Geochemical processes controlling concentrations of Al, Fe, and Mn in Nova Scotia lakes

Abstract

Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes in Halifax County, Nova Scotia, to determine the factors controlling the behavior of these metals in acidic waters with relatively high concentrations of organic matter. Concentrations of trace metals ranged from <50 to 700 µg liter−1 (Al), <20 to 960 µg liter−1 (Fe), and <10 to 240 µg liter−1 (Mn). Concentrations of Fe and Al were highly correlated with H’ ion and DOC concentration, whereas Mn was correlated most strongly with H+ ion concentration and size of catchment relative to lake surface area. Computation of trace‐metal speciation indicated that lakes with pH > 5 were in equilibrium with gibbsite and amorphous iron hydroxide but highly supersaturated with respect to manganite and Mn(IV) oxides. Concentrations of Mn appear to be determined by the magnitude of the source (i.e. the catchment area) relative to the size of the lake. In lakes with pH < 5, concentrations of Al and Fe seem to be regulated by interactions with DOC. Although the metals and organic C enter the lake separately, we propose that precipitation of metal humates is the major process regulating concentrations of Al and Fe and, to a lesser extent, DOC in these lakes.

The Bioavailability and Toxicity of Aluminum in Aquatic Environments

“…The added DOM was likely able to keep Al off the gills through complexation because the DOM binds Al better than do the gills. Previous estimates of Al-DOM binding strengths are log K Al-FA = 7.8 (Shuman, 1992), log K Al-FA = 6.4 at pH 4.5 (Browne and Driscoll, 1993), and log K Al-DOC = 6.2 (Urban et al, 1990), and log K AlA = 8.4, log K AlHA = 13.1 (Driscoll and Postek, 1996;Section II.A.4 and Figure 3C), so DOC at high enough concentrations should be able to bind Al well enough to keep Al off fish gills, with log K Al-gill~6 .5 (Wilkinson et al, 1990).…”

Effect of dissolved organic carbon on 59Fe scavenging

“…However, DOC could enhance or inhibit adsorption to particles and incorporation into biota depending on the reactivity or availability of the metalo-organic complex to particles or biota, respectively. It has been hypothesized that DOC could be removed from water by scavenging (Urban et al 1990), but it can also be mineralized by microbial metabolism (Tranvik and Hiifle 1987).…”

“…8, Table 4), while measured scavenging followed the opposite trend. Lastly, Fe-and Alinduced flocculation DOC was rejected, because the DOC in lake water and stream water was undersaturated with Fe and Al (0.27 and 0.69 pmol mg-l) compared to levels hypothesized to induce flocculation (1.2 pmol mg-l ; Urban et al 1990) and could not have been saturated by combined wet and dry atmospheric deposition (Curtis 199 1). Furthermore, the Fe in lake water was scavenged more quickly and was thus less stable than in stream water, even though the DOC was less saturated with Fe and Al.…”

“…Vezina and Cornett (1990) showed that coagulation of colloidal Fe limited the rate of scavenging for Fe in enclosures. Urban et al (1990) hypothesized that coagulation of Fe (and Al) occurs when the binding sites of humic materials (DOC) are saturated by Al and Fe.…”

“…Fe dissolved in freshwaters is mostly associated with organic material (Shapiro 1964;Laxen and Chandler 19 8 3;Leppard et al 198 8) with small proportions of Fe as inorganic species (Urban et al 1990). The organic species of Fe consist of dissolved low-molecular-weight Fe-organic complexes and higher molecular weight colloidal material, all of which are commonly defined as dissolved (Laxen and Chandler 1983;Leppard et al 1988).…”