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Larbkich, Warangkana ¹ ; Neupauer, Roseanna M ²

¹ Department of Civil, Environmental, and Architectural Engineering, University of Å·ÃÀ¿Ú±¬ÊÓƵ at Boulder
² Department of Civil, Environmental, and Architectural Engineering, University of Å·ÃÀ¿Ú±¬ÊÓƵ at Boulder

Groundwater age is a measurement of the amount of time that water particles have been in the aquifer. It also represents the amount of time that conservative contaminant particles have been in the aquifer because they travel through the aquifer at the same rate as water particles. This similarity in movement of water particles and conservative contaminant particles has led to the application of groundwater age as a tool to assess aquifer vulnerability. An older groundwater can lead to increased vulnerability. For example, if an area is contaminated and the source is removed to prevent further contamination, this area will remain contaminated until all of the contaminated water that has already recharged the aquifer passes through the area. The older groundwater represents the contaminated groundwater that recharged the aquifer in the distant past. Thus, in area with older groundwater, it may take a longer time for the contaminant to exit the area.

Sorbing contaminant particles travel at a slower rate than water particles, thus, the amount of time that sorbing contaminant particles have been in the aquifer is greater than the groundwater age. As a result, if an area is contaminated with sorbing particles, and then the source is removed, the area may take a longer time to become decontaminated than if it was contaminated with conservative contaminants. If there is a decaying contaminant, the amount of contaminant will decrease over time, and the contaminant may degrade to sufficiently low levels that do not concern us. Thus, groundwater age is not an appropriate tool for assessing aquifer vulnerability for sorbing or decaying contaminants. Instead, we introduce a new concept, called “solute age", that represents the amount of time that sorbing and/or decaying contaminant particles have been in the aquifer.

We developed a method to determine the solute age in a manner similar to how the groundwater age is determined. A groundwater sample can represent a distribution of groundwater ages, which is regarded as a statistical distribution rather than a singular age (Newman et al, 2010; Sanford 2011). Goode (1996) developed an equation which can be used to determine mean groundwater age in a steady flow field (Fig. 1). We develop the equation to determine mean solute age for linear equilibrium sorption and first-order decay in a steady flow field based on the approach of Goode (1996) (Fig. 2). MT3DMS version 5.2 is added the zeroth-order decay term so we can use it to directly simulate mean groundwater age by setting the zeroth-order decay constant to -1. We are currently determining the approach to directly simulate mean solute age.

Goode, D.J., 1996, Direct Simulation of Groundwater Age: Water Resources Research. v.32, p.289-296.

Newman, B.D., Osenbruck K., Aeschbach-Hertig W., Solomon D.K., Cook P., Rozanski K., and Kipfer R., 2010, Dating of Young Groundwater Using Environmental Tracers; Advantages, Applications, and Research Need: Isotopes in Environmental and Health Studies. v. 46, p. 259-278.

Sanford, W.E., 2011, Calibration of Models Using Groundwater Age: Hydrogeology Journal. v.19, p. 13-16.