The radioactive isotope of argon, ³⁹Ar, is an indicator of the elapsed time groundwater spends in an aquifer isolated from the atmosphere over time ranges from 10-1000 years. The method works by measuring the amount of ³⁹Ar in water and comparing it to the known atmospheric concentration. Given the known, constant radioactive decay rate, the elapsed time since the water was last in contact with the atmosphere can easily be determined using the difference between the measured concentration and atmospheric concentration. Here we provide evidence that subatomic particles (muons) generated from cosmic rays can be a large source of ³⁹Ar which is not commonly accounted for, potentially confounding the method in high-elevation, mountain aquifers.