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

Abstract

Study focus: 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 18 O H 2 O , 15 N NO 3 -, and 18 O NO 3 -modeling to identify groundwater pathways, recharge elevations, and nitrate sources. Fresh and total SGD rates and nutrient fluxes were quantified using 222 Rn mass balance modeling. New hydrological insights for the region: Low nitrate + nitrite (N + N) SGD fluxes (24 mols/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 (3800 mols/d). Despite their much smaller freshwater flux, these SGD sources provide substantially larger N fluxes than the State's largest rivers (avg. 700 mols/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.