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

[摘要] EXECUTIVE SUMMARYBackgroundThis report summarises the field studies at the Macquarie River, one of the six field sites for the project Interconnection of Surface and Groundwater Systems - River Losses from Losing / Disconnected Streams (or “Losing Streams”). The objectives of the field program at the Macquarie River were to:- Determine at two locations (Woodlands and Macs Reserve, near Narromine, 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 and the infiltration rate in 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.The streambed at the two Macquarie sites was a mixture of clay-lined banks interspersed with sand, gravel and cobble bars and riffles. A combination of “wet” bank tests, lower hydraulic head in piezometers relative to the river, and positive fluid pressures in thestreambed indicated that the river at Woodlands and at Macs Reserve was losing-connected. However, at Woodlands, the water level in pits within 1 m of the river along clay banks was 30 - 50 cm below stream level. This indicated either a very steep water table in the clay banks or the presence of a small unsaturated zone at the edges of the river along some clay banks (possible under either transitional or losing-disconnected conditions).The water table in the riparian piezometers was usually ~2 m below stream level at Woodlands and ~0.5 m below stream level at Macs Reserve. At Woodlands the water level varied by up to 5 m at monthly timescales, presumably because of pumping from the alluvialaquifer. As a result, it was not clear how the hydraulic head in the piezometers responded to flow events (floods and irrigation releases) in the river. At Macs Reserve, hydraulic heads tracked changes in river level, but in a delayed and subdued fashion. Following twosuccessive floods in February 2010, the river was gaining relative to the riparian piezometers for approximately two weeks because of bank discharge.The vertical hydraulic conductivity of the streambed along sand, gravel, cobble bars and riffles was relatively high, ranging from 10-5.25 to 10-2.5 m s-1. No measurement of the hydraulic conductivity of the clay-lined sections of the river was made, but it is likely to be significantly lower.Three different techniques (differential gauging, Darcy flux in the streambed, and streambed Rn-222 profiles) were used to estimate the infiltration rate from the river. Although made at different scales, all three techniques gave comparable results. The range in infiltration rate was 0.67 - 5.3 ML km-1 day-1 by differential gauging, 1.6 - 6.8 ML km-1 day-1 using the Darcy flux, and 0.38 - 1.2 ML km-1 day-1 using Rn-222 profiles. It is expected that these rates are only representative for the conditions at the time of study (September 2009) and could vary significantly over time.At the piezometer transects, groundwater was fresh but tended to slightly increase in salinity with distance from the river. Similarly, there was a tendency (as per the other Losing Streams sites) for groundwater near the river to have an evaporation signal in the stableisotopes of water. This indicated that there is some recharge from the river under low discharge conditions, when river water tends to have an evaporation signal. Due to suspected partial degradation in the aquifer, it was not possible to use CFCs to age groundwater along the piezometer transects. As a result, it was not possible to estimate the infiltration rate using environmental tracers in the piezometers. However, as demonstrated at the Border Rivers sit...

[发布日期] 2011-05-16 [发布机构] CSIRO

[学科分类] 地球科学(综合)