MULTI-TRACER APPROACHES FOR GROUNDWATER DISCHARGE AND ANTHROPOGENIC POLLUTION IN THE PACIFIC

Abstract

Submarine groundwater discharge (SGD), or groundwater that flows to the coastal ocean, is a significant source to both water and dissolved chemical budgets. While SGD fluxes frequently rival or exceed those associated with river discharge, it remains poorly characterized along most coastlines. SGD is also a frequently overlooked contaminant flux pathway, despite being a well-documented vector for excess nutrients or other contaminants derived from urban, agricultural, or industrial land-use to reach the coastal ocean. Commonly, local-scale SGD studies consider the coastal ocean in isolation from stream inputs, particularly stream baseflow, despite the direct connection between one another. Wastewater discharge is a common source of poor water quality. Aging wastewater infrastructure (WIS) that often uses antiquated technology leads to leakage to the groundwater that is difficult to detect. Onsite sewage disposal systems (OSDS; e.g., cesspools, septic tanks) are a common alternative to municipal wastewater treatment, while also a frequent source of groundwater pollution. This is particularly the case in areas with a high density of OSDS constructed along the coast, such as in Hawaiʻi. In addition to OSDS, fractured sewer lines are another potential source of wastewater leakage to groundwater. Wastewater discharge to natural waters remains a major issue globally, in part because it can be difficult to isolate the source and cause of the pollution. Contaminants of emerging concern (CECs; e.g., pharmaceuticals, industrial chemicals, pesticides) are pervasive in the environment, but can be used as tracers due to their uniquely anthropogenic source. Sea level rise (SLR) can also indirectly threaten water quality in coastal areas. In addition to surficial flooding, SLR will lead to groundwater inundation (GWI) of WIS and underground tanks or lead to increased salinization of water resources. To date, most SLR impact studies either focus on surface water impacts or are modeling-based studies, meaning few direct observations of GWI and its linkage to water quality decline exist. Chapter 2 of this dissertation links poor coastal water quality and nutrient pollution to total groundwater (stream baseflow + SGD) discharge along the steam-coastal continuum in a watershed with a high density of OSDS (Kāneʻohe, Hawaiʻi) using radon as a groundwater tracer. Additionally, SGD was also compared between perigean spring (king) and spring tides. Increased SGD and nutrient fluxes were observed during the king tide, implying worsening water quality with SLR. Chapter 3 demonstrates that SGD is a source of wastewater contamination to the coastal ocean in a highly urbanized embayment (Sydney Harbour, Australia) using radium isotopes as groundwater tracers and CECs are the primary tracer for for wastewater. Major findings include: (1) SGD is a pathway for CECs to reach the coastal ocean; (2) increasing CEC inventories are related to increasing water residence time; and (3) two of the measured CECs – dioxins and ibuprofen – were in concentrations that pose a risk to the ecosystem. Chapter 4 provides field-based observations of tidally driven GWI of WIS using radon and CECs as tracers during spring tides in Honolulu, Hawaiʻi. Two pathways were studied: (1) direct GWI of coastal WIS that flows to the coastal ocean as SGD, and (2) indirect inundation of WIS through flooded storm drains. For the direct pathway, CEC fluxes increased at high tide, reflecting additional inundation of WIS with rising water levels. In comparison, CEC concentrations decreased at high tide via the indirect pathway, signifying dilution of constantly leaking sewer lines by the rising water table. This chapter demonstrates a tidal connection between groundwater discharge and water quality and has implications for worsening water quality with SLR. This dissertation examined groundwater as a contaminant vector to streams and the coastal ocean using a combination of groundwater (radon and radium) and contaminant (CECs, nutrients, dissolved organic carbon) tracers. Radon was used in two innovative ways in this dissertation: (1) separation of groundwater and surface water along the stream-coastal continuum – leading to a better understanding of contaminant pathways in a polluted watershed; and (2) during spring tides linking GWI of coastal WIS to groundwater discharge to the coastal ocean and storm drains. The results also demonstrate promising use of CECs as wastewater tracers in novel environments and under transient conditions. Future work can build upon this dissertation by conducting further studies that consider groundwater discharge ridge to reef, increasing the number of CECs analyzed as tracers (particularly in groundwater and non-freshwater environments), and running additional studies in coastal areas that add direct evidence for tidally-driven GWI.