Use of 1-D subsurface thermal profiles to characterize the groundwater flow in the Central Nile Delta region

Borehole temperature logs of twelve wells were analyzed to clarify the connection among groundwater and the subsurface thermal conditions in the central part of the Nile Delta as an irrigated land in an arid area. The annual average surface air temperature amid the interval from 1997 to 2017 was 22.74 degrees C, and the annual increment is 0.0629 degrees C year(-1). This surface warming influenced the subsurface thermal system. Thermal gradient of the conducted temperature-depth (T-D) profiles varies from - 0.009 degrees C m(-1) (well 4) to 0.0098 degrees C m(-1) (well 12). The vertical change in the thermal gradient values lower than zero was detected in wells 1, 3, 4, 5, 6, 7, 9, and 11 while wells that have gradient higher than zero were wells 2, 8, 10, and 12. The T-D logs were numerically examined and the outcomes compared with standard curves to predict the vertical groundwater stream direction and speeds. The logged wells could be divided into three groups. Wells were characterized by downward groundwater flow as wells 1, 4, 7, 8, and 10. Wells were of upward groundwater flow as in wells 2, 3, 5, 9, 11, and 12. Wells were characterized with upward and downward groundwater flows as well 6. Vertical downward groundwater flow velocities in the study area ranged from 11.53 cm year(-1) in well 10 at a depth of 200-236 m to 69.20 cm year(-1) in well 1 at a depth of 32 m. The upward groundwater flow velocities varied from 498.26 cm year(-1) in well 11 at a depth below 60 m to 10.65 cm year(-1) in well 12 at a depth below 115 m. Seawater intrusion effect was estimated from the upward groundwater movement in the northern part (well 11), and the downward movement of the freshwater was reflected from well 10 in the southern part of the study area. Downward flow noticed in wells 7 and 6 reflects the recharge effect of Damietta and Rosetta Nile branches, respectively. The groundwater flow velocity and flow direction change with depth could be related to the aquifer heterogeneity, groundwater pumping, and surface water-groundwater interaction.