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 Table 1. Description of wastewater management scenarios. ...... No. Scenario name Land use 1 Present Current land use 2 Future permitted (inaction) Addition of permitted dev, 3 All ATU u rade (+ WWTP„uprade)„ P Current land use ......, ............ e, 4 WWTP upgrade Addition of permited dev( S Target low efficiency Addition of permitted dev, 6 Target high -efficiency Addition of permitted dev, 7 Target low efficiency + WWTP upgrade Addition of permitted dev, 8 Target hieh efficiencv + WWTP upgrade Addition of permitted dev -Cesspool conversion efficiencies are described in'FoWe 2. It.C'Itas://doli.oirg/R13/�/jr. iirin411.lpone,0257125ti7QlI assumption that the permitted build out occurs in the future. For the options with targeted conversion (Scenarios 5-8 in ")Cable 1), risk scores determine whether cesspools in a particular area are converted to either septic or ATU systems. Risk scores were calculated with a weight and rating method using geographical mapping and numerical modeling [25]. Various factors, such as proximity to the shoreline, distance to groundwater, slope, and soil type were weighted based on their relative contribution to the OSDS risk. The efficiency ratings (low or high) denote how effective the upgrade technology is assumed to be at removing nutrients. Assump- tions underlying each scenario are described in greater detail in the remainder of this section. 2.3.1. Land and water use. Current land cover is shown in Fig ➢ A. Current water con- sumption is based on pumping rates reported to the Commission on Water Resource Manage- ment (CWRM). On average from 1990-2017, approximately 42,000 m3/d of water was pumped from the Keauhou basal aquifer, while approximately 16,310 m3A was pumped from high-level wells located upgradient of the basal aquifer and outside of the model domain. Future development (Fig q IfB) is based on allocated water permits, as indicated in the Kona Water Use Development Plan (WUDP), where each new residential unit was designated a water demand of 1.5 m3/d [37]. In all of the future scenarios, future water demand is assumed to be supplied entirely by pumping from high-level wells. This results in a 64,080 m3/d reduc- tion in influx from the upland boundary condition of the groundwater model. We further assume that 50% of the future water demand will be used for outdoor purposes [38], while the remaining water (32,040 m3/d) is used indoors before being sent to the WWTP. Nutrient con- centrations were applied to water flowing through each land parcel based on development type and intensity. Based on groundwater measurements from Keauhou pumping wells, nutri- ents released from natural land were assigned N and P concentrations of 1.0 mg/L and 0.1 mg/ L, respectively. Groundwater recharge affected by golf courses (locations obtained from the Hawaii Statewide GIS Program: http://pllairiiiiig,lhawaii.gov/gis/clownpoad-gis-data/) were assigned N and P concentrations of 7.59 mg/L and 0.54 mg/L, respectively [39]. Urban devel- opment was further divided into three intensity levels (low, medium, high) according to the 2005 Hawaii Coastal Change Analysis Program [40]. The low, medium, and high urban devel- opment intensities were assigned N concentrations in groundwater recharge of 1.13, 1.07, and 1.02 mg/L, respectively, and P concentrations of 0.107, 0.104, and 0.101 mg/L, respectively [41]. Where new development is anticipated in areas of previously -assigned natural land, nutrient concentrations associated with medium intensity development were assigned to recharging groundwaters. New development anticipated in previously developed areas were assigned new nutrient concentrations for groundwater recharge based on anticipated water demand. If anticipated water demand was 1.5 m3/d (i.e., one residential unit), nutrient PLOSONEIIhuttlps://doaa.org/n0.1371/�OLurirualll,lpoine,025712a September8,2021 6/26