Pollution by Urban Submarine Groundwater Discharge from the Jepara Coastal Region and Its Implications for Local Water Management

Abstract

Submarine groundwater discharge (SGD) is known to transport terrestrial nutrients and other potential pollutants to coastal areas around the world. However, SGD studies in tropical developing regions, such as Southeast Asia, are scarce, even though this area is hypothesized to be an SGD hotspot due to favorable meteorological and hydrological conditions. Jepara, a coastal city in northern Java, is characterized by a number of environmental and anthropogenic steering factors (e.g., precipitation rates, volcanic geology, coastal population density, and urban land use) that may support the notion of considerable SGD rates and its associated pollutant fluxes. Therefore, this thesis investigates SGD volumetric estimation with nutrient and microbial community composition to analyze the scale of land-based pollutants delivered by SGD to the nearshore water. Additionally, environmental and health impacts of SGD and suggested coastal water management in this region are also discussed. 222Rn was employed as a groundwater tracer in the coastal water to estimate SGD rates. Spatial and temporal 222Rn surveys were conducted in the estuaries and along the coastline of Jepara. The results indicate that terrestrial groundwater was discharged more at the estuaries than at the coastline. Fresh groundwater comprised up to 42% of total river discharge and 40% of total SGD at the coastline. SGD in this area was driven primarily by tidal pumping, with additional hydraulic gradient-driven fresh groundwater discharge at low tide. A combination of estuarine and coastal SGD results in total volumetric SGD rates of 6.6 x 105 m3 d-1. SGD rates in this area were comparable with those of other volcanic SGD studies and were higher than those in other sub-tropical or temperate region studies. SGD was confirmed to deliver terrestrial dissolved inorganic nitrogen (DIN) and dissolved silica (DSi) to the coastal system, and it also potentially acted as one of the landa ocean delivery pathways for fecal indicators and bacterial pathogens. It was also found that salinity and temperature were the most determinant variables that shaped microbial community composition in an SGD cross section. Nutrient and land-use analyses suggest that high nutrient pools in the coastal hydrological system originated from human activities, i.e., agriculture, livestock, and the sewage system. This result was also supported by microbial community analysis, where identification of fecal indicators and potential pathogens in the SGD compartment confirmed the occurrence of biological contamination. Nutrient levels and potential pathogens lead to coastal eutrophication and waterborne illnesses, which were reported from this area. From these observations, it was concluded that suitable coastal water pollution prevention at this study site should include terrestrial nitrogen containment along the riverbank and estuaries (e.g., a constructed wetland or riparian zones) and the development of a sewage system and a centralized wastewater treatment plant. Overall, this thesis shows a significant amount of contaminant discharge in the coastal area via SGD due to a combination of both environmental and anthropogenic factors. Moreover, it can be inferred that a combination of interdisciplinary geoscience research (e.g., hydrosphere, biosphere, anthroposphere) can provide a deeper understanding and assessment of SGD in a specific environment. Even though it is a local study, the methodology and results of this thesis can be replicated and thus provide assistance in other coastal urban cities in tropical regions and hence facilitate better evaluation and monitoring of tropical coastal water ecosystems in the future.