A hierarchical approach on groundwater-surface water interaction in wetlands along the upper Biebrza River, Poland

Abstract

Abstract. Groundwater-surface water exchange studies on natural rivers and wetlands dominated by organic soils are scarce. We present a hierarchical approach to quantitatively investigate and interpret groundwater-surface water interaction in space and time by applying a combination of different field methods including piezometer nests, temperature and seepage measurements. The numerical 1-D heat transport model of STRIVE is used in transient mode to calculate vertical fluxes from thermal profiles measured along the upper Biebrza River, Poland over a period of nine months. The calculated fluxes show no clear spatial pattern of exchange fluxes unless an interpolation of the point estimates on a reach scale is performed. Significance of differences in net exchange rates versus morphological features are investigated with statistical tests. Time series of temperature and hydraulic head of the hyporheic zone are used to estimate the temporal variability of the groundwater-surface water exchange. Seepage meter measurements and slug tests were used for cross validation of modelled fluxes. Results show a strong heterogeneity of the thermal and physical soil properties along the reach, leading to a classification of these parameters for modelling purposes. The groundwater-surface water exchange shows predominantly upward water fluxes, however alternating sections of recharge exist. The exchange fluxes are significantly different dependent on the position of the river in the valley floor and the river morphology where fluxes are more dependent on hydraulic gradients than on river bed conductivity. Sections of higher fluxes are linked to the vicinity of the morainic plateau surrounding the rivers alluvium and to meanders, indicating that a perspective on the fluvio-plain scale is required for interpreting the estimated exchange fluxes. Since the vertical component of the exchange fluxes cannot explain the magnitude of the change in river discharge, a lateral flow component across the alluvial plain has to be responsible. The hierarchical methodology increases the confidence in the estimated exchange fluxes and improves the process understanding, however the accuracy of the measurements and related uncertainties remain challenges for wetland environments.