7. Hydrogeochemical Processes During Riverbank Filtration and Artificial Recharge of Polluted Surface Waters: Zonation, Identification, and Quantification

The hydrogeochemical processes acting during riverbank filtration (RBF) and artificial recharge using basins (BAR), aquifer storage recovery (ASR) and deep well injection with remote recovery (ASTR) are reviewed, and quantified by the chemical mass balance approach, also referred to as inverse modeling. The various systems are classified and described, together with the various hydrogeochemical zones and processes within them. The zones discerned include the surface water compartment, the recharge proximal, distant aquifer and discharge proximal zone in general, and a typical redox zonation for RBF, BAR, ASR and ASTR in particular. Mass balances are drawn up by the EXCEL spreadsheet code REACTIONS+ version 6, which is a significant upgrade of an earlier version and explained in detail. The model only requires data entry for the chemical composition of the input (infiltration water), the output (sample of infiltrate in the aquifer, taken from an observation or pumping well), and admixed local groundwater (if relevant), and may necessitate a change of default settings for specific reactions. The output consists of a concise list of the hydrogeochemical reactions in terms of losses and gains (in mmol/L) by the following reactions: oxidation of \({{\rm{NH}}_4}^ +\) and DOC in the input; oxidation of organic material, pyrite or desorbing Fe²⁺ and \({{\rm{NH}}_4}^ +\) from the aquifer; dissolution of various carbonate minerals, silica, gypsum or halite; reductive dissolution of ferrihydrite; and exchange of cations and anions. Examples of application include five different RBF samples with differing water sediment interaction. They illustrate the importance of the redox environment and additional gas inputs and gas outputs.