Chemical, bacteriological and biological examination and evaluation of sediments from drinking water reservoirs--results from the first sampling phase.

Between September 1992 and April 1993, samples of sediment were taken from a total of 16 drinking water reservoirs from seven water suppliers. Five different types of raw water were examined, samples being taken with varying frequency. The comparison of surface reservoir water and ground water constituted a central aim in this first series of investigations. The composition of the sediment was determined in terms of its chemical, bacteriological and biological characteristics. It was established that the deposits that normally accumulate in the course of an operating period contain, in addition to the familiar mass-determinant inorganic constituents such as iron, manganese and calcium compounds, in some cases considerable quantities of organic substances. Total organic carbon (TOC) contents of up to 11 weight-% were found. Besides an increased concentration of heavy metals, in particular cadmium, nickel and zinc, were measured in some sediments. The bacteriological examination of the sediments, in accordance with the German drinking water regulation (TrinkwV) showed, in part, very high colony counts. However, there was apparently no influence on the effluent water as the reservoir was emptied. The microscopical examination showed independent of the raw water source the sediments being colonized by invertebrates. Up to 9000 organisms per litre of fresh sediment were recorded. The predominant group of animals, in terms of numbers, was the rotifers followed by threadworms. The comparison of water from impounding reservoirs and ground water made evident that the presence of organisms in reservoir sediments is not restricted to the use of surface water.