Developing a Methodology for Selecting Wells Sampled for the Evaluation of Groundwater Quality

摘要

The Coastal Aquifer in Israel, and particularly in the area of Tel Aviv, Israel, supplies a great deal of the drinking water needed in the Coastal Zone of the country. Various sampling operations that have taken place over the last few years in the Tel Aviv area have shown it to be contaminated with organic micropollutants (chlorinated hydrocarbons) in concentrations of the order of magnitude of, and even higher than, their threshold values (Ronen et al. 2000a,b). However, none of the sampling operations covered all of the relevant wells in the area, due to limitations in sampling resources and particularly in laboratory capacity. Therefore, each one of the sampling operations presented a totally different description of the contaminant distribution in the aquifer, together providing different pictures of the possible contaminant plumes. rnThis study concerns the development of a new methodology for devising a monitoring plan for groundwater contamination. The objective of the research is to locate accurately and quickly the contaminant sources and plumes by taking a limited number of samples during each sampling operation. Furthermore, the results’ reliability should be maximized for a given number of sampling operations per year, and a specified number of sampled wells per operation. rnThe algorithm developed in this study incorporates the coverage of the area of interest by a grid of cells. Their number is calculated according to the number of available samples, i.e. wells that should be selected for each sampling operation. Data relevant to well selection are produced for each cell, thus being accumulated faster than by considering “isolated” wells. It is then weighted and used to sort cells according to their different priorities for sampling. Within each selected cell, the number of selected wells for sampling is determined. In the following step, the most desirable wells for sampling within each cell are identified by a utility function that accounts for various well parameters, like the well discharge, contaminant concentration (last measured or interpolated from other wells), pollutant discharge, sampling history, and possibly additional factors.rn The developed algorithm was implemented both on real data from the Coastal Aquifer in the area of Tel Aviv, and on various artificial computer-generated aquifers. The results have indicated a gradual convergence of the description of contaminant distribution in the aquifer to its actual image. Parameters measuring the efficiency of the algorithm and its possible improvements have been identified, and thereby codes for improvement of the algorithm operation under different sets of physical parameters have been obtained. rnDuring the first stage of the study, which is reported in this paper, the research concerns comparatively minor time dependence of contaminant distribution in the domain.