Arsenic is a carcinogen known for its acute toxicity to organisms. Geothermal waters are commonly high in arsenic, as shown at the Bjarnarflag Power Plant, Iceland (∼224 μg/kg of solvent). Development of geothermal energy requires adequate disposal of arsenic-rich waters into groundwater/geothermal systems. The outcome of arsenic transport models that assess the effect of geothermal effluent on the environment and ecosystems may be influenced by the sensitivity of hydraulic parameters. However, previous such studies in Iceland do not consider the sensitivity of hydraulic parameters and thereby the interpretations remain unreliable. Here we used the Lake Mývatn basaltic aquifer system as a case study to identify the sensitive hydraulic parameters and assess their role in arsenic transport. We develop a one-dimensional reactive transport model (PHREEQC ver. 2.), using geochemical data from Bjarnarflag, Iceland. In our model, arsenite (H 3 AsO 3 ) was predicted to be the dominant species of inorganic arsenic in both groundwater and geothermal water. Dilution reduced arsenic concentration below ∼5 μg/kg. Adsorption reduced the residual contamination below ∼0.4 μg/kg at 250 m along transect. Based on our modelling, we found volumetric input to be the most sensitive parameter in the model. In addition, the adsorption strength of basaltic glass was such that the physical hydrogeological parameters, namely: groundwater velocity and longitudinal dispersivity had little influence on the concentration profile.