Abstract
Abstract. 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 (v ∼ 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.
Highlights
Multilevel sampling (MLS) has been found useful for improving hydrochemical and hydraulic characterizations of aquifers relative to other sampling methods (Ronen et al, 1986; Cherry et al, 2007; Muller et al, 2010; Kurtzman et al, 2011)
In the present study we explored deep aquifer dynamics using a unique set of observations from two sequential MLS campaigns
Two sequential MLS campaigns were conducted in a 150-m deep observation well penetrating the entire section of the Israeli Coastal Aquifer in Tel Aviv
Summary
Multilevel sampling (MLS) has been found useful for improving hydrochemical and hydraulic characterizations of aquifers relative to other sampling methods (Ronen et al, 1986; Cherry et al, 2007; Muller et al, 2010; Kurtzman et al, 2011). Depth-dependent samples from deep aquifers are usually obtained during drilling (Williams and Chou, 2007; Hendry et al, 2011); interpretation of aquifer dynamics from these single-time data sets is limited. In the present study we explored deep aquifer dynamics using a unique set of observations from two sequential MLS campaigns. Significant vertical variability in the chemical composition of groundwater at a single point in time had been described by Netzer et al (2011). We analyzed the aquifer dynamics that might explain differences in the observed hydrochemical vertical distributions obtained in consecutive MLS campaigns. Principal component analysis (PCA) was employed to examine the depth- and time-dependent chemical variables from the two multilevel profiles. Groundwater electrical conductivity (EC) and head measurements provided the necessary background for interpreting the flow dynamics that were responsible for the temporal changes in hydrochemical profiles
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