Quantification of long-term wastewater impacts on karst groundwater resources in a semi-arid environment by chloride mass balance methods

• Application of chloride as an environmental tracer in a semi-arid karst environment.
• Evaluation of long-term chloride time series 1938–2012.
• Rising chloride concentration trend for many karst springs shows wastewater impact.
• Groundwater recharge calculated by chloride mass balance ranges from 25% to 50%.
• Wastewater contribution to spring discharge of up to 20%.

SummaryKarst groundwater resources in semi-arid environments are highly vulnerable with respect to anthropogenic impacts, such as wastewater infiltration, because of focussed recharge and the rapid transport of dissolved substances in highly permeable conduit systems. In this study, groundwater resources of the western margin of the Lower Jordan Valley and their respective water qualities were investigated. The region is characterised by a carbonate aquifer system, which predominantly discharges via karst springs. The area experiences a large population growth and consequently a rising chloride concentration due to effluent infiltration is observed in many springs. Long-term records of chloride concentration in spring water were assembled in an extensive literature study. For some springs, larger parts of the period of 1938–2012 were covered. Chloride was used as an all-in-one environmental tracer for the objectives of (1) groundwater recharge estimation and (2) anthropogenic impact quantification. The investigations required the determination of the natural background concentration of chloride in groundwater. It was estimated by a combined use of historical data and a trend analysis for larger springs in the area and a range between 20 and 40 mg l−1 was found. Together with chloride concentration data for local precipitation, the long-term mean natural groundwater recharge was calculated with the chloride mass balance method. Recharge ranged between 25% and 50% of the precipitation. The proportion of wastewater-borne spring flow was quantified by a chloride end member mixing model. The springs exhibited a wastewater-borne flow fraction between 0% and 20% calculated from recent samples. The successful application of these methods underlines the value of long-term monitoring, even at a comparatively low time resolution.