CHARACTERIZING THE SOURCE OF SURFACE WATER ALONG THE HAMAKUA AND HILO COASTS, HAWAII

The Hamakua and Hilo coasts lie on the northeast side of Hawaii's Big Island. Large streams appear related to major geological structures, whereas small streams flow in slightly incised channels that occur on the largely unaltered topographic surface. The hypothesis that larger streams will have greater contribution from groundwater and therefore a larger dissolved mineral content was tested by surveying electrical conductivity (EC) of streams. Along these coasts, 22 streams were sampled in July 2014 and analyzed for EC. Additional data on stream flow and morphology were collected and GIS was used to determine the size and characteristics of the watersheds above sample locations. Waters ranged from 10 to more than 200 µS/cm. Along the Hamakua coast, results suggested two groups: high EC (large streams and a spring) and low EC (small streams). Generally, smallest watersheds of the latter group corresponded to higher EC. The large streams have headwaters in steep valleys that likely formed within a prehistoric mega-landslide headwall. The results suggest that EC may be used to distinguish a variable degree of groundwater interaction and evapotranspiration in the island's watersheds. Weather conditions are an important factor to consider in analyzing EC values, as a second sampling following a tropical storm was observed to lower EC significantly.