviate water quality problems in the Leschenault estuary and its rivers.

Suggested best management practices (BMPs) that will result in water quality improvement include:

• better fertiliser management across the catchment
• reduction of effluent contamination from dairy sheds
• restoration of the fringing vegetation to restore the ecological function of waterways
• connection of septic systems to the deep sewerage system
• strategic retrofitting of water sensitive urban design in existing urban areas
• an upgrade of Kemerton wastewater treatment plant (WWTP) was recommended to meet the demand. The Water Corporation has since diverted excess sewage from Kemerton WWTP to Bunbury WWTP.

WQIPs identify sources of contaminants (particularly nitrogen and phosphorus) and provide solutions in the form of management actions supported by cost/benefit analyses. It is important to recognise that these plans require extensive consultation, coordination and action from key government agencies, local government authorities, industry-representative bodies and the community. The plan mainly seeks to improve water quality by reducing nutrient and organic matter pollution, however, ensuring that industrial contaminants are minimised, acid sulfate soils are prevented and river health and function is improved are other important components of the plan.

The WQIP outlines a range of management actions which, if taken together, have the potential to improve current water quality.