Richard Carignan

Potential artifacts in the determination of metal partitioning in sediments by a sequential extraction procedure

The partitioning of trace metals in sediments, as obtained with a sequential extraction procedure, may be affected by (i) the techniques used to preserve the sediments before analysis and (ii) the presence/absence of atmospheric oxygen during the extraction steps. No storage method tested completely preserved the initial chemical and physical characteristics of the sediments. Drying of the sediment (freeze-drying; oven-drying) should be avoided; acceptable preservation techniques include freezing or short-term wet storage (1-2/sup 0/C). Among the different metals (Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe, and Mn), copper, iron, and zinc were particularly sensitive to sample pretreatment. For anoxic sediments, the maintenance of oxygen-free conditions during the extractions is of critical importance.

Trace metals in oxic lake sediments: possible adsorption onto iron oxyhydroxides

Abstract Apparent overall equilibrium constants for the adsorption of Cd, Cu, Ni, Pb and Zn onto natural iron oxyhydroxides have been calculated from the partitioning of these trace metals in oxic lake sediments and the in situ measurement of trace metal concentrations in the associated pore waters. Such values obtained from lakes of various pH located on the Precambrian Shield, in the area of Sudbury, Ontario, are compared with equilibrium constants obtained for the adsorption of the trace metals onto iron oxyhydroxides in well-defined media. The field data are consistent with laboratory experiments reported in the literature and with theory. Both the influence of pH upon adsorption and the binding strength sequence observed for the field data agree with theory. At high sediment pH values, the partitioning of Cd, Ni and Zn between the pore waters and the natural iron oxyhydroxides is similar to those reported in the literature for the adsorption of these metals at low surface coverage onto amorphous iron oxyhydroxides in a NaNO3 medium; deviation from this simple model is however observed for Cu and Pb, presumably due to the competitive action of dissolved ligands. At low sediment pH values, the adsorption is much higher than predicted by the simple model and can be explained by the formation of ternary complexes with the iron oxyhydroxide surface.

Geochemistry of trace metals associated with reduced sulfur in freshwater sediments

Porewater concentration profiles were determined for Fe, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn), sulfide, SO4 and pH in two Canadian Shield lakes (Chevreuil and Clearwater). Profiles of pyrite, sedimentary trace elements associated with pyrite and AVS were also obtained at the same sites. Thermodynamic calculations are used, for the anoxic porewaters where sulfide was measured, to characterize diagenetic processes involving sulfide and trace elements and to illustrate the importance of sulfide, and possibly polysulfides and thiols, in binding trace elements. The ion activity products (IAP) of Fe sulfide agree with the solubility products (Ks) of greigite or mackinawite. For Co, Ni and Zn, IAP values are close to the KS values of their sulfide precipitates; for Cu and Pb, IAP/Ks indicate large oversaturations, which can be explained by the presence of other ligands (not measured) such as polysulfides (Cu) and thiols (Pb). Cobalt, Cu, Ni and Zn porewater profiles generally display a decrease in concentration with increasing ΣH2S, as expected for transition metals, whereas Cd, Pb and Zn show an increase (mobilisation). The results suggest that removal of trace elements from anoxic porewaters occurs by coprecipitation (As and Mn) with FeS(s) and/or adsorption (As and Mn) on FeS(s), and by formation of discrete solid sulfides (Cd, Cu, Ni, Pb, Zn and Co). Reactive Fe is extensively sulfidized (51–65%) in both lakes, mostly as pyrite, but also as AVS. Similarities between As, Co, Cu and Ni to Fe ratios in pyrite and their corresponding mean diffusive flux ratios suggest that pyrite is an important sink at depth for these trace elements. High molar ratios of trace elements to Fe in pyrite from Clearwater Lake correspond chronologically to the onset of smelting activities. AVS can be an important reservoir of reactive As, Cd and Ni and, to a lesser extent, of Co, Cu and Pb. Overall, the trace elements most extensively sulfidized were Ni, Cd and As (maximum of 100%, 81% and 49% of the reactive fraction, respectively), whereas Co, Cu, Mn, Pb and Zn were only moderately sulfidized (11–16%).

Sediment porewater sampling for metal analysis: A comparison of techniques

Abstract The concentrations of Ca, Mg, Fe, Mn, Cr, Co, Ni, Cu, Zn, Cd and organic carbon were measured in a sediment porewater sampled by two different techniques: in situ dialysis, and centrifugation followed by filtration. Sediment centrifugation at 5,000 rpm followed by filtration (0.45 μm membrane) was equivalent to dialysis for Co, Ni, Cr, Fe, Mn but gave higher and more variable concentrations for Cu, Zn and organic carbon. Concentrations comparable to those obtained by dialysis were found when centrifugation speed was increased to 11,000 rpm and when 0.2 or 0.03 μm membranes were used to filter the supernatant. This procedure was also found equivalent to dialysis for Cr, Co, Ni, Cd and organic carbon. A reduction of the dialysis membrane pore size to 0.002 μm did not produce any apparent change in the porewater composition; however, a further reduction to 0.001 μm indicated either incomplete equilibration after two weeks, or exclusion of medium sized metal-organic complexes. Because of its inherent simplicity, in situ dialysis appears particularly well adapted to the study of trace constituents in sediment porewaters under field conditions.

