Restoration of the Rivers Brede, Cole and Skerne: a Joint Danish and British EU-LIFE Demonstration Project, IV - Implications for Nitrate and Iron Transformation

1. The possible effects on the hydrological and biogeochemical processes in the River Brede valley were studied from August 1994 to August 1996 based on measurements in piezometers installed along four transects across the river valley and two river monitoring stations located immediately upstream and downstream of the restored reach.

2. Groundwater discharge to the river varied considerably both along the restored river reach and from bankside to bankside. Comparison of the water balance derived from two river monitoring stations and the groundwater balance for the restored part of the river valley, based on Darcy's equation, indicated that a deep-lying regional aquifer probably discharges to the river in the restored area.

3. The nitrate balance for the floodplain revealed that 92 kg NO₃-N ha^-1 year^-1 was removed during passage through the river valley, probably as a result of pyrite oxidation. In contrast, iron leaked from the floodplain to the river at the rate of 400 kg Fe ha^-1 year^-1.

4. A prolonged dry period, starting four months after completion of the restoration work and lasting for the remainder of the study period, makes it difficult to conclude whether the results obtained are reflective of river and floodplain restoration. Nitrate concentration measurements at the two river monitoring stations revealed no overall significant changes when comparing a pre-restoration period with two similar post-restoration periods. However, comparison of nitrate losses from an upstream control catchment and the restored reach catchment indicated enhanced removal of nitrate along the restored river reach during a three month period of flooding immediately following completion of restoration work (January to March 1995).