Evolution of subterranean groundwaters throughout the ahupua‘a of West Hawaii and its socio-economic impacts

Although nearly a desert without streams, the dry west side of the Big Island of Hawai‘i delivers large quantities of nutrient-enriched groundwaters through its wetland and coastal ecosystems to the ocean via a multitude of large point-sourced and numerous diffuse groundwater discharge sites. Groundwaters deliver significant quantities of nutrients, pollutants, heavy metals, and bacteria directly to the coastal environment, and these “unseen” inputs can pose serious health risks to humans and upset coastal wetland and adjacent marine ecosystems. However, virtually nothing is known about aquifer flow paths or how their nutrient chemistry evolves and is modified along those paths towards the coast. This research thus seeks to understand the sources, sinks and bio-geochemical evolution of groundwater nutrient pathways along the central Kona coast of Hawai‘i, with its main thrust being the differentiation and interaction between pristine, anthropogenic and invasive species nutrient-reservoir components. Scientific results will be integrated into an environmental-economic framework to address important policy issues, such as identifying cost-effective instruments (e.g., improved wastewater treatment or the removal of nitrogen-fixing invasive plants) for reducing terrestrial nutrient inputs to estuarine and marine ecosystems.

To address these goals our research mission initiates a new matrix of baseline groundwater data analyses approaches applicable to coastal zone ecosystem management throughout the Hawaiian Islands, as based on investigation of two highly contrasting environmental study sites. These are (1) The arid Kīholo Bay coast and its watershed to the north, a woodland/savanna ecosystem with locally massive stands of non-native, N-fixing kiawe trees, and dry upland watersheds that are little affected by human influences, except perhaps the latent effects of extensive cattle ranching in the past. (2) The anthropogenically-impacted Kaloko-Honokōhau (KAHO) coastal system and its very wet watershed to the south, an ecosystem surrounded by dense stands of coastal kiawe trees adjacent to rapidly expanding human influence (e.g., residential, tourist and commercial development, regional waste water treatment facilities, and the central Kona Coast's only boat harbor).

The goals of this research require developing variable-density groundwater models within the watershed areas, and most of the extensive information and data needed to build these models are now compiled and organized. This has allowed us develop the first conceptual modelsof the hydrogeologic framework of the groundwater system upon which data related to subsurface hydrology can be considered to describe groundwater flow directions, sources and sinks of water, identify sources and types of nutrient loading, and estimate the distribution of hydraulic properties. For the near coastal areas, the conceptual models also describe or estimate submarine groundwater discharge, fresh water/seawater transition zones, tidal pumping, and groundwater salinity profiles. Samples from 22 wells and from coastal ponds and piezometer stations throughout North Kona and South Kohala districts have been collected and analyzed for temperature, salinity, conductivity, pH, Total N, Total P, phosphate, nitrate, nitrite, silica, and ammonium for tracking of nutrient source areas to sinks. Water samples from eighteen locations were collected in triplicate and have been used for apparent age determination by the chlorofluorocarbon method and it is estimated that ~60-90% of the waters from these are, remarkably less than 50 years old. A statistically rigorous environmental-economics pollution control model has also been developed to determine the lowest cost means of achieving a desired nutrient concentration standard in nearshore marine/estuarine ecosystems.

Based on the conceptual groundwater models described above, cross-sectional, two-dimensional groundwater models are beingdeveloped for the region using variable-density groundwater computer modeling to: (1) Assess the flow of groundwater and nutrients through the subsurface to the marine environment, (2) assess variability and sensitivity of physical parameters, (3) investigate whether some or all of the submarine groundwater discharge nutrient flux may be due to kiawe inputs, increasing the N and P availability, (4) investigate whether the water balance is influenced by usage by the kiawe trees, decreasing the water availability, and (5) investigate whether additional nutrient loading is due to a variety of anthropogenic input sources, including fertilizers, sewage and industry. Beginning 2012, the ENDER team will also initiate deployment of new, mobile, ground penetrating electrical resistivity geophysical equipment for subsurface imaging for refined measurement and modeling of groundwater dynamics, and also establish cumulative collectors for precipitation analysis. Analyses for δ18O and δ2H of the groundwaters (via cavity ringdown mass spectroscopy) are underway to identify groundwater recharge areas and constrain the terrestrial source area inputs to coastal waters. Analysis of all nutrient loads, common ions, and the δ13C of dissolved carbon and the δ15N and δ18O individually-stripped from nitrate via complex bacterial laboratory methodologies are underway as a powerful set of combined biogeochemical tools for discerning the specific origins and contributions of different nutrient source loadings (e.g. rock weathering and soils, fertilizers, manure and septic waste, precipitation, kiawe litter, etc.) as well as the microbial degradational pathways that effect the nutrient loadings that occur in our groundwaters and which are flushed to the sea. This information will then be integrated into the environmental-economics model to determine the least-cost means of abating nutrient pollution in groundwater. Various finance mechanisms (e.g. increased water/sewage fees, general revenue) will be examined to address feasibility and equity concerns. In these regards, our team and its agenda works closely and in concert with the effort and research objectives of the ENDER Submarine Groundwater Discharge and Associated Nutrient Fluxes Project.