Dynamics in surface water chemistry during spring flooding in a temperate river floodplain. Findings from the Biebrza mires, Poland.

Abstract

Dynamics in surface water chemistry during spring flooding in a temperate river floodplain were studied in the Biebrza fens (Poland). Variation in hydrochemistry between years and variation during yearly occurring spring flooding were analyzed using surface water sampling. Meteorological data were used to relate observed dynamics to weather conditions before and during flooding. For analysis of dynamics between years, data was used of 2001-2010. For determination of small-scale variation, a weekly sampling campaign was set up in transects with a high sampling density, during the spring 2012 flooding. The objective was to determine zones with different water types: river water, rain- or snowmelt water and groundwater, and dynamics therein. Additional evidence was gathered with groundwater samples and Electro Conductivity and temperature profiles of the peat soil up to a depth of 1.60 m below the water surface. Results showed that dynamics between years were highest where river water flooded the floodplain and where groundwater discharge occurred. The location of the border between river water and rainwater varied spatially over 250 m and depended on flood magnitude and amount of snow present before flooding. A zone could be identified which was flooded with river water each year independent of flood magnitude. Zones with groundwater discharge showed high variation because of mixing with rainwater at the discharge location. Zones were horizontally stable over the course of four weeks during the flood peak and subsequent drawdown. Only the border between river water and rainwater moved riverwards over 50 m during a rain period. Identification of groundwater discharge zones using surface water sampling is problematic in some cases because of high similarity of local groundwater and rainwater. For these cases, EC and temperature profiles of the peat soil provide enough evidence to determine groundwater discharge zones. Groundwater discharge zones are narrow (< 100 m) and flow of groundwater from these zones on the surface and in the shallow soil enables to characterize significant parts of the Biebrza Lower Basin as throughflow mire with stable water quality instead of inundation mire. The current results show that dynamics in water quality between years are substantial and that variation during flooding can be as large as between-year variation. This should be taken into account when characterizing the system and drawing relations between vegetation and hydrology. Use of small scale sampling and EC and temperature profiles makes identification of groundwater discharge zones possible and proves a useful technique.