Modelling spatial variability and uncertainty of cadmium leaching to groundwater in an urban region

Over the last century, soils in the region of Nordenham in northern Germany received high loads of heavy-metals by air-borne immissions from a close-by metal smelter. Based on measured soil properties and cadmium contamination data the leaching of Cd to groundwater was predicted for Nordenham using a numerical transport model based on the convection–dispersion equation. The main objective in this study was to account for the spatial variability and uncertainty of Cd sorption controlling soil properties (pH, organic carbon content) and to analyze their propagation into the variance of area-related model outputs, i.e. Cd breakthrough concentrations at the groundwater surface. For this purpose a nested Monte-Carlo method was combined with deterministic numerical 1D simulations of Cd leaching. The transport model was parameterized without any parameter fitting involved. The validity of the model was verified by retrospective simulations from the initial operation of the smelter until the year of soil sampling. Forecast simulations were run for a period of 200 years. Predicted local scale Cd breakthrough concentrations at the groundwater surface were evaluated by spatial aggregation for single blocks at the field scale, yielding area-related concentrations with associated uncertainties from imprecise knowledge on local soil properties. Significant exceedance of the limit of the German drinking water ordinance of 5 μg L−1 is observed on approximately 90% of the study area with the average point in time of limit value exceedance being the year 2066 and a 90% prediction interval of 2049–2092. At the end of the simulation period, Cd concentrations at the groundwater surface still increase on large parts of the study area. The spatially averaged Cd concentration is 19.89 μg L−1 with a 90% prediction interval of 15.28–24.69 μg L−1. Locally, however, concentrations larger than 60 μg L−1 may be reached. Prediction uncertainty is only moderate and does not question the exceedance of the limit value on the majority of the regarded plots even for spatially averaged concentrations, unless measures to prevent leaching are taken, such as an increase of soil pH by liming.