200-Year Record of Metals in Lake Sediments and Natural Background Concentrations

Abstract

Lake sediments conserve important information about past conditions of a lake and its environments. We present analyses of calcium, manganese, iron, copper, zinc, cadmium, mercury, and lead in dated (210Pb, 137Cs, varve-chronology) sediment cores from Zürichsee (Lake Zurich), Switzerland, covering a time span of the last 200 years and an additional time interval between 13 500 and 15 000 years before the present time. The concentrations of these elements varied very little in pre-anthropogenic sediments and represent geochemical background concentrations. With the beginning of industrialization in the early 19th century and the corresponding growth of population, significant changes occurred in the concentrations of the investigated metals. Copper, zinc, and cadmium increased until about 1960 and decreased sharply afterwards. The increase correlates to the growth of local industrial productivity (and population growth), whereas the decrease after about 1960 was caused by (i) the introduction of sewage treatment plants, (ii) more stringent legal restrictions for releases of pollution to the environment, and (iii) a growing public awareness toward environmental conservation. Contrary to world-wide observations, the increase of lead by automobile exhausts was not evident in the sediments of Zürichsee. Here, its concentrations remained almost constant between 1900 and 1975. Then they began to decrease due to the reduced use of leaded fuel. Thus, the lead input to the lake by automobile exhausts was nearly compensated by (i) continuously decreasing industrial releases to the environment and (ii) the mentioned improvements of environmental conservation techniques. Despite the consider able improvements in recent times, the present-day concentrations of the investigated heavy elements in sediments of Zürichsee are still much higher than their natural background values.