Geochemical Assessment of Potential Sources for Nitrate in the Wasia Aquifer, Al Kharj Area, Central Saudi Arabia
Geosciences Department, College of Petroleum Engineering and Geosciences, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
Geology Technology Team, EXPEC Advanced Research Center (EXPEC ARC), Saudi Aramco, Dhahran 31311, Saudi Arabia
Ministry of Environment, Water, and Agriculture, Riyadh 11195, Saudi Arabia
GeoZentrum Nordbayern, Universität Erlangen, Schlossgarten 5, 91054 Erlangen, Germany
Author to whom correspondence should be addressed.
Water 2020, 12(5), 1479; https://doi.org/10.3390/w12051479
Received: 1 April 2020 / Revised: 6 May 2020 / Accepted: 10 May 2020 / Published: 22 May 2020
(This article belongs to the Section Aquatic Systems—Quality and Contamination)
Nitrate (NO3−) represents one of the major groundwater constituents with increasing distribution and concentration in the Kingdom of Saudi Arabia. This study aimed to determine potential sources of nitrate in the Early to Late Cretaceous Wasia aquifer system at the Al Kharj area (Central Saudi Arabia) by an integrative approach using groundwater geochemistry, nitrate isotopes (15N–NO3 and 18O–NO3), and tritium (3H) measurements. The lowest saline groundwater samples (TDS = 1400–2000 mg/L) from the peripheral zone were representative for pristine groundwater from the Wasia aquifer with nitrate concentrations below 20 mg/L and low 18O–NO3 ratios (8.7–20.6‰) but enriched 15N–NO3 values (up to 10.8‰). In contrast, 11 out of 34 analyzed water samples from irrigation wells and cattle watering wells exceeded the World Health Organization (WHO) drinking water guideline value for nitrate of 50 mg/L with maximum concentrations of up to 395 mg/L. Nitrate fertilizers and atmospheric deposition are the main sources of nitrate in groundwater in the eastern and northern sections of the study area. The combination of elevated salinities (4940–7330 mg/L), NO3 (111–395 mg/L), boron (516–1430 μg/L), and enriched 18O–NO3 (21.7–25.8‰) ratios with depleted 15N–NO3 (5.7–7.6‰) confirm the local influx of evaporated irrigation water with remnants of dissolved fertilizer into the Wasia groundwater system. There was no evidence for the influx of animal or human wastes from adjacent dairy, poultry, and housing infrastructures. Tritium concentrations below the detection limit of 0.8 TU for most borehole samples implied the absence of recent natural recharge. The estimated annual average N influx of 3.34 to 6.67 kg/ha to the Wasia aquifer requires a combination of atmospheric deposition and anthropogenic sources (mainly nitrate fertilizers) to increase the nitrogen content of the Wasia aquifer.