Microcystins (MCYSTs) are a group of structurally similar toxic peptides produced by cyanobacteria (“blue-green algae”) which occur frequently in surface waters worldwide. Reliable elimination is necessary when using these waters as drinking watersources.Bankfiltrationandartificial groundwaterrecharge utilize adsorption and degradation processes in the subsurface, commonlythroughsandandgravel aquifers, for the elimination of a wide range of substances during drinking water (pre-) treatment. To obtain parameters for estimating whether MCYST breakthrough is likely in field settings, we tested MCYST elimination in laboratory experiments (batch experiments, column experiments) under a range of conditions. Adsorption coefficients (kd-values) obtained from batch studies ranged from 0.2 mL/g for filter sand to 11.6 mL/g for fine grained aquifer materials with 2% fine grains (<63 µm) and 0.8% organic matter. First order degradation rates in column studies reached 1.87 d-1 under aerobic conditions and showed high variations under anoxic conditions (<0.01-1.35 d-1). These results show that, next to sediment texture, redox conditions play an important role for MCYST elimination during sediment passage. Biodegradation was identified as the dominating process for MCYST elimination in sandy aquifer material.

The project “Organic Trace Substances Relevant for Drinking Water – Assessing their Elimination through Bank Filtration (TRACE)” aims at giving an up-to-date overview of the potential risk resulting from the occurrence of chelating agents, perfluorinated compounds (PFCs) and selected pesticides in surface waters and to show if there is a potential for the substances to persist during bank filtration and artificial recharge. During the first phase of the project which is subject of this paper, a literature study was conducted addressing their occurrence (in the Berlin region and elsewhere), amounts produced as well as data on their persistence in the subsurface. This was the basis for a decision on the substance applied in the field scale experiments at the UBAs experimental field during the following project phase. Using freely available databases (e.g. ULIDAT, DIMDI, Tiborder) 1148 references were screened for their relevance to these topics, and 450 of these were classified as relevant. Of these, so far the 223 most important references have been compiled in an ACCESS database which comprises data on the data origin as well as on specific values (e.g. measured concentrations, amounts produced, use, main metabolites, sources, pathways in the environment). The database links this information so that output forms (“fact sheets”) can be created that summarize all data for one specific substance. The regarded substances were subsequently classified according to the criteria: usage / production, occurrence in surface water (if possible also in groundwater and bank filtrate), degradation potential, biological degradability, production of relevant metabolites and toxicity. For the chelating agents three substance groups were examined closely: aminocarboxylates, hydrocarboxylates and phosphonates (all other substance groups were found to be irrelevant due to total biodegradability). The aminocarboxylates are produced in highest numbers and occur most frequently (especially EDTA, PDTA, NTA and DTPA). There are, however, already extensive investigations on this field so that few knowledge gaps were identified. Hydrocarboxylates are produced in lesser amounts and for some ready biological degradability has been shown. For these reasons further investigations were not seen as a priority. Phosphonates produce relevant metabolites (phosphates that enhance eutrophication) and are produced in high amounts (> 1000 t/a). This substance group was therefore recommended for further investigations. Currently a variety of research projects cover the field of perfluorinated compounds (PFCs) that occur in aquatic environments world wide and whose toxicity and persistence is not yet clearly determined. Most investigations aim at the main substances of this group: PFOA and PFOS. These are, however, currently being replaced by shorter chained PFCs on which investigations are lacking. This substance group is therefore also of interest for further investigations. For the pesticides glyphosate and isoproturone high production rates and frequent occurrence in surface and groundwater world wide were determined. Due to this fact and to the presence of relevant metabolites (e.g. AMPA) as well as to limited knowledge on their fate during underground passage these substances were classified as highly interesting for further investigations.

In Berlin – wie auch in anderen Regionen Deutschlands – wird ein Großteil des Trinkwassers durch Uferfiltration gewonnen. Durch eine Untergrundpassage mit einer Dauer von meist mehreren Wochen erhält es eine Aufreinigung, die den Aufwand der konventionellen Trinkwasseraufbereitung verringert und eine zusätzliche Barriere gegenüber Schadstoffen darstellt (Kühn 2001). Das Ziel eines interdisziplinären Forschungsvorhabens mit dem Titel NASRI (Natural and Artificial Systems for Recharge and Infiltration) war, die Reinigungsprozesse für verschiedenste Substanzen zu ermitteln und Empfehlungen für das zukünftige Wassermanagement in Berlin abzuleiten (Fritz 2003). Aufgabe der Arbeitsgruppe des Umweltbundesamtes war dabei zu klären, wie wirksam Microcystinen (MCYST) als wichtigste Gruppe der Cyanobakterientoxine durch die Bodenpassage eliminiert werden. Im Folgenden werden einige Schlüsselergebnisse berichtet. Für eine ausführliche Ergebnisdarstellung siehe Grützmacher et al. (2006). MCYST sind in der Regel überwiegend (> 90 %) zellgebunden, so dass die physikalische Filtration der Zellen an der Sedimentoberfläche als Eliminationsprozess im Vordergrund steht (Grützmacher et al. 2003). Das extrazelluläre MCYST wird dagegen überwiegend biologisch abgebaut (Lahti et al. 1998, Grützmacher et al. 2005a). Um unter naturnahen Bedingungen Extremfälle für den biologischen Abbau zu simulieren, wurden i) Freilandversuche unter variierenden Redoxbedingungen und ii) Laborsäulenversuche bei unterschiedlichen Temperaturen durchgeführt. Ferner wurde die Freisetzung von MCYST aus sedimentierten Zellen untersucht.

