Steady state plumes : transverse mixing in aquifers, numerical experiments and field scale predictions for natural attenuation

Scenario specific modelling to predict the length of steady state contaminant plumes in groundwater was performed. Small transverse vertical dispersivities [agr ]tresult in extremely steep concentration profiles of solutes in groundwater and were found to be a controlling mechanism for the external supply of electron acceptors to plumes of easily biodegradable organic compounds. Scenario specific modelling using the codes MIN3P and BIONAPL as well as a simpler 1-D model yielded an empirical relationship for the plume length, depending on [agr ]t, the aquifer thickness M and reaction stoichiometry. For simple conditions (homogeneous aquifer, fully contaminated aquifer thickness), the simulation of the 2-D scenario in a vertically oriented 1-D model was facilitated. Thus, if [agr ]t is known, the length of plumes of easily biodegradable contaminants in groundwater can be predicted. These results were tested at the field site "Osterhofen", situated on periglacial gravel deposits from the last alpine glaciation period in southwest Germany. An ammonium plume extending from a former landfill was investigated using integral measurements at two consecutive control planes. Fitting the measured mean concentrations yielded [agr ]t = 3.2 cm and the plume was found to cease after 570 m, proving Natural Attenuation as an appropriate remediation measure at the site.