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

摘要

Two sequential multilevel profiles were obtained in an observation wellopened to a 130-m thick, unconfined, contaminated aquifer in Tel Aviv,Israel. While the general profile characteristics of major ions, traceelements, and volatile organic compounds were maintained in the two samplingcampaigns conducted 295 days apart, the vertical locations of highconcentration gradients were shifted between the two profiles. Principalcomponent analysis (PCA) of the chemical variables resulted in a firstprincipal component which was responsible for ∼60% of thevariability, and was highly correlated with depth. PCA revealed threedistinct depth-dependent water bodies in both multilevel profiles, whichwere found to have shifted vertically between the sampling events. Thisshift cut across a clayey bed which separated the top and intermediate waterbodies in the first profile, and was located entirely within theintermediate water body in the second profile. Continuous electricalconductivity monitoring in a packed-off section of the observation wellrevealed an event in which a distinct water body flowed through themonitored section (v ∼ 150 m yr−1). It was concluded that theobserved changes in the profiles result from dominantly lateral flow ofwater bodies in the aquifer rather than vertical flow. The significance ofthis study is twofold: (a) it demonstrates the utility of sequentialmultilevel observations from deep wells and the efficacy of PCA forevaluating the data; (b) the fact that distinct water bodies of 10 to 100 mvertical and horizontal dimensions flow under contaminated sites, which hasimplications for monitoring and remediation.