Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Using appropriate methods to understand groundwater recharge: a case study in semi-arid areas (131)

Shovon Barua 1 , Ian Cartwright 1 , Edoardo Daly 2 , P. Evan Dresel 3
  1. School of Earth, Atmosphere & Environment, Monash University, Clayton, VIC, Australia
  2. Department of Civil Engineering, Monash University, Clayton, VIC, Australia
  3. Department of Economic Development, Jobs, Transport, and Resources, Agricultural Research Division, Bendigo, VIC, Australia

Choosing appropriate recharge methods is important for understanding how groundwater recharge is impacted by successive land-use changes in semi-arid areas. This study examines two contrasting catchments in western Victoria, Australia, that were cleared following European settlement ~180 years ago, one of which was subsequently replanted with plantation eucalypt forest (~15 years ago). Both catchments comprise deeply weathered Devonian ignimbrite overlain by ferruginous duricrust and laterite regolith. Major ion geochemistry suggests that evapotranspiration is the dominant process controlling groundwater geochemistry. The 3H activities range from <0.02 to 4.10 TU and are generally lower than those of present-day rainfall (~2.8 TU) in this region. The 14C activities of dissolved inorganic carbon range from 70.7 to 103.6 (pMC) in the pasture and 29.5 to 100.8 (pMC) in the forest. The spatial variation of 3H and 14C activities imply that groundwater flow is heterogeneous, present-day recharge occurs across both catchments and that mixing between younger and older groundwater has happened locally. The residence times of groundwater calculated using a variety of lumped parameter models and the 14C activities are up to 24,700 years. The typical recharge rates estimated from chloride mass balance are 0.2 to 8.8 mm/year, and 3H renewal rates are 0.01 to 8.0 mm/year; these are generally lower than those estimated from water table fluctuation (15 to 500 mm/year). However, the pre-land clearing recharge rates from chloride mass balance with longer residence times are typically consistent with short-lived 3H activities and annual 3H renewal rates, suggesting that water table fluctuation overestimates the annual to decadal groundwater recharge and most likely an inappropriate method to document groundwater recharge in semi-arid areas, where the recharge rates are generally low due to present-day distribution of rainfall over higher evapotranspiration that has been successively affected by land-use changes.