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EXECUTIVE SUMMARY
The influence of dissolved chemicals, especially nutrients
such as nitrogen (N) and phosphorus (P) on water quality of the Great
Barrier Reef (GBR) lagoon is assuming greater importance as pressure for
development, both on adjacent mainland and islands in the lagoon increase.
Considerations must be given to marine water quality around the main reef
structure itself and also around numerous finding reefs often associated
with islands throughout the region. The local environment around these
fringing reefs will be influenced by discharges from the islands. Given
that coral reef ecology is sensitive to relatively small elevations of
nutrient concentrations above natural background levels, appropriate management
of water and wastewater in resorts on these islands will rely on accurate
predictions of the eventual fate of those nutrients.
This report describes numerical studies that have the
objective of quantifying the significance of N flows from effluent irrigation
practice on resort islands. Our study concentrates on N because, in land
disposal of effluent, P will be adsorbed by the soil, reducing mobility
and the environmental risk posed. As effluent irrigation schemes are becoming
increasingly popular alternative to ocean outfalls, their relative success
in retarding N leaching was of considerable interest.
In order to estimate upper limits for potential N discharges
at sea, numerical modelling of the fate of N within the unsaturated zone
under lawn has been undertaken for three different resorts of climatically
and geologically different characteristics. A series of hypothetical effluent
irrigation regimes was considered. These regimes were based on assumed
fractions of the actual wastewater produced daily, coupled with data obtained
for N measured intermittently in treated sewage over four years.
The parameters required for this study were primarily
obtained from literature sources supplemented by some laboratory measurements
with simulations driven by historic meteorological data and experimentally
determined soil hydraulic properties.
Sensitivity studies were also undertaken on the effect
of spatially variable soil hydraulic properties on the soil water flux
model, and parameter variations on the N cycling sub-model.
The first sensitivity model tool account of stochastic
variations in measured soil hydraulic properties ranging from ±
1 standard deviation from the mean and defined an envelope of probable
outcomes for deep percolation output at each site. A second sensitivity
analysis focusing on the effects of changes to the N model inputs highlighted
which parameters were most likely to influence the predicted downward
movement of N from the soil. An indication of the sign and magnitude of
N model output sensitivity was also given for all major parameters.
A broad-based validation process was also conducted
to ascertain the applicability and relevance of the simulated values for
annual nitrate leaching. The predicted soil solution nitrate concentration
measurements were in reasonable agreement with measured values by the
standards commonly found when modelling g complex biological systems,
and also considering the uncertainty in algorithms and parameter values
for a majority of the processes.
The predicted mean annual losses of nitrate below the
root zone at Great Keppel and Dunk Islands ranged from 30-502kg N/yr and
28-126kg N /yr respectively, across four daily wastewater irrigation scenarios
applied to the golf courses at each resort. Brampton Island which currently
employs effluent irrigation could expect to generate only between 7-38kg
N /yr flow beyond the root zone of the golf course.
Simulation results for resorts at Dunk and Brampton
Islands showed that the transfer of N from the unsaturated zone was reduced
substantially for all cases evaluated. Of note, however, is the significance
of rapid transport of N via surface runoff and possible short circuit
subsurface flow paths at Dunk Island – intensified by the wet tropical
conditions experienced there.
For minor levels of effluent N applied to Great Keppel
Island, the soil and vegetation was shown to be reasonably effective in
minimising the progress of N to the groundwater system. However, at moderate
to high rates of applied N the inherent soil properties and N transformation
processes resulted in a more pronounced level of sub-surface N transport.
For irrigating large fractions of the total available sewage effluent
in this case, it would be advisable to distribute over an area larger
than 1.2 ha (golf course) to lower hydraulic loading.
The effect of recycling turfgrass clippings after cutting
was also investigated. A significant addition of N to each island system
occurred as a consequence. This diminished the efficiency of N usage by
the soil and vegetation in all cases, although N flows from below the
profile were not dramatically increased at Brampton and Dunk islands.
Introducing clipping removal practice could further lessen the potential
leaching risks from moderate to high wastewater reuse particularly on
sandy areas.
For both Dunk and Brampton Islands, the maximum reduction
in the potential N to flow to the sea was achieved when 100% of the current
daily effluent production was distributed over a turfgrass area corresponding
to the size of each golf course area. A reduction of 85% and 93% w3as
simulated for each island respectively for such a case. As wastewater
application rates rose, steady increase in the simulated reduction of
N available for discharge to the sea was predicted.
The reduction in flow of N below the root zone of Great
Keppel Island also reached a value for maximum level of wastewater irrigation
loading, The degree of effectiveness of land utilisation of N was hot
as high here as the other islands studied however, reducing the available
N for discharge to sea by 44%. Additionally, relatively small increases
in N usage by the land system were associated with much larger increases
in the irrigation rate pointing to a maximum threshold of N uptake by
turfgrass being approached or surpassed. It also bears mentioning that
as the prospective irrigation areas of each island were assumed to be
no greater in extent that the golf course, larger areas within the resort
environs such as airstrips and gardens are likely to be available for
use.
IN general, predicted outputs should be considered
a lower estimate of N reduction in terms of the level of N available for
discharge to the local marine environment. This is substantiated by the
assumption of N as a conservative (non-interactive) solute entering an
aquifer (with no dilution) which freely discharges to the Great Barrier
Reef lagoon.
Based on the degree of leaching predicted for Great
Keppel Island at high levels of applied N, increases in the nitrate concentration
of surrounding waters sufficient to be detrimental to the marine ecology
cannot be discounted. Further detailed studies of resort island nutrient
mass balances are warranted in conjunction with more detailed work on
the groundwater system transporting N from the unsaturated zone to sea.
To complete the prediction of the influence on N export
from resort islands on local marine water quality, reliable data on currents
and on marine transport processes will be required in the immediate vicinity
of the islands. These will govern the actual residence times and control
volumes for calculations of local marine nutrient concentrations.
From a wastewater management perspective, and presuming
that high rates of applied effluent irrigation are logistically and economically
viable, considerable benefit can be drawn from the high efficiency of
N assimilation by the soil-plant-atmosphere continuum in reducing potential
N discharge. It is imperative however, that associated health concerns
are adequately addressed, which is outside the scope of this study.
The configuration and implementation of an available
numerical model for the nutrient cycling in Great Barrier Reef resort
island turfgrass systems irrigated with sewage effluent represents a major
step towards a better understanding of the gross outputs expected from
such practice.
For a hard copy (or pdf file) of the report contact CRC Reef on info@crcreef.com.
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