Use of one-dimensional subsurface temperature profiles to characterize the groundwater flow system in the northwestern part of the nile delta, Egypt

Abstract

The temperature-depth profiles of 47 boreholes were investigated to characterize the groundwater flow system in the northwestern part of the Nile Delta, Egypt (hereafter referred to as the study area). A vertical subsurface thermal system was used for the investigation. The groundwater was recharged in the reclaimed area located in the southwestern direction of the study area, where high subsurface temperatures were recorded. The discharge regions were located in the old agricultural lands (northern and northeastern areas) and were characterized by low subsurface temperatures compared with those in the recharge area. This abnormal thermal system was attributed to contrasts in the surface air temperature, where higher values were recorded in the recharge area (Wadi El-Natrun station; annual average temperature 23.15°C) and lower values were detected in the old agricultural lands (Damanhour station; annual average temperature 20.37°C). Regardless of this unusual framework, the geothermal gradient was low in the recharge territory (average 0.0198°C/m) and high in the discharge area (average 0.0343°C/m). The effect of irrigation canals on the thermal system was detected from the constructed vertical two-dimensional (2D) cross-sections, where the gaining streams were underlined by a warm zone, while the losing streams were underlined by a cooler zone. Vertical groundwater fluxes in the study area were assessed from a comparison between measured and simulated one-dimensional (1D) temperature profiles. All temperature profiles related to wells in the Wadi El-Natrun station area were of the recharge type, with groundwater flux (U) values ranging from 0.3 to 1.5 m/year, except for well 21, which was the discharge type (U range −0.1 to −0.5 m/year). The temperature profiles related to Damanhour station were of the discharge type (U range −0.1 to −1.5 m/year), except for wells 22, 47, and 39, which were the recharge type (U range 0.1 to 0.5 m/year). We concluded that the geothermal gradient is superior to the thermal system for following the groundwater stream framework in regions such as those we examined.