Drought contingency monitoring at Tareena Billabong, NSW

"June 2009".

Project Number: Drought Contingency Monitoring Tareena Billabong & Salt Creek - M/BUS/251.

MDFRC item.

Tareena Billabong is a ≈44ha terminal oxbow wetland on the Chowilla floodplain in south-western NSW. The permanently-inundated wetland was selected for disconnection to achieve water savings and prevent saline outflows from the wetland as part of drought contingency planning in 2007/08. The Murray-Darling Freshwater Research Centre (MDFRC) was engaged by the then Murray-Darling Basin Commission (now the Murray-Darling Basin Authority) to monitor Tareena Billabong to observe changes in environmental condition in order to allow detection of, and response to, risk of environmental damage. The ecological indicators monitored at Tareena Billabong included water levels and quality, blue-green algae, fish, river red gum, understorey vegetation, waterbirds and groundwater. Six 3-monthly surveys were completed during the monitoring period of 27 November 2007 to 10 March 2009. Monitoring results were assessed against pre-determined ‘trigger values' for management intervention based on Wallace et al. (2007) and DWE (2007). Water levels and quality Tareena Billabong was disconnected from Salt Creek in April 2007 with a temporary blockbank. Water levels in the wetland decreased through evaporation and the majority of the wetland had dried by November 2007 when MDFRC commenced its monitoring program. Low inflows continued into Tareena Billabong in December 2007 and January 2008 from a leak in the blockbank that maintained a small pool of surface water near the inlet. In February 2008 a management decision was made to refill the wetland by removing some of the sand bags, and by mid-March 2008 the water level within the wetland had equilibrated with Salt Creek. The electrical conductivity (EC) of surface water ranged from 252 – 55,860 μS.cm-1@25oC and exceeded the 5,000 μS.cm-1 (5 ds.m-1) trigger value of DWE (2007) on 29 of 38 sampling occasions, and the 1000 mg.L-1 (or ≈1,500 μS.cm-1) trigger value of Wallace et al. (2007) on 30 of 38 sampling occasions. Surface water pH levels remained above the pH< 6 trigger value for all sites and times (minimum pH 6.87) and was alkaline on 36 of 37 occasions. However, pH levels exceeded 9 on 38% of sampling occasions due to high rates of photosynthesis within the wetland. Water temperatures remained above the trigger value of <6oC on all sampling occasions. However, water temperatures exceeded the trigger value of >30oC on 5 occasions (maximum 34.3oC) on hot days due to the low thermal buffering capacity of the shallow wetland. Dissolved oxygen (DO) concentrations remained above the 5 mg.L-1 trigger value for all sampling times and sites, ranging from 6.43 - 17.52 mg.L-1. Surface water turbidity was relatively high (311 – 567 NTU) at Tareena Billabong when the wetland was isolated from Salt Creek and comprised only a small and shallow pool near the inlet. Turbidity decreased to below 200 NTU from March 2008 following refilling. Blue-green algae Blue-green algae (BGA) counts exceeded the trigger value of 15,000 cells.mL-1 of Wallace et al. (2007) for 5 of 18 site-times. However, no confirmed toxin producers (or suspected but unconfirmed toxin producers) were sampled at counts above 15,000 cells.mL-1. Fish Four native and two exotic fish species were sampled at Tareena Billabong in relatively low abundance from all fish surveys. No rare or threatened fish species were sampled. Large numbers of common carp (Cyprinus carpio) were trapped within Tareena Billabong following its disconnection in June 2007. Some fish were harvested commercially at this time, although many thousands more perished in September 2007 as surface water EC increased. River red gum River red gum (RRG) condition at Tareena Billabong was extremely poor. 26 of the 30 monitored trees contained no foliage throughout the monitoring period and were considered to be dead. Their deaths are attributed to the extensive soil salinisation in the region around Tareena Billabong. Of the four live RRG trees monitored, the trigger point for management intervention was not met as there was no clear deterioration in tree condition during the monitoring period as a result of wetland disconnection (Wallace et al. 2007). Understorey vegetation A total of 60 plant taxa (51 native and 9 exotic) from six functional groups were sampled at Tareena Billabong. The sampled understorey vegetation community of Tareena Billabong did not contain rare or threatened plant species, and hence did not meet the trigger value of Wallace et al. (2007) relating to the maintenance of aquatic habitat for rare or threatened vegetation species. Waterbirds A total of 28 waterbird species were recorded at Tareena Billabong. The lowest abundance (n=26) and richness (n=5) of waterbirds occurred in December 2007 as the wetland had mostly dried and consisted of a small fresh pool near the inlet. Following wetland refilling in February and March 2008, waterbird abundances increased progressively to December 2008 when 4,256 individual waterbirds from 15 species were recorded. High abundances of micro- and macroinvertebrates were observed within Tareena Billabong following refilling and provide an important food source to waterbirds. Two waterbird species of conservation significance were recorded at Tareena Billabong including Caspian Tern (n=4) and Great Egret (n=2). Groundwater Groundwater was monitored from two bores located 300-500m to the north-east of Tareena Billabong. In May/June 2008, the groundwater level was 19.12 mAHD (≈4m below ground surface) and 18.80 mAHD (≈3m below ground surface) at bores GW087581 and GW087582, respectively. These groundwater levels are considerably higher than the wetland bed level of ≈18.0-8.3 mAHD indicating that the wetland may receive saline inflows (groundwater EC was ≈80,000 μS.cm-1). Achieving the study objective The monitoring program was successful in achieving its objective of assessing the ecological condition of Tareena Billabong following its disconnection from Salt Creek and during its drying and refilling phases. One important aspect of the study was the close interaction between the survey team and managers throughout the period of monitoring. Monitoring results were interpreted against pre-determined trigger values and communicated regularly to key managers whereupon they were distributed more broadly to other managers and stakeholders. This communication was important to ensure that managers were able to make timely decisions if required based on recent and appropriate environmental data. Future management The occurrence of high salt loads in the landscape surrounding Tareena Billabong and within the wetland itself is, and will continue to remain, the dominant threat to the ecological values of this site. Management actions such as drying the wetland are problematic due to potentially greater salinisation issues. In addition, there are threats to other downstream areas from saline outflows from the wetland. A further issue that may limit management options at Tareena Billabong is the occurrence of sulfidic sediments. Reduced sulfur was present in relatively high concentrations at Tareena Billabong (Baldwin et al. 2007), although the drying event of 2007 did not result in wetland acidification following refilling. However, it is unclear whether additional drying events may generate quantities of acid that exceed the buffering capacity of the wetland. Therefore, it is recommended that any future drying event at Tareena Billabong be accompanied by a more spatially-comprehensive soil monitoring program to better assess the risks of acidification and allow for a timely management response. There is no simple solution for the future management of Tareena Billabong. A key challenge is to manage the threats to the ecological values at this site so that the wetland can continue to provide ecological services in its salinised landscape, whilst taking into consideration the effects of this management upon downstream wetlands. As demonstrated in this study, regular monitoring and effective communication with managers can reduce the risks of environmental damage when implementing management interventions. However, managing the saline groundwater levels remains the dominant long-term challenge for this region.