Linking Land-Use to Submarine Groundwater Discharge Nutrient Fluxes on Maui, Hawaii

Abstract:

Fertilized agricultural lands, wastewater injection, and areas with high septic system density each have substantial potential for contributing excess nutrients to the coastal waters of islands via submarine groundwater discharge (SGD). We evaluated the coastal impacts of such land-use around the island of Maui using stable isotopes (δ¹⁵N of NO₃^-; δ¹⁸O and δ²H of H₂O) and nutrient concentrations from wells, springs, beach seeps, SGD, and coastal waters, and coupled these to coastal water SGD and nutrient fluxes using ²²²Rn mass balance. Flowpaths and recharge elevations for groundwater samples were determined using the δ¹⁸O_H2O of samples, recharge data, modeled groundwater head, and published local meteoric water lines. Coastal groundwater samples whose flowpaths transect sugarcane plots showed highest dissolved inorganic nitrogen (DIN - NO₃^- + NO₂^-) concentrations, ranging from 225 - 450 µM, and the δ¹⁵N of those samples was on average 3.3 ± 0.6 ‰ (n = 15), as expected for urea fertilizers applied to commercial sugarcane. Samples whose flowpaths transect large amounts of septic sources showed moderate DIN concentrations (max. 100 µM), but had higher δ¹⁵N values (9.7 ± 4.8 ‰, n = 5), reflecting their sewage influence. Due to nitrate reduction during subterranean transit, groundwater and coastal waters proximal to deep, near-coast wastewater injection (Kahului) display the highest average δ¹⁵N values encountered (δ¹⁵N_NO3- 18.8 ± 11.1 ‰, n = 19) and moderate DIN concentrations (max 90 µM). Average SGD nutrient fluxes among the different field sites ranged from 700 - 1,500,000 µmol/d/m of shoreline for DIN and 600 – 22,000 µmol/d/m for orthophosphate. These results indicate that areas with the highest N and P fluxes are areas where coastal groundwater flowpaths transect large tracts of sugarcane production. Although highest SGD rates occur near the Kahului wastewater injection wells (~ 2800 m³/day) the relatively low nutrient concentrations in the groundwater end member lead to comparatively modest nutrient fluxes of 270,000 and 13,000 µmol/d/m for DIN and orthophosphate, respectively. This study demonstrates the utility of coupling stable isotope and SGD-radioisotope tracers to (1) determine the source of nutrients in SGD as well as (2) the trajectories and specific fluxes of those nutrients to the coast.