Partitioning of zinc between the water column and the oxic sediments in lakes

Zinc concentrations were measured in oxic sediments and in the associated interstitial and overlying waters (vertical profiles) at 40 littoral stations of various lakes; the stations were chosen to represent a large range of lake pH (pH 4–8.4) and of zinc concentrations in the sediments and the overlying waters. Large concentration gradients of dissolved zinc (leading presumably to large downward fluxes) were found at the sediment-water interface of acid lakes (pH < 6). Concentrations of dissolved zinc [Zn]in the overlying waters were found to be highly undersaturated with respect to Zn(OH)2 (s) and ZnC03 (s), and in general with respect to ZnSiO3 (s), ruling out solubility equilibrium with pure solid phases as a mechanism of control of zinc concentrations in the overlying waters. The partitioning of zinc between the overlying water and the surficial sediments was described by sorption processes, using either the distribution coefficient model or a simplified version of the surface complexation model. In relation with this latter model, binding intensity values, ka, for the sorption of zinc to natural iron oxyhydroxides have been estimated using the field data, I.E., the partitioning of Zn in oxic lake sediments and their dissolved concentrations in the associated overlying waters. The following empirical relationship between ka and lake pH has been found: log ka = 1.21 pH - 2.83 (r2 = 0.89). The pH dependence of KA is in agreement with recent theory on the adsorption of trace metals at oxyhydroxide surfaces; however, the binding intensity values derived from field measurements differ markedly in some cases from those of the equilibrium constants obtained for zinc in laboratory experiments. As several observations suggest that Fe-bound zinc is mobile, the distribution of labile zinc between the water column and the sedimentary iron oxyhydroxides has been calculated as a function of pH for typical shallow lakes with the log KA-pH empirical relationship. This model predicts that most of the mobile Zn should be present in the water column at pH < 5 and in the surficial sediments above pH 7. The pH interval 5.5–6.5 (into which fall a large number of poorly buffered lakes receiving acidic deposition) defines a region where changing pH in the water column should have the most pronounced effect on the partitioning of Zn between the water column and the sediment.

Trace metal deposition and mobility in the sediments of two lakes near Sudbury, Ontario

The accumulation and mobility of Fe, Mn, Al, Cu, Ni and Pb in the sediments of two lakes (Clearwater, pH 4.5; and McFarlane, pH 7.5) near Sudbury, Ontario have been investigated. The Al, Cu and Ni concentrations are expectedly relatively high in the overlying waters of Clearwater Lake and much lower for Al and Cu in McFarlane Lake. The low trace metal concentrations found in the anoxic porewaters of Clearwater Lake could be explained by a sharp increase in porewater pH concomitant with SO42 reduction and H2S production within the first 1–2 cm of the sediments, which has conceivably led to the precipitation of mineral phases such as AL(OH)3, NiS, and CuS. In both lakes, Fe concentrations in anoxic porewaters appear to be controlled by FeS and/or FeCO3 formation. Solubility calculations also indicate MnCO3 precipitation in McFarlane Lake. In Clearwater Lake, however, both porewater and total Mn were relatively low, a possible result of the continuous loss of Mn(II) through the acidic interface. It is suggested that upwardly decreasing total Mn profiles resulting from the removal of Mn from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification. The downward diffusion of AI, Cu and Ni from the overlying water to the sediments has been estimated from their concentration gradients at the interface and compared to their total accumulation rates in the sediments. In both lakes the diffusion of Al is negligible compared to its accumulation rate. However, diffusion accounts for 24–52% of the accumulation of Cu in the sediments of Clearwater Lake, but appears negligible in McFarlane Lake. The downward diffusive flux of Ni is important and may explain 76–161% of the estimated Ni accumulation rate in Clearwater Lake, and 59% in McFarlane Lake. The porewater Cu and Ni profiles suggest that the subsurface sedimentary trace metal peaks observed in Clearwater Lake (as in other acid lakes) may not be caused by sediment leaching or by a recent reduction in sedimentation but may have a diagenetic origin instead. Diffusion to the sediments thus appears to be an important and previously overlooked trace metal deposition mechanism, particularly in acid lakes.

