ESTIMATING INDIANA GROUNDWATER AGE FROM TRITIUM ACTIVITIES

As part of a groundwater susceptibility evaluation the Indiana Department of Environmental Management (IDEM) has generated a database of groundwater tritium activities throughout the state. This database provides an opportunity to study the distribution of groundwater ages throughout Indiana. In order to interpret the distribution of groundwater ages; however, the tritium activities had to first be converted to groundwater ages based on a geographically representative tritium input function and an appropriate mixing model.

Separate tritium input functions were developed for recharge in Northern, Central, and Southern Indiana. The input functions were based on records of tritium in precipitation over the period of 1953 to 1998 in St. Louis, MO; Chicago, IL; and Ottawa, ON; as well as limited data for tritium in surface water and rain in Lafayette, IN. Yearly average tritium activities were calculated for the stations by weighting summer and winter months according to the fraction of recharge expected for those months. That fraction was previously determined by another researcher using stable oxygen and hydrogen isotopes.

Apparent groundwater age can be estimated directly from tritium precipitation records corrected for first order decay. However, some amount of hydrodynamic dispersion is likely to occur. Therefore, a mixing model was used to more accurately estimate groundwater ages by accounting for attenuation of tritium levels due to advective dispersion and diffusion. The mixing model approximates the dispersion and diffusion processes by assuming a gaussian distribution of tritium activities. The decay corrected precipitation input function served as model input and groundwater ages were estimated by comparing model calculated activities with groundwater tritium activities.

Finally the estimated groundwater ages can be compared with hydrogeology and interpreted in terms of recharge patterns. The results of that analysis will be presented in a later paper.