Can karst aquifer for public supply threatened by lead smelting be sustainably managed? Part II: Evidence from groundwater modeling.

This study investigates the possibility to sustainably manage two public supply well clusters situated perilously close to one of Asia's largest lead (Pb) smelters, a site that has approximately 10 km of overlying soil contaminated by multiple toxins including Pb (>500 mg/kg). One cluster of wells tap into a deep buried karst aquifer (n = 15 wells, 163-296 m depth) while the other into a shallower Quaternary gravel and sand aquifer (n = 4 wells, 100-150 m depth). Because both aquifer types are highly vulnerable to contamination, this study seeks to develop tools that can guide how to ensure long-term water security and safety against this high-risk backdrop. First, a regional groundwater flow model with simplified contaminant transport processes simulated by particle tracking is constructed. The models are used to estimate the risk of contamination (R) for a given duration of pumping by supply wells and the minimum amount of time for the pollutant particles to reach the supply wells (T). These two risk factors are then combined to arrive at a sustainability index (SI), estimated after setting an acceptable level of R of 1 % for 100 years of pumping by supply wells and T of 5 years. To be sustainable (SI>0) means that both risk factors must be within the acceptable levels, which demands a roughly halving the current pumping rates of these public supply wells, to 3 × 10 m/d for the buried karst and 0.5 × 10 m/d for the Quaternary aquifers, respectively, in order to supply water for 100 years. A key factor in the surprisingly "good" sustainability is that a deep buried karst aquifer (depth: 230-500 m) located in the higher elevation area to the north is channeling water upward due to the fortuitous presence of a major fault zone FMK that acts as a "hydraulic barrier" to delay the arrival of surface soil sourced pollutants. However, the response of this buffering effect to increasing pumping rates by the supply wells is non-linear, resulting in the conclusion that the current pumping rates are not sustainable. In addition to adopting water conservation measures, replacing groundwater pumping by tapping into numerous springs in the southern part of the study area is also a likely solution. Because current groundwater sustainability assessments mostly address water quantity imbalances at regional scale which would have categorized current pumping rates by these supply wells as "sustainable", this study illustrates the value of integrating groundwater quality risks into this important task, and provides a method to do so.