Evidence of Submarine Groundwater Discharge (SGD) Along the Northern Persian Gulf: Insights from Physicochemical Profiles and Structural Geology

Abstract

Submarine groundwater discharge (SGD) plays a significant role in the hydrological and environmental balance of arid coastal systems like the Persian Gulf. This study investigates the presence of SGD signals along the northern coastline of the Persian Gulf using seawater physicochemical data from the 1992 Mt. Mitchell oceanographic survey—prior to major anthropogenic impacts such as desalination activities. A total of 74 vertical seawater profiles were analyzed, each containing high-resolution measurements of salinity (36.59–43.93 psu), temperature, and dissolved oxygen (1.31–5.85 ppm) from the surface to seabed. The results reveal unusual stratification patterns—such as freshening near the seabed and increased oxygen levels—particularly near the Kazerun-Qatar (KQF), Hendijan-Bahregansar (HBF), and Karehbas (KMF) fault zones. These anomalies strongly suggest offshore freshwater inflow, likely controlled by deep fault systems extending from the Zagros Mountains. By using historical, pre-desalination datasets, this study minimizes confounding from modern effluents and introduces a novel, regional-scale approach for detecting SGD in the Persian Gulf. The findings highlight the possible influence of deep-seated fault zones in offshore SGD, especially in the western basin. These insights offer a novel approach to water resource assessment and SGD monitoring across the Zagros domain.

Keywords: submarine groundwater discharge; SGD; fault zone; ROPME; Persian Gulf

 

1. Introduction

In general , wherever the coastal aquifer is hydraulically connected to the sea, terrestrial groundwater may discharge into the sea. Therefore, the submarine groundwater discharge (SGD) is a phenomenon through which water and solutes may be transferred to the surface water bodies (Taniguchi et al., 2002; Burnett et al., 2003). Although this phenomenon has been studied mainly from an environmental point of view, in arid areas, it can make significant amounts of fresh water inaccessible. So studying the SGD in the coasts of arid regions may also have applications for fresh water management. The water shortage crisis is one of the most critical issues in the Middle East and the arid countries surrounding the Persian Gulf. The northern coasts of the Persian Gulf are bordered by the Zagros Mountains. Recent droughts, have minimized the streams flow rate, dried up many lakes, and have raised serious alarms not only for drinking water but also for creating major environmental problems. This study investigates the presence of SGD signals along the northern coastline of the Persian Gulf using available seawater physicochemical data (Appendix 1).

Ancient climate studies indicate that in the last ice age, the global water level was about 100 to 130 meters lower than the current level, so that until 14,000 years ago, the Persian Gulf was a wide valley (Rose, 2010). The old rivers of the Tigris and Euphrates and other small and large rivers were flowing there. Archaeological evidence also shows that the Persian Gulf basin has been the settlement of human until 8000 years ago (Rose, 2010). Then, the water level rose gradually and submerged the Persian Gulf. In fact, the springs within the Persian Gulf basin could form when the absolute basal level of erosion (sea) was lower than the current level (Milanovic, 2005). The first official submarine groundwater discharge report in the Persian Gulf is related to groundwater artesian springs (Judd & Hovland, 2007). Additionally, fresh submarine groundwater discharge (FSGD) reported from shallow waters of the Bahrain (Farzin, 2017). It has also been said that Bahraini pearl fishermen could spend very long time at sea as they could provide potable water from submarine springs (Chapman, 1981).

Farhoudi & Poll (1992) discussed that a significant part of groundwater resources (especially karst water resources) are likely directed to the Persian Gulf from Zagros Mountains, Iran, through large faults. The evidences that Farhoudi & Poll (1992) used to present this theory are as follow: (1) the Persian Gulf has an asymmetric shape, so that the slope of the Iranian side is five times greater than the slope of the southern parts; (2) the northern part of the Persian Gulf receives considerable more rainfall in comparison with the southern parts, but high flow rate springs are emerged at aquifers located south of the Persian Gulf; (3) the aquifers of southern Iran have spread under the Persian Gulf and are expanded to the north of the Arabian homocline; (4) the average depth of the Persian Gulf is ~35 meters and the groundwater can flow into the Persian Gulf from northern aquifers; (5) the water salinity of some Dubai oil reservoirs is eight times less than the Persian Gulf water.

