This report attempts to give a survey from literature on the microorganisms involved, on the factors and mechanisms potentially relevant for the susceptibility of drinking water wells to health related microbial contamination. The habitat groundwater accommodates a rich diversity of microorganisms, which has only begun to be identified since the development of molecular detection methods in addition to the conservative cultivation techniques. Characteristics of the subsurface are darkness, low spaces, low nutrient and low oxygen content. Indigenous microorganisms have adapted to these oligotrophic conditions and are able to proliferate in this environment permanently. Other incoming microorganisms generally cannot reproduce under these conditions, but have developed strategies to survive. They can grow only, when the parameters turn favourable. Pathogenic microorganisms comprise bacteria, viruses, and protozoa, which can also survive a certain time in groundwater. Most microorganisms in the subsurface are attached to surfaces and survive best within biofilm populations. Pathogenic microorganisms originate from human or animal faeces. These organisms are not easily detected. The methods are very time and labour consuming. Therefore, other microorganisms regularly present in the faeces are used for detection. Their presence indicates the possibility of a contamination with pathogens. As indicator microorganisms mostly coliform bacteria, E. coli, enterococci and clostridia are used. Contamination with pathogens is reported to derive essentially from communal sources: defects in wastewater treatment plants, sewage tanks, pipes, and waste deposits; from agricultural sources: animal wastes, liquid manure, and grazing; and from point sources like faeces from animals, birds, and humans. Entrance into the subsurface occurs via rainwater and surface waters, as well as by direct contamination of wells. The transport of the microorganisms into the subsurface is influenced by the geologic conditions of a specific site: soil and rock type, presence of fissures, heterogeneity. In sand, microbial movement is less far than e.g. in Karst regions, thus the susceptibility to contamination of groundwater and wells is lower. Pore sizes are crucial for sedimentation and filter efficiency of the soil. Also important is the extent of the unsaturated zone, the flow velocity of the groundwater, the geochemistry and mineralogy of the site. Wells receive their water from the groundwater reservoir of the surrounding soil. The quality of the well water is therefore essentially dependent on the properties of the groundwater and all the factors influencing the groundwater may also be relevant for the well water. The wells represent, in addition, a separate complex system with specific conditions and influencing parameters. This specific habitat involves additional variable adsorption surfaces, more space, higher flow velocity of the water, a mixing of waters from different groundwater layers and thus a different chemical composition. Contamination may also arise from microbial introduction at the open wellhead. Two main processes have been identified which are essentially responsible for the elimination of pathogens during their pathway from top of the soil to the extraction well: inactivation of the microorganisms and their adsorption to the soil particles in the subsurface. Both processes are influenced by a variety of factors and conditions present at a given site. To mention are here properties of (i) The soil: consistence and texture of surfaces, electric charge, hydrophobicity, degree of moisture, coating with organic material. (ii) The groundwater: temperature, pH, presence of cations and ionic strength, presence of organic substances, dissolved oxygen content, activity of indigenous microorganisms. (iii) The microorganisms: forming of flagella, fimbria, hydrophobicity of the cell surface, forming of extracellular polymeric substances, forming of cysts and spores as survival strategies. In addition to the description of the microbial diversity in the subsurface, the sources of pollution and the factors controlling the microbial pathways into groundwater and wells, main methods for the detection of a variety of contaminating microorganisms are given at the end of the report.

The overall project WellMa, which stands for well management, aims at the optimization of the operation and maintenance of drinking water abstraction wells. For this purpose, in addition to a statistical analyses of well data (report D 1.2) and first field investigations to compare various diagnosis methods (report D 1.3), a review of literature during the preparatory phase WellMa1 should answer the following questions: (1) Which processes affecting the well performance and conditions can occur? (2) Which correlation exists between well ageing and well characteristics? (3) How can such well ageing be recognized at an early stage? (4) What is the state of the practice to restore a good performance and condition? (5) What can be done during well design and construction to prevent well ageing? (6) How can well operation be adjusted to slow-down well ageing processes? Based on textbooks, standards and professional articles published in large number since the middle of the nineties, the state of the art was gathered and compared to current practice at BWB and Veolia to identify possibilities for improvement and specify the need for further investigations to be proposed for WellMa2. 1) Three well ageing types involving different processes could be identified. These are chemical, biological and physical clogging. They are closely linked to the characteristics of the exploited aquifer, such as the physical properties of the formation or the chemical composition of the groundwater. 2) The evaluation of these site-specific aquifer characteristics, the impacts from well design and the observed effects on the well performance and condition and their development with time of operation should be used to specify the individual ageing potential for each well site. 3) The early recognition of well ageing implies the need to monitor wells (1) regularly and (2) with comparable methods. As suitable indicators, the development of water levels and discharge rates to calculate the specific drawdown and specific capacity, the pump surveillance and the visible condition of the well interior could be identified. 4) Both, the assessment of the ageing potential and the monitoring of a reference value describing the state of the well lead to the specification of maintenance requirements. Generally, three strategies could be identified, ranging from sheer operation, over reactive maintenance to regular condition assessment and preventive treatment. Concerning the choice of maintenance method, key criteria must always be the well design, its state of construction, the well ageing type and location. Up to now, patterns linking well characteristics and the success of maintenance could not be identified. Thus, maintenance relies on practical experience and the willingness to discuss limitations and disadvantages of methods as open as the advantages on side of the rehabilitation companies. 5) For well design and construction, the technical standards were summarized, describing the necessary steps for proper dimensioning, drilling, choice of materials and final well development. Not only the avoidance of nonconformities and the careful evaluation of the advantages, but also the restrictions of different well design alternatives, e.g. for the accessibility of rehabilitation, assure an optimal well ageing prevention and well operation. 6) Furthermore, well operation could be identified as a key element and critical factor codetermining the lifetime, but at the same time the economic efficiency of a well. It is always a compromise between demand, technical possibilities and economic considerations, for which reason general standards or technical guidance are not available so far. They need to be developed individually considering present well ageing processes and the quantification of impacts. Comparing the state of the art with current practice at BWB and Veolia, room for improvement could primarily be identified for monitoring and subsequent data processing for both, operational parameters (to assess well performance and condition), and maintenance (to evaluate the success of applied treatments). Based on the recommendations derived on this state of the art review, within WellMa2 the effects of measures for preventing and treating well ageing shall be quantified so that the benefits can be assessed for future optimized well management.