Description: 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).

Description: Precipitation chemistry data provide important information for environmental studies on large-scale element cycling and anthropogenic impacts on our atmosphere, but also for hydrochemical models and groundwater recharge estimations via the Chloride Mass Balance method. Such recharge data play a crucial role in groundwater management, particularly in (semi-)arid areas. Unfortunately, precipitation analyses are often scarce in such regions. This also applies to the Arabian Peninsula, including southern Oman. To overcome this lack of rain chemistry data, we developed a strategy for automatic weekly bulk precipitation sampling, using recently designed automatic rainwater samplers. The integral samples were gathered along an elevation gradient from the Salalah coast to the Dhofar mountains during the Indian Ocean Monsoon seasons 2017 and 2018. Our major ion analyses of the rainwater samples revealed considerable temporal and spatial heterogeneity, in terms of ion proportions and absolute concentrations. Samples from the coast were relatively salty (EC mostly 〉3000 μS cm−1) and rich in Na+ and Cl−, reflecting small rain amounts and a sea spray effect. Further inland, solute concentrations were lower, partly due to more precipitation, and ions such as Ca2+ and SO42− gained importance, probably due to calcite and gypsum dust. This pattern reflects the interplay between solute availability (influenced by regional geology, wind direction at different altitudes, and wind speed) and precipitation amounts. Cl−/Br− ratios were fairly uniform and scattered around the seawater value. Combining ion concentrations and rain amounts yielded bulk depositions that showed an erratic pattern along the elevation gradient, i.e., depositions did not decrease steadily in inland direction, as one may assume. This suggests that the occasionally reported approach of collecting a few opportunistic grab samples at a single site is unlikely to yield data that are representative for a larger coastal study area.

Description: Groundwater discharge into the sea occurs along many coastlines around the world in different geological settings and constitutes an important component of global water and matter budget. Estimates of how much water flows into the sea worldwide vary widely and are largely based on onshore studies and hydrological or hydrogeological modeling. In this study, we propose an approach to quantify a deep submarine groundwater outflow from the seafloor by using autonomously measured ocean surface data, i.e., 222Rn as groundwater tracer, in combination with numerical modeling of plume transport. The model and field data suggest that groundwater outflows from a water depth of ∼100 m can reach the sea surface implying that several cubic meters per second of freshwater are discharged into the sea. We postulate an extreme rainfall event 6 months earlier as the likely trigger for the groundwater discharge. This study shows that measurements at the sea surface, which are much easier to conduct than discharge measurements at the seafloor, can be used not only to localize submarine groundwater discharges but, in combination with plume modeling, also to estimate the magnitude of the release flow rate.