A Multitracer Approach to Detecting Wastewater Plumes from Municipal Injection Wells in Nearshore Marine Waters at Kihei and Lahaina, Maui, Hawaii

Municipal wastewater plumes discharging from aquifer to ocean were detected by nearshore wading surveys at Kihei and Lahaina, on the island of Maui in Hawaii. Developed in cooperation with the Hawaii State Department of Health, the survey methodology included instrument trolling to detect submarine groundwater discharge, followed by analysis of water and macroalgae for a suite of chemical and isotopic constituents that constitute a 'multitracer' approach. Surveys were conducted May 6-28, 2008, during fair-weather conditions and included: (1) wading and kayak trolling with a multiparameter water-quality sonde, (2) marine water-column sampling, and (3) collection of benthic algae samples. Instrument trolling helped guide the water sampling strategy by providing dense, continuous transects of water properties on which groundwater discharge zones could be identified. Water and algae samples for costly chemical and isotopic laboratory analyses were last to be collected but were highly diagnostic of wastewater presence and nutrient origin because of low detection levels and confirmation across multiple tracers. Laboratory results confirmed the presence of wastewater constituents in marine water-column samples at both locales and showed evidence of modifying processes such as denitrification and mixing of effluent with surrounding groundwater and seawater. Carbamazepine was the most diagnostic pharmaceutical, detected in several marine water-column samples and effluent at both Kihei and Lahaina. Heavy nitrogen-isotope compositions in water and algae were highly diagnostic of effluent, particularly where enriched to even heavier values than effluent source compositions by denitrification. Algae provided an added advantage of time-integrating their nitrogen source during growth. The measured Kihei plume coincided almost exactly with prior model predictions, but the Lahaina plume was detected well south of the expected direct path from injection wells to shore and may be guided by a buried valley fill from an ancestral course of Honokowai Stream. Nutrient concentrations in upland wells at Lahaina were comparable to concentrations in wastewater but originate instead from agricultural fertilizers. A key factor in detecting and mapping the wastewater plumes was sampling very close to shore (mostly within 20 m or so) and in very shallow water (mostly 0.5 to 2 m depth). Effluent probably discharges somewhat offshore as well, although prior attempts to detect an injected fluorescent tracer at Lahaina in the 1990s were inconclusive, having focused farther offshore in water mostly 10-30 m deep. Sampling of benthic porewater and algae would offer the best chances for further effluent detection and mapping offshore, and sampling of onland monitor wells could provide additional understanding of geochemical processes that take place in the effluent plumes and bring about some degree of natural attenuation of nutrients.