Föroreningstransport i grundvatten : En modelljämförelse

Sammanfattning: This thesis compares different methods for risk assessment of a creosote contaminated site in Vansbro, Dalarna. In a complementary study the importance of using representative data for calibration and validation of stationary groundwater models is discussed. The time dependency between surface water levels and groundwater levels on the site in Vansbro is evaluated by step response analysis. The results show that the groundwater levels reflect only long-term variation in the surface water levels. Hence the use of such long-term means for calibration and validation of the stationary groundwater models is the only theoretically defendable alternative. As a first step in the comparison of risk assessment methods the degree of contamination is evaluated on the basis of generic guideline values for contaminated soils, developed by the Swedish Environmental Protection Agency. The guidelines specifically developed for contaminated petrol stations are also taken into consideration. Secondly, a site-specific evaluation is carried out. The transport of contaminants from the soil to the on-site groundwater and nearby surface water is calculated using the mathematical models behind the aforementioned guideline values. The calculated groundwater concentrations show a poor agreement with observed data and the thesis points out several possible explanations of this deviation. A sensitivity analysis is carried out and the influence of biodegradation during the transport in the aquifer is evaluated. The results indicate that the deviation between observed and calculated groundwater concentrations cannot be explained by the absence of biodegradation in the mathematical models, but rather by other simplifications and uncertainties in estimated model parameters. Finally, a risk assessment is made on the basis of observed groundwater concentrations instead of observed soil concentrations as in the assessments above. Observed groundwater concentrations are compared with guideline values for groundwater on contaminated sites. The risk of negative environmental effects in the surface water is evaluated based on the estimated inflow of contaminated groundwater and its concentration of contaminants. The inflow is estimated from the interpolation of observed groundwater levels as well as with two numerical groundwater models of varying hydrogeological complexity: a three-dimensional flow model developed in Visual MODFLOW and a two-dimensional flow model developed in TWODAN. The models are compared with respect to the calculated inflows of contaminated groundwater into the surface water body as well as their ability to reproduce observed groundwater levels. The results indicate that the differences between the models, with respect to the calculated inflows, are of lesser importance to the risk assessment in comparison with the uncertainties associated with other parameters. The choice to base the assessment on observed contaminant concentrations in either the soil or the groundwater is shown to be of crucial importance.