Fw. Leaney et Al. Herczeg, REGIONAL RECHARGE TO A KARST AQUIFER ESTIMATED FROM CHEMICAL AND ISOTOPIC COMPOSITION OF DIFFUSE AND LOCALIZED RECHARGE, SOUTH AUSTRALIA, Journal of hydrology, 164(1-4), 1995, pp. 363-387
The relative importance of different recharge mechanisms has been inve
stigated for a karstic area in a subhumid to semi-arid region of South
Australia. The chemical and isotopic signature (delta(2)H and delta(1
8)O) of regional groundwater, sampled from shallow bores in the study
area, as well as water from sinkholes, swamps, drainage bores and wate
r extracted from soil in the unsaturated zone has been measured. Point
estimates of recharge, based on data from the unsaturated zone, have
been combined with temporal and spatial trends in the isotopic and che
mical signature of the groundwater to provide a regional estimate for
recharge for different land elements. For approximately half of the st
udy area, which has predominantly clay soils, recharge is low (< 10 mm
year(-1)) and the groundwaters within the study area are derived from
lateral flow from the east. For the remaining area, recharge has sign
ificantly increased since agricultural development, and has affected t
he groundwater system. Groundwater within the vicinity of irrigation a
reas has shown marked increases in salinity over the past 30 years owi
ng to the impact of irrigation drainage waters. Other areas not under
irrigation, but cleared of native vegetation and replaced by shallow r
ooting pastures, also show increased salinity in parts where there are
sandy soils. In these areas, saline soil water from the unsaturated z
one has been displaced by the increased recharge since clearing. The e
ffect of clearing on areas of clay surface soils has not increased rec
harge to the same extent and groundwater salinity levels are largely u
nchanged. Areas where salinity levels are rising correlate well with i
ncreased C-14 activity and depleted delta(13)C concentrations of the g
roundwater suggesting that these isotopes may be an early indicator of
areas of enhanced recharge.