Nutrient attenuation by a riparian wetland during natural and artificial runoff events

Due to chronic nutrient enrichment of surface water, wetlands adjacent to land managed with fertilizer have been studied to determine their role in nutrient dynamics. We sampled golf course runoff and determined the loads of NO3- and PO4(-3) transported during storms and the attenuation of those loads when runoff passed through a riparian wetland. All sampled storm events contained NO3- (2 to 1470 g NO3-N per event) and PO4(-3) (1 to 4156 g PO4-P per event). Extensive nutrient attenuation occurred when water passed through the riparian wetland. In 11 events, NO3- and PO4(-3) attenuation averaged 80 and 74%, respectively. In subsequent experiments, we created a stream of water flowing into the wetland and amended it with NO3-, PO4(-3) and Br-, creating an artificial runoff event. The experiments were conducted using conditions similar to those of natural runoff events. We observed rapid and complete attenuation of PO4(-3) immediately after runoff water infiltrated into the wetland subsurface. No PO4(-3) was observed in discharge from the wetland. Nitrate attenuation occurred following a lag phase of several hours that was probably due to reactivation of denitrifying enzymes. Nitrate attenuation was initially less than 60% but increased to 100% in all experiments. We observed extensive dilution of runoff water in the wetland subsurface indicating mixing with pre-event ground water in the wetland. The results indicated that intermittent inputs of NO3- and PO4(-3) could be successfully attenuated in the wetland on the time scale of natural storm events.