Numerical delineation of transient capture zones

Delineating capture zones plays an important role in the protection of drinking water wells. In the recent literature most of the related work has dealt with the uncertainty of location and extent of the capture zone due to the heterogeneity of the hydrogeologic parameters of the aquifer. This paper is about the impact of considering transient aquifer conditions when computing the capture zone of a well. The presented approach alternates for predefined time steps between the groundwater flow and a particle tracking module enabling the management of a vast number of individual particles. Thus, the development of the capture zone can be monitored more precisely and it can be differentiated between parts of it. Specific results are presented for an aquifer in South-East Austria in terms of the percentage of particles of any starting location that are terminated by a well and of the minimum travel time a particle needs from the same starting location to reach a well. The transient delineated capture zone is compared to those derived from steady state computations for specific groundwater flow conditions. Though the results are site specific the significance of considering the transient system boundary conditions is demonstrated since a superposition of the steady state case results clearly overestimates the area to be protected. Additionally, in order to address sensitivity and computational efficiency the effect of increasing the release interval and the starting pattern of particles on the development of the capture zone is analyzed.