Sediment reference concentrations of seldom monitored trace elements (Ag, Be, In, Ga, Sb, Tl) in four Swedish boreal lakes — Comparison with commonly monitored elements

This paper presents reference and recent acid-leachable concentrations of some seldom monitored trace elements (SMTE; Ag, Be, Ga, In, Sb and Tl) in sediments from four boreal oligotrophic lakes in a south to north transect in Sweden. For comparison commonly monitored trace elements (As, Cd, Co, Cr, Cu, Ni, Pb and Zn) are included as well as those of relevance for redistribution processes (Al, Ca, Fe, Mg and Mn). Pore water pH and the corresponding solid/solution distribution coefficients (Kd) indicated that redistribution of the elements due to acidification is minor. The depth of impact was defined as the level in the sediment where the mean values became constant on successive exclusion of metal concentrations in overlying strata. Reference concentrations were calculated below the depth of impact. The present concentration changes are estimated by the ratio between the average concentration above the depth of impact and the reference concentration. Reference concentration ranges for the SMT-elements are (mg/kg, dry wt.): Ag 0.16–0.66; Be 1.6–3.7; Ga 2.0–5.1; In 0.05– 0.22; Sb 0.05–0.11 and Tl 0.17–0.70. The concentration ratios for these elements ranged in the two most southern lakes from 1.5 to 4.5 and in the two northern ones from 0.6 to 1.6. A high correlation between Kd for the SMT-elements, and iron, except for Sb and Tl, infer that the biogeochemistry of iron is quantitatively important for the accumulation of these elements. The reference concentration ranges for the commonly monitored trace elements are (mg/kg, dry wt.): As 7.0–29.6; Cd 0.33– 0.98; Co 5.7–23.8; Cr 15.2–26.1; Cu 27.6–58.4; Ni 5.4–20.8; Pb 44–96. The corresponding concentration ratios were 1.2–18 (second highest 3.9) in the two most southern lakes and 0.8–1.6 in the two northern ones. Declining ratios were found from south to north, most obvious for cobalt and zinc. The copper ratios did not show a regional pattern, partly because of the impact from old mine waste. Increased concentrations of Ag, Be, Ga, In, Sb and Tl in recent sediments up to 4.5 times the reference levels in combination with the geographical pattern infer an elevated loading of these elements.