Integrating transient behavior as a new dimension to WHPA delineation

Abstract

Abstract The protection of groundwater abstraction is primarily based on the delineation of “time-of-travel capture zones” known as Well Head Protection Areas (WHPAs). Their delineation is subject to several uncertainties: the scarcity of hydrogeological data due to limited budgets for exploration and additional uncertainty from time-varying flow conditions. While the former has lead to the use of probabilistic strategies, no existing approaches for WHPA delineation consider jointly geological uncertainties, transient flow conditions and the associated additional uncertainties. We propose to extend existing steady-state probabilistic approaches for WHPA delineation to account for transient flow conditions and their related uncertainties. In our synthetic scenario, transiency is represented through four selected transient drivers: (I) regional groundwater flow direction, (II) regional strength of the hydraulic gradient, (III) natural groundwater recharge and (IV) pumping rate. As a result of our proposed probabilistic framework, we obtain probabilistic catchment maps that consider the joint effect of (uncertain) transient flow conditions and geological uncertainty. To represent hydraulic conductivity we use a random space function using a Matern covariance function. In order to reduce computational costs, so that we can conduct Monte Carlo analysis over both sources of uncertainty, we represent transiency as a dynamic superposition of steady-state flow solutions. Overall, we show that (1) each transient driver defines a distinctive pattern of temporal catchment membership, with the ambient flow direction driving most of the changes. (2) The transient analysis enhances decision support for probabilistic WHPA delineation by providing additional information in terms of time reliability levels. Now, a WHPA can be defined from this analysis by selecting a reliability level for time and one for geological uncertainty. (3) Time reliability addresses a different kind of risk when compared to geological reliability: While a lack of time reliability implies a known risk to the well for a fraction of time, a lack of geological reliability means that one does not know whether the well is at risk. (4) In the presence of uncertain transient and geological conditions, high time reliability levels are cheap compared to high reliability levels against geological uncertainties. (5) The combined use of time-geological reliability information with land use information leads towards more targeted risk reduction measures that align with the two different types of risk addressed by time reliability and geological reliability.