Eight sediment cores recovered from Tamaki Estuary were analysed for Cu, Pb, Zn, and Cd using downward cored sub-samples. The results indicate a significant upward enrichment in heavy metals with the highest concentrations found in the uppermost 0–10 cm layer. Assessment of heavy metal pollution in marine sediments requires knowledge of pre-anthropogenic metal concentrations to act as a reference against which measured values can be compared. Pristine values for the cored sediments were determined from flat “base-line” metal trends evident in lower core samples. Various methods for calculating metal enrichment and contamination factors are reviewed in detail and a modified and more robust version of the procedure for calculating the degree of contamination is proposed. The revised procedure allows the incorporation of a flexible range of pollutants, including various organic species, and the degree of contamination is expressed as an average ratio rather than an absolute summation number. Comparative data for normalized enrichment factors and the modified degree of contamination show that Tamaki Estuary sediments have suffered significant systematic heavy metal contamination following catchment urbanization. Compared to baseline values the uppermost sediment layers show four-fold enrichment averaged across eight cores and four analysed metals.

Assessment of heavy metal enrichment factors and the degree of contamination in marine sediments from Tamaki Estuary, Auckland, New Zealand.

Historic and ongoing sediment contamination adversely affects estuaries, among the most productive marine ecosystems in the world. However, all estuaries are not the same, and estuarine sediments cannot be treated as either fresh or marine sediments or properly assessed without understanding both seasonal and spatial estuarine variability and processes, which are reviewed. Estuaries are physicochemically unique, primarily because of their variable salinity but also because of their strong gradients in other parameters, such as temperature, pH, dissolved oxygen, redox potential, and amount and composition of particles. Salinity (overlying and interstitial) varies spatially (laterally, vertically) and temporally and is the controlling factor for partitioning of contaminants between sediments and overlying or interstitial water. Salinity also controls the distribution and types of estuarine biota. Benthic infauna are affected by interstitial salinities that can be very different than overlying salinities, resulting in large-scale seasonal species shifts in salt wedge estuaries. There are fewer estuarine species than fresh or marine species (the paradox of brackish water). Chemical, toxicological, and community-level assessment techniques for estuarine sediment are reviewed and assessed, including chemistry (grain size effects, background enrichment, bioavailability, sediment quality values, interstitial water chemistry), biological surveys, and whole sediment toxicity testing (single-species tests, potential confounding factors, community level tests, laboratory-to-field comparisons). Based on this review, there is a clear need to tailor such assessment techniques specifically for estuarine environments. For instance, bioavailability models including equilibrium partitioning may have little applicability to estuarine sediments, appropriate reference comparisons are difficult in biological surveys, and there are too few full-gradient estuarine sediment toxicity tests available. Specific recommendations are made to address these and other issues.

https://setac.onlinelibrary.wiley.com/doi/pdf/10.1002/etc.5620200102

Assessing sediment contamination in estuaries.

Suspended particles are instrumental in controlling the reactivity, transport and biological impacts of substances in aquatic environments, and provide a crucial link for chemical constituents between the water column, bed sediment and food chain. This article reviews the role of suspended particles in the chemical and biological cycling of trace constituents (trace metals, organo-metallic compounds and hydrophobic organic micropollutants; HOMs) in estuaries, with particular emphasis on the effects of and changes to particle reactivity and composition.

The partitioning (or distribution coefficient,KD ) and bioavailability of chemical constituents, and assimilation efficiency (AE) of such by bivalve suspension feeders, are identified as key parameters requiring definition for accurate biogeochemical modelling, and the discussion centres around the determination of and controls on these parameters. Particle-water interactions encompass a variety of physical, biological, electrostatic and hydrophobic effects, and are largely dependent on the character and concentration of suspended particles and salinity. The salinity-dependence results from the competing and complexing effects of seawater ions for trace metals, and the compression of water in the presence of dissolved seawater ions and consequent salting out of neutral solute (HOMs, organo-metallic compounds and some trace metal complexes). The extent of biological solubilization of chemical constituents from suspended particles is dependent on the nature of chemical components of the gastro-intestinal environment and their interactions with ingested particles, and the physiological (e.g. gut passage time) and chemical (e.g. redox conditions and pH) constraints imposed on these interactions. Generally, chemicals that associate with fine, organic-rich particles (or, for some HOMs, fine inorganic particles), and desorb at pH 5–6 and/or complex with digestive enzymes or surfactants are most readily solubilized in the gut. The extent of assimilation of solubilized chemical is then determined by its ability to pass the gut lining and partition into cytosolic material.

