Why Wadi Al Bih Limestone is the Most Sustainable Aquifer in the United Arab Emirates?
Authors: RIZK Zeinelabidin E. 
Published: August 2021

Results of several research studies conducted on Wadi Al Bih Basin in Ras Al Khaimah area between 1996 and 2015 were used to answer the question: why Wadi Al Bih limestone is the most sustainable aquifer in the United Arab Emirates (UAE)? Extensive field work, climatic data, hydrogeologic studies, results of chemical analysis of 193 groundwater samples, records of stable (2H and 18O) and radioisotopes (3H and 14C) in 52 rainwater samples and 312 groundwater samples, analysis of aquifer recharge to discharge and findings of a numerical model were used in this investigation. Results show that Wadi Al Bih basin receives an average annual rainfall of 155 mm, which is higher the UAE annual average (119 mm). Annual rainfall in excess of 400 mm is common in the study area. The percentage of aquifer recharge from the total annual rainfall (74 million m3 ) was ≈9% (6.7 million m3 ), increased to 13% (17.6 million m3 ) after construction of Wadi Al Bih dam in 1982. In the meantime, the reduction of average annual groundwater production from Wadi Al Bih limestone aquifer decreased from 9 million m3 during the period 1991-1995 to 4.5 million m3 during the period 2010-2014. The decrease in groundwater pumping from the aquifer paralleled the construction of Al Burayrat, Al Humraniah, Rafaq, Ghalilah and Al Nakheel desalination plants, with a total production capacity of 93 thousand m3 per day. These conditions has eased pressure on aquifer, increased groundwater storage, raised of hydraulic heads by 1 m in Al Burayrat area and 16 m near Wadi Al Bih main dam and decreased the average groundwater salinity from 2,122 milligrams per liter (mg/L) and 3,901 mg/L in Wadi Al Bih and Al Burayrat well fields, respectively, during the period 1991-1995, to 1,497 and 2,145 mg/L in Wadi Al Bih and Al Burayrat well fields, respectively, during the period 2010-2014. The decrease in aquifer’s salinity due to the aquifer recovery was 30% in Wadi Al Bih well field and 45% in Al Burayrat well field. The karstic nature and structural setting of Wadi Al Bih limestone aquifer increases secondary porosity and hydraulic conductivity (K ranges from 32.65 to 67.30 m/d and averages 50 m/d) and enhances aquifer recharge. The temperature of the groundwater in Wadi Al Bih limestone aquifer varied between 32.8 and 43.3°C, with averages of 36.1°C during winter and 36.3°C during summer. The average groundwater temperature decreases from 36.5 and 36.4°C during winter to 36.4 and 35.8° C during summer in Wadi Al Bih and Al Burayrat well fields, respectively. The decrease of groundwater temperature in the aquifer is associated with decreasing salinity and indicates aquifer recharge from rains falling on the mountain peaks surrounding the basin. Stable isotopes (2H and 18O) suggest water in Wadi Al Bih limestone aquifer is recharged at an average elevation of 1,050 m. Tritium (3H) data are consistent with a local source of precipitation and that the aquifer has a small residence time of a few years. Isotope and carbonate chemistry suggest that a significant amount of the groundwater in the Wadi Al Bih well field is recharged behind the dam. This is consistent with the observation that most of the groundwater samples collected in Wadi Al Bih limestone aquifer suggest recent recharge by exhibiting concentrations of a few tritium units. Results of a groundwater flow model for Wadi Al Bih limestone aquifer reveal that the average annual groundwater production in the present (4.5 million m3 ) is reasonably less than average annual aquifer recharge (6.7 million m3 ), does not violate the aquifer’s safe yield and ensures aquifer sustainability for years to come.