The use of bank filtration for drinking water treatment in Europe dates back to the days of beginning industrialization in the 19th century. With regard to improved source water quality in Europe, the millennium development goals and global climate change, aquifer recharge (AR) and bank filtration (BF) need to be reassessed in terms of sustainability and their role within an integrated water resource management. Based on the IC-NASRI study comprising 194 drinking water facilities worldwide integrating aquifer recharge techniques in their treatment system, an average AR/BF site would be located in Central Europe alongside a river and is characterized by: a sandy gravel aquifer with a hydraulic conductivity of 2x10-3 m/s, a maximum aquifer thickness of 30 m, 175 m travel distance from bank to well, a travel time of 70 days and by vertical well operation with a daily capacity of 55.000 m³. A literature survey conducted within the TECHNEAU project demonstrated that for substances highly relevant to newly-industrialized or developing countries (e.g. pathogens) the removal efficiency is good. Hydro-chemical analyses from three study sites in Delhi support these results. However, it was also shown that poor surface water quality, saline groundwater or subsurface conditions leading to mobilization of trace metals like iron, manganese or arsenic may limit the applicability of AR / BF without further post-treatment. Climate change might affect the performance of AR / BF worldwide, impairing source water quality and influencing removal efficiency. However, other factors like changes in demography or land-use can impact the systems by far more severely.

While climate change is an emerging hazard to water supply, literature on the vulnerability of bank filtration (BF), a proven technique of drinking water production in Central Europe and North America, is yet scarce. The Intergovernmental Panel of Climate Change (2007) has projected a global temperature increase between 1.1 and 6.4 °C by 2100. This will affect vital factors for water supply such as precipitation regime, groundwater recharge, run-off, river discharge and raw water quality. Projections on climate change and the implications are difficult because of the uncertainties associated with climate scenarios and modelling. However, in Europe and North America where BF is in operation, the projected increase in seasonal floods and droughts has already been experienced. In addition, site-specific considerations (e.g. land use, demographic trends) are to be taken into account to evaluate the potential impacts on water supply. To fill the current gap in literature, this report provides a first overview on how changing environmental conditions may affect BF operation.

Riverbank filtration (RBF) denotes the process whereby river water is induced to infiltrate into a groundwater system by well operation adjacent to banks. In Central Europe, RBF has been common practice for 100 years to produce drinking water. Due to the easy implementation and little maintenance necessary, BF has been suggested to be a useful drinking water treatment for developing and newly-industrialised countries. Experience from Europe has demonstrated that RBF is suitable to remove a range of organic and inorganic contaminants while an exhaustion of cleaning capacity has not been observed. RBF systems can mitigate shock loads and are particularly known for the efficient removal of pathogens, suspended solids and algal toxins from surface water, all being water quality parameters of high relevance in developing and newly-industrialised countries. Another benefit of RBF operation is the storage capacity which may help to balance freshwater availability in areas experiencing high variations of precipitation and run-off. This report aims at evaluating the relevance and opportunities of RBF systems to provide safe water to these countries. In order to evaluate the relevance and opportunities of RBF systems to developing and newly-industrialised countries, the report is structured to address key considerations and (i) identify prerequisites for successful RBF operation based on the experience in Central Europe and the United States, (ii) assess the removal potential of RBF for various water contaminants based on available literature, the TECHNEAU investigations in India and NASRI data from Berlin and (iii) evaluate the sustainability and relevance of RBF operation with regard to the particular needs in developing and newly-industrialised countries.

In summer 2007 & 2008, 100 water samples were collected from 10 freshwater reservoirs with cyanobacteria issues. Phytoplankton was determined according to the Utermohl method [1]. Intra- and extracellular CYN, ATX-a, STX were analyzed by LC-MS-MS or HPLC-PDA at UBA, and in addition, Veolia tested Abraxis ELISA kits for total CYN and total STX on the 2008 water samples (n=45). Cyanobacterial abundance was comparably low in 2007 & 2008 for all reservoirs, probably because of cooler summer months, with less sunlight, more rain and quickly decreasing fall temperatures (except in reservoir 10, which had low incoming nutrient charges). For instance, average chlorophyll content was 12 µg/L in 2007 and 35 µg/L in 2008 in Western France, when 60-80 µg/L concentrations are usually measured. In spite of these environmental conditions, cyanobacteria were detected in 97% of the samples and cyanotoxins in 55%. WHO level 3 for drinking water (>100 000 cell/mL) was reached for 20-25% of the samples. Among the species observed in the water samples, the following potential CYN, ATX-a, STX producers were observed: Cyanotoxin LC-MS-MS and HPLC-PDA results are given on the right. ELISA results for CYN and STX of the 2008 samples only partially agree with the LC-MS-MS data. This might be due to the differences in extraction procedures of the two methods, cross-reactivity issues of the ELISAs for derivatives, in combination with overall very low concentrations of the toxins.

