Interconnection of surface and groundwater systems - river losses from losing/disconnected streams. Lachlan River site report

[摘要] EXECUTIVE SUMMARYThis report summarises the field studies at the Lachlan River, one of the six field sites for the project Interconnection of Surface and Groundwater Systems - River Losses from Losing / Disconnected Streams. The objectives of the field program at the Lachlan River were to:- Determine at two locations (Hillston Bridge and Gonowlia Weir, near Hillston, NSW) whether losing-connected or losing-disconnected conditions were present;- Instrument and monitor a piezometer transect at each location to estimate the depth to water table and evaluate the piezometric response to changes in river stage;- Measure the hydraulic conductivity of the streambed;- Sample on one occasion groundwater from the piezometer transects for a range of environmental tracers (stable isotopes of water, CFCs, radon-222, major ions) to evaluate the sources of water to the aquifer and the infiltration rates from the river; and,- Develop a methodology to estimate infiltration rates through the streambed.The two locations investigated on the Lachlan River had losing-disconnected conditions at the time of the study, with the regional water table being more than 20 m below the streambed. At Hillston Bridge, the streambed consisted of 1 - 2 m of clay over sand.Moisture and matric potential profiles collected by augering in the alluvial aquifer through the streambed confirmed that an unsaturated zone was present under the river. The hydrogeological environment was more complex at Gonowlia Weir, where the alluvial aquifernear the stream contained several sand and clay layers. In addition to the regional water table, up to two perched aquifers may have been present underneath the river. However, due to the presence of one of the perched aquifers, it was not possible to collect a profile ofthe alluvial aquifer below the streambed at Gonowlia to confirm that an unsaturated zone was present.While intact cores of the streambed clays were collected at Hillston Bridge, attempts to estimate their hydraulic properties using pressure plate techniques failed. The vertical hydraulic conductivity (Kv) of the clays was also below the practical detection limit using fieldpermeameters (Kv2·10-7 m s-1). Preliminary estimates of infiltration rates below the streambed were attempted at Hillston Bridge using the moisture and matric potential profiles collected by augering. The infiltration rates were apparently higher in the underlying sands than through the streambed clays. This indicated that the infiltration process at Hillston Bridge is more complex than the simple vertical 1-D approach used here. In other words, higher infiltration rates probably occurred through the streambed in the vicinity of where the profiles were collected.The pattern in environmental tracers suggested that there is some recharge of river water under low flow conditions, as demonstrated by the presence of an evaporation signal in the stable isotopes of water in shallow groundwater. All groundwater samples were fresh (350 mg L-1) which is also consistent with significant recharge from river water. CFC-11 and CFC-12 concentrations in groundwater were usually at or near detection limit, suggesting that groundwater was recharged prior to 1965. However, it is likely that CFC degradation occurred because of the anoxic conditions in the aquifer. Thus, it is probable that recharge occurred earlier than 1965. Because of the uncertainties associated with CFC dating at the transects, infiltration rates using environmental tracers could not be estimated.In general, there was a gradual one meter drop in the water table at both transects over the course of the monitoring period (April 2009 to March 2010). The only exceptions were the two piezometers presumably located in perched aquifers near the river at Gonowlia Weir, with the shallower one following the trends in river stage while the deeper one remained stable. Unlike at the Billabong transects, the lack of a strong instanta...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)