Estimating groundwater mixing ratios and their uncertainties using a statistical multi parameter approach

Rueedi, Joerg; Purtschert, Roland; Beyerle, Urs; Alberich, Carmen; Kipfer, Rolf (2005). Estimating groundwater mixing ratios and their uncertainties using a statistical multi parameter approach. Journal of hydrology, 305(1-4), pp. 1-14. Elsevier 10.1016/j.jhydrol.2004.06.044

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The estimation of groundwater mixing ratios is a crucial task in groundwater research and management. The quantity as well as the quality of groundwater resources are sensible to, e.g. the admixture river water, the interaction between different groundwater layers and the spatial distribution of such mixing zones. In this paper, we present a procedure to calculate mixing ratios and their uncertainties using a set of chemical and isotopic parameters. The advantages compared to estimations based on only few parameters are better-constrained mixing ratios and the limitation of systematic errors due to the parameter choice. The statistical consistence of the resulting mixing ratios is checked with a χ2-test.

In particular in larger aquifers, where sampling resolution is often coarse, the composition of end members may vary considerably from one borehole to another due to rock water interaction or varying recharge conditions. After selecting sampling sites where no mixing occurs, the spatial distribution of the end member composition was calculated using Kriging. This provides the best estimates of end member compositions and their uncertainties at any location of the area of investigation.

The method is, in principle, applicable for any number of end members. In this paper, the spatial distribution of the interaction of two layers of the Continental Terminal aquifer (Niger, Africa) was investigated. Mixing between the deep CT2 and the shallow CT3 aquifer occurs where the separating layers are thinning out or due to improper sealing of some boreholes. Radiogenic 4He concentrations, which differ significantly in the two aquifers, were used as independent parameter to crosscheck the mixing calculations.

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