Managed Aquifer Recharge (MAR) comprises a wide variety of systems in which water is intentionally introduced into an aquifer and subsequently recovered, e.g. for drinking water or irrigation purposes. The objective is i) to store excess water for times of less water availability and / or ii) to introduce an additional barrier for purification of water from different sources (e.g. surface water, treated waste water) for a specific use. Common MAR techniques in Europe are (Figure 1): river bank filtration (RBF) and artificial groundwater recharge – usually via ponded infiltration (AR). Riverbank filtration (RBF) has a long history as a process for generating safe water for human consumption in Europe. During industrialization in the 19th century drinking water facilities in England, the Netherlands and Germany started using bank filtered water due to the increasing pollution of the rivers. The systematic production of bank filtrates started around 1870-1890 (BMI 1975, 1985). Since then, RBF and in case of insufficient quantity, artificial groundwater recharge (AR) have been generally applied as a first barrier within the drinking water treatment chain. The most common and widely used method for artificial groundwater recharge (AR) are infiltration ponds (Asano, 2007). These simple surface spreading basins provide added benefits of treatment in the vadoze zone and subsequently in the aquifer. Advanced pretreatment of the infiltration water by coagulation, and advanced post-treatment of the recharged water, e.g. with activated carbon or ozonation became necessary in many cases after the 1960’s as the quality of the source water further decreased. Today the water supply of many European cities and densely populated areas relies on riverbank filtration or artificial recharge. Following Castany (1985), in France, the proportion of bank-filtered water reaches approximately 50% of the total drinking water production (Doussan et al., 1997). In the Netherlands 13% of drinking water is produced from infiltration of surface water, such as bank filtration and dune infiltration (Hiemstra et al., 2003). In Germany riverbank filtration and artificial groundwater recharge are used in the valleys of the rivers Rhein, Main, Elbe, Neckar, Ruhr, and in Berlin along the Havel and Spree (Grischek et al., 2002). In Berlin 75% of the drinking water is derived from riverbank filtration and artificially recharged groundwater (Schulze, 1977). Riverbank filtration is also applied in the United States as an efficient and low cost drinking water pre-treatment technology (Ray et al., 2002), also to improve the removal of surface water contaminating protozoa. In most applications, MAR is intended to act as a buffer in terms of water availability (quantity) and water quality. In general, the level of knowledge of natural treatment systems, notably in aquifers, is not as high as in engineered systems, because the biogeochemical environment in aquifers that modify water quality for sure, will vary in space and time (Dillon et al. 2008). The heterogeneity of the system, strengthens its buffer potential on the one hand, but makes it more difficult to describe and control on the other hand. Key parameters that determine the quantitative storage capacity of the system are the specific hydrogeology of the aquifer (e.g. transmissivity and porosity) and the clogging potential at the entry point of the recharge water (infiltration pond, well or river bank). Clogging occurs due to physical, chemical and biochemical processes and needs to be regarded carefully as it may reduce the systems performance substantially. From literature it is known, that increased clogging reduces the oxidation state of the clogging layer. At a bank filtration site at Lake Tegel, Berlin, it was observed that intensity and spatial distribution of clogging strongly depends on the extent and thickness of the unsaturated zone. Geochemical observations suggest, that atmosperic oxygen induces redox processes which lead to a reduction of the clogging layer (Wiese & Nützmann 2008). This is possibly due to the complex interaction of hydrochemical and biological processes within the uppermost centimetres of the aquifer (Hoffmann et al., 2006). If these processes are likewise found in AR system, they may be influenced as to minimize basin-cleaning efforts. This needs to be further investigated. Water quality aspects of MAR are governed by i) the quality of the infiltrated / injected water ii) physical straining of particulate and particle-bound substances, iii) adsorption and desorption, iv) biogeochemical degradation / deactivation processes within the aquifer, iv) the geochemical composition of the aquifer, and v) the quality of the ambient groundwater. The process most important for MAR applications is usually the physical straining of particulate and particlebound substances, lessening the effort for subsequent drinking water treatment. In Berlin, e.g. disinfection of drinking water can usually be avoided due to complete removal of pathogens during underground passage of up to 6 months. Cyanobacterial toxins (e.g. microcystins) that are primarily cell-bound are efficiently removed as well (Grützmacher et al. 2007). On the other hand there is still a lack of understanding under which circumstances microcystins or other cyanobacterial toxins like cylindrospermopsin (currently observed in growing quantities in Germany) are released, thus becoming potentially more mobile in the subsurface. Adsorption to the aquifer matrix contributes to the elimination of organic substances and heavy metals. Although this does not remove the substances completely, peak loads – e.g. from oil spills – are retarded and maximum concentrations reduced. In addition, sorption prolongs the detention time in the aquifer which multiplies the time for biodegradation. Biological degradation in the subsurface is responsible for the elimination of dissolved organic carbon (usually resulting from natural organic matter, NOM) and organic trace substances that occur at varying extent. Investigations have shown that the redox potential in the aquifer is decisive for the degree of elimination (Stuyfzand, 1998; Massmann et al. 2007). Due to increasingly sensitive analytical methods trace organics present in surface waters (e.g. pharmaceutical residues) have been detected in many MAR systems e.g in Berlin and the Netherlands (Massmann et al, 2007, Stuyfzand et al. 2007). Advanced numerical models including reactive flow and transport can simulate the complex interactions between the hydrogeochemical environment and degradation of trace organics (Greskowiak et al. 2006). However, so far this has only been applied for a limited number of compounds at very few sites. Further research is needed to apply these methods for risk assessment. A second method for predicting the removal of organic micropollutants is the more statistically based approach of linking substance properties (molecular weight, number of double bonds, number of aromatic rings, etc.) to biodegradation via quantitative structure-activity relationship (QSAR) type models. This has been applied successfully to other water treatment methods – a transfer to MAR is lacking so far. As MAR is a technology that relies on the interaction of natural processes framework conditions like climate and hydrogeology play an important role. There is a need for testing the transferability from central European conditions to other regions, and for an assessment, how temperature changes affect the system’s elimination capacity. With ongoing climate change, reducing precipitation in some regions of Europe and increasing peak flow events in others, MAR is the ideal technology to act as a buffer for quantity and quality. The European Water Supply and Sanitation Platform (www.wsstp.org) for example has identified MAR as a technology potentially fit for future challenges.

