The hyporheic zone composition of a mining-impacted stream: Evidence by multilevel sampling and DGT measurements

Abstract

The importance of the hyporheic zone in influencing stream solute loads and stream ecosystems has been increasingly recognised. The hyporheic zone physicochemical composition of a stream stretch of the Rookhope Burn, a tributary of the River Wear in the North Pennines, UK, affected by historical mining of Pb ore, was characterised at two contrasting flow and temperature regimes. Vertical element concentration gradients were obtained using multilevel samplers down to a depth of 40cm below the water–sediment boundary. Additional in situ Diffuse Gradients in Thin films (DGT) measurements of surface water and pore water were obtained.Circumneutral pH and oxidising conditions characterised the hyporheic zone of the study reach, composed of coarse-textured bed sediments. The surface hyporheic zone (top 15–20cm) was dominated by the chemistry of the overlying water, while interactions with the solid phase were important for some elements in the deeper section of the hyporheic zone.Manganese was attenuated in the hyporheic zone as shown by SEM analysis of Mn-rich grain coatings from the bed sediment. Fine resolution data obtained through the use of the DGT probe deployed in the sediment at one sampling location, showed localised loss in Mn and Zn concentrations at 1.5–4.5cm below the water–sediment interface. There was clear evidence for hyporheic pore water enrichment in Pb, which was unaffected seasonally, while Zn was greater in the summer sampling. These temporal variations of the hyporheic zone composition warrant consideration when accounting for the contribution of disperse inputs to mining impacted catchments, highlighting the need for hyporheic zone studies taking into account differences in seasons.The significance of the observed sediment-scale hyporheic processes on the reach-scale geochemical mass balance was estimated by using surface water geochemical loading calculations. Water metal mass balance from four previous sampling events indicated a constant loss of Mn load, a continuous gain of Pb load and a more temporally variable loss of Zn load for a 700m stream stretch which included the study site. These results closely agreed with the present observations at the stream bed sediment scale, supporting the importance of hyporheic dynamic solute exchanges in affecting surface water quality for the study stream.