GROUNDWATER DERIVED NUTRIENT INPUTS TO WESTERN COASTAL WATERS OF SAIPAN, COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS

Submarine groundwater discharge (SGD) is frequently a significant source of nutrient inputs to coastal waters. Saipan, the largest of the islands in the Commonwealth of the Northern Mariana Islands (CNMI), has experienced degradation of seagrass beds and corals over the past 50 years, due in part to nutrient pollution. This study aimed to characterize and quantify the spatial and temporal variability in SGD and associated nutrient fluxes along the western coast of Saipan. Radon was measured along the western shore during dry (March) and rainy (August) seasons with a RAD7 radon detector. These surveys indicated one consistent source of SGD and another seasonally variable region of SGD. Four cross-shore transects were sampled for radium, another natural tracer, in regions with elevated radon. Radium activities were measured using a delayed coincidence counter (RaDeCC) system. 223Ra flux was used to calculate SGD; of the measured Ra isotopes, 223Ra had the best fit log-linear decrease along cross-shore transects and declined to negligible levels 100 m from shore. The activities of 223Ra were highest at the shoreline (2.0 – 6.9 dpm/100 L) and decreased in a log-linear fashion to a minimum measured activity 100 m from shore (0.4 – 2.1 dpm/100 L). Nutrients (nitrate, nitrite, ammonium, and phosphate) were analyzed on an EasyChem Plus autoanalyzer. Total dissolved inorganic nitrogen (DIN, sum of nitrate, nitrite, and ammonium) in the coastal ocean ranged from below detection to 0.36 ppm in March and below detection to 0.65 ppm in August. Groundwater DIN ranged from 0.17 – 1.42 ppm in March and below detection – 6.66 ppm in August. The tidal prism method was used in conjunction with a box model approach to quantify SGD. SGD estimated for the region with consistent discharge was 0.06 – 10.9 m3/m/day in March and 0.07 – 10.4 m3/m/day in August. The region of seasonal discharge had a smaller magnitude of 0.02 – 0.57 m3/m/day. Estimated nutrient fluxes, using coastal wells as the groundwater endmember, indicated that SGD can contribute up to 2000 mol N/day in Garapan Lagoon. These results suggest that SGD is an important source of nutrients to Saipan’s coastal waters.