Groundwater quality monitoring network design and optimisation based on measured contaminant concentration and taking solute transit time into account

Abstract

Groundwater quality monitoring networks are designed to give the best possible overview of the quality status and trend of aquifers from a limited number of measurement stations. Ideally, the stations of the network should (i) allow the detection of significant changes in water quality, either improvements due to successful mitigation measures, or deteriorations caused for instance by land use change or other increasing pressure and (ii) approximate closely the true unknown mean concentration of the considered aquifer. If these two features are met, the monitoring network is representative of the entire aquifer. In this article, we present a method with which to assess and improve the representativeness of a regional groundwater monitoring network. The method is designed specifically to optimize the selection of monitoring stations from a larger choice of fixed locations such as springs or existing observation wells using one contaminant for the optimisation procedure. Firstly, the empirical distribution of selected solutes (contaminants and age tracers) is calculated from a large dataset consisting of as many sampling points as possible and compared to the distributions obtained solely from the stations of the existing or planned monitoring network. If differences are large, the network is modified by adding new stations or replacing some of them, using one contaminant for the optimisation. Lastly, the distribution of the optimised network is compared to the total distribution for other contaminants and one groundwater age tracer.The use of the method is illustrated for the main aquifer of the country of Luxembourg, where it is shown that the representativeness of the current monitoring network can be improved by replacing two stations. In the case study, the optimisation was based on desethyl atrazine, a ubiquitous pesticide transformation compound which was assumed to be an indicator of diffuse agricultural pollution. Although the largest improvement was obviously obtained for the optimised compound, improvement was also observed for other contaminants, including nitrate, probably showing that at the regional scale, the pattern of diffuse pollution of widely used agrochemicals is similar between compounds. The temporal evolution of the major contaminants could also be reproduced by the optimised network, even though the range of tritium values used as age tracer was narrower for the modified network than for the original network, thus indicating some bias towards faster reaction times.