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 treatment (SAT), the upgrade is projected to reduce N and P by up to 90% [43]. Assumed nutrient concentrations in released effluent for the current and proposed WWTP upgrade are presented inTable blle 2. For management scenarios with increased future urban development, we assume that all new residential units are connected to the WWTP and that the amount of sew- age entering the WWTP is consequently increased by 50% of projected future water demand, while the remaining 50% is allocated to outdoor water uses [37]. 2.4 Groundwater model This study utilized a three-dimensional, density -dependent, multi -species numerical ground- water model that was previously developed [32] with the simulation program SEAWAT [44, 45] within the GMS software interface (hit.�ip,,,://www.aqu qqu vcro,con_ii/^,,ofwaire/griis-mruode�s utilities). The numerical model spans over the Keauhou basal aquifer and offshore, and con- sists of 21 layers (26,323 cells total) with a top elevation that follows local topography and bathymetry and a flat bottom elevation of 550 m below mean sea level (see S2 Fig the Support- ing Information section). The bottom elevation of the first layer is located 1 m below mean sea level to ensure dry cells were not produced and the remaining layers increase in thickness, where the uppermost layers are thinnest. The horizontal spacing of the grid cells are a consis- tent 490 m by 490 m (Fig i E). Recharge was estimated based on the results from the USGS Hawaii Island water budget model [all ]. The top two layers of the eastern boundary condition were assigned a constant flow rate of 388,399 m3/d to account for the upland recharge and pumping, and specified salinity, N, and P concentrations of 0.26, 0.001 g/L, and 0.0001 g/L, respectively, as measured in groundwater wells, to account for upland inflow conditions. The land -use nutrient concentrations previously described were intersected with the recharge spa- tial distribution and applied as recharge concentrations. Aggregated OSDS and the Kealakehe WWTP were applied as point -source mass loads. Efforts to simulate SGD as a diffusive ocean -interface process as a default approach in the model were not successful and produced results that were drastically different from observed discharge patterns. SGD mainly occurs as coastal springs with no evidence of significant diffu- sive fluxes [45]. To improve on results, and following similar approaches taken in previous studies [30, 46], discrete SGD springs identified by [45] were simulated as point drains (head - dependent sinks). Here, it is assumed that the conductance of each spring is proportional to spring discharge. Therefore, each spring has an individual conductance related to the mea- sured discharge, which varies across the complex aquifer coastline. Nineteen of the 27 springs were assigned conductance values based on measured discharge rates, with a total simulated discharge of — 100,000 m3/d. Discharge rates were not measured for the remaining eight springs, so average rates were applied, thus increasing the total simulated discharge to 140,000 m3/d. The model was calibrated with 54 well water -level measurements that were obtained from CWRM and were measured between 1944 and 2008. Salinity, N, and P concen- trations were calibrated with 20 well measurements [4f7] and 13 SGD measurements (]Pig l B). 2.5 Marine water quality model To link the groundwater nutrient loads to the potential impact on coral reef habitat, the nutri- ent flux (kg/yr) was diffused into the marine environment (60 x 60 m) from the coastal springs, from here on referred to as pourpoints for the purpose of the water quality modeling. The pourpoints represent the SGD for each management scenario. A previously developed marine water quality model tested in Hawaii [39] was applied, which was adapted to account for diffu- sion from point sources (fig 1E). First, a diffusion factor layer was created that represents the impedance of moving planimetrically through each cell from each groundwater pourpoint PLOSONE IIhuttlps://doaa.oirg/n0.t:171/�oauirirualll,lpoine,0257125 September8,2021 8/26