Chapter 9 - Hydrologic fluxes of iron in groundwater ecosystems: Implications for global risks and challenges

Abstract

Iron (Fe) is ranked as the second most abundant metallic element in the Earth’s crust and functions as an essential micronutrient for all living organisms. Both bioavailability and the redox sensitivity of iron are well documented in a variety of environmental ecosystems. The cycling of iron influences various global reservoirs including sediments, soil, lakes, and groundwater via abiotic and biotic processes. In groundwater, the occurrence of iron is predominantly in the form of complexes, such as ferrous bicarbonate, Fe(OH)3, and Fe3O4, which form as a result of dissolved iron content and percolation from underlying soil and rock structures. In groundwater systems, iron speciation is shaped by critical factors including chemical composition, addition and removal of iron, and internal recycling. Several studies conducted around the world highlighted that in comparison to WHO standards, concentrations exceeding 0.3μg/mL level pose a serious threat because groundwater is contaminated by iron. Long-term assessments suggest that the reductive dissolution of iron (hydr)oxide minerals and complexes in groundwater contribute to sources of dissolved Fe concentrations. The reducing conditions, residence time, depth, and salinity contribute to dissolution and migration of Fe to groundwater. Moreover, diverse microbial species representing heterotrophic iron-oxidizing (FeOB) and iron-reducing (FeRB) phylogenetic groups greatly influence the fate of ion cycling and bioavailability in groundwater. With these issues in mind, this chapter seeks to outline global iron flux, highlighting microbial and biochemical intervention in the transition processes and possible issues and challenges that need to be resolved for combating iron contamination in groundwater systems.