Basic parameterization of Schleswig–Holstein’s shallow geological formations for numerical reactive transport simulations: representative groundwater compositions

Groundwater protection has to remain ensured in spite of the ambition to apply more and various types of subsurface usages in future. In this context, numerical simulations using “Virtual Aquifers” can be suitable for evaluating the general effects of complex induced process interactions, while meaningful simulation results require the appropriate parameterization of scenario analyses, such as regarding representative groundwater compositions. Therefore, this study reviewed the hydrochemical groundwater compositions of the different aquifers in the German state Schleswig–Holstein. To evaluate what aquifers exhibit statistically different compositions, the nonparametric Kruskal–Wallis test, the analysis of variance, the discriminant analysis, and the hierarchical cluster analysis were applied. These showed that between the free aquifers at pH < 6, the free aquifers at pH > 6, and the group of confined aquifers, significant differences in the dissolved constituents exist, but also that among the confined aquifers these differences are not significant, except for saline groundwaters that can be present near underground salt structures and the North Sea. Furthermore, the two methods applied for deducting representative compositions were the nearest neighbor method, where the monitoring wells accessing the groundwater most similar to the median compositions in the respective aquifers were identified, and the cluster center analysis. The calculated representative groundwater compositions for four aquifer groups (“Acidic” shallow aquifers, “Neutral” shallow aquifers, confined freshwater aquifers, saline aquifers) using both methods were very similar. Thus, this study provides a methodology and a basis for parameterizing Virtual Aquifer studies and discusses the limits of representativeness based on the regional data set.