Groundwater mixing in shallow aquifers stressed by land cover/land use changes under hyper-arid conditions

The impact of land cover/land use (LCLU) changes on surface runoff and groundwater contamination have been widely investigated, yet their impact on groundwater mixing is still poorly understood. The stress on resources in the Nile Delta triggered a vast migration of urban, agricultural, and industrial activities to its desert fringes exposing the Quaternary alluvial aquifers to contamination and introducing new sources of recharge. Here, we integrated remote sensing, chemical, and isotopic data with multivariate statistical analysis to identify groundwater sources and the mixing dynamics in response to LCLU changes. LCLU analysis indicated an increase of agricultural and urban areas from 84 km² to 470.5 km² between 1972 and 2018. These changes introduced new recharge sources including wastewater and irrigation return. Three major water sources were defined including: (1) modern precipitation (cluster 3 and Miocene aquifer; δ¹⁸O: −4.59 to −0.47‰, and δ²H: −34.4 to 3.1‰) with an estimated annual recharge of 11–13.5 × 10⁶ m² and 5.1–6.2 × 10⁶ m² for El-Gafra and El-Watan catchments, respectively, (2) old Nile water (subcluster 1.1; δ¹⁸O: −0.87 to −0.53‰, and δ²H: 1 to 3.7), and (3) modern Nile water (cluster 2; δ¹⁸O: 0.49 to 2.71‰, and δ²H: 10.5 to 24.4‰). Moreover, three mixed groups were defined including: (1) a mixture of modern precipitation, wastewater and irrigation return (cluster 4; δ¹⁸O: −1.02 to 0.37‰, and δ²H: −9.3 to 6.4‰), (2) a mixture of old Nile waters and modern precipitation (subcluster 1.3; δ¹⁸O: −1.74 to −1.38‰, and δ²H: −6.8 to −2.3‰), and (3) a mixture of more than two sources with wide ranges of isotopic and chemical compositions (subclusters 1.2 and 1.4). These findings provide a good basis for further studies of groundwater mixing and assessment of contamination sources in shallow aquifers stressed by LCLU changes in hyper-arid environments.