Parameter Sensitivity  in  the  Predication of DNAPL Infiltration and Redistribution in Heterogeneous Porous Media
Creators: name: Illangasekare, Tissa
name: Mathew, Mini
name: Fagerlund, Fritjof
name: Niemi, Auli
name: Illangasekare, Tissa
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: Subsurface heterogeneity has a significant influence on infiltration and distribution of  DNAPLs. Numerical 
models of multiphase flow are limited by their ability to predict behavior of DNAPLs in the field due to lack of 
accurate data on characterization of heterogeneity.  However, these models help to obtain insights into the 
complex behavior that will be useful in designing effective remediation schemes.  A number of multiphase 
flow codes that have the ability to capture the fundamental processes that control the migration of NAPLs 
exist. However, the issue of which constitutive model and parameters need to be used in simulating DNAPL 
behavior at various scales of interest is not studied satisfactorily.  With the goal of contributing to the 
knowledge needed to address this issue, an experimental study was conducted to generate an accurate data 
set on the migration of DNAPLs in a synthetic aquifer with various combinations of heterogeneous/
homogenous configurations. This paper presents the results of a study where the data from these 
experiments was used in combination with an existing multiphase code UTCHEM9 to evaluate constitutive 
model sensitivity to prediction of both infiltration and redistribution.  In the experiment, a tank with 
dimensions 0.71 m x 0.53 m x 0.04 m was packed with two layers. The layer in which the spill occurs was 
packed using five well-characterized sands to represent a spatially correlated random field with geostatistical 
parameters; Lnk of 22.5 (k in m2) and variance of 1. Second layer was homogenously packed with a single 
sand. The interface between the two layers had a slope of 3.5º.  NAPL was injected as point source and 
saturation distribution was accurately monitored using an automated x-ray attenuation system.  Modifications 
to the code were made to incorporate hysteresis and accommodate layer inclinations. The model simulations 
were conducted for two phases of the experiment; during NAPL injection and re-distribution for the following 
cases: (1) Brooks Corey drainage model during the injection and redistribution, without hysteresis and 
entrapment, (2) Brooks and Corey drainage model during injection and imbibition during redistribution, 
without hysteresis and entrapment,  (3) Parker and Lenhard model with hysteresis, (4) Parker and Lenhard 
model without hysteresis, and (5) Brooks and Corey drainage model with trapping during injection and 
redistribution. The analysis shows during injection period, all of the above models were able to match the 
experimental observations reasonably well. The models without trapping and hysteresis effect were also able 
to match the experimental results. During the redistribution period, none of the five models were able to 
reproduce the experimental results.  This preliminary analysis suggests that further detailed study is needed 
to determine which parameters control redistribution of NAPL. Also it is observed that entry pressure is the 
most important parameter of the constitutive relations; residual saturations of wetting and non-wetting fluids 
have minor effects during infiltration and redistribution.