Interaction between shallow groundwater, saline surface water and nutrientdischarge in a seasonal estuary: the Swan-Canning system

The Swan and Canning Rivers converge to form an estuary that is seasonally, forced by wet winter and dry summer conditions. The estuary is also tidall y forced due to its contact with the Indian Ocean. The perception that the occurrence of nuisance algal blooms has increased in frequency and severity in recent years has prompted the present investigation into the interactio n of the shallow groundwater system with the Swan-Canning Estuary. The exte nt to which this interaction contributes to nutrient delivery to the river is a focus of the work. Groundwater interaction with the upper reaches of the Swan River is shown t o occur at three length scales: (i) the scale of the river-bed sediments (i .e. < 10 m); (ii) the scale over which tidal forcing of the river is transm itted into the adjacent aquifer (10 to 100 m); and (iii) the scale of the r egional groundwater field (> 1000 m). Two-dimensional groundwater flow mode lling in plan covering the regionally advected groundwater flow domain of t he upper Swan River Estuary from the Causeway to Guildford shows that there is a net groundwater discharge to the Swan River of groundwater discharge of about 80000 m(3)/day, or about 29 million m(3)/year. Between 1987 and 19 96, the average surface tributary inflow to the Swan River was about 460 mi llion m(3)/year. Thus groundwater discharge contributed approximately 6% of the total annual river flow. This percentage is clearly small in compariso n to the total river flow. However, in the six months from November to Apri l in summer, tributary flow into the Swan River declines sharply to an aver age total of approximately 12 million m(3). Groundwater discharge during th is six-month period is approximately 14 million m(3) or about 55% of the su rface tributary flow, and thus groundwater is a significant component of th e total inflow to the Swan-Canning Estuary during this period. Nutrient con centrations, particularly ammonium, within the sediment pore fluids underly ing the river are very high relative to concentrations in the river, such t hat groundwater discharge rates of this magnitude are capable of introducin g significant nutrient loadings to the river. The nitrogen load to the Swan River derived from regionally advected groundwater discharge is conservati vely estimated at between 30 and 60 t/year. This ranks groundwater as one o f the highest single inputs of nitrogen to the Swan-Canning Estuary when co mpared to surface water flows. It contributes about 10% of the total nitrog en load entering this reach of the river. Seasonal and tidal variations inc rease the complexity of the system and may act to increase the presence and availability of groundwater-derived nutrients in the river and estuary sys tem. Field observations and results from numerical groundwater flow modelling; s how that groundwater preferentially discharges into the Swan River along th e outside of river meanders. Along the inside of meanders, groundwater disc harge is either very low or at times the flow direction reverses such that saline river water recharges the aquifer during the late spring, summer and early autumn months. Short-term, high-frequency fluctuations such as wave- induced displacement, seawater intrusion, hyporheic flux and tidal fluctuat ions may cause small spatial scale mixing and perturbation of the nutrient- rich groundwater and sediment porewater with either the river water directl y or with fresher groundwater as it discharges to the river. These short-te rm fluctuations appear to cause nutrient release from the sediment pore flu ids, particularly in summer low flow periods such that they are a significa nt contributing factor in triggering algal blooms. Copyright (C) 2001 John Wiley & Sons, Ltd.