E. Wolanski . B. King . S. Spagnol
7.6
Water Quality
Typically water quality may involve nutrients and faecal coliforms from sewage discharges and heavy metals from mining and industries. In tropical waters nutrients are
rapidly consumed and predictions of the directly impacted zone can be limited to the
first few days of residence after discharge. Over longer periods the impact is on benthic
communities. Distinguishing natural from pollution-related environmental changes in
the benthos is very difficult (Boesch et al. 1990), yet this is a key to coastal management because of the confounding effects of natural and pollution variability (Ferraro
et al. 1991).
Nevertheless, the first impact to be studied is the spatial extent of the water-borne
poVutant plume. Simple analytical plume models (e.g. Fischer et al. 1979) are elegant
but unrealistic along a rugged coastline. A typical example is Malakal harbour, Palau
(Fig. 17). It has a deep water harbour surrounded by shallow waters and a coral reef to
the east. The currents were studied by Hamner et al. (1997). They are primarily tidal
and are complex because channelled by the topography (Anim. 27). As a result the
impacted zone from a sewage discharge varies enormously with small changes (100 m
only) in the discharge point. For instance (Anim. 28), if the discharge is located just
offshore of a small headland, as is actually the situation in the harbour, the plume
spreads over much of the outer harbour and the southern shipping channel while the
inner harbour remains relatively uncontaminated. The sewage plume is patchy and
intermittent over the coral reef and since it is 2 days old by the time it reaches the reef
its impact on the coral reef is thus likely to be small (Hamner et al. 1997). If the discharge occurred just inside the headland (Anim. 29), the inner harbour and the northern shipping channel would be contaminated. In both cases the plume breaks up in
patches in the outer harbour. Occasional water quality sampling at a few scattered points
in the harbour without synchronising the sampling with the expected location of the
plume as a function of the tide, as is the usual practice, is clearly meaningless.
In turbid estuaries mud is a major sink for pollutants such as heavy metals. Predictions of the fate of these pollutants necessitate understanding not only the dynamics of water and fine sediment but also the chemistry of the heavy metals in the estuary (Salomons and Forstner 1984). In particular, different forms of particulate metals
may exist and the metals may be exchanged between the particulate and the diffused
states and between different forms of particulate states (Fig. 18). These reaction rates
are a priori unknown but may be derived from laboratory experiments. The results are
generally very difficult to interpret because several parameters are important, including salinity, suspended sediment concentration, pH, POC and DOC. Nevertheless, for
practical applications it may be necessary to simplify these relations to incorporate
only the dominant parameters. For instance, a decay model could be used to
parameterize the transformation of releasable metal into exchangeable metal, and a
Kd model could be used for the reaction between particulate and dissolved metals. The
estuarine mobile mud can be assumed to be ultimately saturated with metal at steady
state. With all these simplifying assumptions the resulting distribution of predicted
dissolved (Anim. 30) and particulate (Anim. 31) metals can be predicted. Further
modelling shows that the zones of maximum concentrations vary continuously with
varying tidal amplitudes and with the wind. The size of the impacted zones, the maxi-
7.6
Water Quality
Typically water quality may involve nutrients and faecal coliforms from sewage discharges and heavy metals from mining and industries. In tropical waters nutrients are
rapidly consumed and predictions of the directly impacted zone can be limited to the
first few days of residence after discharge. Over longer periods the impact is on benthic
communities. Distinguishing natural from pollution-related environmental changes in
the benthos is very difficult (Boesch et al. 1990), yet this is a key to coastal management because of the confounding effects of natural and pollution variability (Ferraro
et al. 1991).
Nevertheless, the first impact to be studied is the spatial extent of the water-borne
poVutant plume. Simple analytical plume models (e.g. Fischer et al. 1979) are elegant
but unrealistic along a rugged coastline. A typical example is Malakal harbour, Palau
(Fig. 17). It has a deep water harbour surrounded by shallow waters and a coral reef to
the east. The currents were studied by Hamner et al. (1997). They are primarily tidal
and are complex because channelled by the topography (Anim. 27). As a result the
impacted zone from a sewage discharge varies enormously with small changes (100 m
only) in the discharge point. For instance (Anim. 28), if the discharge is located just
offshore of a small headland, as is actually the situation in the harbour, the plume
spreads over much of the outer harbour and the southern shipping channel while the
inner harbour remains relatively uncontaminated. The sewage plume is patchy and
intermittent over the coral reef and since it is 2 days old by the time it reaches the reef
its impact on the coral reef is thus likely to be small (Hamner et al. 1997). If the discharge occurred just inside the headland (Anim. 29), the inner harbour and the northern shipping channel would be contaminated. In both cases the plume breaks up in
patches in the outer harbour. Occasional water quality sampling at a few scattered points
in the harbour without synchronising the sampling with the expected location of the
plume as a function of the tide, as is the usual practice, is clearly meaningless.
In turbid estuaries mud is a major sink for pollutants such as heavy metals. Predictions of the fate of these pollutants necessitate understanding not only the dynamics of water and fine sediment but also the chemistry of the heavy metals in the estuary (Salomons and Forstner 1984). In particular, different forms of particulate metals
may exist and the metals may be exchanged between the particulate and the diffused
states and between different forms of particulate states (Fig. 18). These reaction rates
are a priori unknown but may be derived from laboratory experiments. The results are
generally very difficult to interpret because several parameters are important, including salinity, suspended sediment concentration, pH, POC and DOC. Nevertheless, for
practical applications it may be necessary to simplify these relations to incorporate
only the dominant parameters. For instance, a decay model could be used to
parameterize the transformation of releasable metal into exchangeable metal, and a
Kd model could be used for the reaction between particulate and dissolved metals. The
estuarine mobile mud can be assumed to be ultimately saturated with metal at steady
state. With all these simplifying assumptions the resulting distribution of predicted
dissolved (Anim. 30) and particulate (Anim. 31) metals can be predicted. Further
modelling shows that the zones of maximum concentrations vary continuously with
varying tidal amplitudes and with the wind. The size of the impacted zones, the maxi-