Contamination exposure assessment of a well field through Null-Space Monte Carlo method

An application of the Null-Space Monte Carlo (NSMC) method is presented for the evaluation of exposure to contamination of two aqueduct well fields located in Vara-Magra Valley (La Spezia) on the right (FRX) and left riverbank (FLX). The study involved the implementation of a MODFLOW model, and the next optimization of permeability parameters by assimilating the hydraulic head records collected. The optimization of the parameters was carried out using PEST adopting a high parameterization approach using pilot points and Tikhonov regularizations. Using the NSMC method, a set of permeability realizations was generated and employed to simulate an accidental pollution spill through a particle tracking algorithm, in a point located on the right bank side about 1800 meters upstream from the FLX well field. The results show that the FRX well field is the relatively most exposed target, being reached by contamination in 48% of the simulations, while Magra River and FLX well field were exposed to contamination in 42% and 9% of the simulations, respectively.