Groundwater Resource Sustainability in Wadi Watir Watershed, Sinai, Egypt
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The Wadi Watir delta is a tourist area in the arid southeastern part of the Sinai Peninsula, Egypt, where development and growth of the community is constrained by the amount of groundwater that can be withdrawn sustainably. To effectively manage groundwater resources in the Wadi Watir delta, the origin of groundwater recharge, groundwater age as related to the timing of groundwater recharge, groundwater chemistry, upwelling of a deep saline groundwater and seawater intrusion along the coast of the delta need to be understood. Mineral identification, rock chemistry, water chemistry, and isotopes (2H, 18O, 14C, 13C, 87Sr/86Sr, 37Cl and 81Br) were used to identify recharge sources, mixing of different waters (including a saline groundwater and seawater), and groundwater age. The chemical evolution of groundwater as it flows from the upland areas in the watershed to the Wadi Watir delta was evaluated using inverse geochemical modeling. The geochemical model NETPATH was used to identify possible geochemical processes that account for the chemical and isotopic changes in groundwater throughout the watershed and in alluvial aquifers of the delta. Geochemical models are presented that identify and quantify the amount of water-rock reactions, mixing of different waters, and evaporative concentration of shallow groundwater. The geochemical and isotopic data in combination with the modeling show that groundwater in the Wadi Watir watershed is primarily recent recharge within the watershed and that groundwater salinity is controlled by mixing of chemically different waters and dissolution of minerals and salts within the aquifers. The main channel area of Wadi Watir, up gradient from the delta, receives groundwater flow primarily from the El Shiekh Attia area. Groundwater from the El Shiekh Attia and main channel areas is the primary source of groundwater supplying alluvial aquifers of the Wadi Watir delta. The Wadi El Ain area, which is also up gradient of the main channel area, does not supply much groundwater to the main channel area based on isotopic data. The same storms that recharge the El Sheikh Attia area also recharge the Wadi El Ain area, but the isotopic signature of the groundwater in the Wadi El Ain area is more depleted in ä18O and ä2H than the El Sheikh Attia area groundwater because of the rainout effect and altitude effect. This isotopically depleted groundwater is not evident in the main channel area or the alluvial aquifers of the Wadi Watir delta.Groundwater in alluvial aquifers of the Wadi Watir delta can be divided into two groups, based on their isotopic and chemical compositions. Group I is characterized by relatively depleted 18O values ranging from -3.82 to -2.58 / as compared to the other groundwater (Group II). Most of the Group I groundwaters are located away from the coast with many of the sites along the mountain front, particularly where the Wadi Watir drainage enters the delta. This groundwater is isotopically similar to groundwater in the upper watershed recharge areas of El Sheikh Attia and the main channel. Most Group II groundwaters are located in shallow wells along the coast and near wetted Sabkha areas. The ä18O values of these groundwaters range from -0.66 to +6.86 /. The majority of Group II groundwater has undergone some evaporation as indicated by an isotopic evaporation trend from winter rain and Group I groundwaters to Group II groundwaters. Two wells located near the coast have elevated ion concentrations and isotopic values that indicate seawater intrusion. Group I groundwaters in the community well field have a component of deep saline water as indicated by 37Cl and 81Br. The deep saline water is depleted in 81Br and enriched in 37Cl suggesting a marine origin. Over pumping of the well field has resulted in upwelling of this deep saline groundwater. Geochemical modeling indicates that 6 to 12% of deep saline groundwater mixes with the well field water and 8 to 12.5% of seawater mixes with groundwater along the coast.Groundwater level and salinity data were used to develop a three-dimensional, variable-density, groundwater flow and salinity transport model using the SEAWAT modeling environment. The model was developed to estimate average annual groundwater recharge to the alluvial aquifers of the Wadi Watir delta and to simulate annual groundwater pumping, evaluate upwelling of saline water from beneath the well field, and quantify the extent of seawater intrusion along the coast for different pumping scenarios. The model was calibrated using groundwater level and salinity changes. Modeling results for the period 1982 to 2009 showed that: the extent of seawater invasion along the coast was about 200 m; the main factors controlling groundwater salinity are pumping stresses and the availability of recharge; the daily extraction rate from individual deep drilled wells of the main well field ranged from 200 to 1400 m3/day with an annual average rate of 3100 m3/day for all the wells; and, the estimated annual average recharge to the delta using different modeling scenarios ranges from 4200 to 6000 m3/day.