Characterizing a Sewage Plume Using the 3H‐3He Dating Technique

Abstract

An extensive 3 H −3He study was performed to determine detailed characteristics of a regional flow system and a sewage plume over a distance of 4 km in a sand and gravel aquifer at Otis Air Base in Falmouth, Massachusetts. 3 H −3He ages increase with depth in individual piezometer clusters and with distance along flowpaths. However, the age gradient with depth (Δt/Δ 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 in precipitation is archived longitudinally along a flowline through the main axis of the plume and vertically in individual piezometer clusters. On the eastern side of the sampling area, where water from Ashumet Pond forces plume water deeper into the flow system, 3 H −3He ages are young at depth because the 3 H −3He “clock” is reset due to outgassing of helium in the pond. A reconstruction of the tritium input functions for the regional and plume samples shows that there is no offset in the peak [3H]+[3Hetrit] concentrations for the plume and regional water, indicating that the water from supply wells for use on the base is young. The 3 H −3He ages and detergent concentrations in individual wells are consistent with the beginning of use of detergents and the time period when their concentrations in sewage would have been greatest. Ages and hydraulic properties calculated using the 3 H −3He data compare well with those from previous investigations and from particle‐tracking simulations.

Noble Gases in Lakes and Ground Waters

“…Yet, as long as the value of Lex is restricted to the range allowed by the CEmodel, the differences in the calculated quantities are usually small. In the study of DUhkle Shapiro et al (1999) practically no changes result, because 4Heter = 0 was assumed in the calculation of most ages. In principle, however, for samples with a large and strongly fractionated excess air component the usual atmospheric excess air correction can result in significantly underestImated ages.…”

“…For example, in the large 3H)He study of a shallow sandy aquifer on Cape Cod of Dunkle Shapiro et al (1999), 55 out of a total of 91 samples yielded negative 4Heter concentrations, clearly indicating that Lex was overestimated. For each sample, the upper limit for Lex is defined by the physical requirement 4Heter 2: O.…”

“…An example for the use of 3 H-3 He data to determine the origin and spreading rate of pollutants is the extensive study of a sewage plume spreading in an unconfined aquifer in Cape Cod, Massachusetts (USA) by Dunkle Shapiro et al (1999). 3H)He ages at the center of the plume were found to increase linearly as a function of distance from the source (Fig.…”

“…Apparent 3HPHe age (years) the estimation of hydraulic conductivity~effective porosity, and dispersivity on a range of scales . Another important application of the method is to provide chronologies for records of past environmental change, in particular histories of groundwater contamination (Bohlke et al 1997;Aeschbach-Hertig et al 1998;Johnston et al 1998;Schlosser et al 1998;Dunkle Shapiro et al 1999), 3H_ 3 He data can also provide constraints on mixing of different water components, e.g., in aquifers affected by river infiltration (Plummer et al 1998b;Holocher et al 2001). …”

Effects of intraborehole flow on groundwater age distribution

“…Depending on the tracer used, age dating techniques can be used to date relatively young groundwater (e.g., Busenberg and Plummer 1992;Plummer and Friedman 1999;Shapiro et al 1999) or can target longer, basin-scale transport times (Castro et al 1998;Sanford et al 2004).…”