Cyanobacterial toxins are substances produced by cyanobacteria that occur in surface waters world wide. The most common group of cyanobacterial toxins is the group of structurally similar microcystins (MCYST). Sand passage as used in slow sand filtration, artificial recharge and bank filtration has shown to be effective in eliminating microcystins in many cases. For secure drinking water production from surface waters infested by microcystins removal has to be ensured in a wide variety of cases met in the field. It was therefore the aim of experiments in technical and semitechnical scale on the UBA’s experimental field in Berlin to test some worst case scenarios for the reliability of microcystin elimination during sand passage. Experiments were conducted with virgin sand (no previous contact to MCYST) and high filtration rates as well as under anaerobic conditions. The results show that the greatest problem for MCYST elimination can be found under anaerobic conditions as degradation is not complete and may lead to harmful residual concentrations.

Microcystins (MCYST) are a group of toxic substances produced by cyanobacteria (‘blue-green-algae’). In case of cyanobacterial blooms microcystin concentrations in surface waters may reach values far above the value proposed as provisional guideline for drinking water by the WHO of 1 µg/L for MCYST-LR. For drinking water production via underground passage it is therefore necessary to ensure removal to a large extent. For this reason experiments with extracellular microcystins were conducted in the laboratory as well as in a natural setting on the UBA’s (German Federal Environmental Agency) experimental field for simulation of underground passage. Laboratory batch experiments showed that adsorption of microcystins can be neglected in sandy material (kd < 1 cm³/g). Batch and column experiments identified biodegradation as the predominant elimination process in these sediments. The degradation rates derived from laboratory column experiments as well as semi-technical scale enclosure experiments varied between 0.2 d–1 and 18 d–1. In the worst case this means a half life of 2.8 days, so that under aerobic conditions contact times of several days should be sufficient to eliminate MCYST to an extent safe for use as drinking water.

The present report characterizes the field sites Lake Tegel and Lake Wannsee as well as the artificial recharge site GWA Tegel in terms of their clogging layer, sedimentary, hydraulic and hydrochemical properties. As a result, a solid basis for the interpretation of specific compounds evaluated within NASRI and for subsequent modeling and quantification of the data is given. Major problems or difficulties where identified, in order to focus investigations on aspects not fully understood to date in the next project phase. The combination of different tracers enables the interpretation of the flow regime. With the help of T/He analysis, ages of different water bodies can be estimated. The analysis of tracer showing distinct seasonal variations is used to estimate travel times while water constituents which are either mainly present in the bank filtrate or the background water are used for mixing calculations. The proportions of treated wastewater in the surface water were estimated in front of the transects. The surface water composition varies largely both in time and space, which is a problem at Wannsee, where the surface water sampling point is not representative for the bank filtration input. Estimates for travel times of the bank filtrate to individual observation and production wells are given and vary between days and several months. The production wells are a mixture of bank filtrate and water from inland of the wells and deeper aquifers, proportions of bank filtrate are given where possible to differentiate between contaminant removal and dilution. They vary between < 20 and > 80 %. The new observation wells enable a vertical differentiation of the infiltrate. It becomes clear that at Tegel and Wannsee, there is a strong vertical succession towards larger proportions of considerably older bank filtrate with depth. At the Wannsee transect, the observation wells deeper than the lake do not reflect the surface water signal at all. It will be important to combine the new information with hydraulic information of existing flow models (mainly of the IGB “model” group). The evaluation of the redox conditions shows that redox successions proceed with depth rather than (only) in flow direction. In addition, the redox zoning (as characterised by the appearance or disappearance of redox sensitive species) is very transient. The zones are much wider in winter than in summer, in particular at the artificial recharge site GWA Tegel, probably due to temperature effects. This poses a challenge for the desired modelling and the interpretation of data from redoxsensitive substances.