Transport and Variability of Fecal Bacteria in Carbonate Conglomerate Aquifers by Nadine Goeppert 1,2 and Nico Goldscheider 3 Abstract Clastic sedimentary rocks are generally considered non-karstifiable and thus less vulnerable to pathogen contamination than karst aquifers. However, dissolution phenomena have been observed in clastic carbonate conglomerates of the Subalpine Molasse zone of the northern Alps and other regions of Europe, indicating karstification and high vulnerability, which is currently not considered for source protection zoning. Therefore, a research program was established at the Hochgrat site (Austria/Germany), as a demonstration that karst-like characteristics, flow behavior, and high vulnerability to microbial contamination are possible in this type of aquifer. The study included geomorphologic mapping, comparative multi-tracer tests with fluorescent dyes and bacteria- sized fluorescent microspheres, and analyses of fecal indicator bacteria (FIB) in spring waters during different seasons. Results demonstrate that (1) flow velocities in carbonate conglomerates are similar as in typical karst aquifers, often exceeding 100 m/h; (2) microbial contaminants are rapidly transported toward springs; and (3) the magnitude and seasonal pattern of FIB variability depends on the land use in the spring catchment and its altitude. Different groundwater protection strategies that currently applied are consequently required in regions formed by karstified carbonatic clastic rocks, taking into account their high degree of heterogeneity and vulnerability. Introduction Karst aquifers are known to be generally more vulnerable to contamination than aquifers with fractured or intergranular porosity. Microbial pathogens can easily enter karst aquifers through thin soils and the epikarst or via swallow holes. In the conduit network, they are rapidly transported toward springs, often associated 1 Corresponding author: Department of Hydrogeology, Institute for Applied Geosciences, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76128 Karlsruhe, Germany. 2 Currently at Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel; (972) 8934-4230; fax: (972) 8934-4124; nadine. goeppert@weizmann.ac.il 3 Technische Universit¨ at M¨ unchen (TUM), Faculty for Engineering, Geo- and Environmental Sciences, Hydrogeology and Geothermics Group, Arcisstrasse 21, 80333 Munich, Germany. Received May 2009, accepted June 2010. Copyright © 2010 The Author(s) Journal compilation © 2010 National Ground Water Association. doi: 10.1111/j.1745-6584.2010.00741.x with suspended sediments (Dussart-Baptista et al. 2003; Mahler et al. 2000; Pronk et al. 2007, 2009). Clastic sedimentary rocks are generally not expected to form highly vulnerable karst aquifers. However, conglomerates (and sandstones) of predominantly carbon- atic composition occur in many orogenic basins, such as the foreland basins of the alpine fold belts in Europe (Figure 1a), and in similar sedimentary environments. They are characterized by carbonate rocks in the source area, high relief energy, and climatic conditions allow- ing for the formation and fluvial transport of carbonate gravel (Nichols 1999). The type locality of foreland basins is the Molasse basin north of the Alps, which is sub- divided into Foreland Molasse and Subalpine Molasse (Figure 1b). The latter has been affected by late-alpine fold-and-thrust tectonics and thus became a part of the Alps. The coarsest sediments with the highest carbonate contents occur at the southern margin of the basin, often within the Subalpine Molasse (Berge and Veal 2005). Several studies have reported karstification of carbon- ate conglomerates in orogenic basins near the margins of NGWA.org Vol. 49, No. 1 – GROUND WATER – January-February 2011 (pages 77 – 84) 77