Hydrogeochemical Evaluation of Contaminant Release from Drilling Equipment During Aquifer Testing

Groundwater quality assessment during well drilling could be biased by transient contamination from drilling equipment and fluids. To address this issue, batch leaching tests, sequential filtration experiments, and field validation were combined. Batch leaching experiments were performed on painted and unpainted drilling rods. Unpainted carbon-steel rods promoted marked Fe and Mn release through oxidative corrosion, whereas painted rods released Zn through progressive dissolution of the zinc coating and As under transiently reducing conditions. Leaching tests performed on fresh and exhausted drilling muds revealed elevated concentrations of SO42−, Cl− and Na+, attributable to the drilling additives rather than the drilling muds. Meanwhile, exhausted drilling muds showed a limited enrichment in trace elements, likely due to sorption onto the aquifer matrix. Stepwise filtration showed that 0.15 µm membranes effectively removed colloid-associated trace elements, while major ions remained unaffected by filters’ pore size. Field data from two wells confirmed that Fe, Mn, Zn, and Al anomalies detected during drilling-phase sampling were consistent with release by drilling equipment and muds. The coupled laboratory–field approach provides a practical framework for identifying and minimizing drilling-induced artifacts, supporting the use of in situ 0.15 µm filtration and extended purging to obtain representative groundwater quality data.