Hygiene standards and parasites have been a special focus of drinking water utilities for several years. In this context the development of new, high-tech water treatment methods is often taken into consideration. However, we have been applying riverbank filtration as an inexpensive, natural method in Düsseldorf for over 130 years. Indeed it had been introduced for “hygiene reasons” at the time and, according to our experience, riverbank filtration is well suited to meet these “new” hygiene challenges. We have intensively examined the infiltration of river water into the aquifer. We view this core process as the prerequisite for the sustained function of riverbank filtration. It is closely linked with the retention of turbid matters in the riverbed and the shearing forces on the subsurface. In addition, we have investigated the effectiveness of bank filtration as regards the elimination of microorganisms over recent years. According to these examinations, bacteria are reduced by an average of 3 log orders by bank filtration; individual breakthroughs correlate with high water events. According to our measurements Giardia and Cryptosporidium have been completely eliminated in riverbank passage. The retention of three examined types of viruses was also found to be almost completely accomplished.
In a gasoline-contaminated site in Düsseldorf, Germany a two-year monitoring program was carried out to determine the presence, behavior, and fate of 12 gasoline additives in a total of 96 samples from 14 groundwater wells. The origin of contamination was suspected to be a gasoline spill at a gas station. Target compounds were methyl-tert-butyl ether (MTBE), its main degradation products, tert-butyl alcohol (TBA) and tert-butyl formate (TBF); other gasoline additives, oxygenate dialkyl ethers: Ethyl-tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and diisopropyl ether (DIPE); aromatics: Benzene, toluene, ethylbenzene and xylenes (BTEX), and other compounds causing odor problems: Dicyclopentadiene and trichloroethylene. Purge and trap coupled with gas chromatography-mass spectrometry permitted detection of ng/L concentrations. Ninety of the 96 samples analyzed contained MTBE at levels varying between 0.01 to 645 microg/L. Five contaminated hot spots were identified with levels up to U.S. Environmental Protection Agency (U.S. EPA) drinking water advisory values (20-40 microg/L) and one of them doubling Danish suggested toxicity level of 350 microg/L at a depth of 11 m. No significant natural attenuation was found in MTBE degradation, although samples with high levels of MTBE contained 0.1 to 440 microg/L of TBA. These levels were attributed to its presence in the contamination source more than MTBE degradation. tert-Butyl alcohol was found to be recalcitrant in groundwater. In all cases, BTEX were at low concentrations or not detected, showing less persistence than MTBE. The monitoring of the contamination plume showed that the distribution of the MTBE and TBA in the aquifer formed a similar vertical concentration profile that was influenced by the groundwater flow direction.
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