Hilo Bay Planning Assistance to States (PAS)

On April 29, 2020, the Honolulu District, U.S. Army Corps of Engineers’ (Corps) Civil and Public Works Branch met with the County of Hawaii (County) Planning Department and, then Mayor, Harry Kim to discuss the County’s vision for long term water resource management and economic development within the Hilo Bay Watershed. The Corps and the County entered into a cost shared partnership agreement on November 6, 2020, under the Planning Assistance to States (PAS) program, as authorized under Section 22 of the Water Resources Development Act of 1974, as amended (42 U.S.C. §1962d-16). Under this agreement, the Corps would provide technical assistance to theCounty related to the management of water resources in the Hilo Bay watershed.

The objective of this PAS study is to investigate the problem of water quality impairment in Hilo Bay and identify possible solutions. The first phase of the study involved a scientific literature review to establish the state of the current knowledge base and account for efforts that have already been undertaken towards implementing water quality solutions. The second phase of the study formulated specific measures to address the sources of water quality impairment, mitigate their impacts, or to advance the existing knowledge base in areas that are lacking information.

Past studies and discussions with community stakeholders indicate that the pollutants causing water quality impairment in Hilo Bay are primarily terrigenous sediments and nitrogenous compounds, and secondarily, heavy metals such as arsenic that are present, but appear to be neutralized. The main pathways for pollutants entering the bay are via natural surface drainage e.g. Wailuku and Wailoa Rivers and stormwater discharge across land. Due to the coarsely porous basalt landforms surrounding Hilo Watershed, groundwater discharge may also convey pollutants to Hilo bay, to an unknown degree.

High levels of nitrates and phosphates cause eutrophication, making waterways uninhabitable for aquatic flora and fauna. Previous research by the Corps indicates that the Hilo breakwater, designed to protect Hilo Harbor by reducing wave energy, consequently, increases residence time of pollutants in Hilo Bay, further contributing to water quality impairment. It is important to note that studies evaluating potential for modification to the breakwater indicated that the magnitude of effects to water quality from inputs of pollutants is significantly greater than poor circulation. In other words, as a matter of priority, the initial effort should focus on watershed management to reduce the conveyance of pollutants into Hilo Bay, followed by detailed analysis to identify the level of circulation necessary and measures to implement.

The PAS study team identified several measures, both structural and non-structural, both in Hilo Bay and across the entire watershed study area, that could address the County’s concern of impaired water quality in Hilo Bay. Structural measures were considered solely based on feasibility of implementation and general effectiveness; no concept design or detailed cost estimates were developed. Non-structural measures include regulatory and policy recommendations, future research recommendations to address critical knowledge gaps, and in situ projects such as bioremediation and aquaculture applications. The measures were evaluated to ensure they meet the study objective and using The Water Resources Council’s National Evaluation Criteria: completeness, effectiveness, efficiency, and acceptability (see ). In total, four (4) structural and eleven (11) non-structural measures were examined and qualitatively prioritized based on the evaluation criteria. A common goal among stakeholders and reiterated in this report, is the need to address pollutants at their source, as opposed to in the bay.

The recommendations of the study are as follows: address the lack of data and knowledge via non-structural measures by 1) analyzing water quality and sediments accumulated in Hilo Bay to comprehensively identify pollutants of concern and 2) identifying pollutant sources through flood mapping the entire watershed and conducting a shear-stress analysis to identify erosion hot spots. Once this critical information has been collected, a watershed management plan could provide all stakeholders with a roadmap of efforts and avenues of coordination among all concerned parties.