Interaction between shallow groundwater, saline surface water and nutrient discharge in a seasonal estuary: the Swan–Canning system

Abstract

AbstractThe Swan and Canning Rivers converge to form an estuary that is seasonally forced by wet winter and dry summer conditions. The estuary is also tidally 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 interaction of the shallow groundwater system with the Swan–Canning Estuary. The extent 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 to 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 transmitted into the adjacent aquifer (10 to 100 m); and (iii) the scale of the regional groundwater field (>1000 m). Two‐dimensional groundwater flow modelling in plan covering the regionally advected groundwater flow domain of the 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 80 000 m3/day, or about 29 million m3/year. Between 1987 and 1996, the average surface tributary inflow to the Swan River was about 460 million m3/year. Thus groundwater discharge contributed approximately 6% of the total annual river flow. This percentage is clearly small in comparison to the total river flow. However, in the six months from November to April in summer, tributary flow into the Swan River declines sharply to an average total of approximately 12 million m3. Groundwater discharge during this six‐month period is approximately 14 million m3 or about 55% of the surface tributary flow, and thus groundwater is a significant component of the total inflow to the Swan–Canning Estuary during this period. Nutrient concentrations, particularly ammonium, within the sediment pore fluids underlying the river are very high relative to concentrations in the river, such that groundwater discharge rates of this magnitude are capable of introducing significant nutrient loadings to the river. The nitrogen load to the Swan River derived from regionally advected groundwater discharge is conservatively estimated at between 30 and 60 t/year. This ranks groundwater as one of the highest single inputs of nitrogen to the Swan–Canning Estuary when compared to surface water flows. It contributes about 10% of the total nitrogen load entering this reach of the river. Seasonal and tidal variations increase the complexity of the system and may act to increase the presence and availability of groundwater‐derived nutrients in the river and estuary system.Field observations and results from numerical groundwater flow modelling show that groundwater preferentially discharges into the Swan River along the outside of river meanders. Along the inside of meanders, groundwater discharge 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 fluctuations may cause small spatial scale mixing and perturbation of the nutrient‐rich groundwater and sediment porewater with either the river water directly or with fresher groundwater as it discharges to the river. These short‐term fluctuations appear to cause nutrient release from the sediment pore fluids, particularly in summer low flow periods such that they are a significant contributing factor in triggering algal blooms. Copyright © 2001 John Wiley & Sons, Ltd.