Twenty-four sites were surveyed, split into four groups – some exposed to drains, some not exposed (controls), some acidic, some near-neutral or slightly alkaline. There was no consistent difference in the structure of aquatic invertebrate communities between controls and exposed sites, although there were some differences between acidic and neutral sites. A clarification: nearly half of the lakes and wetlands examined were acidic, including many that had never been exposed to drains. Some of these ‘naturally’ acidic waters may have been acidic even before clearing; others have clearly become acidic only recently – probably through rising groundwater tables.
The Yarra Yarra invertebrate fauna is broadly similar to other invertebrate faunas in the wheatbelt. Just as in other parts of the wheatbelt, salinity is by far the most dominant environmental determinant. Salinity has an over-riding effect. There is a suggestion that sites exposed to acidic drainwater have higher concentrations of metal pollutants and less diverse faunas than naturally acidic sites with similar levels of salinity. However, there are not enough sites in the survey to be able to conclude this with confidence.
Report Conclusion: To the best of our knowledge, there is no glaring threat to aquatic invertebrate communities from the drains, but also no definitive answer. If drains were doing something really bad, it would have been picked up in this survey – the fact that it didn’t is good for the project and overall this report does not have a negative outlook for drainage.