Algal δ15N values detect a wastewater effluent plume in nearshore and offshore surface waters and three-dimensionally model the plume across a coral reef on Maui, Hawai‘i, USA

The coral reef at Kahekili, Maui is located ∼300 m south of the Lahaina Wastewater Reclamation Facility which uses four Class V injection wells to dispose of 3–5 million gallons of wastewater effluent daily. Prior research documented that the wastewater effluent percolates into the nearshore region of Kahekili. To determine if the wastewater effluent was detectable in the surface waters offshore, we used algal bioassays from the nearshore region to 100 m offshore and throughout the water column from the surface to the benthos. These algal bioassays documented that significantly more wastewater effluent was detected in the surface rather than the benthic waters and allowed us to generate a three-dimensional model of the wastewater plume in the Kahekili coastal region. Samples located over freshwater seeps had the highest δ15N values (∼30–35‰) and the effluent was detected in surface samples 500 m south and 100 m offshore of the freshwater seeps (∼8–11‰).