Environmental Engineering Reference
In-Depth Information
sediments are bound with native phosphorus (Olley & Wallbrink
2004
), which
is remobilised when riparian surfaces are exposed or when changes to the
hydrological response of the catchment elevates the erosive potential of storm
runoff. This change acts to further advantage productive phytoplankton
reinforcing the state shift from a previous clear-water condition. The same
catchment changes driving erosion also alter the surface-groundwater balance
and can lead to the increased accession of precipitation to recharge zones.
The resultant increased hydraulic pressure can expand upslope the region of
groundwater discharge (Macumber
1991
) and, where this is overlain by strata
rich in connate salts, can lead to surface secondary salinisation. This leads to
increased flux of solutes to aquatic systems elevating surface water salinity. The
associated increased sodicity leads to soil efflorescence and makes the soil
surface more dispersive and erodable (Neave & Rayburg
2006
). Furthermore,
in provinces rich in sulphate salts, the burial of sulphates in sites of sediment
accumulation can lead to the rapid production of sulphides. These, if exposed
under conditions of drought, for example, can lead to the release of sulphurous
acids in the manner of acid sulphate soils in coastal contexts where sediments
and salts have accumulated over millennia. In all, the sum of many diffuse
sources of pollution, delivered at very low loads over long periods, can result
in a cocktail of acute salinity, acidity, turbidity or eutrophication. Ultimately,
due to the interrelatedness of the drivers of pollution, several pollution symp-
toms can emerge synchronously in wetland systems (Gell et al.
2007a
). Here, the
multifaceted suite of drivers in action makes cause and effect particularly
difficult to unravel.
Commitments to restore wetlands
The restoration of wetland systems is now a high profile ideal of many developed
and developing nations. Restoration or rehabilitation of wetlands affected by
direct sources of pollution is more easily affected as the degraded condition is
often attributable to few pollution sources. Depending on the nature of the
system, rehabilitation can occur rapidly, e.g., in fast flowing streams, or can take
decades, e.g., endorheic (closed) crater lakes (Gell, unpublished data) and estuaries
(
Chapter 14
). The restoration of wetlands subject mostly to diffuse pollution
sources, particularly if situated within large catchments such as the Murray
Darling Basin (MDB) of Australia or the Mississippi River basin in the USA, is a
vastly greater challenge as the degradation may have occurred over decades,
is driven by a myriad of causes and sources of pollution and continues to be
subjected to a pervasive shift inhydrological, ecological and socio-political regime.
South Australia's wetland strategy
There are now emerging strategies and policies to preserve, rehabilitate and
restore wetlands, including rivers and lakes. Across Australia, the Federal
