Spatiotemporal Variability of Groundwater Iodine in the Northern Arid Basins: Significance for Safe Water Supply
Kunfu Pi, Junxia Li, Xianjun Xie, Philippe Van Cappellen, Duo Zhang, Kun Qian, Yanxin Wang
Abstract
The genesis of geogenic iodine (I)-contaminated groundwater poses a significant threat to long-term water exploitation. Safe and sustainable water supply, particularly in the northern arid basins, demands a quantitative prediction of the high variability of I distribution over hydrogeological timescales. Here, bioenergetics-informed reactive transport modeling was combined with high-resolution molecular characterization of fueling organic matter to decipher the time-controlled interactions between vertical flow and (bio)geochemical processes in I transport within the Datong aquifers. The declining reactivities of I-bearing organic matter and Fe oxides in the 15–40 m depth decreased the rate of I release, while a growing number of pore volumes flushed through the aquifers to leach out I– and organic I. This removal effect is compensated by the desorption of I– from Fe oxides and secondary FeS generated from the concurrent reduction of Fe oxides and SO42–. Consequently, peak concentrations of groundwater I– may have appeared, depending upon the vertical recharge rate, at the first several pore volumes flushed through the aquifers. The current vertical distributions of the various I species likely represent a quasi-steady state between I mobilization and leaching. These new mechanistic insights into the dynamic hydrogeological–(bio)geochemical processes support secure groundwater use in the I-affected northern arid basins.