Transmission losses through effluent-fed ephemeral streams: a case study from the Canale Reale River (Brindisi)”

Climate change and anthropogenic pressures are the main drivers affecting the sustainability of freshwater resources, worldwide, in term of hydrological cycle. Rising in temperature and droughts events, have a deep negative impact on the amount of water infiltration and hence groundwater recharge; moreover, the reduction in precipitation forecasted by climate-change models will intensify this process, leading furthermore to a dramatic shift from hydrological perennial regimes to ephemeral. Population growth imposes an additional stress on water resources, especially groundwater, often overexploited for irrigation purposes, meanwhile urbanization processes move together with the tendency to produce waste and therefore the need to build-up new wastewater treatment plants. In many urban environments, discharging treated wastewater from wastewater treatment plants into surface water bodies represents a water resource and ecosystem services management tool to tackle different environmental issues. Although this practice may raise some concerns among people but also scientific community, about possible sanitary and ecological side-effects, as well as the impairment of water quality of receiving streams, the reuse of treated effluent is crucial to support ecosystem quality and urban amenities, to contribute to the environmental baseflow of ephemeral streams and to enhance groundwater recharge processes, especially in semi-arid and arid regions. In arid environments these effluent fed streams are likely to become crucial for guaranteeing a vital equilibrium in the hydrological cycle since aquifer recharge through ephemeral streambeds is believed to be a major source of groundwater storage and replenishment. Groundwater recharge processes are enhanced by transmission losses that preferentially occur trough losing reaches along the streambed, which create a connection between the river and the underling aquifer. Interactions between surface and ground water bodies are extremely complex since several factors may affect the infiltration rate, such as spatial distribution of streambed sediments and hydraulic properties. Owing to the peculiar nature of ephemeral rivers, they often flow within ungauged basin, leading to a significant lack of in situ hydrological data, such as precipitation, streamflow, and evaporation time series. Furthermore, not all the methods proposed in the scientific literature for estimating transmission losses in perennial streams can be applied to ephemeral ones, although they can provide some proxy for measuring groundwater recharge from hydraulically connected surface water bodies. The study case is the ideal framework in which all these natural features and hydrological issues converge. The Canale Reale River is an effluent-fed river located nearby the city of Brindisi, on the south-eastern side of the Apulia Region, in Italy. Four wastewater treatment plants discharge within the river a wastewater volume which contributes for about 16.5% of the annual volume of channel drainage (i.e., 3.82 Mm3 out of 23.02 Mm3 distributed along its path, about 50 km) and that partially feeds the Torre Guaceto protected wetland, along the Adriatic coast. Within a complex geological setting, the Canale Reale River flows throughout different lithologies, which reflect in different streambed hydraulic conductivity values. The aim of the study was to investigate the transmission losses occurring between the ephemeral watercourse and the underlying aquifers and to estimate the volume infiltrating which in this specific study case identify with treated wastewater discharge. By adopting the Reach Length Water Balance method, the estimation of a spatially average value of the riverbed’s infiltration rate applicable to the whole river course was investigated, as well as Potential Transmission Losses (TLP) from the river to the underlying groundwater systems. Combining the estimated TLP values and the Flow Duration Curve (FDC), it was possible to draw the Transmission Loss Duration Curves (TLDCs) and finally, to estimate the water volume infiltrating during an average hydrological year, equal to 6.25 Mm3, 61% of which consist in treated wastewater. The obtained outcomes confirm that the practice of increasing the river flow rates with effluents can be considered a sustainable management tool for both surface and groundwater resources, since in the first case, this allow to reduce the riverbed periods of zero-flow, with potential improvements to the river's ecological sustainability and in the second case, relevant increasing of groundwater recharge is possible.