Solute dynamics during bank storage flows and implications for chemical base flow separation

Abstract

[1] Chemical base flow separation is a widely applied technique in which contributions of groundwater and surface runoff to streamflow are estimated based on the chemical composition of stream water and the two end-members. This method relies on the assumption that the groundwater end-member can be accurately defined and remains constant. We simulate solute transport within the aquifer during and after single and multiple river flow events, to show that (1) water adjacent to the river will have a concentration intermediate between that of the river and that of regional groundwater and (2) the concentration of groundwater discharge will approach that of regional groundwater after a flow event but may take many months or years before it reaches it. In applying chemical base flow separation, if the concentration in the river prior to a flow event is used to represent the pre-event or groundwater end-member, then the groundwater contribution to streamflow will be overestimated. Alternatively, if the concentration of regional groundwater a sufficient distance from the river is used, then the pre-event contribution to streamflow will be underestimated. Changes in concentration of groundwater discharge following changes in river stage predicted by a simple model of stream-aquifer flows show remarkable similarity to changes in river chemistry measured over a 9 month period in the Cockburn River, southeast Australia. If the regional groundwater value was used as the groundwater end-member, chemical base flow separation techniques would attribute 8% of streamflow to groundwater, as opposed to 25% if the maximum stream flow value was used.

Investigating the spatio-temporal variability in groundwater and surface water interactions: a multi-technique approach

“…This would result in the fluxes from Cl mass balance being too low (McCallum et al, 2012). Bank infiltration will vary as a function of river morphology, aquifer characteristics and changing flow conditions (Chen and Chen, 2003;Chen et al, 2006;Lambs, 2004;McCallum et al, 2010;Woessner, 2000) and is difficult to characterise accurately. While this study sampled near-river groundwater, it is possible that the near-river groundwater along the Tambo River has variable Cl concentrations, resulting in a level of uncertainty in the characterisation of groundwater end members.…”

“…Discrepancies between the Cl and 222 Rn mass balances may also reflect the discharge of relatively young groundwater that has been stored for a period of weeks, either as recently infiltrated rainfall, bank return flow or parafluvial flow ( McCallum et al, 2010;Woessner, 2000). Chemically, such groundwater would have low Cl concentrations but elevated 222 Rn activities through ingrowth .…”

“…It is likely that some discrepancies between the tracer methods result from uncertainties in groundwater end member characterisation, and the sensitivity of the mass balance models to this parameter. It has been shown that interaction between groundwater and surface water near rivers is likely to increase the variability of groundwater chemistry near rivers, making accurate characterisation of the groundwater end member difficult (Lambs, 2004;McCallum et al, 2010). For example, infiltration of river water into the banks at high river discharges may result in near-river groundwater having lower Cl concentrations than the regional groundwater.…”

Channel Representation in Physically Based Models Coupling Groundwater and Surface Water: Pitfalls and How to Avoid Them

“…A major control is the relationship between stream flow and the wetted area (i.e., the area of overland-flow). For example, any deviation from a vertical representation of banks will increase the area of exchange during a stream flow event (Doble et al 2012), and possibly the area adjacent to the unsaturated zone, which tends to slow down the propagation of a flood wave in the shallow subsurface (McCallum et al 2010) (see Figure 3a). As stream flow recedes, smaller bedforms increasingly affect the surface flow field (Horritt et al 2006;Casas et al 2010b;Legleiter et al 2011) (Figure 3b).…”

Using <sup>14</sup>C and <sup>3</sup>H to understand groundwater flow and recharge in an aquifer window

“…Episodic recharge of aquifers by large over-bank floods is also locally important (Moench and Barlow, 2000;Cendón et al, 2010;Doble et al, 2012), particularly in arid areas (Shentsis and Rosenthal, 2003); however, the potential for over-bank events to recharge aquifers in temperate areas is still poorly understood. Additionally, during high flow, water from rivers is likely stored temporarily in the banks (McCallum et al, 2010;Unland et al, 2014); however, the depth and lateral extent to which bank exchange water infiltrates the aquifer is not well documented. Lastly, knowledge of residence times of groundwater in close proximity to the river can provide important information on groundwater-river interactions (Gardner et al, 2011).…”