Downstream ecological impacts of engineering interventions for salinity control in the wheatbelt of Western Australia: Summary Report

[摘要] Executive SummaryManagement of dryland salinity by engineering interventions is becoming common in the WA wheatbelt as vegetative and agronomic solutions have not yet provided a viable alternative.Over 4,500 km of deep (1.5 m) groundwater drains have been constructed in the eastern wheatbelt within the last few decades in an attempt to control salinity and waterlogging, with a discharge, often highly saline and acidic, usually into the natural creeks and streams.Little consideration has previously been given to the ecological impacts of these discharges on aquatic ecosystems.For the first time in the wheatbelt of WA, the hydrological and ecological impacts of groundwater drainage discharge from several engineering interventions on the receiving streams are reported.The project aims were phrased as the Null Hypothesis that “discharge of groundwater drainage and pumped groundwater does not cause degradation of receiving stream ecosystems”.The objectives of the study were to:1.Assess the effects of changed flow and chemical composition on in-stream and hyporheic zone biota.2.Determine the changes to water quality and sediment loads of streams receiving groundwater drainage and pumped groundwater disposal, in comparison to nearby streams with similar characteristics in untreated catchments.3.Where possible, make recommendations of options for mitigation of downstream impacts of engineering interventions for dryland salinity control.4.Assess gaps in this project and make recommendations for follow on activities.The study analysed data from four sites within the wheatbelt, three with deep groundwater drains and one at which groundwater was pumped from a palaeo-river sediment and discharged to a local stream.At each site flow and water quality measurements were taken in streams un-impacted by groundwater drainage discharge, as well as upstream and downstream of the drain discharge point.Drains were found to have high levels of metals, and during periods of low flow, exhibit extreme acidity (pH3) and salinity (up to 200,000 mg L-1).While clearly degraded from the salinity experienced over past decades, there remain ecological values in these streams.This study confirmed that the aquatic systems in the wheatbelt are highly seasonal.Observations on the ecosystems showed a rapid biological response to rain indicating that the flora and fauna of these systems have evolved to withstand the prevailing harsh environmental conditions.Small Ruppia sp. seedlings were observed only two weeks after the onset of rain, and adult invertebrates and tadpoles were found four days after rain.The degree to which groundwater drainage discharge was diluted or buffered within the receiving stream environment significantly affected the impact of the discharge on the ecosystems within the streams.In some cases the impact of drainage waters additional to that of salinity appeared limited, with a recovery in water chemical conditions downstream, and with winter rainfall.In other cases, there was a clear degradation resulting from the discharge of the acidic and hypersaline waters and there were also situations where the receiving stream was so damaged that additional discharge did not appear to contribute to further degradation.It is clear, that if groundwater drainage discharge is limited, primarily through a relatively lower density of drains within a catchment, damage to in-stream ecology will be limited, whereas ecological impact can be much more severe in streams with greater discharge of drainage waters. This conclusion obviously depends on the quality of drainage discharge, but holds for the majority of drainage tested in which the quality was fairly uniformly poor.Water pH was found to be a better ecosystem discriminator than salinity, with aquatic taxa more resilient to variations in salinity than pH. Although both pH and salinity were important as taxa richness increased with increasing pH le

[发布日期] 2010-11-01 [发布机构] CSIRO

[效力级别] Agricultural Land Management [学科分类] 地球科学(综合)