Apparent 85Kr ages of groundwater within the Royal watershed, Maine, USA

Specific 85Kr activity is mapped from 264 domestic and municipal wells sampled during 2002–2004 in the Royal watershed (361 km2), Maine. Gas samples are collected at 20 m, 40 m, and >50 m interval depths within the unconfined aquifers. Gas extraction for 85Kr from wells is obtained directly via a wellhead methodology avoiding conventional collection of large sample volumes. Atmospheric 85Kr input to the recharge environment is estimated at 1.27 Bq m−3 by time-series analyses of weighted monthly precipitation (2001–2004). Numerical simulation of Kr gas transport through the variable unsaturated zones to the water table suggests up to 12-year time lags locally, thus biasing the 85Kr groundwater ages. Apparent 85Kr ages suggest that ≈70% of groundwater near 20 m depth was recharged less than 30 years BP (2004). Mass–age transport modeling suggests that post mid-1950s recharge penetrates to part of the basinʹs floor and that older groundwater seeps from the underlying fractured bedrock may occur.