Water Resources Research Act Program

Details for Project ID 2017HI466B

IDENTIFYING GROUNDWATER FLOW AND CONTAMINATION TO STREAMS: KAHALUU WATERSHED, OAHU

Institute: Hawaii
Year Established: 2017 Start Date: 2017-03-01 End Date: 2018-02-28
Total Federal Funds: $21,741 Total Non-Federal Funds: $43,495

Principal Investigators: Craig Glenn

Abstract: The surface waters in the Kahaluu Lagoon on the windward side of Oahu fronted by Kaneohe Bay have been shown to have very high levels of wastewater related bacteria. Watersheds feeding this lagoon contain over 1,600 cesspools or other On-Site Disposal Systems (OSDS). While some introduction of sewage contamination may be due to overland flow during storm events causing OSDS to overflow, there is also chronic introduction of sewage contamination by the discharge of groundwater to streams. To identify the areas and more importantly the OSDS that are the primary source of wastewater contamination to the streams and the lagoon, the specific stream reaches where groundwater discharges to the streams need to be identified. This knowledge is critical to fully characterize the problem of wastewater contaminating surface water. Therefore, this research will combine and refine new methods to identify and quantify the locations and flux of groundwater and subsurface contaminant pollution to streams via continuous measurements of groundwater/surface water interaction by using four independent approaches that combine: 1) infrared mapping of groundwater seepage to streams and lagoon with thermal infrared imagery, 2) traditional seepage runs, 3) radon-222 and radon-220 activity surveys and continuous monitoring, and 4) continuous stream flow monitoring at multiple locations. Using these four independent methods to assess groundwater–surface water interaction in combination will allow cross-validation between the individual methods as well as the refinement of each method. The temperature of groundwater is relatively cool and constant through time whereas stream surface water temperatures are warmer and vary on seasonal and diurnal cycles. Using these thermal contrasts we will employ high-resolution thermal imagery obtained by remote-controlled drones to help locate and map the groundwater seepage to the major streams that feed the Kahaluu Lagoon estuary, as well as the lagoon itself. The thermal contrast allows groundwater discharge to be located at the stream’s surface and along its banks. We will further measure the gain (or loss) of groundwater to the streams using seepage runs by measuring streamflow at the upstream and downstream ends of stream-reach segments; the stream’s loss to or gain from groundwater being the algebraic difference between consecutive stream flow measurements when corrected for the contribution from tributaries and reduction due to diversions. As the relationship between surface water is not static, making repeated seepage runs under varying hydrologic conditions is critical. We will measure radon, an inert gas naturally present in the groundwaters, but occurring only in trace quantities in streams. We will utilize radon-222 and radon-220 as groundwater tracers during surveys and time-series measurements to identify groundwater outcrops and quantify groundwater flow into the streams and lagoon. A radon mass balance will be constructed to derive groundwater discharge rates, and we will use short half-life radon-220 for finding exact locations of groundwater seeps. In the lagoon, we will deploy an autonomous gamma-spectrometer for short-term (tidal, storm-related) changes and long-term seasonal patterns in groundwater discharge. Multiple stream gaging stations will be installed based on identified gaining reaches of streams to measure the contribution of groundwater to total stream discharge.