Effect of land use and groundwater flow path on submarine groundwater discharge nutrient flux

•Groundwater pathways, recharge elevations, and nitrate sources are identified.•Numerical groundwater modeling stable isotopes and Rn mass balance are used.•N flux to coastlines via SGD can contribute more N than Hawaii's largest rivers.•N fluxes from sugarcane and pineapple are greater than all other land uses.•Septic, cesspool and wastewater injection N flux to the coast are relatively low.