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

[摘要] EXECUTIVE SUMMARYBackgroundThis report summarises the field studies at the Dumaresq River in the Border Rivers region, 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 Dumaresq River were to:- Determine at two locations downstream from Texas (Qld) 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, SF6, Radon-222, major ions, noble gases) 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 resultsThe Dumaresq River was connected at the two sites visited in October 2009, but one site was gaining (Site 1) and the other losing (Site 2) to the alluvial aquifer. The presence of terrigenic He-4 (4Heterr) in surface water and alluvial groundwater indicated that the river reach received some old regional groundwater discharge. Thus, the Dumaresq River is either a gaining or a flow-through system atthe regional scale at the study reach. The source of the regional groundwater is unclear at present but could include either rockaquifers abutting the alluvial aquifer or the underlying Great Artesian Basin (GAB). The gaining transect had significant bank recharge - discharge cycles during floods but the losing transect was apparently consistently losing.Alluvial groundwater at the transects was fresh and had a large range in stable isotopes of water, with groundwater closer to the river having a distinct evaporation signal (that is, similar to river water during low flow conditions). Chlorofluorocarbons (CFCs) and sulfurhexafluoride (SF6) dating indicated that alluvial groundwater was relatively young (a few decades-old or less). However, discrepancies between CFC-12 and SF6-derived ages suggested that some CFC degradation occurs in this aquifer and that the SF6-derived ageswere more reliable. The few SF6 dates available indicated that the alluvial groundwater was 15 years-old or less. Overall, the patterns in the environmental tracers indicated two recharge mechanisms for the alluvial aquifer. The presence of groundwater with a depleted isotopic signature, high SF6 concentrations and low 4Heterr concentrations is consistent with recharge during floods. On the other hand, groundwater with high SF6 and 4Heterr concentrations and with an evaporation signal represents river infiltration under low flow conditions.The unusual tracer signature in near-stream groundwater highlighted that the recharge process is complex, including features of both piston flow for a single water source and mixing between different sources of recharge water. Thus, further interpretation ofenvironmental tracers to estimate infiltration rates from the river will requires a careful consideration of the different spatial and temporal scales of groundwater - surface water interaction between the river and the alluvial aquifer.Attempts were made at the Dumaresq River to estimate the reach-scale infiltration rates by differential gauging. The stretch of river between Texas Bridge and Site 1 was gaining but no clear trends emerged downstream from Site 1 to Cunningham Weir. However, conditions for differential gauging were not ideal because the field trip coincided with the tailing end of an irrigation release (that is, discharge was not steady at daily time scales along the studyreach). Thus, the differential gauging results must be considered with some caution.RecommendationsThe Losing Streams project was successful in devel...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)