Detecting Submarine Groundwater Plumes in the Salalah Area: a Radon Tracer and Numerical Integral Plume Model Approach

The inflow of fresh or brackish groundwater into the sea is referred to as Submarine Groundwater Discharge (SGD). The SGD is enforced by a terrestrial component which mainly depends on freshwater extraction and recharge by meteoric water and on aquifer permeability. And a marine component that is mainly controlled by the spatial distribution of outflows and water depth (hydraulic gradients between land and sea). This studyis motivated by the importance of freshwater in arid regions and, in particular, by the continuous challenges posed by the exploration and exploitation of fresh water sources inthe Sultanate of Oman. Moreover, there is a lack of studies on SGD phenomena along the 1000 km coastline in the South of Oman. The objective of this study is to develop a method to detect SGD spots in the offshore region, autonomously, and understanding the hydrodynamics of the discharge seepage for future backtracking, quantification and coastaland groundwater management. The study area Salalah, Dhofar Governorate, South of Oman is known to have a high natural groundwater recharge during the monsoon seasonand a karstic coastal seafloor, which results in a high potential of submarine groundwater discharge spots. A geochemical tracer (radon-222) was selected to detect SGD using anRTM1688-2 radon sensor instrument. This sensor underwent experimental design performance tests to adopt to mobile offshore platform monitoring i.e Wave Glider (WG). Groundwater characteristics (i.e.Rn222, salinity, temperature) aquifers were first deter-mined by measuring Salalah’s coastal onshore groundwater wells. The preliminary radonactivity results offshore Salalah demonstrated a distinctive radon concentration gradient between groundwater and seawater with an enrichment factor of up to 4000, which is idealfor signal preservation in a freshwater plume until reaching the sea surface. Accordingto the collected morphological data and the hydrological data from literature, Salalahis found to be the best potential location for SGD. A numerical integral plume model(TAMOC - Texas A&M Oil spill Calculator) was used to investigate the detectability of single-phase freshwater plumes. The model considers the local freshwater characteristics, geomorphological and oceanographic constraints at different discharge rates anddischarge buoyancy. After validating the plume model with literature data derived from laboratory experiment, detection limit guidance for autonomous monitoring of SGDs was established from each parameter: ambient cross-flow velocity, initial discharge salinity/temperature, initial discharge velocity, initial diameter discharge and initial dischargeradon activity concentration. Moreover, groundwater flow rates of a recently investigated SGD (Dhalkut SGD) in the area could be estimated using the plume model. It is shown that measured physical and chemical oceanographic parameters’ combined with ground-water well data provide a well constrained data set to simulate SGD off Salalah quite well and could give realistic values for SGD plumes in the area. The outcome of the model was utilized to find best practices for SGD detection by using autonomous surface vehicles (i.e. Wave Glider)