Effect of Land Use and Groundwater Flow Path on Submarine Groundwater Discharge Nutrient Flux

Abstract

Fertilized agricultural lands, wastewater injection, and areas with high septic system and cesspool density each have potential to contribute excess nutrients to coastal waters of Maui via submarine groundwater discharge (SGD). We investigated the connection between such land uses and coastal waters and quantified their respective impacts around the island of Maui, Hawai‘i using a numerical groundwater model, O isotopic composition of H2O, and N and O isotopic compositions of dissolved nitrate to identify the groundwater pathways, recharge elevations, and nitrate sources. Fresh and total SGD rates and nutrient fluxes were quantified using 222Rn mass balance modeling. Low nitrate + nitrite (N+N) SGD fluxes (24 moles/d) were measured where groundwater flowed beneath primarily undeveloped land on transit to the coast. By contrast, sugarcane and pineapple fields discharge the largest amount of N to coastal waters via SGD of any land use type (4900 moles/d), and despite the much smaller freshwater SGD flux these rates are substantially larger than N fluxes from the State's largest rivers (avg. 700 moles/d). Septic systems, cesspools, and near coast wastewater injection wells also contribute N+N to groundwater and coastal waters, though in much smaller quantities. This study demonstrates that numerical groundwater modeling combined with geochemical modeling can be used to determine sources and flux of nutrients in SGD and provides a unique, original, and practical framework for studying the effect of land use and its impact on nutrient delivery to coastal waters.