Development of a quantitative transport-time-based groundwater vulnerability model for non-point-source pollution in karst aquifers: a conceptual approach and example from the Tanour and Rasoun spring catchment, north-western Jordan

A quantitative transport-time-based vulnerability assessment approach for non-point-source pollution using a spatially distributed geographic information system (GIS)-generated topographic model was developed for the karst aquifer system feeding the Tanour and Rasoun springs in NW Jordan. The approach implies the assessment of the groundwater residence times in different compartments of the karst system, i.e. the epikarst, the vadose zone and the phreatic system. Groundwater transport times within the epikarst zone were determined by employing two main variables: (1) the length of the flow path, and (2) the transport velocity between the assumed point of subsurface infiltration into the soil and the location for the actual groundwater recharge at the ephemeral streams. Sub-horizontal lateral flow is assumed for the epikarst. The base-flow recession curve method was employed to estimate the effective, average and catchment-wide transmissivity of the epikarst. Based on the calculated transmissivity, three different hydraulic conductivity values were assumed to assess the effect of the unknown average saturated thickness of the epikarst. The developed method was validated by independent measurements of the transport-time-based on the δ¹⁸O breakthrough signal in the spring catchment discharge, following a recharge event. The transport-time distribution map, calculated based on flow-path length towards the streams and average solute transport velocity, shows high correlation with the catchment-averaged stable isotopes’ breakthrough curves. Because of the intensive karstification below the stream bed, residence times in the vadose and the phreatic zones can be assumed to be negligible compared to the transport times in the epikarst.