MT12 Salinity Revegetation

The historic data for the Hunter Valley from 1975 to1999 shows evidence of a background rising trend in groundwater pressures across geologies and the catchment as a whole. Although the number of bores analysed is small in proportion to the area of the whole catchment, rising trends that were identified previously were confirmed by further fieldwork in the course of this study. An analysis of stream salinity trends for 10 locations across the catchment did not, on the whole, indicate a worsening stream salinity problem in the Hunter Catchment. However, analysis of trends
within the stream salinity data for the base assessment period is confounded by the paucity of data and the very significant changes imposed on the catchment hydrology by development. Therefore conclusions regarding positive, negative or nil trends in the historic stream salinity are difficult to
make with confidence. Recent rising trends in the upper Hunter River at Muswellbrook may support a
link with rising water tables, although falling trends at Liddell and Greta may be the result of several
factors.
For example, the following factors may all play a part:
• falling groundwater trends in alluvial aquifers in the lower catchment;
• changes to river regulation following the commissioning of the Glennies Creek dam; or
• the effect of the introduction of the Hunter River Salinity Trading Scheme (HRSTS).
DLWC (2000) in the State of the Rivers Report shows rising trends in the Hunter River at Singleton
for the period 1970 to 1979, and falling trends from 1980 to 1998. Falling trends in the Goulburn
River at Sandy Hollow are at odds with rising water tables in the catchment, but may be influenced by
groundwater pumping in the alluvial aquifers.
Assuming that rising groundwater trends will lead to increased stream salinity, this study has
undertaken to quantify the likely impact of increased salt export from groundwater on stream salinity
in the Hunter River and its tributaries to the nominal end of system at Greta.
Salt load and salinity predictions have been calculated for the target dates 2010, 2020, 2050 and
2100. The groundwater analysis covered the whole of the Hunter River catchment, but the surface
water analysis covered only the contributing area upstream of Greta.
The audit should be considered in four parts.
1. Discrete and/or continuous flow and salinity data existed for most tributaries for varying
periods from 1975. Relationships were established between salt load and flow using observed
data for tributaries where it existed. Salt load parameters were regionalised for tributaries
without observed salinity data, producing time series of salinities for all the tributaries for 1975
to 1998.
2. The river system comprises unregulated tributaries feeding into mainstream reaches that are
regulated via storage and release from two supply dams. The tributary contributions were input
and the groundwater contributions were adjusted to calibrate the Hunter Integrated Quantity and
Quality Model (IQQM) on observed flow and salinity data on the mainstream for 1993 to 1998.
3. These contributions were then input into the IQQM ’current’ conditions model which applies
river operation and development as at 2000 for the entire 1975 to 1998 climatic period. By
C H A P T E R O N E
EXE C U T I V E S U M M ARY
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definition, the unregulated tributary and groundwater contributions are unchanged by the
’current’ conditions scenario. In the mainstream reaches ‘current’ conditions specifically refers
to modelled flow and salt loads from the Hunter IQQM scenario, reflecting the variability of the
base climatic signature, whilst complying with the most up to date flow rules and development.
The results from the ’current’ conditions model for the 1975 to 1998 climatic period were then
used as a base case to which future increase scenarios could be compared.
4. The future conditions scenarios are the ‘current’ conditions scenario, with the addition of future
increases in salt loads derived from observed groundwater trends at the target dates 2010, 2020,
2050 and 2100. There are limitations to this approach. The evidence for its likelihood is
concrete but the size of the groundwater data set is small compared to the area to which it has
been extrapolated. Although landuse change is generally thought of as the cause of rising water
tables, no clear link has been established with the groundwater rates of rise examined in this
study. The rates of rise have been extrapolated uniformly across all sub-catchments by
association with geology; despite the fact that catchments vary in land use, vegetation and
climate: and therefore in recharge and discharge potential.
Perhaps the most important lesson to be learned from the study of this scenario is that the solutions
to the problem of the trends identified in this study must be addressed in the tributary and residual
catchments since they are the source of the trends. The study has also identified that salt wash off from
the tributaries is not the main driver of high salinity in the currently observed salinity distribution in
the mainstream. Groundwater fluxes from the major fault zones are the prime determinant of high
salinities observed during periods of low flow. If groundwater pressures continue to rise in the future,
salt loads from fault zones may also rise. The magnitude of such an impact could be very significant,
emphasising the need to address the rising trends at their source. The study shows that dilution flows
via dam releases are a significant modifying factor to the observed salinity distribution in the
mainstream.
Since 1995, reported discharges of salt to the river from coal mining in adherence with the protocols
of the HRSTS have amounted to approximately 11,000 t a year. In this study it is predicted that an
additional 5,000 t a year will pass through the Hunter River at Greta by 2020 as a result of rising
groundwater pressure and dryland salinity processes in the tributary catchments. The simulated load
passing beyond Greta represents only approximately 60% of the salt inputs to the model generated in
the catchment as a whole. That is, the whole of the predicted additional salt load arising from dryland
salinity is likely to reach a similar magnitude to that contributed by the HRSTS. (Total additional salt
from groundwater = 8,800 t a year at 2020). Although the impact of this additional load on salinity is
relatively small it may restrict the window of opportunity of the HRSTS currently and limit expansion
of the scheme in future.
As further development of mining in the Hunter Catchment occurs additional pressure on the
trading scheme will result as both the amount of salt to be discharged increases and the window of
opportunity for such disposal shrinks. Although the trends in median and 80th percentile salinities
reported in this study are unlikely to shrink that window radically, the amount of salt coming onstream
is set to increase both as new mines are commissioned and old ones are decommissioned. No
account of the impact of mine closure and the fate of salt within voids on future salt pollution has been
attempted in this study.
Overall the trends in salinity predicted in the study are not great. In the mainstream, salinity values
are predicted to rise by no more than 10% over the next 100 years. Change in some tributaries will be
greater with a 10%, 13% and 33% change over 100 years predicted for Wybong Creek, the Goulburn
NSW coastal rivers salinity audit: predictions for the Hunter ValleyIssue 1: December 2000
NSW Department of Land and Water Conservation
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River and Dart Brook respectively. Water users across the catchment are already experiencing the
management risk implications of the salinity levels identified in the study. Surface water salinity
already presents threats to the wine industry, power generation and town water supplies. The trends
show a gradual worsening of these current threats.