The US National Oceanic and Atmospheric Administration (NOAA) surveyed totally 13 logs through the Sea of Oman, Strait of Hormuz, and the Persian Gulf using the Mt Mitchell vessel in 1992 (Reynolds, 1993). Results from this extensive campaign provided several vertical profiles for seawater salinity and temperature which does not indicate any abnormality in the salinity of the Persian Gulf water bed and the water salinity increases gradually with increasing depth. In a latter investigation, Alessi et al. (1999) studied hydrography of the Persian Gulf by using the following data to investigate the physicochemical properties of the waters of the Oman Sea and the Persian Gulf: (1) archived data at the US Naval Oceanographic Office; and (2) the information obtained from the marine scientific surveys of the NOAA research vessel Mt Mitchell. In this research, the area of the Persian Gulf was divided into 7 parts and the depth profiles of salinity and water temperature were plotted. Although the purpose of this research was not to investigate the possibility of submarine groundwater discharge; investigations show that the profiles of Persian Gulf water salinity around the Kazerun-Qatar Fault (KQF) are separated in two distinct groups of graphs. Therefore, although these charts do not confirm the presence of SGD in the desired part; they indicate a kind of abnormality that should be investigated more carefully. Jafari and Fuladi (2024) assessed the potential of water outflow from the folds of Zagros Mountains to the Persian Gulf by evaluating the relationship between precipitation and elevation in several basins in the Zagros Mountains. They estimated that approximately 110,695 billion cubic meters of water were channeled to the Persian Gulf via the tidal currents of the Zagros Mountains. Rausch and Dirks (2024) did a review on the hydrogeology of the upper mega aquifer system on the Arabian Platform. They highlighted that the Persian Gulf borders the mega aquifer system on the Arabian Platform to the east. They added that the flow to the Persian Gulf is estimated at 12 m3/s (378 MCM/a) for the predevelopment state, but due to the high groundwater abstraction rate in the inflow area, the current discharge rate has reduced to 10 m3/s (315 MCM/a). McGirr et al. (2024) mentioned that the significant reductions in continental hydrology contributions to ocean mass in the Persian Gulf and several other basins were caused by decreased strength in the direct gravitational attraction due to declining terrestrial water storage in Asia since 2002. They added that anthropogenic intervention, such as extraction of groundwater resources, increased far-field manometric sea level, but caused decreased local sea level of up to ∼1 mm/yr in the case which rates of near-field sea-level fall were comparable in magnitude to the longer-term contributions of the polar ice sheets and mountain glaciers, at times masking ∼80% of the sea level increase caused by melting of ice-covered regions. They concluded if this extraction of groundwater ceases, then near-field regions such as the Persian Gulf would see an increase in the rate of local sea-level rise of up to 1 mm/yr.

In a recent research conducted by Farzin (2017) and Farzin et al., (2019) on the Bushehr province coastline, Iran, areas with SGD potential were identified. This study was based upon the following data: (1) sea surface temperature (SST) in the vicinity of karst aquifers; (2) the geomorphometric indices of land areas near the studied coastline; and (3) structural geologic features. Although the results of this research showed the possibility of SGD along the studied coastline, there is still no evidence and useful documentation about the deeper parts of the Persian Gulf (along the KQF). Bhandary and Sabarathinam (2020) used the naturally occurring radioactive isotopes to identify and estimate the SGD in the coastal strip of Kuwait along the Persian Gulf. They were able to determine the loss of brackish groundwater from inland of Kuwait. Samani et al. (2021) applied Landsat 8 thermal sensor data to identify potential sites of SGD at a regional scale in the northern coasts of Persian Gulf. They analyzed the relationships between the remotely-sensed sea surface temperature (SST) patterns and geo-environmental variables of upland watersheds using logistic regression model for the first time. They found that the percentage of karstic lithological formation and topographic wetness index were key variables influencing SGD phenomenon in the northern coasts of the Persian Gulf. Heydari et al. (2021) studied temporal and spatial changes of SST and STA in the Persian Gulf to find anomalies occurred in the Bandar Magham, Bandar Nakhiloo, and coasts of Bandar Divan, Bandar Shenas, Bandar Lengeh, and Bandar kong indicate that these areas have a very high probability of underground aquifers. Meanwhile, the presence of SGD evidence in the deeper parts of the Persian Gulf and along KQF indicates the flow of groundwater from the high areas of the Zagros Mountains under the influence of the bedrock faults in the region. Moreover, the probability of SGD occurrence has always been a scientific challenge in Iran and hypothesizes about the possibility of SGD into the Persian Gulf from northern coastline are still unclear