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

[摘要] EXECUTIVE SUMMARYBackgroundThis report summarises the field studies at Billabong Creek, 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 Billabong Creek were to:- Determine at two locations (Billabong East and Billabong West, near Jerilderie, 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.Field and lab resultsThe streambed at Billabong East and West consisted of a clay layer 0.5 to 1.2 m thick overlying sand, silty sand and clay sediments. Field observations and moisture content profiles from below the streambed indicated that the river at both transects was losing-disconnected.The water table was approximately 9 -12 m below the streambed and the hydraulic gradients were away from the river at both sites. The piezometric surfaces (when corrected for barometric effects) remained relatively stable during the monitoring period but the variations in stream stage were also relatively small (~1 m). Most of the piezometers had relatively elevated barometric efficiencies (0.4 - 0.9), consistent with either confined or semiconfined conditions.The vertical hydraulic conductivity of the streambed (Kv) was usually at the practical measurement limit for the field permeameters used (5·10-7 m s-1). Lower Kv (10-8 to 10-10 m s-1) were estimated on intact cores in the laboratory using evaporation experiments,although only few measurements were made. Overall, the Billabong Creek streambed had a range in hydraulic conductivity consistent with a clay porous medium. However, because of significant variations in the thickness of the clay cover, localised areas with a higher transmissivity could also occur where the clay is thin or absent.The environmental tracer data suggested that groundwater at the two piezometer transects was not from a single source or from a single recharge event. However, there was apparently little or no water originating from infiltration from the river under low flow conditions (that is, when the water originated from upstream releases from reservoirs). This source of water is quite distinct based on its stable isotopes of water composition. Alternative sources of recharge could include bank infiltration during floods, vertical recharge in the floodplain during overbank floods, diffuse rainfall recharge, and irrigation recharge. Because of apparent degradation in an anoxic alluvial aquifer environment, it was not possible to reliably age groundwater using CFCs. Overall, the patterns in the environmentaltracers indicate that the infiltration rates were relatively low at Billabong Creek relative to the other sites.Due to the added complexity associated with the presence of an unsaturated zone, a twostep methodology was developed to estimate the infiltration rate through the streambed. The hydraulic properties of the clays were first determined from intact cores using evaporation experiments. Using realistic boundary conditions, these properties were then used to estimate the infiltration rate using the WAVES model. This approach suggested that the infiltration rate was 1 mm day-1 under low flow conditions. However, it is not recommended to use these modelled infiltration rates to estimate recharge to the alluvial aquifer until some of the assumptions of the methodology are further evaluated...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)