Summary of key issues:
|Description||Techniques to establish the transport pathways of sediments and the way in which contaminants are adsorbed and released from the sediments.|
|Temporal applicability||Typically applied to the short to medium-term (single tide up to several months).|
|Spatial applicability||Varying from a single point to estuary wide including the open coast.|
|Links with other tools||Numerical modelling can support these assessments by linking both local hydrodynamics and sediment dispersion with water quality parameters in a computational domain (see also water quality).|
|Data sources||Sediment information, hydrodynamic information, estuary volumes, areas, hypsometry, sediment budget/availability.|
|Necessary software tools / skills||Sediment transport and hydrodynamic modelling, good understanding of the sediment pathways and estuary processes.|
|Typical analyses||Changes in estuary morphology, erosion and deposition patterns.|
|Limitations||Requirement for calibration data (flow, water levels) and sediment characteristics (type, consolidation, thickness, density).|
Naturally occurring and synthetic contaminants such as heavy metals, hydrocarbons, organotins (TBT), pesticides, organochlorides, organophosphates, solvents and polychlorinated biphenyls (PCBs) tend to be associated with fine sediments (silts and clay) due in part to their greater surface area for geochemical binding.
The re-suspension and bioavailability of sediment bound contaminants in the water column is often an important consideration by regulators and statutory agencies when determining the acceptability of capital and maintenance dredging operations. The risks associated with such operations may be a determining factor when stipulating developmental controls such as seasonal restrictions and monitoring thresholds.
The potential risks for marine environmental receptors (biology and water quality) can be inferred initially through comparison with sediment quality standards. In the absence of statutory sediment quality values that define thresholds for acceptability (for licensed disposal for instance) the quality of material for dredge and disposal operations is typically assessed through comparison with Dutch (IADC/CEDA, 1997) and Canadian Standards (CCME, 1999).
Whilst such standards are based on biological effects for a range of organisms, the testing conditions used to derive the standards are unlikely to be representative of different dredging and construction techniques in the field. In particular, the degree of re-suspension of host suspended sediments is likely to vary.
Understanding particle-water interactions provides a means of estimating the degree of contamination of the water column (near and far-field effects) through the process of geochemical modelling.