An evaluation of approaches used to determine the distribution and biomass of emergent and submerged aquatic macrophytes over large spatial scales

We compared the performance of various approaches to determine the distribution and biomass of submerged and emergent aquatic plants in a large fluvial lake. Three empirical models linking local macrophyte biomass to single and multiple environmental variables were applied in a GIS-framework to estimate the spatial distribution and biomass of aquatic macrophytes in Lake St. Pierre, a large (300 km 2 ), shallow (mean depth: 3 m) and complex widening of the St. Lawrence River (Quebec, Canada). The resulting maps and emergent and submerged macrophyte distributions obtained independently by remote sensing and echo sounding techniques were compared to field data collected in 2000. Maps derived from echo sounding, from a biomass versus depth regression and from a four-variable model (i.e. exposure to wind and waves, plant growth form, water depth and transparency) were the most accurate (55–63% overall agreement with field data). Remote sensing techniques were the least accurate for determining underwater macrophyte distribution in Lake St. Pierre due to the limitations of image-based methods for detecting submerged aquatic vegetation in coloured, turbid waters. This study demonstrates that environmental models in combination with GIS can be used to estimate aquatic macrophyte distribution over larger spatial scales and to examine potential change in macrophyte growth form assemblages arising from different environmental conditions.

Nutrient dynamics in the floodplain ponds of the Paraná River (Argentina) dominated by the water hyacinth Eichhornia crassipes.

Some aspects of nutrient status and dynamics prevailing during low and high water conditions in the fringing floodplain ponds of the Paraná River dominated by the floating macrophyte Eichhornia crassipes are described. During summertime low water conditions, low DIN:DRP ratios (0.16–1.0) and low DIN (0.5–4.8 μmol.liter−1) in the root-zone of the floating meadows suggest that macrophyte growth is limited by nitrogen. DRP concentrations appear to be controlled more by abiotic sorption-dissolution than by biological reactions. Preflood nutrient fluxes from the sediments, as estimated from porewater profiles, show that a minimum of 1.19 and 0.38 mmol.m−2.d−1 of DIN and DRP were regenerated from the sediments, respectively. Heterotrophic N2 fixation is primarily associated with decaying litter (0.4 to 3.2 μmolN2.g−1.d−1). Nutrient recycling from sediments and meadow-litter, and heterotrophic N2 fixation (1.4 mmolN.m−2.d−1) appear sufficient to sustain high floating macrophyte productivity for long periods of time, without invoking large inputs from the river. The high water and early isolation periods are characterized by a very dynamic behavior of DIN, reflecting marked imbalances between N supply and demand by the biota. After hydrologic isolation of the ponds, DIN rapidly decreases to undetectable levels and stays low for the following 3 weeks, presumably as a result of high demand by phytoplankton and sediment bacteria. DIN increases again to high values 3–8 weeks after the flood, following the re-establishment of NH4+ fluxes from the sediments. Compared to DIN, DRP concentrations remain relatively high and change little during and after the flood. Because of their small amplitude and short duration, floods do not appear to stimulate floating macrophyte production in the Paraná.

The co-diagenesis of sulfur and iron in acid lake sediments of southwestern Québec

Reduced inorganic sulfur species constitute the major fraction of total sulfur in the recent sediments of eight acid lakes of southwestern Quebec. Nearly all of the anthropogenically derived excess S present in the sediments has accumulated as reduced inorganic forms, showing that organic S formation plays a very minor role in long term anthropogenic S retention in these lakes. These results appear different from most other published studies on S fixation in acid lake sediments. Respectively 74% and 76% of the variance in total S and reduced inorganic S burden in the recent sediments can be explained by their Fe content alone, indicating that the reduced S is associated with iron, and that the capacity of the sediments to retain S may be limited by available Fe supply. When indexes of organic C supply (winter O2 deficit and sediment C) are included with Fe in a multiple regression analysis, respectively 97% and 98% of the variance in total S and reduced inorganic S accumulation can be explained. The origin of the pronounced subsurface Fe maximum observed in most cores is attributed to the reduction and recent burial, as iron sulfides, of a surficial diagenetic Fe(OH)3 reserve. In some sediments, this Fe(III) reserve presently appears to be nearly or completely exhausted, and the lakes may have lost their capacity to generate permanent alkalinity via SO2−4 reduction. The present rate of alkalinity production due to SO2−4 reduction and fixation of reduced S in the sediments of one lake were estimated from several 210Pb-dated S profiles and averaged to 163 μeq · m−2 · d−1; this value is comparable to those reported for other acid lakes.

Macroinvertebrates on Eichhornia crassipes roots in two lakes of the Paraná River floodplain

Changes in the abundance, biomass and in the relativeproportions of functional feeding groups ofmacroinvertebrates were studied for 17 months duringlow and high water periods in two floodplain lakeswith indirect connections to the Paraná River.At low water, Eichhornia crassipes root clusterswere colonized by comparatively low densities ofmacroinvertebrates, especially in the lake moredistant to the river. Collector-gatherers andpredators were the most abundant functional feedinggroups, but different taxa of these groups dominatedin the two lakes during low water.At high water, macroinvertebrates densities increasedas a result of increased flow through the roots. Inthis period, collector-filterers dominated at bothlakes and constituted over 34% of themacroinvertebrates. Four independent variables(conductivity, hydrological periods, dissolved oxygenand site) explain 87% of the variability inmacroinvertebrates density expressed as the number ofindividuals per m2. No significant relationshipwere found between macroinvertebrate biomass and anyof the independent variables.