Land use practices, subterranean groundwater pollution, and coral reef sustainability
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Land use practices, subterranean groundwater pollution, and coral reef sustainability
We propose to apply an innovative, interdisciplinary approach that combines remote sensing, GIS, geochemical, hydrological and biological techniques, to explore the effect of different land-use practices on non-point source pollutants associated with submarine groundwater discharge into the coastal environment, and their effect on coral reef health and sustainability. A key to this approach is determining the flux of submarine groundwater to the coast, as well as its associated nutrient and other pollutant loads, and to determine if there is a relationship between these inputs, onshore land cover, land-use and watershed characteristics, and measures of coral reef health. The latter will be assessed by documenting herbivory, coral cover and diversity, and pollutant levels in coral mucus. We will also conduct a risk analysis and suggest potential solutions to curtail harmful SGD.
By Alexei Koseff
The wonder of Hawaii's coral reefs brings thousands of awed tourists to the islands every year. But pollution from a variety of human activities is threatening to ruin this economic and ecological lifeblood of the Hawaiian Islands.
Alexandria Boehm, a Stanford University researcher who focuses on coastal water quality, has long been interested in examining the source and effect of the pollution. "I grew up in Hawaii, so of course I have an interest in what's going on there," said Boehm, an associate professor of civil and environmental engineering at Stanford. She is especially concerned over how population growth and rapid development will affect the vibrant ocean life of the islands.
In 2005, Boehm and Adina Paytan, now a marine biogeochemist at the University of California-Santa Cruz, were awarded an Environmental Venture Projects (EVP) grant from Stanford's Woods Institute for the Environment to investigate the relationship of land use, groundwater quality and submarine groundwater discharge on Kauai and the Big Island of Hawaii. Funding for the EVP project was provided by the Giles W. and Elise G. Meade Foundation. "The goal of the project is to understand how groundwater discharging from the land to the sea affects coastal water quality and coastal ecosystems," Boehm said.
The EVP groundwater study focused on two common coastal pollutants - fecal bacteria and nutrient contamination, specifically nitrogen and phosphorous runoff. Fecal bacteria from sewage and septic systems can sicken swimmers, surfers and other recreational water users. Nutrient contamination, often caused by fertilizer runoff, can have a detrimental impact on coastal water quality and damage coral reef ecosystems.
Kona groundwater
The EVP study initially focused on the Kona Coast of the Big Island. "It's a really interesting place, because there aren't really any streams on the leeward side of the island," Boehm said. "All of the water is discharged from the mountains to the sea through groundwater."
Lab tests of seawater samples collected at Kona revealed high nitrogen concentrations along the shoreline. To determine if groundwater was the source of the pollution, Boehm, Paytan, and Stanford gradate student Karen Knee collected fresh- and groundwater samples from a variety of locations-on the island, in the coastal ocean and offshore. The researchers used radium, a naturally occurring element, to identify the location and amount of groundwater in each area. "Radium is a tracer of groundwater, because it is desorbed from rocks in the subsurface and then becomes enriched in groundwater," Boehm explained. "When that groundwater discharges into the sea, radium concentrations increase."
The radium measurements allowed Boehm and Paytan to map the general flow of freshwater from the island to the ocean. "Using those measurements at different locations and times gives us an idea of the spatial-temporal variability in groundwater discharge," Boehm said. The measurements also served as a point of reference for documenting the behavior of pollutants in the water. "We could look and see if increased radium and increased groundwater discharge correlated to increased nitrogen, for example," she said.
A similar experiment was conducted at a second research site on the north shore of Kauai. The results were the same. "We found that there is definitely submarine groundwater discharge happening on the Big Island and on Kauai," she said. "The groundwater is a significant source of nitrogen and phosphorous to coastal waters."
Kauai rivers
At the research site on the Big Island, nitrogen-contaminated groundwater was traced to fertilizer runoff from a nearby a golf course, while on Kauai, the source was agricultural. There were other differences as well. Unlike Kona, Kauai's north shore has many rivers and streams that run directly into the ocean. "These are very interesting environments that are opposite of one another," Boehm said.
That contrast played out, not only in the geography of the two research sites but also in the day-to-day fieldwork. On the Big Island, the local community left the scientists alone to do their research and even allowed them to collect samples from areas considered somewhat sacred. But things were a bit more complicated on Kauai, where there has been a strong effort to preserve the island's culture and customs.
To conduct fieldwork on Kauai, the research team sought permission from the Hanalei Watershed Hui (HWH), an environmental organization that practices ahupua'a, a traditional Hawaiian approach to sustainable water management. "HWH has a cultural expert," Boehm said. "We met with him before we conducted research to learn about the watershed and about what's kapu-what's not allowed and what is allowed. The cultural expert taught us to be good guests' of the watershed."
As a result, Boehm and Paytan became much more involved with the people of Kauai, whose input helped shape some of the research goals of the EVP study. For example, when members of HWH expressed concern that near-shore waters were being contaminated with high levels of fecal indicator bacteria, the research team decided to investigate the source of the bacterial pollution.
It turned out that rivers and streams, rather than groundwater, were the main conduits of the bacteria. This finding has helped HWH focus its efforts on addressing the problem of coastal pollution and is playing a part in an ongoing public debate over whether to install a new sewage treatment plant. Many residents of Kauai worry that the septic tanks they currently use are polluting the groundwater with fecal bacteria, and that the contaminated groundwater is getting into the rivers and streams that are the source of the coastal pollution. Boehm and Paytan plan to conduct future experiments to investigate whether septic tanks are a significant problem.
Coral reef conservation
Boehm and Paytan have shared their findings with other organizations in Hawaii, including the National Park Service. "They're interested in managing the coastal waters and keeping them pristine, so they were really interested in our results," Boehm said. The Park Service staff, which runs the Kaloko-Honokohau National Historical Park on the Big Island, is using the results to assess several planned development projects and minimize their environmental impact on coral reefs and other aquatic resources.
Boehm hopes that, in the long run, the EVP study will have a major impact beyond Hawaii-especially the findings on groundwater discharge, which often goes unregulated because it is so difficult to trace.
"If we can show that groundwater is an important part of the water cycle all over the world, and that it can be a source of pollutants to the coastal oceans in various places, then we'll increase the awareness that we should be trying to monitor, and maybe even regulate, these discharges to coastal waters," she said.
Alexei Koseff is a writer-intern at the Woods Institute for the Environment at Stanford University.