A novel treatment was tested with groundwater to investigate its arsenic removal under natural conditions. The system utilised in-line electrochlorination to oxidise water constituents without the need for external chemical supply. The oxidised arsenic and iron co-precipitated and were filtered via Greensand Plus™. The filter was catalytically active and provided an emergency oxidant. The system had only a few maintenance requirements due to online water quality monitoring. The contaminant removal during the field test in Costa Rica was impaired by strong fluctuations in water quality including low iron concentrations. However, the system removed on average 68% of the arsenic. Mean values of arsenic were 40 ± 23 μg/L in groundwater and 13 ± 6 μg/L in treated water. Iron was removed from an average of 2.8 ± 2.4 mg/L to 0.2 ± 0.2 mg/L (93% removal). Free chlorine produced and available in the treated water tank had a mean concentration of 1.25 mg/L and 0.64 mg/L, respectively.