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Comparison of variance-reduction and space-filling approaches for the design of environmental monitoring networks

Abstract:

This article focuses on the design of groundwater monitoring networks to detect contamination with nitrates from agricultural origin. This is a problem that has been in the minds of the general public, scientists, governmental agencies, and legislators for some time now. If one looks at European statistics, despite still incomplete data, in 13% of the regions the 50 mg/l European water quality standard for drinking water (Drinking Water Directive, 98/83/EC) is exceeded in more than 25% of the monitoring stations. A compliance groundwater nitrate-monitoring network is developed for a case study in the south of Portugal (Gabbro of Beja aquifer system), using both variance-reduction and space-filling approaches. In the first the variance of the error of estimation obtained by ordinary kriging is used, after building a covariance model, and the objective is to minimize the average kriging variance. In the second approach a criterion for the quality of spatial coverage is used, usually based on a metric. The objective here is also usually to minimize a function of the metric (criterion). The search for a solution is made in an iterative manner, by replacing one station, analyzing the result, and deciding whether to keep the solution or not. Due to the enormous number of possible combinations even for small networks, a structured search method, the simulated annealing method, is used to obtain the final network. © 2007 Computer-Aided Civil and Infrastructure Engineering.