In practice, KD and AE are determined experimentally by means of radiotracers added to contained suspensions or mesocosms, while operational measurement of bioavailability relies on in vitro chemical or biological (enzymatic) extraction of particles. What is lacking, however, and is identified as an ultimate goal of future research, is the ability to predict these parameters from theoretical principles and thermodynamic constants. Since many of the inherent interactions and mechanisms are controlled by particle composition and reactivity, a more immediate objective would be better characterization of the biogeochemical properties of suspended particles themselves. This includes chemical resolution of the bulk organic matter, definition of the abundance and synergistic effects of component sorbent phases, and determination of the effects of particle-seawater ion interactions on the reactivity of the particle surface.

Suspended Particles: Their Role in Estuarine Biogeochemical Cycles

Evaluation of metal contamination in coastal sediments of the Bay of Bengal, India: geochemical and statistical approaches.

https://www.ldeo.columbia.edu/~avangeen/pdf/Hornberger_MarChem_99.pdf

Historical trends of metals in the sediments of San Francisco Bay, California

Sabine-Neches Estuary, near Beaumont, TX, receives wastewater effluents from over 160 industrial and municipal treatment plants. The concentrations of trace metals (Co, Cr, Cu, Ni, Pb, and Zn), Al, Fe, Mn, and organic carbon were determined in four dated sediment cores. A reliable geochronology and reconstruction of the history of trace metal inputs of these sediments was possible because the 239,240 Pu profiles closely trace the bomb fallout history into the environment. Down-core variations of aluminum-normalized enrichment factors for these metals demonstrate that the sediments of this estuary have remained relatively 'pristine' with respect to trace metal contamination since 1860. While the concentrations of Pb and Zn at various depths in the sediment column are slightly enriched, Co, Cr, Cu, and Ni are depleted. The sedimentary and biogenic particles that are presently being deposited are also depleted in trace metals. Lack of strong enrichment for trace metals like Cu can be attributed to the short residence time of water, low salinity conditions, and possibly strong complexation of these metals with organic matter.

History of trace metal pollution in sabine-neches estuary, beaumont, Texas.

Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

The Sabine-Neches estuary is a shallow, turbid estuary in south-east Texas with high concentrations of dissolved organic carbon (DOC). The sediment inventory of210Pb and239,240Pu indicates that only a fraction of the particle-associated nuclides that reach the estuary were retained in the sediment. To better understand the cause for this low-sediment inventory of particle-reactive nuclides,7Be and210Pb concentrations have been measured in the dissolved and particulate phases, in addition to the DOC and suspended particle concentrations. The ratios of dissolved to particulate concentrations of7Be and210Pb were generally higher than in most other coastal waters.

The dissolved residence times of7Be and210Pb (accounting for riverine input) varied between 0·6 and 9·6 days and 1·7 and 9·8 days, respectively. Distribution coefficients (Kd)ranged between 1500 and 87 100 cm3 g−1for7Be and 2600 and 37 000 cm3 g−1for210Pb. TheseKds are lower than those reported for most coastal waters. There was no significant correlation between suspended particle concentration andKdof either7Be and210Pb; this has been observed for many other particle-reactive nuclides, suggesting that particle concentration is not the primary controlling variable for the removal of particle-reactive nuclides in these high DOC waters. The average particle residence time in this estuary is ∼2 days. The relatively lowKdvalues, longer dissolved residence times of7Be and210Pb, longer particle residence times and shorter hydraulic residence times compared to other coastal areas, result in only a partial removal of particle-reactive radionuclides in this estuary.

Mahalingam Ravichandran

Papers

History of trace metal pollution in sabine-neches estuary, beaumont, Texas.

Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

Cycling of7Be and210Pb in a High DOC, Shallow, Turbid Estuary of South-east Texas

Distribution of rare earth elements in sediment cores of Sabine-Neches Estuary