Characterization of deep aquifer dynamics using principle component analysis of sequential multilevel data

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 (VOC) were maintained in the two sampling campaigns conducted 295 days apart, the vertical locations of high concentration gradients were shifted between the two profiles. Principle Component Analysis (PCA) of the chemical variables resulted in a first Principal Component (PC) 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 be shifted vertically between the sampling events. This shift cut across a clayey bed which separated between 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 (EC) monitoring in a packed off section of the observation well revealed an event in which a distinct water body flowed through the monitored section (v ∼ 150 m yr-1). Compilation of the aforementioned data and analysis lead to a conclusion that the observed changes in the profiles resulted 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 this data. (b) The fact that distinct water bodies of 10-100 m vertical and horizontal dimensions flow under contaminated sites has implications for monitoring and remediation.