title: On the use of mean groundwater age, life expectancy and capture probability for defining aquifer vulnerability and time-of-travel zones for source water protection.
abstract: Protection and sustainability of water supply wells requires the assessment of vulnerability to contamination and the delineation of well capture zones. Capture zones, or more generally, time-of-travel zones corresponding to specific contaminant travel times, are most commonly delineated using advective particle tracking. More recently, the capture probability approach has been used in which a probability of capture of P=1 is assigned to the well and the growth of a probability-of-capture plume is tracked backward in time using an advective-dispersive transport model. This approach accounts for uncertainty due to local-scale heterogeneities through the use of macrodispersion. In this paper, we develop an alternative approach to capture zone delineation by applying the concept of mean life expectancy E (time remaining before being captured by the well), and we show how life expectancy E is related to capture probability P. Either approach can be used to delineate time-of-travel zones corresponding to specific travel times, as well as the ultimate capture zone. The related concept of mean groundwater age A (time since recharge) can also be applied in the context of defining the vulnerability of a pumped aquifer. In the same way as capture probability, mean life expectancy and groundwater age account for local-scale uncertainty or unresolved heterogeneities through macrodispersion, which standard particle tracking neglects. The approach is tested on 2D and 3D idealized systems, as well as on several watershed-scale well fields within the Regional Municipality of Waterloo, Ontario, Canada.
authors.affiliation: Canada Research Chair in Quantitative Hydrogeology of Fractured Porous Media, Université Laval, Dept. of Geology & Geological Engineering, Québec City, Québec Canada. john.molson@ggl.ulaval.ca
authors.forename: J W
authors.initials: JW
authors.lastname: Molson
authors.forename: E O
authors.initials: EO
authors.lastname: Frind
mesh_terms: D060587:Groundwater / Q000737:chemistry*; D008962:Models, Theoretical; D009864:Ontario; D013997:Time Factors; D035501:Uncertainty; D014872:Water Movements; D060753:Water Quality