Understanding the Priority Areas

This story map is intended to provide additional information about the Priority Areas seen in the 2021 Hawai‘i Cesspool Prioritization Tool (HCPT) Report. The 15 risk factors used to determine the Priority Areas have been split into five risk categories: surface waters, drinking water, coastal impacts, soils, and marine ecological impacts.

The  HCPT  links each cesspool to its most likely effluent (human waste) flow path to the coastline, determines areas that will likely be most affected by effluent discharge, and assigns a risk score based on where these flow paths fall in the risk factors.

Each cesspool was given a risk score based on its proximity to 15 risk factors:

Cesspools are sewage disposal methods that are widely recognized to harm human health and the environment. The HCPT is a geospatial tool that was created in order to help meet the State Legislature's Act 125, which mandates that all of Hawai'i estimated 80,000+ cesspools be replaced by 2050. In order to achieve this goal, this project provided updated information and data that assisted the  Cesspool Conversion Working Group (CCWG)  and the  Hawai'i State Department of Health (DOH)  in making informed, long-term planning and preparation decisions (HCPT Report, 2021).

Average annual rainfall datasets were collected from the  Hawai‘i Climate Atlas  ( Giambelluca et al. 2013 ) as statewide grids of annual rainfall totals. In the model, increasing rainfall is synonymus with worsening conditions for wastewater treatment. Rainfall rates were extracted at each cesspool location to create a raster layer in ArcGIS Pro that overlays the islands.

The complexity of environmental processes makes it difficult to quantify specific risks to streams and wetlands from wastewater pollution. Instead, hazards are identified through a basic geographic distance calculation to the nearest  stream  or  wetland  by using data from the statewide  Hawai‘i GIS Portal  in order to understand the potential risks to these systems from wastewater effluent. Streams were ultimately joined with wetland features, including emergent ponds, to simplify the score. The HCPT assumes that streams and wetlands are equally important even though certain streams or wetlands may be of greater importance based upon location, cultural significance, development, and ecosystem services provided.

According to the  Environmental Protection Agency (2020) , a failing onsite sewage disposal system or cesspool that is located too close to a drinking water well can contaminate the source. Locations of pumping wells were acquired from the state well inventory from the  Commission on Water Resources Management (CWRM). 

To most accurately convey potential risks to drinking water supplies, the HCPT evaluated if a cesspool was located within a municipal well capture zone. This is largely because cesspool effluent has the potential to contaminate drinking water supplies with viruses and other pathogens that can withstand long travel times. Data on capture zone locations was provided by the DOH and CWRM.

The shorter the distance to groundwater under a wastewater treatment system, the greater the risk for wastewater effluent entering the environment. Rises in the groundwater table are already impacting subsurface infrastructure in Hawai'i, such as cesspools and sewer lines ( Habel et al. 2020 ). Land surface elevations were calculated along the coastline using a statewide high-resolution (<1 meter)  LiDAR Digital Elevation Model (DEM)  obtained from Hawai‘i GIS Online Portal. Where the LiDAR DEM was missing elevation data, it was filled with lower resolution (10 meter) data. Statewide-coverage  10 meter-DEMs  are readily available from the  UH Coastal Geology Group .

Groundwater flow paths were not considered as an independent risk factor and were used to link each cesspool location to a corresponding location along the coastline where the cesspool’s effluent is estimated to be discharged. These linked flow paths were calculated using the MODPATH code within the  Groundwater Modeling System (GMS)  graphical user interface (Pollock, 2012). In theory, a cesspool located on the land surface can be linked directly to the coastline location to which its effluent will eventually discharge after traveling through the aquifer.

Cesspools adjacent to the coastline face numerous challenges such as sea level rise, erosion, and shallow depth to groundwater. Studies such as  Abaya et al. (2018)  have demonstrated that distance to the coast and geology can have dramatic effects on the travel time of wastewater pollution entering the ocean. The HCPT uses a basic geographic distance calculation to the nearest point on a  Hawai‘i coastline GIS shapefile  to assign a distance to each cesspool in the inventory.

The available data was based on the methodology/modeling used in the Hawai‘i Sea Level Rise Vulnerability and Adaptation Report and the  Hawai‘i Sea Level Rise Viewer . The products have undergone peer review and publication in the Scientific Reports Journal: Nature.

