National Institute for Water Resources, Water Resources Research Institute Program
9/1/2012 - 10/31/2013
This study was aimed at assessing the effects of stream valleys that are filled with alluvium below the water table that act as hydrologic barriers to cross-valley groundwater flow. Weathered basalt underneath the streambed contributes to the permeability contrast under the valley fill with respect to the otherwise high-permeability basalt aquifer. The scope of work included: (1) developing a regional numerical groundwater model that quantifies groundwater fluxes to the Pearl Harbor Aquifer from adjacent areas, and (2) analyzing groundwater-level data to evaluate the hydrologic effectiveness of valley-fill barriers, including those associated with Waimano, Waimalu, and Kalauao Streams.
The study included developing a three-dimensional island-wide MODFLOW model with the focus on groundwater areas adjacent to the Pearl Harbor Aquifer. The numerical model is capable of simulating groundwater flow and the freshwater-saltwater interface using the Saltwater-Intrusion (SWI) package. The steady-state model of the recent hydrologic conditions was calibrated using observed groundwater levels, vertical salinity profiles, and estimated base flows of streams. Upon successful calibration, groundwater fluxes into the Pearl Harbor Aquifer were determined for recent conditions. The effects of predevelopment conditions on the location of the groundwater divide between leeward and Pearl Harbor side in the Koolau high-level water area were tested.
Recent synoptic water-level surveys in the Pearl Harbor Aquifer by the USGS and water levels measured on opposite sides of valley-fills were used to characterize the effectiveness of the alluvium as a hydrologic barrier. Moreover, continuously measured water levels were analyzed to evaluate the cross-boundary effects of groundwater withdrawals. After removing environmental stresses that influence water levels other than groundwater withdrawals (e.g., barometric pressure, recharge events), the water-level time series were investigated for signs of drawdown and recovery across valley fills.