Kurtzman, D., Institute of Soil, Water and Environmental Sciences, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Netzer, L., Department of Hydrology and Microbiology, Sde Boker Campus, Ben Gurion University of the Negev, Negev 84990, Israel
Weisbrod, N., Department of Hydrology and Microbiology, Sde Boker Campus, Ben Gurion University of the Negev, Negev 84990, Israel
Nasser, A., Institute of Soil, Water and Environmental Sciences, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Graber, E.R., Institute of Soil, Water and Environmental Sciences, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Ronen, D., Department of Hydrology and Microbiology, Sde Boker Campus, Ben Gurion University of the Negev, Negev 84990, Israel, Hydrological Service, Water Quality Division, Israel Water Authority, P.O. Box 20365, Tel Aviv 61203, Israel

Two sequential multilevel profiles were obtained in an observation well opened to a 130-m thick, unconfined, contaminated aquifer in Tel Aviv, Israel. While the general profile characteristics of major ions, trace elements, and volatile organic compounds were maintained in the two sampling campaigns conducted 295 days apart, the vertical locations of high concentration gradients were shifted between the two profiles. Principal component analysis (PCA) of the chemical variables resulted in a first principal component which was responsible for ∼60% of the variability, and was highly correlated with depth. PCA revealed three distinct depth-dependent water bodies in both multilevel profiles, which were found to have shifted vertically between the sampling events. This shift cut across a clayey bed which separated the top and intermediate water bodies in the first profile, and was located entirely within the intermediate water body in the second profile. Continuous electrical conductivity monitoring in a packed-off section of the observation well revealed an event in which a distinct water body flowed through the monitored section (ν ∼ 150 m yr -1). It was concluded that the observed changes in the profiles result from dominantly lateral flow of water bodies in the aquifer rather than vertical flow. The significance of this study is twofold: (a) it demonstrates the utility of sequential multilevel observations from deep wells and the efficacy of PCA for evaluating the data; (b) the fact that distinct water bodies of 10 to 100 m vertical and horizontal dimensions flow under contaminated sites, which has implications for monitoring and remediation. © 2012 Author(s) CC Attribution 3.0 License.