The Kulnura-Mangrove Mountain plateau consists of
the catchments of Mangrove, Narara, Mooney Mooney,
and Ourimbah Creeks, and Wyong River. Groundwater
plays a key role in sustaining stream flow within these
catchments. Estimates indicate up to 50% of annual
stream flow is derived from baseflow.
The local community water supply relies on the
groundwater within the elevated Hawkesbury-
Narrabeen sandstone plateau. Furthermore, the
Gosford-Wyong Councils’ Water Authority (WSA) is the
third largest in NSW and utilises many of the streams
flowing from the sandstone plateau for municipal water
supply. It is anticipated that the WSA will provide
municipal water for 319 000 persons by the year 2010.
The increasing volumes of groundwater being extracted
and changing land use have the potential to cause
damage to the fresh water aquifer through
contamination and aquifer depletion.
A hydrogeochemical survey (2006-2009) has been
conducted in NSW Dept of Water and Energy (DWE)
monitoring wells across the plateau in order to
determine groundwater residence times. Groundwater
was analysed for major ions, minor and trace elements,
H2O 18O and 2H, 13CDIC, 87Sr/86Sr, 14CDIC, and 3H,
and complemented with mineralogical and isotopic
information obtained from soil and drill chips collected
during well construction. Water stable isotopes confirm
the meteoric origin of the groundwater with most values
plotting on the local meteoric water line. Localised
evaporative trends suggest recharge with evaporated
groundwater stored in ponds.
Shallow groundwaters have 3H and 14C activities
consistent with modern recharge (Fig 1). Carbon “bomb
pulse” signatures of up to 116.8 pmC are found in the
central areas of the plateau. The thin soils, lack of
carbonates in the intensely weathered near-surface
Hawkesbury sandstone, and the shallow depth of the
water samples is consistent with the 3H results
measured, suggesting minimal dilution of the original
14C. Input of this data into a southern hemisphere bomb
pulse model [1] suggest potential recharge during the
1990´s, coinciding with sustained wet conditions and
above average rainfalls experienced during this period.
Fig. 1. 14C vs 3H plot of groundwater samples in the Kulnura-
Mangrove Mountain Plateau
Deeper groundwaters have lower 14C and 3H activities
in some cases close to background level (Fig. 1). The
quantifiable 3H suggests residence times of <70 a.
However, non-corrected 14C residence times are submodern
(>500 a). This apparent discrepancy can be
explained by either mixing with older waters or
dissolution of carbonates. The good correlation of total
dissolved inorganic carbon (TDIC) and Ca (R2=0.8),
13CTDIC in groundwater and mineralogy results from
drill chips suggest that dissolution of dispersed
carbonates is taking place.
The deepest groundwaters show the most difference in
residence time across the study area. The eastern and
western plateaus yield old groundwater with 14C
corrected residence times of around 9 ka and 4 ka
respectively. However, the groundwater at equivalent
depths in the central plateau was found to be
considerably younger with residence times of <70 a
