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

[Display omitted] •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. Maui, Hawaii, United States. We investigated connections between land uses and submarine groundwater discharge (SGD) nutrient fluxes to coastal waters of Maui, Hawai'i. Nutrient contributions from agricultural lands, wastewater injection, and septic-cesspool systems were examined by combining a numerical groundwater model with δOH2O18, δNNO3−15, and δONO3−18 modeling to identify 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 (24mols/d) were measured where groundwater flowed beneath primarily undeveloped land on transit to the coast. By contrast, of all land use types, sugarcane and pineapple fields contributed the largest amount of N to coastal waters via SGD (3800mols/d). Despite their much smaller freshwater flux, these SGD sources provide substantially larger N fluxes than the State's largest rivers (avg. 700mols/d). Septic systems, cesspools, and near coast wastewater injection wells also contribute N+N to groundwater and coastal waters, although 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.