Nutrient flux through soils and aquifers to the coastal zone of Guam (Mariana Islands)

Abstract

Nitrification enriches terrestrial soil waters with nitrate, and other solutes also leach rapidly (within hours to days after saturating rain) through Guam’s northern karst plateau into a largely unconfined carbonate aquifer system. Nutrient chemistry and discharge of these enriched aquifer waters into the inter‐ and subtidal zone was measured to evaluate the importance of this flux to the coastal nutrient regime of the island. Aquifer waters mix with seawater in the coastal transition zone, produce about a 10% seawater mixture, and leak year‐round through cracks and fissures and from seeps in beaches around the entire perimeter (57 km) at rates of 2.2–110 m3 (m of shoreline)−1 d−1 (avg, 5.1 m3 m−1 d−1). The theoretical maximum rate can average. 14 m3 m−1 d−1, which is equal to net annual aquifer recharge. At the shoreline, the discharged aquifer waters average 3.14‰ salinity, 96±21 µM NO3−, 28±6.8 µM Si, 0.85±0.26 µM P, and 0.18±0.19 µM Fe (n = 238). Deep seawater under the aquifer averages 33.8‰ salinity, 8.5±2.1 µM NO3−, 7.2±1.8 µM. Si, and 0.05±0.04 µM Fe (n = 47). The aquifer system can potentially discharge 1,340, 390, 12, and 2.5 µmol m−1 d−1 of NO3−, Si, P, and Fe out to a distance of 1 km from shore, which enriches surface seawater by about 20 times the ambient concentration per day. In general, natural flux of nutrients in aquifer waters from tropical carbonate islands will increase as a function of island diameter, aquifer recharge, head, the thickness of the soil layers in which remineralization occurs, and as the acquisition of solutes from subterranean volcanics above sea level increases.