The infiltration of untreated wastewater into aquifers highly endangers the availability of fresh-water for human consumption in semi-arid areas. This growing problem of potable water scarcity urgently requires solutions for groundwater protection. Decision support systems for local wastewater treatments in settlements already exist. However, the main challenge of implementing these for regional groundwater protection is to identify where wastewater treatments are most efficient for the whole region. In this paper, we addressed this scale-crossing problem with an interdisciplinary approach that combines regional risk assessment and assessment of local wastewater treatment scenarios. We analysed the impact of polluting the groundwater using vulnerability, hazard, and risk assessments. Thus, we identified the need for semi-arid and karst-related adjustments, defined more suitable standards for wastewater hazard values, and accounted for the groundwater dynamics beyond the vertical flow paths. Using a lateral groundwater flow model, we analysed the impact of the pollution sources and linked the regional and local scale successfully.

Furthermore, we combined the geoscientific results with the urban water engineering methods of area and cost assessments for local wastewater scenarios. Based on the example of the Wadi al Arab aquifer in Jordan, we showed that implementing an adapted treatment solution in one of the heavily polluted suburban settlements could reduce 12% of the aquifer pollution, which affects 93% of the potential aquifer users. This novel method helps to identify settlements with significant pollution impact on the groundwater, as well as the users, and also gives specific guidelines to establish the most efficient locally tailored treatment solution.