Groundwater Discharge and Hydrological Connections in a Coastal Fishpond
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The Heʻeia Fishpond is an 800-year old Native Hawaiian aquaculture system that has relied on stream and groundwater inputs of silica, nitrogen, and phosphorus for productivity. Decades of land-use and water management changes in the watershed altered water and nutrient fluxes threatening the pond’s water quality and food-web. In order to quantify fluxes during current variable hydrological and tidal conditions, we contrasted stream and groundwater discharge and nutrient inputs in dry and wet seasons, during and after intense rain events and droughts as well as perigean spring tides. We used spatial radon surveys and temporally continuous radon measurements to better understand groundwater discharge patterns. The largest uncertainty in the radon mass balance models was the pond’s spatially and temporally variable residence time and resulting atmospheric evasion and radon decay correction. Using a triangulation method, we were able to subdivide the pond and evaluate SGD spatial patterns and changes across the different sampling periods in each parcel. We identified SGD hotspots along the landward margin of the pond but also on the outer, ocean side of the pond wall. In addition, temporal analysis revealed that SGD is higher at rising tides and is also higher at spring tides/higher ocean water levels. Contrary to how tidal pumping has been described in the literature, SGD at low tide is limited. Not until the coastal aquifer is lifted by tides do we observe increased SGD both on daily tidal and on neap-spring tidal time scales. The connection of the tidal cycle and the water level in the coastal aquifer affects the water flow paths and seems to be an important driver of SGD into the pond.