FLOW AND TRANSPORT SIMULATIONS OF UNSATURATED ZONE FOR QUANTITATIVE GROUNDWATER VULNERABILITY ASSESSMENTS

The hydrogeologic properties of unsaturated zone are the most important factor for groundwater deterioration induced by surface contamination. Overlay-and-index type approaches are the most widely applied technique for the assessment of the vulnerability. In the most conventional methods (ie. DRASTIC, SINTACS, etc.), the ratings and weights related to the unsaturated zone properties are apt to be qualitative by choosing an arbitrary integer between 1 through 10 based on the order of permeability, which may cause biased results. The objective of this study is to build up the quantitative database where the behavior of water flow and the contaminant transport is simulated based on the unsaturated zone characteristics. For the purposes, models under saturated and unsaturated conditions are composed and the unsaturated zone is assumed to be composed of soil which properties are acquired from a previously developed random function. The depth to water of all models is set to 1 m. For each soil type, twenty random models are generated and simulated, and the time when the groundwater concentration reaches to 10% surface concentration (t_0.1) is set up as an effective contamination measure. The results show perfect linear correlation between the permeability and t_0.1 in the saturated soil. From the unsaturated soil, the correlation is non-linear and the models of high clay containing soil do not reach to t_0.1 within valid time. Excluding the high clay containing models, the results also show that silty loam required the longest time of 10.8 days and sand, sandy loam, loamy sand takes less than 0.02 days. Loam and silt takes about 1.5 and 7.7 days, respectively, to reach t_0.1. We expect that the result of this study may contribute to the complement of rating index for the conventional vulnerability models conditioned on the homogeneity assumption used.