IDENTIFYING POLLUTANT SOURCES ALONG GROUNDWATER FLOWPATHS IN KANEOHE, OAHU, HAWAII

Nutrients and other pollutants of anthropogenic origin that are fluxed to Kaneohe Bay, Oahu, Hawaii through streams and submarine groundwater discharge (SGD) impair water quality and feed algal blooms that cause cascading degradation to its coral reef ecosystems. It is now well established that nutrient concentrations are highly elevated in Hawaiian SGD relative to seawater, and recent results predict continuous SGD to be 2-4 times the volume of surface runoff fluxing to the Bay. Here we outline objectives, strategies, and initial results used to (1) identify vectors of groundwater flow from recharge zones at high elevations to coastal SGD by coupling a MODFLOW model with a local meteoric waterline (LMWL), and (2) determine the quantity and source of nutrients leaching into the groundwater system along flowpaths, derived from onsite sewage disposal systems (OSDS) or fertilized agricultural lands. There are over 176,000 OSDS cesspools in Hawaii, and approximately 4,500 OSDS septic tanks; more than 1,300 OSDS exist in our principal target area surrounding Kahaluu alone. Our groundwater model of east Oahu thus considers groundwater elevation gradients determined by reported wellheads across the island in addition to modeled nitrate concentrations extrapolated for both (OSDS)-equipped households and agricultural land plots. Isotopic compositions of precipitation measurements are being used to create the first Local Meteoric Water Lines (LMLWs) for Oahu; the island’s modest elevation gradient makes spatial distribution an important factor in changes to composition island-wide. Comparing the LMWL to isotopic composition of groundwater corrected for salinity will show flow vectors from precipitation recharge zones to SGD. Nutrient measurements along flowpaths are being developed for extent and spatial distribution of nutrient loading from both streams and SGD to the Bay. The isotopic composition of nitrate (NO₃^-) has characteristic values for agricultural fertilizer vs. human waste from OSDS and thus differentiates nitrate sources. When coupled with total alkalinity, d13C values in dissolved inorganic carbon (DIC) can predict extent of denitrification and account for variation in the isotopic composition of NO₃^-.