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

[摘要] EXECUTIVE SUMMARYBackgroundThis report summarises the field studies at the Gwydir 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 Gwydir River were to:- Determine at two locations (Yarraman Bridge and Brageen Crossing, near Moree 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,- Estimate infiltration rates through the streambed.Field and lab resultsAt the time of the site visit (7 - 11 December 2009), the Gwydir River at Yarraman Bridge and Brageen Crossing was losing-disconnected. Both sites had a clogging layer (a clay unit 0.5 m or more in thickness) in the streambed and relatively deep water tables in the riparian zone (6 and 12 m below the streambed at Yarraman Bridge and Brageen Crossing, respectively). Streambed profiles at Yarraman Bridge also confirmed the presence of a vadose zone below the streambed. However, the clogging layer was below a thin gravel unit at Yarraman Bridge (0.3 to 1 m in thickness) and the river was also losing-connected with respect to the gravel unit.Hydraulic heads in the piezometer network and in surface water showed limited variations over time during the monitoring period (March 2009 to March 2010). However, the river remained losing at all times and one deep piezometer apparently located in a confinedaquifer showed evidence of pumping-induced drawdowns.Groundwater at the two transects was fresh and appeared to originate mainly from flood recharge. There was little evidence for recharge under low flow conditions. However, it was not possible to confirm the mechanism for recharge or evaluate the recharge rates because of the uncertainties in groundwater dating with CFCs in the aquifer. Because of the very low flow conditions, it was not possible during the site visit to estimate infiltration along the river by differential gauging.RecommendationsThe Losing Streams project developed and successfully applied a methodology to identify whether a river is losing-connected or losing-disconnected using simple field and laboratory measurements. The project was less successful in estimating infiltration rates from the rivers in the field because of difficulties with the use of hydrometric techniques (like differential gauging) in this environment and the uncertainties encountered with groundwater dating with CFCs. Nevertheless, the design of the riparian piezometer network used was satisfactory and the application of an alternative groundwater dating technique should enable the estimation of infiltration rates in future studies. In summary, the recommendations from the Gwydir River site studies are to:- Use the field measurements, especially the identification of losing-disconnection, to help constrain existing groundwater models for the Gwydir River;- Monitor the piezometric and surface water levels for a longer period of time at the two transects to better evaluate the near- stream hydraulic head response for losing-disconnected rivers;Re-assess the connectivity at the two sites following a prolonged wet period to evaluate if it could change over time;- Determine and apply an appropriate hydrometric technique to measure infiltration rates at the reach scale in the Gwydir River and identify potential “hotspots” for recharge;- Attempt to date the alluvial groundwater in the riparian piezometer transects with an alternative t...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)