G.E. Millward

Trace Metals in the Central and Southern North Sea

Measurements of both dissolved and particulate trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) were made on samples collected simultaneously in the central and southern North Sea (below 56°N), during four cruises covering the seasonal cycle. Analyses of trace metals in both phases were undertaken using similar analytical protocols, and the data subjected to rigorous quality controls. Clean techniques were used throughout sampling and analysis. Trace metal concentrations ranged from those characteristic of waters entering the shelf region from the North Atlantic Ocean to values which in some cases were two orders of magnitude higher. Marked increases in concentrations were often associated with waters directly influenced by riverwaters, but the results showed that the distributions could be influenced by processes other than fluvial inputs. The redox-sensitive metals, Fe and Mn, were influenced at some sites, including those overlying areas of fine-grained sediments, by benthic inputs associated with seasonal changes in oxidation-reduction conditions in the benthic interfacial zone. The behaviour of metals which are not redox-sensitive (Cd, Zn) in some cases paralleled those of Fe and Mn, suggesting an association with Mn-or Fe-rich solid phases, or with organic matter. However, seasonal variations in concentrations of Cd and Zn were not directly linked to the cycle of biological utilization and regeneration of nutrients, suggesting that their geochemistry is more influenced by boundary inputs. Resuspension of Pb-rich sediments, long-range transport of Pb-rich suspended particulate matter (SPM), and enhanced, though diffuse, atmospheric inputs of lead were considered to have contributed to the elevated concentrations of particulate Pb (mass metal/mass sediment) in the water column during winter. Estimates of partitioning (K D s) were derived from the dissolved and particulate data for each of the metals over the seasonal cycle. For the more particle-reactive metals (Co, Fe, Mn, Pb), transport in the suspended particulate phase is a significant, sometimes dominant factor. In contrast, for Cd, Cu and Zn, transport in solution will dominate under almost all conditions in the southern North Sea. Furthermore, the values of K D , for a particular metal, varied widely, partly in response to seasonal differences in the relative contributions of lithogenous and biogenous particles to the particles in suspension. Only for Cu and Zn during the winter could the partitioning be described by a narrow range of K D values covering the observed concentrations of SPM. The data from this study give unique information concerning the contemporaneous distribution of dissolved and particulate trace metals in the southers North Sea and they provide a baseline against which the effects of reductions of inputs into the North Sea may be more accurately assessed. Viewed in combination, the dissolved and particulate data have yielded new insights into mechanisms affecting trace metal cycling and transport, essential in the development of coupled hydrodynamic-geochemical models.

Metal behaviour in an estuary polluted by acid mine drainage: The role of particulate matter

The concentrations of dissolved and suspended particulate Cd, Cu and Zn have been determined in water samples obtained during two axial transects of the Rio Tinto-Huelva Ria system in south-west Spain, which is severely impacted by acid mine drainage. Although the metal concentrations in both phases were elevated, dissolved metals were dominant and, in the upper estuary, constituted > 99% of total metal in the water column. Dissolved metals behaved non-conservatively on each transect, with maximum concentrations in the low salinity region. There was no evidence of metal adsorption within the turbidity maximum zone, despite the high specific surface areas of resuspending particles. Measurements of electrophoretic mobility showed that the suspended particulate matter (SPM) had a positive surface charge in the salinity range 0-4, where the waters had a pH < 3. Desorption experiments were carried out in which SPM from the turbidity maximum zone was resuspended in coastal seawater. The desorption of the metals was monitored for 24 h, using anodic stripping voltammetry (ASV) to detect the variation in total dissolved Cd, Cu and Zn and the species of Cu and Zn. Total dissolved Cd concentrations doubled during the incubation period, whereas the concentration of total dissolved Cu declined and that of Zn remained rather constant. The ASV-labile fraction of dissolved Cu and Zn showed an initial sharp release followed by a slower uptake. However, desorption was shown to be a minor source of dissolved metals and made little contribution to the non-conservative behaviour in the low salinity zone. The results are used to predict the effects of acid mine drainage on estuarine ecology.

