Characterizing a sewage plume using the H-3-He-3 dating technique

An extensive H-3-He-3 study was performed to determine detailed characteris tics of a regional flow system and a sewage plume over a distance of 4 km i n a sand and gravel aquifer at Otis Air Base in Falmouth, Massachusetts, H- 3-He-3 ages increase with depth in individual piezometer clusters and with distance along flowpaths, However, the age gradient with depth (Delta t/Del ta z) is smaller in the plume than that in the regional waters, due to the intense recharge in the infiltration beds, The 1960s bomb peak of tritium i n precipitation is archived longitudinally along a flowline through the mai n axis of the plume and vertically in individual piezometer clusters, On th e eastern side of the sampling area, where water from Ashumet Pond forces p lume water deeper into the flow system, H-3-He-3 ages are young at depth be cause the H-3-He-3 "clock" is reset due to ontgassing of helium in the pond . A reconstruction of the tritium input functions for the regional and plum e samples shows that there is no offset in the peak [H-3] + [He-3(trit)] co ncentrations for the plume and regional water, indicating that the water fr om supply wells for use on the base is young. The H-3-He-3 ages and deterge nt concentrations in individual wells are consistent with the beginning of use of detergents and the time period when their concentrations in sewage w ould have been greatest. Ages and hydraulic properties calculated using the H-3-He-3 data compare well with those from previous investigations and fro m particle-tracking simulations.