2021-10-27 EMC Agenda item 5-b(i) Identifying wastewater management tradeoffs ... in Kona, Hawaii (RG)

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PLOS ONE Identifying wastewater management tradeoffs in Kona, Hawai'i current paper were developed through an iterative discussion process with the CCWG with the goal of generating research results that provide utility to decision makers. The State Department of Health identified 6,500 cesspools in the coastal Kailua-Kona area of Hawaii Island as having the potential to negatively impact sensitive coastal waters [24, 25]. While not expected to pose a significant risk to drinking water sources, these cesspools were identified as having negative impacts on nearshore marine water quality and linked groundwa- ter -dependent ecosystems (GDE). GDEs have historically served as important sources of water and food to Kona's coastal communities and continue to be highly valued today for their cul- tural uses and provision of a wide variety of ecosystem services [26, 27]. The importance of GDEs and the SGD that supports them is not unique to Hawaii, however, as a number of stud- ies have documented the many societal values of these systems worldwide [28]. Thus, develop- ing a framework to evaluate the potential impacts of land use and wastewater management decisions on nearshore ecosystems will not only help to inform local policy, but will also have broader applications for management and ecosystem implications. To that end, this paper discusses a systems approach that was developed to link land use decisions (development, water use, wastewater treatment), nutrient fate and transport pro- cesses in groundwater, and nearshore marine health. Whereas a single model built for any of these individual components would likely be sufficient to test various hypotheses about that particular component, our developed ridge -to -reef approach provides a means for assessing decisions that have cascading effects across multiple elements of the system. Several terrestrial management scenarios are evaluated in terms of the nutrients nitrogen (N) and phosphorus (P) discharged to nearshore waters, potential impacts to marine habitat quality, and costs. Results are then presented in three ways: (1) maps that show the spatial distribution of impacts for each scenario, (2) spider diagrams that illustrate tradeoffs between multiple management objectives based on aggregated spatial outputs, and (3) a cost -benefit analysis, where benefits are measured in physical units and costs in dollars. 2. Materials and methods 2.1 Study site description The Keauhou aquifer (426 km2) encompasses the southern flank of Hualalai volcano, located in the northern Kona district of Hawaii Island (Fig 1). Like most volcanic island aquifers, groundwater is the major source of fresh drinking water for residents of the Kona community [29]. The Keauhou aquifer has an anomalous hydraulic head gradient approximately six kilo- meters inland, where water levels drastically increase from 1 m to > 100 m above mean sea level. A geologic subsurface structure, referred to as the high -low divide, is believed to prevent high-level groundwater from quickly flowing to the coast, therefore creating a productive high-level aquifer that supplies approximately 25% of Kona's major pumping wells [30]. The basal aquifer is considered to be the area where water levels are G5 m above mean sea level (150 km2), extending from the high -low divide to the western coastline. 2.2 Modeling approach overview The ridge -to -reef framework consists of three models coupled with an assessment of potential impacts to the marine habitat (see Mekhods sections below for greater detail). F^ g 2 provides a high-level graphical overview of how the different elements of the ridge -to -reef framework are connected. Groundwater recharge (Fig 2A) [31] feeds into the groundwater model (Fig 213), while land use and wastewater management scenarios (Fig 21)) are concurrently applied to the groundwater model domain to generate groundwater nutrient concentrations (F'ig 2C) [32]. Using the simulated nutrient concentrations [32] coupled with spatial information about PLOSONE IIhu�t1lws://doaa.oirg/n0.t371/�OLuirirualll,lpoine,0257125 September8,2021 3/26