Algal bioassays detect modeled loading of wastewater-derived nitrogen in coastal waters of OʻAHU, HAWAIʻI

Amato, Daniel W.; Whittier, Robert B.; Dulai, Henrietta; Smith, Celia M.

Algal bioassays detect modeled loading of wastewater-derived nitrogen in coastal waters of OʻAHU, HAWAIʻI

2020

Amato, Daniel W. | Whittier, Robert B. | Dulai, Henrietta | Smith, Celia M.

Previous studies indicate coastlines are at risk of wastewater contamination from injection wells, cesspools, and septic systems. In this study, common marine algae were used to ground-truth modeled loading of wastewater-derived N to coastlines of Oʻahu, Hawaiʻi. Macroalgae were collected and/or deployed at 118 sites and analyzed for tissue δ¹⁵N and N %. Wastewater source locations were used to estimate wastewater-derived N in groundwater with the modeling software MT3DMS/MODFLOW. Algal bioassays identified six coastal regions subjected to elevated wastewater-derived N loading. In a case study, submarine groundwater discharge (estimated by ²²²Rn mass balance) was related to wastewater loading from onsite sewage disposal systems (OSDS) and municipal wastewater injection wells in Waimānalo. The highest ²²²Rn-derived SGD rate and N flux were 21.4 m³/m/d and 62.6 g/m/d, respectively. The results of this study suggest that OSDS and injection wells discharge substantial volumes of wastewater and N across broad regions of coastal Oʻahu.

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