Investigation of the adulterating effect of vertical convection in groundwater monitoring wells on in-situ measurements or water samples
Summary
Monitoring of Natural Attenuation (MNA) requires a long-term representative, from the measurement process unaffected groundwater sampling and in-situ measurement with high sensitivity regarding geochemical variations and trend indicators. Available are special monitoring wells that are, however, linked with high costs and maintenance problems, not part of monitoring networks and mostly limited to a depth interval and small sample quantity.
Conventional groundwater monitoring wells do not underlie the above mentioned constraints, however, they are subject to vertical convection, even if developed and equipped under the recent regulations and are thus not usable for MNA without restrictions. Hence, the aim was to investigate and quantify vertical convections occurring verifiably in monitoring wells according to their measurement adulterating effects.
The investigation of the underlying processes and their relevance was carried out by theoretical estimations, numerical multiphysics simulations and systematic experiments in a medium scale emulated monitoring well. In numerous real monitoring wells, on KORA testing sites amongst others, free convective flows and its effects on the mass transport (e.g. dissolved oxygen) were proven by adapted borehole geophysical methods.
Formation and characteristics of density driven free convection in wells were clarified. It became apparent, that partly extensive vertical gradients in temperature and salinity occur in monitoring wells, which can interfere with each other and start many specific phenomena of free convection and double diffusion already in 2-inch diameter wells. Vertical transport of oxygen, incorporated over the air-water-interface, through free convection is a very efficient process compared to diffusion, as verified by simulations and medium-scale experiments.
For the appraisal of monitoring wells concerning existing sample-adulterating vertical flows, a data processing algorithm was developed for geophysical logs, which enables the differentiation of the water column in sections with free, forced or without convection.
As a possible solution the idea of an adulteration-free sampling system following the permanent-packer-shuttle principle (patent issued) was developed. The system is applicable for implementation in new and already existing monitoring wells, as well. Furthermore, suggestions for a modified construction of monitoring wells and for a modified testing procedure with borehole measurements are made.
Literature
Berthold, S., 2010: Synthetic Convection Log - Characterization of vertical transport processes in fluid-filled boreholes. Journal of Applied Geophysics, 72, S. 20-27, doi:10.1016/j.jappgeo.2010.06.007.
Berthold, S. 2009: Geophysikalischer Nachweis freier Konvektion in Grundwassermessstellen und Bohrungen. Proceedings of the Dresdner Grundwasserforschungszentrums e.V., Issue 39 (ISSN 1430-0176), Dresden, Germany. 254 p.
Börner F., Berthold S. 2009: Vertical flows in groundwater monitoring wells. In: Kirsch, R. (ed.), Groundwater Geophysics – A Tool for Hydrogeology, 2. ed. (ISBN 978-3-540-88404-0), Springer-Verlag, Berlin, Heidelberg, doi:10.1007/978-3-540-88405-7, S. 367-389.
Berthold, S., Börner, F. 2008: Detection of free vertical convection and double-diffusion in groundwater monitoring wells with geophysical borehole measurements. Environmental Geology 54(7), S. 1547-1566. doi:10.1007/s00254-007-0936-y.