Human usage of the coastal zone is a driver of prioritization as it is both part of public and environmental health. The methodology of  Wood et al. (2013)  was chosen to calculate the current amount of visitors to the coastline through photo-sharing data scraped from the popular photo-sharing website  flickr . The  INVeST model  is used to calculate current amount of by using the specific latitude/longitude of the image, the photographer’s user-name, and the date that the image was taken to count the total photo-user-days for each grid cell or polygon of annual person-days of photographs uploaded to flickr. One photo-user-day at a location is one unique photographer who took at least one photo on a specific day.

 Placement of lifeguard towers  may be determined due to the number of incidents responded to at sites, visitation levels, or requests by the state government. Because it is assumed that lifeguarded beaches have a higher in-water activity usage (swimming, surfing, diving, wading) than unguarded beaches, cesspools discharging to these beaches are ranked a higher priority in the assessment. County websites and databases were examined to compile a statewide inventory of lifeguard towers. Any area of coastline within 500 meters on either side of a lifeguard tower was considered to be a swimming beach.

Soil is essential for wastewater treatment systems to function properly. In typical onsite wastewater treatment fields, soil provides space for biological activity and filters pathogens and chemicals through its physical characteristics ( Hygnstrom et al. 2011 ). Statewide soil data was extracted from the  Natural Resources Conservation Service (NRCS)  database. Each cesspool was assigned a single soil-suitability score based on the factors and thresholds defined by NRCS:

1. Depth to bedrock: A measurement from the ground surface to the contact with continuous bedrock or cement pan;
2. Flood frequency: The degree to which the soil is subject to flooding or ponding;
3. Filtering characteristics: How well the soil filters out particulates and bacteria;
4. Water infiltration rate: How well water moves through the soil;
5. Bottom seepage rate: How quickly water will move from the lowest soil layer to the bedrock;
6. Slope: Measurement of the direction and the steepness of the ground surface. A slope of more than 15% is considered problematic for OSDS installation;
7. Rock fragmentation: Measurement of the fraction of rock fragments in the soil. A percentage of 3-inch rock fragments of more than 50% is problematic for OSDS pollution.

To develop the density calculation for the HCPT, the authors investigated current and past literature.  Bicki and Brown (1991)  conclude that groundwater monitoring and modeling demonstrate a correlation between contamination and septic system density, suggesting a minimum lot size of one-half (0.5) to one (1) acre is needed to prevent groundwater contamination.

The current live coral information was provided by  Arizona State University’s Global Airborne Observatory  (GAO). Historical coral data are provided by the  Hawai’i Monitoring and Reporting Collaborative  (HIMARC), which were derived from in-water observations from a broad network of monitoring programs and agencies, including the  Division of Aquatic Resources  (DAR).

This dataset represents a subset of fish biomass that directly supports fishing and feeds local communities, and does not represent total fish biomass on the reef. The Reef Advisory Group (Jamison Gove, Ph.D., Joey Lecky, Greg Asner, Ph.D., Mary Donovan, Ph.D., Tom Oliver, Ph.D., Eric Conklin, Ph.D., and Kim Falinski Ph.D.) produced predictive maps of standing resource fish biomass and the theoretical recovery potential of resource fish biomass if effluent from cesspools were eliminated.

Available data did not meet the standards and format needed for use in the HCPT. After consulting with oceanography experts, the most feasible way to incorporate an element of ocean circulation was to use wave power as a best-available proxy.

Wave power (kW/m) is a major forcing mechanism in Hawai'i, which can influence many marine ecosystems, including coral reef development. Wave power drives the mixing of the upper water column and can also play a role in nutrient availability and ocean temperature reduction during warming events. Wave forcing is also highly seasonal in Hawai'i: winter months usually experience much greater wave power than the summer months.

 Wedding et al. (2018)  developed a statewide long-term mean wave power dataset, which was made publicly available through the  Pacific Islands Ocean Observing Systems PacIOOS   Ocean Tipping Points Project . This dataset was determined by the authors to be the best available proxy for determining if a coastal area was considered geographically sheltered versus exposed.

Wave power data were obtained from the  University of Hawaiʻi at Mānoa (UH) School of Ocean and Earth Science and Technology (SOEST) SWAN model (Simulating WAves Nearshore)  from 1979-2013.