DNAPL TRANSPORT THROUGH MACROPOROUS, CLAYEY TILL COLUMNS

This paper provides the first experimental determination of the rates and distribution of transport of a dense, nonaqueous phase liquid (DNA PL) through a naturally bioporous and fractured clayey till deposit. U ntil now, assessment of DNAPL behavior in this type of deposit has rel ied on theoretical studies. Predictions of DNAPL transport have proven to be uncertain as a result of difficulties in measuring critical par ameters such as DNAPL entry pressure and flow behavior in response to natural fracture/biopore apertures and the degree of interconnection o f these structures, In the present investigation, the migration of fre e product trichloroethylene (TCE) was studied by means of two undistur bed clayey till columns (dimensions: 0.5 m diameter by 0.5 m long) und er in situ effective soil stress conditions. The experiments revealed that transport of TCE was restricted to biopores in one column and fra ctures in another column, bypassing the low-permeability clayey matrix , Effective porosities of the columns, i.e., biopores and fractures, w ere two to three orders of magnitude lower than total porosities (25% and 32%, respectively, for the two columns), i,e,, macropores and matr ix, Single phase water flow rates through the columns at water-saturat ed conditions followed a linear relationship with hydraulic gradient, TCE now could not be predicted from the single-phase calculations beca use of nonlinearity observed between applied TCE injection heads and r esulting TCE flow. TCE flow rates were 24 and 10.3 m/day at TCE gradie nts of 1.18 and 0.91, respectively. The observed flow rates indicate t hat in cases where vertical biopores or fractures fully penetrate clay ey till aquitards, a low-viscosity DNAPL (e.g., TCE) may quickly enter underlying aquifers, The experiments further indicate that 100 liters of a low-viscosity DNAPL are sufficient to contaminate approximately 25 to 100 m(3) of till material because of the small effective porosit y constituted by the biopores and fractures.