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

摘要

Specific ~(85)Kr activity is mapped from 264 domestic and municipal wells sampled during 2002—2004 in the Royal watershed (361 km~2), Maine. Gas samples are collected at 20 m, 40 m, and > 50 m interval depths within the unconfined aquifers. Gas extraction for ~(85)Kr from wells is obtained directly via a wellhead methodology avoiding conventional collection of large sample volumes. Atmospheric ~(85)Kr 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 ~(85)Kr groundwater ages. Apparent ~(85)Kr 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 basin's floor and that older groundwater seeps from the underlying fractured bedrock may occur.