The capacity of drinking water wells, i.e. the yield for a given drawdown, is often decreasing after a certain time of operation. This effect is called well ageing and is due to different processes related to the geology and hydrochemistry at any given well site and to the construction and operation of these wells. The Hydrogeology workgroup and partners investigate wells in Berlin and France in terms of their ageing behaviour with the aim to determine suitable measures helping to slow down well ageing processes and optimise strategies for well operation and maintenance. A precondition for well clogging by iron incrustations is the mixing of different groundwaters with incompatible chemical properties in the well and/or within aquifer and is induced by combined hydrochemical and microbiological processes. The assessment of (i) formation of reduced/oxidized groundwater layering in the aquifer, (ii) localization of mixing zones and (iii) mixing ratios within the well was done by field and laboratory studies. The research reveals that redox condition in the well and the surrounding aquifer are subject to short to long-termed variations. These variations are caused by operation intervals of the wells and by seasonal effects. The results permit a characterization of oxygen enrichment and transport dependent on well operation, location and design and further on an input-output balancing and a modeling of incrustation rates.

The assessment of methods for the diagnosis and distinction of well ageing types and processes with the aim to recommend methods and tools for further fieldwork was part of work package 1 of the preparatory phase WellMa1. Therefore, field tests were carried out at selected well sites with a variety of methods covering standard monitoring methods to assess the constructive state of a well (TV inspections, borehole geophysical methods) and its performance (pump tests) as well as methods aiming at a better process understanding such as the hydrochemical and microbiological analysis of the raw water and clogging deposits. Altogether ten methods were applied at 21 different wells of the Berliner Wasserbetriebe (BWB) covering (i) exposure of object slides during operation and rest periods for microbiological investigations, (ii) BART with test kits for iron-related bacteria (IRB) and slime-forming bacteria (SLYM), (iii) water sampling for the investigation of pristine groundwater organisms, (iv) online measurements of chemical parameters O2, Eh, pH and T and water sampling for chemical analyses (main cations and anions), (v) TV inspections, (vi) three-step pumping tests, (vii) borehole geophysics with Gamma-Gamma-Density scan (GG.D), NeutronNeutron log (NN), Flowmeter (Flow) and Packer-Flowmeter measurement and (ix) Particle countings. The assessment and comparison should originally be completed by a horizontally directed core sampling from different depths from the screen sections of three of the chosen wells. Due to technical difficulties, this was not achieved during this phase of the project. The investigations led to a development and refinement of the methods and approaches. Because of their limited accessibility to the different parts of a well, a combination of methods is always necessary. Especially for the indirect methods like borehole geophysics, an initial assessment of the well condition directly subsequent to construction is essential to provide a basis for the assessment of the well performance development. Generally, the applied standard monitoring methods and diagnosis tools provided the expected identification of a performance deterioration and evidence for the presence of starting materials for clogging processes such as iron, oxygen, iron-related bacteria and particles. Room for improvement could be identified with regard to the reliability, information value and comparability of the tested methods, e.g. by a stepwise combination and extension of the methods to determine the interacting processes from the composition of the deposits. Further investigations should aim at method validation, especially for well monitoring during routine operation (e.g. use of delta h, development of standards for Qs-measurements and TV inspections), and further method development for the ongoing project with scientific investigations to obtain deeper process understanding, e.g. investigating shares of deposits resulting from the different processes (chemical, biological, physical) and relations between the rate of clogging or the location of deposits to well characteristics and site conditions to separate the different well ageing processes. This will then lead to the identification of key parameters that may be influenced to slow down well ageing and keep the well performance and water quality at an optimum.