Identifying wastewater management tradeoffs: Costs, nearshore water quality, and implications for marine coastal ecosystems in Kona, Hawai'i

Multiple proxies to investigate the submarine groundwater discharge into the Arabian Sea, Southwest coast, India: integration of biogeochemical, geophysical, and remote sensing techniques

Submarine Groundwater Discharge (SGD) constitutes a pivotal mechanism for the transference of freshwater, nutrients, and pollutants from terrestrial to marine environments, exerting a profound influence on coastal water quality and ecosystem dynamics. In this investigation, we executed an extensive field sampling campaign along the 650 km coastal expanse of southwest India, employing a 10-km sampling interval, to discern and validate the probable zones of SGD. We have utilized a transect-based methodology for the systematic collection of groundwater, porewater, and seawater samples, employing a suite of proxies to scrutinize SGD). This multifaceted approach encompassed biogeochemical, geophysical, and remote sensing techniques. The in situ physio-chemical parameters, encompassing electrical conductivity (EC), total dissolved solids (TDS), pH, dissolved oxygen (DO), temperature, and salinity, facilitated the delineation of prospective SGD sites. Adjacent continuous probable SGD sites were amalgamated into nine potential SGD zones spanning the 650 km coastal stretch. Comprehensive analyses of major ions and nutrients revealed maximum observed seawater concentrations of nitrate, phosphate, and silica at 22.11 µM/L, 12.5 µM/L, and 11.69 µM/L, respectively, underscoring the SGD signatures and the subsequent transference of nutrients from terrestrial sources to the ocean via subsurface pathways. Furthermore, geophysical investigations employing Electrical Resistivity Tomography (ERT) at the nine potential SGD zones substantiated the groundwater signatures, elucidating subsurface lithology, delineating the aquifer system, and determining the extent of the saline-freshwater interface, including discharge depth. All ERT profiles were meticulously calibrated against available lithological data. Additionally, we executed a comprehensive evaluation of Landsat-8 satellite imagery within the thermal infrared spectral domain (10.6-11.19 μm) to monitor variations in sea surface temperature (SST) and sea surface anomalies across three stratified thermal ranges (21-28 °C, 25-33 °C, and 11-23 °C) encompassing the entire study area. The visual correlation observed between lower SST values and the identified SGD probable zones further substantiates supplementary validation. Ultimately, the verification of these nine prospective SGD zones was reinforced through a meticulous comparison with groundwater level data, which ranged from 0 to 41 m above mean sea level (MSL). This extensive investigation represents the inaugural comprehensive identification and confirmation of SGD zones along the southwest coast of India, spanning a 650-km stretch, resulting in a more precise demarcation of the area into nine SGD probable zones where multiple proxies are mutually corroborative.

The connection between Submarine Groundwater Discharge and seawater quality: The threat of treated wastewater injected into coastal aquifers

Submarine Groundwater Discharge (SGD) delivers nutrients to the coastal sea triggering phytoplankton blooms, eutrophication, and can also serve as a pathway for contaminants. Wastewater treatment plants (WWTP) including injection wells in coastal areas influence coastal aquifers and might impact the composition and magnitude of SGD fluxes. In tourist areas, wastewater treatment may be less efficient and larger in volume during high seasons, potentially impacting nutrient fluxes from SGD and exacerbating environmental impacts. This study analyzes the nutrient transfer from treated wastewater injection in karstic aquifers to the coastal sea via SGD, considering the impacts of tourism seasonality. This study is conducted in Cala Deià, a small cove in the Balearic Islands, a Mediterranean tourist destination. The findings suggest that the seasonality of tourism, leading to variations in the volume of wastewater treated in the WWTP, influences the dynamics of the coastal aquifer. This leads to increased SGD water and nutrient fluxes to the sea in summer, i.e. the peak tourist season. The measured DIN, DIP, and DSi inventories in the cove are much larger in August than in April (3, 10, and 1.5 times higher, respectively) due to higher input of nutrients in summer due to SGD impacted by the WWTP. These elevated nutrient flows can support algal blooms in the cove, compromising water quality for local swimmers and tourists. Indeed, in August, shoreline stations exhibited eutrophic Chl-a concentrations, with peaks reaching approximately 4 mg Chl-a L-1. These elevated levels suggest the presence of an algal bloom during the survey. The anthropogenic origin of SGD-driven nutrients is traced in seawater and seagrass meadows, as evidenced by high ∂15N signatures indicative of polluted areas. Thus, the high pressure exerted on coastal areas by tourism activities increased the magnitude of SGD nutrient fluxes, thereby threatening coastal ecosystems and the services they provide.

Divergent responses of native and invasive macroalgae to submarine groundwater discharge

Marine macroalgae are important indicators of healthy nearshore groundwater dependent ecosystems (GDEs), which are emergent global conservation priorities. Submarine groundwater discharge (SGD) supports abundant native algal communities in GDEs via elevated but naturally derived nutrients. GDEs are threatened by anthropogenic nutrient inputs that pollute SGD above ambient levels, favoring invasive algae. Accordingly, this case study draws on the GDE conditions of Kona, Hawai'i where we evaluated daily photosynthetic production and growth for two macroalgae; a culturally valued native (Ulva lactuca) and an invasive (Hypnea musciformis). Manipulative experiments-devised to address future land-use, climate change, and water-use scenarios for Kona-tested algal responses under a natural range of SGD nutrient and salinity levels. Our analyses demonstrate that photosynthesis and growth in U. lactuca are optimal in low-salinity, high-nutrient waters, whereas productivity for H. musciformis appears limited to higher salinities despite elevated nutrient subsidies. These findings suggest that reductions in SGD via climate change decreases in rainfall or increased water-use from the aquifer may relax physiological constraints on H. musciformis. Collectively, this study reveals divergent physiologies of a native and an invasive macroalga to SGD and highlights the importance of maintaining SGD quantity and quality to protect nearshore GDEs.