Particulate metals in five major North Sea estuaries

Suspended sediment samples collected from the North Sea and during axial surveys of five major North Sea estuaries (Humber, Thames, Scheldt, Weser, Elbe) have been analysed in a consistent manner for leachable metals, specific surface area and carbon. Particulate trace metal (Co, Cr, Cu, Ni, Pb and Zn) concentrations were similar in each estuary and estimates of fluxes to the North Sea based on an empirically derived prediction equation therefore reflected catchment size. Concentrations generally decreased seawards as a result of mixing of riverborne sediment of high metal concentration with material of marine origin of relatively low metal concentration, although in the Humber, substantial anthropogenic inputs at high salinities were responsible for more ambiguous distributions. In contrast, profiles and concentrations of Fe and Mn, implicated as being important to particle surface and compositional properties, exhibited significant yet explicable inter-estuarine variability (e.g. Humber during winter: Fe = 12·0 mg g−1, Mn = 0·99 mg g−1; Elbe during summer: Fe = 6·6 mg g−1, Mn = 3·08 mg g−1). Comparable data such as these serve as valuable baselines for any future international long-term water quality monitoring programme of European estuaries.

Tidal export of particulate mercury from the most contaminated area of Aveiro's Lagoon, Portugal

Mercury pollution is a regional problem in the Aveiros Lagoon. During 4 decades, a remote branch (Estarreja Channel) has received a discharge of mercury from a chlor-alkali plant, that is spread in the Laranjo Basin. Of special interest is the dispersion of mercury from these areas to the Lagoon. This work is a first attempt to quantify the net flux of particulate mercury from the most contaminated basin (Laranjo). Measurements of current intensity, suspended particulate matter and particulate mercury were carried out at two sampling stations over a spring tidal cycle, in April 1994. Based on a mass balance calculation at two stations, at the end of Estarreja Channel (F1) and near the outlet of Laranjo Basin (F2), the net tidal fluxes of particulate anthropogenic mercury were estimated: 0.4 g of mercury enter the Laranjo Basin and 94.5 g of particulate mercury escape the Basin. These results indicate that mercury from the most contaminated area continues to be dispersed, predominantly through the particulate fraction, to the rest of the Lagoon.

Modelling contaminant geochemistry in estuaries

Abstract A numerical model predicting the transport and fate of contaminants in estuaries has been developed and tested. The modelling framework allows for (1) advection and diffusion of contaminants in the dissolved phase, (2) partitioning of contaminants between the dissolved and adsorbed phases, (3) input of adsorbed contaminants to the water column due to the resuspension of bed sediment, (4) accumulation of contaminants in the bed sediment due to deposition of suspended sediment and (5) in the case of organic contaminants, losses due to volatilization and biodegradation. Particle-water interactions throughout the estuary are encoded using relationships between empirically-derived partition coefficients and salinity, which have been coupled to an established two-dimensional depth integrated hydrodynamic and sediment model configured for the Humber Estuary, England, to simulate the distributions and transport of cadmium and zinc. The model has been run for a range of tidal and river flow conditions, and has been calibrated against field measurements of hydrodynamic parameters, salinity, suspended sediment concentrations and dissolved trace metal conentrations, and validated using independent dissolved trace metal data sets. The application of the model is discussed to a wide range of estuaries where the fate of trace metal contaminants is of immediate concern.

Processes influencing the fate of trace metals in the North Sea

Measurements were made in the southern North Sea of trace metals (dissolved and particulate Mn, Fe, Ni, Co, Cu, Zn, Cd, Pb) with contrasting biogeochemical behaviours. The extent to which distributions of dissolved metals over the survey area were determined by the mixing of river waters with seawater varied among the metals, and also seasonally; even for nickel and copper, which behaved most conservatively, other processes were significant. Specific instances of the effects of scavenging by particles and of inputs from benthic recycling are evident from the data and these processes lead generally to modifications of fluxes of dissolved metals in estuarine plumes. While the effect of atmospheric inputs on dissolved concentrations of metals is, on the basis of other evidence, a factor contributing to spatial and temporal variability, specific features arising from them were not identified; they probably account, however, for increased concentrations of dissolved lead observed at high salinities in the Humber plume. Seasonal variations in concentrations of dissolved metals, paralleling those of micronutrients, were not identified but manganese showed a pronounced seasonal variability related to its reduction—oxidation chemistry. Wide differences occurred in the partition between dissolved and particulate fractions, although for copper and zinc the partition could be described by a narrow range of distribution coefficients under certain conditions.

