Analysis of stream-aquifer relationships: A comparison between mass balance and Darcy's law approaches

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In Mediterranean areas, groundwater contributions are fundamental to maintain stream flow, especially during dry periods. Since groundwater withdrawal may affect stream discharge, understanding the hydraulic connectivity between streams and aquifers is critical to preserve stream runoff. However, assessing stream-aquifer interactions is often complex, due to spatial and temporal heterogeneities and the effect of diverse human pressures on the river system. Potential methods used to estimate this interaction work at different scales. All of them, however, offer complementary results which, in some cases, are difficult to integrate in a conceptual model for the stream hydrological dynamics that becomes useful to assess future water resources management actions.In this study we compare different Darcy's law and mass balance methodologies to analyze their effectiveness in determining these relationships: piezometers drilled in the stream bed were used to qualitatively describe the stream-aquifer relationship and potentiometric maps at small and large scales, respectively; the incremental flow methodology and end member mixing analysis quantified the magnitude and the proportion of surface water, waste water and groundwater contributions to the stream flow; and hydrochemical analysis using Piper diagrams, Cl-/Br- ratios, and δ18O and δ2D identified the main origin of stream water. Finally, principal component analysis satisfactorily describes the hydrogeochemical processes occurring in the stream and aquifer interface and it can be used as a valid approach to support estimations derived from the above mentioned procedures. Different seasonal behaviors, including the effect of bank storage in the stream runoff, could be only recognized by the comprehensive use of all these methods. In this sense, this paper illustrates the need of combining distinct methodologies to achieve an appropriate knowledge on the stream-aquifer interaction ​
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