The potential effects of the proposed salt interception scheme on the aquatic biota of Billabong Creek

"May 2005".

Project Number: Ecological impacts of groundwater disposal to Billabong Creek - M/BUS/69.

MDFRC item.

Background: A field survey of the general water quality, fish, macroinvertebrates, and aquatic/riparian plants was carried out at three sites on Billabong Creek near Walla Walla on the 13 December 2004. Sampling was completed before 1:00 pm. Site 1: The Walla Walla-Henty road bridge, approximately 1.5 km upstream of the proposed discharge. Site 2: Billabong Creek, where the in-stream discharge is proposed to take place. Site 3: Brooklyn Bridge, approximately 2.5 km downstream of discharge point Site assessment. The electrical conductivity (EC) of the river increased between site 1 (1053 μs/cm), site 2 (1175 μs/cm) and site 3 (1430 μs/cm), presumably as a consequence of the previously-described saline intrusion into the creek at site 2. Electrical conductivity above 1500 EC units have been predicted to have adverse impacts on freshwater aquatic biota. The in-stream temperature of the water was approximately 22ºC. Macroinvertebrates were collected using standard sweep net techniques from all sites and identified to Family taxonomic resolution. Twenty one taxa were collected from site1, 19 from site 2 and 15 from site 3. All taxa collected were typical of lowland rivers in the Murray region. Fish were sampled using a back-pack electrofisher. No fish were caught but European carp were observed. Site 2 is a known popular angling hole for Golden Perch and this species is apparently regularly caught from site 2. Riparian vegetation was dominated by mature red gums with little evidence of regeneration. Patches of Phragmites spp. occurred at all sites. Based on our initial survey and reading of associated literature on Billabong Creek, there is little doubt that Billabong Creek is a degraded waterway. Identifiable risks and potential Impacts There are three immediately identifiable potential impacts of the discharge into Billabong Creek on the associated biota: 1. There is a risk that the discharged waters will raise the ambient temperatures, particularly over winter. This may have a negative impact on the biota immediately downstream of the discharge point. However, any impact would decrease as distance increases downstream and would be expected to be undetectable within a few hundreds of meters. 2. Billabong Creek is a naturally ephemeral system experiencing frequent periods of no or low flows. There is a risk that providing increased and more permanent flows down Billabong Creek would result in the loss of wetting and drying cues that form an important part in triggering aspects of the life cycles of some biota. Ground water removal that is not ultimately returned to the Creek, i.e., the proposed scheme continues no pumped water is returned to the creek, will result in the stream experiencing extended periods of lower base flow (extended dry periods). The latter will have negative impacts on biota. 3. A reduction in salinity will potentially be beneficial to the aquatic biota of Billabong Creek, many of which may currently be near their salt threshold, particularly at site 3. The decrease in salinity is likely to have some, albeit small, benefit to the aquatic biota in Billabong Creek. Concluding remarks: The effects of pumping groundwater will depend on the mechanism used for disposal of the pumped water: 1) If all the pumped water is returned to the creek then organisms requiring wetting/drying cycles will be affected as the dry (or low flow) periods will be diminished. The extent of this effect would be determined by the extent and timing of the flow. 2) If no water is returned to the creek then biota will be subjected to extended periods of low flow or longer periods of dry conditions. 3) Given that Billabong Creek already is currently in a degraded state, in our opinion, the best operation would be to return pumped water to the creek in such a fashion as to promote periods of increased flow, but also to retain periods of low flow. We strongly advise introducing a monitoring programme for stream biota and water quality. Determining the impacts of changes to flow requires compiling a comprehensive data set on the ecology of the system prior to changes as well as continuing sampling after the introduction of flow changes. These monitoring programmes should be designed within an adaptive management framework.