Project Overview:

The goal of this project was to measure the impact of various types of land use practices on the quality of coastal groundwater, nearshore marine water, and coral reef health on Maui. Using a wide range of techniques, we determined the sources of nitrogen, groundwater flowpaths, quantified submarine groundwater discharge and nutrient loading, measured N content of water and algal tissues, and assessed  reef health at six locations on Maui. Our results suggest agricultural fertilizer and wastewater are the two largest sources of N to impacted coastlines. This project was the investigation to conclude that treated wastewater injected into coastal groundwater at Kahului Wastewater Reclaimation Facility is present within a large region of Kahului Bay.

Related Publications : 

Marine algae as a bioindicator of nutrient source and loading to coastal zones of Maui, Hawaiʻi

Amato DW, Bishop JM, Glenn CR, Dulaiova H, and CM Smith

Intended for submission to Plos One (in prep)

Abstract:

Submarine groundwater discharge can transport land-based contaminants to coastal ecosystems. In this study, algal bioassays, benthic community analyses, and geochemical methods were used to compare the water quality and health of reefs adjacent to various types of land use and potential nutrient sources on Maui. Three common reef algae, Acanthophora spicifera, Hypnea musciformis, and Ulva spp. were collected and/or deployed at six locations with submarine groundwater discharge. Algal tissue nitrogen (N) parameters (δ¹⁵N, N %, and C:N) were compared with nutrient and δ¹⁵N-nitrate values of coastal groundwater and marine surface waters. Benthic composition was analyzed using digital photographs. The results of this study indicate that input of anthropogenic N to coastal groundwater impacts adjacent marine water quality and coral reef communities in areas with submarine groundwater discharge. Reefs adjacent to sugarcane farms or wastewater injection wells had the greatest abundance of macroalgae, low community diversity, and the highest concentrations of N in algal tissues, coastal groundwater, and marine surface waters compared to relatively unimpacted locations. The highest δ¹⁵N values in water and algal tissues were found adjacent to Kahului Wastewater Reclamation Facility, which suggests wastewater is a source of N to Kahului Bay. Using interpolated δ¹⁵N values of Ulva spp. tissue, we estimate that ~ 310,000 m² of Kahului Bay was impacted by treated wastewater originating from injection wells at this facility. Significant correlations between algal tissue N parameters with adjacent surface and coastal groundwater values indicate these bioassays provided a good measure of nutrient source and loading. A conceptual model that uses Ulva tissue δ¹⁵N and N % to identify potential N source(s) and relative N loading is proposed for Hawaiʻi. In order to reduce the occurrence of opportunistic algal blooms and protect coral reef health in the nearshore environment, anthropogenic nutrient loading to coastal groundwater must be addressed.

 

Effect of Land Use and Groundwater Flow Path on Submarine Groundwater Discharge Nutrient Flux

 Bishop JM, Glenn CR, Amato DW, and Dulaiova H

Journal of Hydrology: Regional studies (in press)

Abstract:

Fertilized agricultural lands, wastewater injection, and areas with high septic system density each have potential for contributing excess nutrients to coastal waters of islands via submarine groundwater discharge (SGD). It has been hypothesized for the island of Maui that excess nutrient loading via SGD is a causal factor fueling the macroalgal blooms that have been smothering corals and fouling beaches since the late 1980’s.  While methodologies for source tracking of nutrients to receiving waters from overland flow are well established, methods for determining nutrient sources in SGD are less well developed. The purpose of this study was to identify the sources of nutrients delivered to coastal waters via SGD using a variety of methods including numerical groundwater models to identify the specific groundwater flow pathways to the coast, δ¹⁸O of H₂O (δ¹⁸O_H2O) to determine groundwater recharge elevations, δ¹⁵N (δ¹⁵N_NO3) and δ¹⁸O (δ¹⁸O_NO3) of dissolved nitrate to determine nitrate sources, and ²²²Rn mass balance modeling to quantify fresh and total SGD rates. With these tools we (1) quantify the flux of nutrients to coastal waters via SGD in different areas of Maui, (2) identify specific land use practices that contribute nutrients to the coastal zone via SGD, and (3) calculate the flux of nutrients delivered to coastal zones from different land use practices. Our study demonstrates that numerical groundwater modeling combined with geochemical modeling is a robust method for determining the sources and flux of nutrients in SGD. The results presented here also illustrate how such work can provide site specific information of value to land use managers and planners regarding the magnitude of nutrients contributed to coastal waters from different land use practices.