Fluxes and retention of trace metals in the Humber Estuary

The Humber Estuary is a physically dynamic system into which industrial wastes contaminated with heavy metals have been discharged for many years. The total fluxes of Cd, Cu, Fe, Ni, Pb and Zn from the Humber Estuary to the North Sea have been estimated for December 1988, May 1990 and August 1990. These fluxes have been compared to inputs into the estuary from rivers, sewage and industry as reported by the Paris Commission. A discrepancy exists between the estuarine inputs and the fluxes to the North Sea, which suggests that the estuary is acting as a trap for fine, metal-contaminated sediments. An alternative approach to flux assessment also indicates that metal retention by sediments is a major feature of the Humber Estuary. The results are discussed in the context of the recovery of the estuary from metal contamination and the consequences for the coastal zone. This critical review allows assessment of effectiveness of current management strategies and new directions for the future to be proposed.

Distribution and partitioning of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and polychlorinated biphenyls in the Humber Estuary, UK

Abstract The distribution and concentration of polychlorinated dibenzo-para-dioxins (PCDDs), polychlorinated dibenzo-furans (PCDFs) and polychlorinated biphenyls (PCBs) in Humber Estuary bed sediment and in suspended particulate matter were extensively studied. Dioxin concentrations in sediments ranged from below detection to 3830 pg g−1 and PCB concentrations were from nondetectable to 84 ng g−1. These concentrations were low in comparison to other estuaries. The concentrations of chlorinated organics were found to be correlated with sediment lipid content, which is a significant factor in determining the contaminant distribution in the Humber Estuary. Laboratory experiments were carried out using 14C-radiolabelled compounds to determine partition coefficients (KD). A KD of 2.5 × 105 was determined for 2,3,7,8-TCDD and that of 6.4 × 104 for 2,2′,5,5′-TCB; these did not vary significantly with salinity. There was no evidence in either the concentration data or the homolog profiles that contamination from tributaries to the Humber Estuary had penetrated the estuary in significant quantities.

Trace metal distribution coefficients in the Weser Estuary (Germany)

Trace metals in the dissolved phase, and in operationally defined available and total particulate associations, have been determined along an axial transect of the Weser Estuary, together with measurements of salinity, temperature, pH, dissolved oxygen, suspended particulate matter (SPM), chlorophyll a, free and attached bacteria, particulate carbon and particle specific surface area. Throughout the estuary the dissolved metals exhibited non-conservative behaviour due to sorption reactions with particle surfaces and sediment-water exchange processes. This reactivity was enhanced at the fresh water-brackish water interface where changes in the master variables were most pronounced. The influence of sorptive processes on total particulate metal distributions was not evident and major compositional variability was ascribed to physical mixing of different particle populations. The relative proportions of trace metals in the dissolved and solid phases were interpreted in terms of conditional distribution coefficients, KDs, whose magnitude depends critically on which fraction of particulate metal is selected (i.e. available or total). The KDs for Fe and Mn increased with increased SPM concentration, whereas those for Cu, Ni, Zn and possibly Pb decreased with SPM; such observations are related to whether particle-water exchange processes at any locality in the system are far from or at (quasi-) equilibrium, and to differences in the sorptive characteristics of suspended particle populations. These results have implications for the behaviour of trace metals in the Weser Estuary and are also discussed in the context of the value of the KD concept for quantitative modelling of dissolved trace metal distribution in estuaries.

The solid-solution partitioning of trace metals in the southern North Sea—in situ radiochemical experiments

Abstract An in situ experimental technique using radioisotopes for studying trace metal behaviour in coastal waters has been developed and results from the southern North Sea presented. Isotopes109Cd,137Cs and65Zn were adsorbed rapidly (within 25 h) by particles from the Humber and Thames estuarine plumes. The reversibility of this sorption was assessed by a series of leaching experiments and decreased in the order109Cd (removed by seawater) >65Zn (removed by 1M HCl) 137Cs (40% resistant to 1M HCl). In contrast,54Mn was deposited onto plume particles by slow oxidative precipitation which proceeded linearly. However, the protracted nature of this reaction meant that particle affinity (as quantified by a solid-solution distribution coefficient, KD) after 5 days was significantly greater than other isotopes studied; thus, 54Mn (5 × 104) >65Zn (5 × 103) >109Cd,137Cs (5 × 102). Moreover, 5-day KDs for109Cd and65Zn were enhanced by plankton compared with plume samples, whereas a137Cs association with plankton was not detectable. These trends were corroborated by KDs derived from ambient dissolved and digested particulate metal concentrations as determined by atomic absorption methods. Ambient KDs were, however, consistently an order of magnitude or more greater than their radiochemical counterparts because of an overwhelming detrital (unavailable) component of particulate metals inherent in their calculation. Accordingly, results from the radiochemical approach are favoured for incorporation into incipient shelf sea pollutant dispersal models and for assessing the potential for accumulation in the aquatic foodchain.