Pollution research on dated sediment cores from Lake Constance

The vertical distribution of eight heavy metals (Hg, Cu, Zn, Cr, CO, Ni, Pb, Cd) and nitrogen and phosphorus in dated sediments of four sedimentary profiles from the central part of Lake Constance was examined. From 1900 to about 1960-1970, a general increase of all heavy metals was observcd. After this time, the concentrations of most of the metals decreased. As compared with pre-1900 "natural" metal concentrations, Cd and Pb have been enriched the strongest (enrichment factors 4.0 and 3.8 resp.), whereas Ni and CO exhibit least civilizational influences (enrichment factors 1.5 and 1.9 resp.). Cu, Cr, Zn and Hg were enriched by a factor between 2-3. Elements which were not influenced by anthropogenic processes - such as K and Li - remained unaffected over the whole vertical sequence. Polycyclic aromatic hydrocarbons (PAH) which were also studied in the same sediment material by Grimmer and Bohnke show a similar pattern. Within a sedimentary profile the concentration ratio benzo (a) pyrene : cadmium remained about constant. Combustion of coal is believed to be the common source of both heavy metals and PAH and seems to be responsible for the general distribution pattern of both groups of pollutants. In addition, local sources - both domestic and industrial - are assumed to have delivered additional heavy metal pollution.
From about 1900 until now a steady increase of N and P is to be observed. Nitrogen is enriched by a factor of about 5, phosphorus by a factor of about only 2. The N/P-ratio shows a continuous increase since 1900. The explanation for this development is the steady increase of phosphates delivered into the lake since about 1900. Thus, p.ex. the total amount of phosphorus increased between 1930 and 1974 from 274 to 1929 tons per year. Most of the sedimentary nitrogen is incorporated into the organic material, the production of which is clearly bound to the availability of phosphorus in the lake water. Phosphorus is mostly found in an adsorbed position on clay minerals. Between N and P on one hand and certain heavy metals (Cd, Zn) and polycyclic aromatic hydrocarbons on the other hand a positive correlation only can be found as long as the curves for phosphate delivery (responsible for N- and P-concentrations) and coal consumption (responsible for certain heavy metals and polycyclic aromatic hydrocarbons) have a parallel development. The decrease in coal consumption in connection with a very strong increase of phosphate delivery during the past 15 years has ceased the parallel evolution of the different groups of pollutants in the sediments, and also demonstrates, that a causal relationship N and P - heavy metals and polycyclic aromatic hydrocarbons does not exist.