The Aquisafe project is a cooperation of the Indiana University Purdue University Indianapolis (IUPUI, USA), the German Federal Environment Agency (UBA, Germany) and the Berlin Centre of Competence for Water (KWB, Germany). The aim of the project is the development of a scheme for natural mitigation zones to protect surface waters from diffuse pollution in rural and semi-rural environments. In particular, key contaminants, applicable management and modelling tools and potential substance removal by constructed wetlands or riparian zones are being studied. Within these frameworks, two case studies are carried out in Brittany, the number one agricultural region in France. A hydrological model is currently being applied on the Ic catchment (92 km2) to test its capability of (i) understanding hydrological, basin-scale regimes, (ii) predicting the effect of mitigation measures and (iii) distinguishing diffusion pathways for different types of contaminants. In the second case study, a constructed wetland in Iffendic on the River Meu is monitored as an example of a natural and inexpensive mitigation option. On the way through the wetland nitrate concentrations from drainage inflows to the river decreased more than tenfold. In the ongoing monitoring, knowledge on hydrological flowpaths is improved to be able to quantify the retention potential of constructed wetlands in Brittany for nitrate and other agriculturally-based pollutants, such as pesticides.

The Aquisafe project assesses the effectiveness of natural mitigation zones in reducing diffuse pollution to surface waters. In one case study on a constructed wetland in agriculturally dominated Western France, nitrate concentrations from drainage inflows to a small river decreased up to tenfold on the way through an intermediary constructed wetland. However, only ~30 % of the total N-load is retained in the wetland, whereas ~70 % enters the river directly during high flow events as a result of low soil permeability. The study underlines the importance of flow paths and infiltration for nitrate removal in natural or constructed wetlands, which is often neglected in practice.

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.