Summary

Age-dating data collected from 12 shallow monitoring wells in the Portland basin yield apparent ground-water ages that generally range from near 0 to about 20 years for wells screened in the uppermost 15 m of the aquifer. The age gradient in the shallow aquifer is consistent with the range of reasonable recharge rates for the aquifer. Estimates of apparent age allow reconstruction of initial tritium concentrations for ground-water samples; the pattern of these reconstructed tritium concentrations matches historical tritium input records, providing support for the interpreted apparent ages. Knowledge of the apparent ages of ground water produced by these monitoring wells could be used to help calibrate flow models or elucidate trends in ground-water quality.