Consequences of Preferential Flow in Cracking Clay Soils for Contamination-risk of Shallow Aquifers

摘要

A simple method is presented to assess, on a regional scale, the contamination-risk of shallow aquifers covered by cracking clay soils, with special emphasis on preferential flow of contaminants through shrinkage cracks. A water extraction area of several hundred hectares is divided into units with homogeneous soil types and hydrological conditions. For each unit, a one-dimensional computer simulation model is used to compute residence times in the unsaturated zone of the cracking clay soils. Analytical equations are used to estimate residence times in the saturated zone. Finally, a classification system is proposed to summarize and visualize the model computations in a simple way. The developed method is illustrated with a case study from the Netherlands. It is known how contamination-risk maps of water extraction areas can be obtained. Policies to prevent contamination of the aquifer can be focused on the most vulnerable areas as indicated on such a map. The contamination-risk maps, obtained using the present method, deviate sharply from the traditional, more or less concentric protection zones, computed with saturated groundwater flow models.