Origin and mobility of humic colloids in the Gorleben aquifer system

The origin and mobility of humic colloids in the Gorleben aquifer system ha ve been examined. For this purpose, the distribution of humic colloids and relevant hydrological and geochemical parameters were examined. An investig ation area was selected where sediments have been disturbed by salt dome up lift and glacial events. It is shown that, on a local scale, considerable g roundwater movement and intermixing takes place from the surface down to th e salt dome. Consequently no effective separation of groundwater layers occ urs. Two different humic colloid sources are identified: influx from the hu mus horizon with recharge water and continuous in situ generation via miner alization of sedimentary organic carbon (SOC). The in situ generation leads to groundwaters with humic colloid concentrations approaching 0.4 g/L, com pared to concentrations of less than 0.005 g/L in recharge waters. Young gr oundwaters (no C-14 decay detected) between approximately 50 and 200 m dept h exhibit these highly elevated humic colloid concentrations. At greater de pth, salt brines are found with low humic colloid concentrations. This can be attributed to precipitation of humic acid and/or hampering of the in sit u generation process due to the high salt content. There is no indication o f retention or decomposition of humic colloids. The fate of in situ generat ed humic colloids cannot be precisely evaluated due to analytical limitatio ns and insufficient understanding of groundwater movement. (C) 1999 Elsevie r Science Ltd. All rights reserved.