THE WEST MAUI MULTI-TRACER INJECTION PROJECT: MODELING AND MONITORING LAND-SEA FLOW, THROUGH THE SUBTERRANEAN ESTUARY AND OUT INTO THE SEA
Session: 060: Submarine Groundwater Discharge - from Ridge to Reef: Groundwater Evolution, Climate, Land-Use,
Abstract Number: 17968
Date: Tue, Feb 25th, 2014
Time: 2:45 PM - 3:00 PM
True SGD travel times, chemical modifications, and dispersion within subterranean estuaries are poorly known. We completed a 2-year study of the land-sea connection between deep-well injection at West Maui Lahaina Wastewater Treatment Plant (LWTP) and the nearshore coastal ocean. Traveling 0.85 km along oblique trajectory in layered basalts, injected Fluorescein (FLT) dye began discharging from small nearshore submarine spring-fields (< 0.6 acre) after 84 days, and displayed an average breakthrough curve travel time of ca. 15 months. Revealed by airborne thermal infrared mapping, discharged anomalously warm SGD mushrooms to cover >167 acres of ocean surface, with lateral dispersal mirrored by variations in macro-algal-tissue 15N and far-field detection of dye. Radon mass balance estimates of spring-field SGD were 2.19-3.33 MGD total-SGD, of which 73-87% was fresh. Based on FLT recovery, 64% of the injected wastewater emerges at the submarine spring-field sites; this agrees with our geochemical/stable isotope source water partitioning analysis that estimated treated wastewater fractions in submarine spring discharge ranged from 12-96%, averaging 62%. Suboxic to anoxic conditions prevail during in-aquifer transit: High nitrogen loss occurs due to extensive and prolonged denitrification (nitrate δ15N up to +90 per mil), abundant N2-bubbles stream from the seafloor, and reduced Mn is precipitated as MnO at SGD-vents. In contrast, spring-discharged phosphorus is enriched relative to the injectate.