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

[摘要] EXECUTIVE SUMMARYBackgroundThis report summarises the field studies at the Namoi 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 Namoi River were to:- Determine at two locations (Old Mollee and Yarral East, near Narrabri NSW) whether losing-connected or losing-disconnected conditions were present;- Instrument and monitor a piezometer transect at each location to estimate the depth to the 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 resultsThe Namoi River was losing-connected along the study reach at the time the river was visited (16 - 20 November 2009). Because of the absence of a clogging layer in the streambed, this section of the river is unlikely to become losing-disconnected. Instead, a decline in the water table would increase the infiltration rate, resulting in the river eventually drying out rather than becoming disconnected. This process was apparently underway during the site visit, with low flows at the upstream location (Old Mollee) and no flow (stagnant pools) at the downstream one (Yarral East).Hydraulic head in the riparian piezometer transects responded in a dynamic and complex fashion to changes in stream stage. At low stream stages and during irrigation pulses, hydraulic heads varied but the river was always losing relative to the riparian piezometers.However, larger floods triggered significant bank recharge - discharge cycles, with the river gaining for weeks to months following the events.The vertical hydraulic conductivity of the streambed was relatively large at both sites (average 2.4 E-4 m s-1), corresponding to a silty sand to sand porous medium. However, the vertical hydraulic gradients in the streambed were variable, with both gaining and losing sections present. This large variability in the hydraulic gradients was attributed to the low flow conditions, under which parafluvial processes become relatively more important in thehydrological exchanges through the streambed. In other words, much of the water in the river at the time of the study originated from the drainage of sand bars and other features in the alluvial plain and significant exchange of water occurred between pools via shallowgroundwater flow paths. As a result, while a net infiltration to the aquifer was measured in the streambed at the Old Mollee and Yarral East sites, these estimates had a large error. For similar reasons, no attempt was made to estimate the infiltration rate by differential flow gauging. However, the possibility to combine hydrometric techniques and in-stream tracer addition experiments to untangle gross and net loss rates from losing streams is discussed.Riparian groundwater had a low salinity (TDS250 mg L-1) and also had a distinct evaporation signal for the stable isotopes of water closer to the river. This indicated recharge during floods but also under low flow conditions (i.e., when the river has an evaporationsignal). However, it was not possible to quantify the magnitude of the recharge rate because of the uncertainties associated with CFC-derived groundwater ages in the alluvial aquifer. The range in signature for stable isotopes in the river is also unknown.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 rat...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)