OSDS) to Human Health and Critical Ecosystems on the Islands of Hawaii, Kauai, Maui, and Molokai
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HUMAN HEALTH AND ENVIRONMENTAL RISK RANKING OF ON-SITE SEWAGE DISPOSAL SYSTEMS FOR THE HAWAIIAN ISLANDS OF KAUAI, MOLOKAI, MAUI, AND HAWAII FINAL Robert B. Whittier and Aly I. El-Kadi September 2014 PREPARED FOR State of Hawai‘i Department of Health Safe Drinking Water Branch Principal Investigator: Aly I. El-Kadi School of Ocean and Earth Science and Technology Department of Geology and Geophysics University of Hawai‘i at M¯anoa Honolulu, Hawai‘i 96822 HUMAN HEALTH AND ENVIRONMENTAL RISK RANKING OF ON-SITE SEWAGE DISPOSAL SYSTEMS FOR THE HAWAIIAN ISLANDS OF KAUAI, MOLOKAI, MAUI, AND HAWAII Final Robert B. Whittier and Aly I. El-Kadi September 2014 PREPARED FOR State of Hawai‘i Department of Health Safe Drinking Water Branch Principal Investigator: Aly I. El-Kadi School of Ocean and Earth Science and Technology Department of Geology and Geophysics University of Hawai‘i at M¯anoa Honolulu, Hawai‘i 96822 This page is intentionally left blank Executive Summary 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. ES-1 OSDS Inventory Results This study and the pilot study done for Oahu (Whittier and El-Kadi, 2009) represent the most comprehensive census of OSDS performed to date for the State of Hawaii. The two studies estimate that the number of the OSDS in the State of Hawaii exceeds 110,000. Table ES-1 gives a breakdown of the OSDS inventory by type and by island. Figures ES-1 through ES-4 show the OSDS distribution for the islands evaluated by this study (Hawaii Island, Kauai, Maui, and Molokai) with density expressed as OSDS per mile squared (mi2). The majority of these OSDS (80 percent) are cesspools where the effluent receives no treatment prior to being released to the environment. It is estimated that statewide OSDS discharge nearly 70 million gallons per day (mgd) of minimally treated effluent to groundwater. This produces an estimated nutrient load to the environment of over 12,500 and 3,500 kilograms per day (kg/d) of nitrogen and phosphorus, respectively. Nearly half of the OSDS in the state are located on Hawaii Island. However, the highest OSDS density is on Kauai where there are approximately 32 units per square mile (mi2). Molokai, which is the least developed of these islands, has the lowest total population (7,345) and OSDS density (approximately 7.5 units/mi2). ES-2 Table ES-1. The OSDS inventory results and effluent discharge totals Effluent N P Total CLASS CLASS CLASS CLASS Discharge FLUX FLUX Island OSDS I II III IV (mgd) (kg/d) (kg/d) Hawaii 58,982 8,951 694 68 49,344 34.6 6,607 1,848 Kauai 18,011 3,107 910 304 13,688 12.5 2,115 607 Maui 16,883 4,015 559 75 12,242 11.6 1,869 554 Molokai 1,956 477 33 4 1,442 1.2 206 59 Oahu* 14,606 2,620 534 199 11,253 9.7 1,732 500 Total 110,438 19,170 2,730 650 87,969 69.6 12,529 3,568 *Oahu OSDS data taken from Whittier and El-Kadi (2009) Class I – OSDS utilizing soil treatment Class II – Septic systems discharging to a seepage pit Class III – Aerobic treatment units discharging to a seepage pit Class IV – Cesspools ES-3 Figure ES-1. The distribution of OSDS density on Hawaii Island shown with district boundaries and sewered areas ES-4 Figure ES-2. The distribution of OSDS density on Kauai shown with district boundaries and sewered areas ES-5 Figure ES-3. The distribution of OSDS density on Maui shown with district boundaries and sewered areas ES-6 Figure ES-4. The OSDS density distribution on Molokai shown with district boundaries and sewered areas ES-7 Summary of OSDS Risk The risk that an OSDS poses to sensitive ecosystems and human health is dependent upon on the likelihood that the effluent can reach a receptor where harm may occur. The points of potential human contact and sensitive ecosystems will be referred to as receptors of concern (ROC) by this study. Many factors govern the transport of OSDS effluent from its point of discharge to where harm may occur. The factors include the proximity of the OSDS to the ROC, the ability of the soil to transmit and remediate the OSDS effluent, the amount of dilution the effluent is subjected to in the saturated zone, and the hydrologic factors allowing the effluent contaminated groundwater to return to the surface and impact the ROC. The distribution of these factors was mapped and a weighting coefficient was assigned as a semi-quantitative method for estimating the distribution of risk to ROCs. The following sections describe the ROC and the method of estimating an OSDS’s relative risk. Characteristics of OSDS That Determine Risk The inherent risk posed by an OSDS varies by the quantity of effluent, the system type, and the method of effluent treatment. This study classified OSDS by the type of treatment the wastewater effluent receives. Table ES-1 below lists the OSDS classes. Table ES-2. OSDS Classes and Corresponding Treatment and Disposal Type OSDS Class Treatment and Disposal Type Class I Any system receiving soil treatment. This includes disposal types listed as bed, trench, and infiltration/chambers. Class II Septic systems discharging to a seepage pit. The effluent receives primary treatment only. Class III Aerobic units discharging to a seepage pit. The effluent receives primary and secondary treatment. Class IV All cesspools where the effluent receives no treatment. Note: A seepage pit is a dry well that disperses effluent from septic tanks. The effluent receives no treatment other than settling of solids that occurs in the septic tank EFFLUENT QUANTITY The risk that OSDS pose to human health and the environment is strongly influenced by the rate of effluent discharge. Therefore, it is critical in any risk evaluation to quantify the amount of effluent being released to the environment. The OSDS effluent discharge rate was estimated using residential dwelling characteristics or by the type of activity occurring at non-residential parcels. For residential units, guidance was provided by Hawaii Administrative Rules Title 11, Chapter 62 that estimates an effluent rate of 200 gallons per day (gpd) for each bedroom served by OSDS. OSDS effluent discharge rates for systems serving non-residential structures needed an alternative ES-8 method estimation method. These non-residential activities included businesses, churches, schools, parks, and condominiums. The OSDS discharge rate for non-residential systems was based on estimates given in Metcalf and Eddy (1991). EFFLUENT QUALITY The mass of nutrients reaching surface or coastal waters determines the degree of impact on the receiving bodies. The risk to human health is driven by the concentration and types of contaminants in drinking water impacted by OSDS. It is beyond the scope of this study to evaluate all of the contaminants in the OSDS effluent. The contaminants of greatest concern are the nutrients that cause excessive bio-productivity in surface waters and, in the case of nitrate, toxic substances. The nutrients evaluated, nitrogen and phosphorous, were considered with nitrogen used as the primary species to evaluate risk. This approach was taken because nitrogen can be a limiting nutrient in aquatic and marine waters, making it a contaminant of concern.