Heavy Metal Concentration Variability Of Pore Water In Sediment And Its Response To Hyporheic Water Exchange

Hyporheic zone is a saturated zone of surface water-groundwater interaction,which regulates the physical and chemical processes of surface water-groundwater systems and can be an important source for transport,degradation and transformation of contaminants and heavy metals in river ecosystems.In this study,upstream and downstream sites in the Juehe River located in Xi'an,Shaanxi Province,China were selected to conduct some experiments for streambed sediment hydraulic conductivity(Kv)and vertical hydraulic gradient(VHG),as well as the determination of heavy metal in pore water,sediment grain size analyze.Moreover,the hyporheic water exchange fluxes and the diffusive fluxes were calculated by Darcy's law and Fick's law,respectively.The spatial-temporal distribution patterns of hyporheic water exchange fluxes,heavy metal concentrations in pore water and diffusive fluxes were analyzed.Combined with sediment grain size distribution curve method and related geostatistical methods,the relationships among the hyporheic upward water fluxes,sediment grain size,heavy metal concentrations in pore water were analyzed to further reveal the response of the concentration variation of heavy metal in pore water to hyporheic water exchange.The conclusions were summarized in this following:1)The spatial-temporal heterogeneity of Cu and Zn concentrations in pore water were derived in hyporheic zone.In summer,Cu and Zn concentrations in pore water were higher than those in surface water and groundwater.The average Cu concentration in pore water of upstream was higher than that in downstream,while the average Zn concentration in pore water in downstream was three times higher than in upstream.In winter,the Cu and Zn concentration in surface water was significantly higher than that in groundwater and pore water.The average Zn concentration in pore water of upstream was higher than that in downstream,while the average Cu concentration in pore water in downstream was significantly higher than that in upstream.Meanwhile,the seasonal variability of Cu and Zn concentrations in pore water were observed in the Juehe River.The Cu and Zn concentrations in pore water in summer were higher than those in winter.2)The vertical variability of Cu and Zn concentration in pore water were obtained in the different reaches of Juehe River.In summer,the Cu and Zn maximum concentration in pore water were observed in the depth of 0-0.15 m,showing a trend of decreasing with the sediment depth.In winter,the Cu concentrations in pore water in upstream were observed in the depth of 0.15-0.30 m,showing a trend of increase firstly and then decrease.The Cu concentrations in pore water in downstream were decreasing with the increase of sediment depth.But the Zn concentration exhibited a trend of increase firstly and then decrease.3)The spatial-temporal variability of hyporheic water exchange fluxes were obtained in the different reaches of Juehe River.The hyporheic exchange patterns of most measurement points were upward flux,and the higher water exchange flux and streambed Kv were observed in upstream,which presented the decreasing trend from upstream to downstream of the river.Meanwhile,the significantly seasonal variability of hyporheic water exchange fluxes were obtained in winter and summer.The higher water exchange fluxes occurred in summer compared with the winter.4)The significant lateral spatial variation of hyporheic water exchange fluxes and streambed Kv values were observed in the channel cross-section perpendicular to the flow direction.The higher upward water exchange fluxes and streambed Kv values appeared in the center closed to the left erosional bank of the channel.The lower upward water exchange fluxes and streambed Kv values occurred in the right depositional bank of the channel in summer.In addition,higher upward water exchange fluxes and streambed Kv values in upstream were obtained in the center near the right erosional bank and the lower upward water exchange fluxes and streambed Kv values in the left depositional bank of the channel in winter.The higher upward water exchange flux and streambed Kv values in downstream were observed in the center of channel,as well as the downward water exchange fluxes and lower streambed Kv values in the right depositional bank.The sediment grain size was important factor affecting the spatial heterogeneity of hyporheic water exchange fluxes and streambed Kv values.The large sediment heterogeneity lead to the higher water fluxes.5)Hyporheic water exchange flux and sediment grain size were important factors affecting the Cu and Zn concentrations variability in pore water.In summer,higher Cu and Zn concentrations occurred near the channel center with coarse sand and gravel and greater upward water exchange fluxes because coarser sediment and greater upward water fluxes cause stronger metal desorption capacity.Lower Cu and Zn concentrations were obtained near the right depositional bank of the two sites,where lower upward water exchange fluxes generally occurred.It exhibited the similar lateral spatial distribution patterns in winter.In addition,there was a significant positive correlation between the upward water exchange fluxes and the metal diffusive fluxes.6)It was examined that the significant positive correlation of the single metal concentration among the river water,pore water and groundwater were obtained,indicating that they were homologous and transformed with each other under certain conditions.The metal concentrations in pore water had significant correlation with the electrical conductivity,redox potential,dissolved oxygen and pH,suggesting that the spatial and seasonal differences of water quality parameters could be one of driving factors to inducing the variation of metal concentrations in pore water.