Groundwater Recharge, Mixing and Salinity across the Angas-Bremer Plains, South Australia:Geochemical and Isotopic Constraints

[摘要] Executive Summary: Water and salt management in the Angas Bremer Plains have undergone a number of changes over the last 150 years.Initially, development of irrigation through diversion of the surface drainage network and construction of dams in the upper reaches of the tributary systems, resulted in reduced flooding and provision of an irrigation industry focussed on lucerne production.After the mid 1950's, three decades of intense groundwater pumping, some artificial recharge, and then, more recently, import of water from Lake Alexandrina for irrigation of wine grapes, has further altered the hydrological and salt balance.Salinisation and water resource development are inextricably linked, and the ABP provides an excellent case example to test the efficacy of the use of integrated geophysics, hydrogeology and geochemistry to better manage these.The objectives of this component of the NAP project was to determine the relative importance of direct (river) recharge versus indirect soil water infiltration recharge, the role of geological structures such as faults in diverting groundwater flow systems, the extent of vertical mixing between the two principal aquifers, and source(s) of solutes in the groundwaters.The dominant recharge mechanism is direct recharge concentrated along the courses of the major creeks.This contributes sufficient recharges waters to rapidly replace extracted groundwater, as was evidenced by almost complete recovery of depressed water levels following the large flood of 1992.Cyclic salts dominate the chemistry of the groundwaters, which have been evapoconcentrated prior to recharge.There is no evidence for significant mineral weathering contributions except for limited dissolution of carbonates in the deeper, confined aquifer.Faulting plays a significant role in defining the extents of the deep aquifer in particular, but has only a minor role in directing recharge to the aquifers, except at the margins of the basin.Some limited recharge occurs across the northern margin of the deep aquifer; which has been thought of as a faulted margin.The fault present at this location may actually be a lithology boundary where the Tertiary limestones onlap onto the Cambrian sediments (Gibson, 2004).Similar major ion chemistry for the shallow and deep aquifers suggests significant inter-aquifer mixing.However much older apparent 14 C groundwater ages in the deeper aquifer indicate that this mixing takes place over thousands of years, except in very localised areas close to the rivers.The shallow, unconfined aquifer contains significant quantities of CFCs and elevated radiocarbon indicating very young ages (less than 50 years) for most of the water.The deeper, confined aquifer has much lower levels of radiocarbon and may be averaging waters over many thousands of years.This would include waters derived locally as well as waters flowing in form the broader Murray Basin to the east.There is no evidence in either aquifer of waters derived from lake Alexandrina (or from the River Murray).

[发布机构] CSIRO Land and water

[学科分类] 地球科学(综合)