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One crucial task in groundwater research and management is the estimation of groundwater mixing ratios. Here, estimations of mixing ratios are presented systematically and spatially for shallow and deep groundwater in some areas of excessive groundwater pumping with different magnitudes of groundwater drawdown. The mixing ratios are estimated using two methods: (1) the total mixing ratio using all parameters, and (2) the mixing ratio using nitrate concentrations. The values for the total mixing ratio indicate that mixing between the shallow and the deep groundwater clearly occurs in all three depression areas, but with different ratios. The spatial distribution map of the total mixing ratio clearly shows that the largest mixing ratio occurs near the center of the cone of depression, and that the ratio decreases gradually away from the center of the depression area. There is a positive correlation among total mixing ratios, CFC-12 concentrations, and modeled vertical flux. Remarkably, the highest correlation is found between the correlation of the total mixing ratio and magnitude of vertical flux in the largest drawdown area. Meanwhile, comparison of the mixing ratio calculations by the different methods showed insignificant correlation which means nitrate is ineffective as the prevailing contaminant tracer for deep groundwater in this basin. Overall, this study concludes that the method of total mixing ratio using all chemical parameters is the most effective and consistent with previous methods. This study provides further proof that groundwater mixing between the shallow and deep groundwater systems has clearly occurred in the Bandung basin as an impact of excessive groundwater pumping.
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