Concentrations and speciation of Cu, Zn, Cd, and Al in mine-polluted Norwegian rivers : influence of main water parameters and consequences to fish

Abstract

This thesis presents concentration and speciation data for Cu, Zn, Cd, and Al in eight highland rivers and streams in the Røros area, central Norway. About 16 sampling campaigns were performed before, during and after floods during spring and autumn of 1997. Due to rain- and snowmelt-induced flushout from weathered mine tailings, the flood episodes were expected to produce high concentrations of metals in the local rivers. The examined river sites represent highly different degrees of pollution, height above sea level, annual discharge, pH, etc., and the project is aimed towards producing general information about the temporal variations of the metal chemistry and parameters important for the metal chemistry in this and similar regions. Dissolved species of the metals were fractionated by dialysis in situ, colloidal species by filtration, and total (more precisely; soluble in 0.1 M HNO3) concentrations were determined directly after acidification. Ca concentration, pH, river discharge, water temperature, and to a lesser extent precipitation and TOC were also monitored. In addition Cu and Zn/Cd metallothioneins were studied in kidney, liver and gills in trout (salmo trutta) populations in two of the rivers characterized by completely different metal concentration fingerprints.

The results showed that Cu and Al, and possibly Zn and Cd as well, were practically completely in particulate or colloidal form at pH values of 7 and above. At pH levels one or a few pH units lower, the trace metals shifted to occur almost completely dissolved. The pH range at which the change from colloidal/particulate to dissolved species occurred, depended on the metal concerned and the TOC in the water. High TOC concentrations (> 8) seemed to accompany low fractions of dissolved metals, probably because the metals adsorbed on high molecular weight organic compounds or organic coatings on inorganic particles. At TOC concentrations lower than 8 mg/L, a 50 % dissolved fraction was estimated at pH ~7.2 and ~5.8 for Cu and Al respectively, whereas for Cd and Zn, a 50 % dissolved fraction was estimated at pH 7.7. The latter is a pH slightly higher than the highest value observed in the present investigation. Higher TOC concentrations (>8 mg/L) increased metal adsorption and made adsorption start up to one pH unit lower than in low TOC waters (<8 mg/L).

Total metal concentrations were generally elevated during flood conditions in the pH neutral rivers, whereas pH was significantly lowered. In spite of the low pH, the dissolved fractions of Zn, Cd, and Al decreased during flood periods, probably due to enhanced particle concentrations. Thus flood conditions apparantly brought metals into a less acute toxic state. However pH may have influenced metal toxicity in other ways as well; e.g. the free metal ion activity in the dissolved fraction could have increased during flood due to the decreased pH. But even if that was the case, metal toxicity would not necessarily be higher since H+ competes with free metal ions for uptake sites on biological membranes.

Alkalinity and Ca reduce negative effects of metals, and both were low during flood conditions. This is obviously unfortunate for aquatic organisms. Generally however, total metal concentration peaks occurred at the beginning of rising floods, followed by a very low pH, alkalinity, and Ca concentration a few weeks later and the spring discharge maximum a few weeks after that. Thus Ca2+ and pH had not yet reached their spring minimum, that is; the most unfavorable condition to protect organisms against metals, at the metal concentration maximum.

The snowcap covering River Orva accumulated and contained huge amounts of Cu, and may have substantially increased the Cu concentration in the river during snowmelt. This also impacts reaches of the large river Glåma which receives water from Orva. It is suggested that the hydroelectric power plant Kuråsfossen in Glåma should regulate river runoff in a different manner in order to smooth out metal concentration peaks.

Gill concentrations of Cu metallothionein (MT) in Rugla and Cd/Zn MT in Naustebekken were appreciably elevated during run-off episodes. The Cu MT and Cd/Zn MT concentrations in gills and kidneys were high enough to account for all or almost all Cu and Cd but only for a minor fraction of the Zn present in these organs. For Zn this indicates that other detoxifying mechanisms may be more important than MT.