Abstract
Abstract. Understanding the effects of major hydrogeological controls on hyporheic exchange and bank storage is essential for river water management, groundwater abstraction, restoration and ecosystem sustainability. Analytical models cannot adequately represent complex settings with, for example, transient boundary conditions, varying geometry of surface water–groundwater interface, unsaturated and overland flow, etc. To understand the influence of parameters such as (1) sloping river banks, (2) varying hydraulic conductivity of the riverbed and (3) different river discharge wave scenarios on hyporheic exchange characteristics such as (a) bank storage, (b) return flows and (c) residence time, a 2-D hydrogeological conceptual model and, subsequently, an adequate numerical model were developed. The numerical model was calibrated against observations in the aquifer adjacent to the hydropower-regulated Lule River, northern Sweden, which has predominantly diurnal discharge fluctuations during summer and long-lasting discharge peaks during autumn and winter. Modelling results revealed that bank storage increased with river wave amplitude, wave duration and smaller slope of the river bank, while maximum exchange flux decreased with wave duration. When a homogeneous clogging layer covered the entire river–aquifer interface, hydraulic conductivity positively affected bank storage. The presence of a clogging layer with hydraulic conductivity < 0.001 m d−1 significantly reduced the exchange flows and virtually eliminated bank storage. The bank storage return/fill time ratio was positively related to wave amplitude and the hydraulic conductivity of the interface and negatively to wave duration and bank slope. Discharge oscillations with short duration and small amplitude decreased bank storage and, therefore, the hyporheic exchange, which has implications for solute fluxes, redox conditions and the potential of riverbeds as fish-spawning locations. Based on these results, river regulation strategies can be improved by considering the effect of certain wave event configurations on hyporheic exchange to ensure harmonious hydrogeochemical functioning of the river–aquifer interfaces and related ecosystems.
Highlights
Surface water–groundwater interfaces have recently received growing research interest (Sophocleous, 2002) and have become the focus of multiple water resources management policies (Klöve et al, 2011)
Bank storage increased with lower bank slope (Fig. 7a)
The results presented here suggest that with decreasing bank slope, the contribution of bank storage to the river extends in time, prolonging the falling limb of the river hydrograph
Summary
Surface water–groundwater interfaces have recently received growing research interest (Sophocleous, 2002) and have become the focus of multiple water resources management policies (Klöve et al, 2011). The hyporheic zone that harbours river–aquifer interactions plays a key role in riverine and riparian ecosystem functioning The reactive nature of this zone maintains exchange and transformation of solutes along the pathways between surface water and groundwater. The hyporheic zone is an appreciated habitat for hyporheos, microorganisms and bacteria occupying the space below and along the river channel (Boulton et al, 1998). Besides having negative impacts on the ecosystem of the zone itself, with changes in hyporheic water composition (Calles et al, 2007; Siergieiev et al, 2014c), alteration of the hyporheic functionality due to surface water–aquifer disconnection can severely modify neighbouring ecosystems. Riparian zones and adjacent wetlands may experience changes in groundwater table, water and nutrient fluxes. Restricted hyporheic exchange limits mobilisation of solutes from the ri-
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