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 of the model's detailed spatial results. To create the spider diagrams, each of four metrics (N reduction, P reduction, potential change in marine habitat quality, present value cost) were ranked ordinally for all scenarios except the current condition, as the current condition is the base against which the nutrient and marine habitat results are compared. For each metric, the scenario with the top ranked output was assigned a value of seven, the second ranked output was assigned a value of six, and so forth. For a given scenario, scores for each metric were then plotted on a set of concentric circles, where the innermost circle represents the minimum score (one), the outermost circle represents the maximum score (seven), and each cardinal direction represents one of the metrics being evaluated. The area of the polygon generated by connecting the plotted points for each scenario represents an aggregate score that can be directly compared. A cost -benefit analysis was also undertaken, where benefits were measured in physical units and costs in dollars, to further inform prioritization of the management scenarios. Simulated N, P, and marine habitat quality for each management scenario were compared to the future permitted scenario, and the difference was divided by the respective scenario's cost in order to estimate the conservation benefit generated per million dollars spent on a particular manage- ment action [60]. That is, each potential future was compared to the inaction scenario, wherein future land development occurs but no action is taken to address the wastewater management problem, to determine the return on investment (ROI) of each candidate management action. As previously noted, caution is warranted when interpreting results generated using aggre- gated values given the spatial nature of the management problem. However, ROI estimates when viewed in conjunction with spatial impact maps and spider diagrams may allow for more concrete policy recommendations. 3. Results 3.1 Groundwater & marine water quality Groundwater nutrient concentrations vary in severity across the Keauhou basal aquifer. Under present conditions, nutrient concentrations are highest surrounding the Kealakehe WWTP (Figs 3A and 4A). Nutrient concentrations are also relatively elevated north and south of the WWTP, in areas with high OSDS quantities. Under future permitted conditions, overall nutrient concentrations generally increase, but the most significant change is around the WWTP due to the drastic increase in nutrient mass load from the additional anticipated efflu- ent discharge (Figs B and 41.1). If the WWTP is upgraded, however, the nutrient concentra- tions surrounding the WWTP will decrease (Figs 3D and 411)). If OSDS are converted to high efficiency ATU and septic units, lower nutrient concentrations are seen across the aquifer in comparison to low efficiency conversions. This is particularly noticeable in areas located south of the WWTP (Figs 3E 31E1 and 4F 41H). This is likely because a higher proportion of OSDS south of the WWTP were assigned risk scores > 12, therefore being converted to ATU rather than septic systems. The higher number of SGD pourpoints representing prolific SGD springs to the south of Kailua Kona and the WWTP results in more nutrient discharge along the southern coast. Because of this spatial variation, upgrading the cesspools across the study area results in more nutrient reduction along the southern coastline of the site and less along the northern coastline (Figs 5E 51:1: and 6Ll , . 611:1). The nutrient load discharge reduction is lower under the target low efficiency (Figs 51? and 6L.'.) compared to the target high efficiency scenario (Figs 5F and 6F). Under both target low and high efficiency scenarios, the models show an increase in nutrient load export downstream from the WWTP due to the increase in sewer connections assumed for the permitted build out (Figs 5E 5F and 6F,..6F). When the WWTP is upgraded and PLOS ONE I Ihuttlps://doaa .oirg/ 10.1371/�oauirirualll,lpoi ne,025712 a September 8, 2021 11 / 26