Human and environmental risk ranking of onsite sewage disposal systems for the Hawaiian islands of Kauai, Molokai, Maui, and Hawaii

Outside of the urban centers and major towns, residences and small businesses dispose of wastewater at the location where it is generated. This on-site disposal of wastewater gives rise to risks to human health and the environment. This study assessed the potential risk posed by on-site sewage disposal systems (OSDS) to human health and critical ecosystems on the islands of Hawaii, Kauai, Maui, and Molokai. To assess this risk, the number and locations of OSDS were estimated based on a search of wastewater and tax databases. The risk posed to critical ecosystems and human health was evaluated based on the volume and water quality characteristics of the effluent discharged and the proximity of OSDS to receiving ecosystems and potential points of human contact. Finally, a cumulative risk severity score was calculated to rank the relative risk posed by each OSDS. Project Goals and Methods The objectives of this study were to: 1. Estimate the quantity, location, and types of OSDS on the islands of Hawaii, Kauai, Maui, and Molokai; 2. Estimate the effluent load added to the environment by these systems; 3. Identify and map the factors influencing the risk posed by OSDS to the environment and to human health; 4. Evaluate the potential risk to the receptors of concern (ROC) that may be impacted by OSDS; 5. Develop a scoring system to map the severity and distribution of OSDS risk factors for each class of ROCs; and 6. Based on the ROC scoring results, compute an overall risk score to rank the severity of the risk posed by individual OSDS. The objectives are met by: • Completing an inventory to estimate the quantity, characteristics, and location of the OSDS (Section 3); • Modeling the impact to the groundwater from the effluent discharged from these OSDS (Section 4); • Using Geographical Information Systems to map the spatial distribution of the hydrologic parameters that affect the vulnerability of the human and environmental receptors to OSDS effluent contamination (Sections 4, 5, 6, and 7); and • Linking the OSDS locations to the OSDS risk factors to compute a relative risk-ranking score for each OSDS parcel. Data developed by this study can be used by planning and regulatory agencies to set policy regarding OSDS, identify areas most suitable for locating OSDS, and delineate those areas where the negative impact from OSDS effluent is most likely to occur. The information can also be used to develop a schedule for OSDS inspections by prioritizing systems based on relative risks.Safe Drinking Water Branch, Department of Health, State of Hawai