Calibration of a mass transport model using environmental tritium

Abstract

Tritium profiles of groundwater were obtained by detailed vertical sampling of a 20-m thick aquifer comprising fluvioglacial sediments in the central part of The Netherlands. Using the known environmental tritium input time series since 1951, these profiles were used to calibrate the Random Walk model of T.A. Prickett et al. To perform this procedure the model was modified to allow vertical use and a time-dependent non-instantaneous input of a decaying radioactive tracer. During this calibration the following parameters were varied: (1) geological geometry (position and extent of a clay lens); (2) permeability variations; (3) ratio of net precipitation and effective porosity; and (4) longitudinal and transverse dispersivity. The calibrated model was hence use to predict the flushing of manure contamination from the aquifer. The applied calibration technique has proved useful to test and roughly calibrate the hydraulic properties estimated from the logs of a limited number of borings, with the accuracy needed for mass-transport modelling. Compared with the very detailed drilling program that is usually required to obtain such detailed information, this tritium-based calibration seems competitive for shallow phreatic aquifers.