Movement and remediation of trichloroethylene in a saturated heterogeneous porous medium 1. Spill behavior and initial dissolution

摘要

An intermediate--scale flow cell experiment was conducted to study the flow of liquid and the transport of dissolved trichloroethylene (TCE) in a saturated, heterogeneous porous medium system. The 1 .67--m long by 1.0-m high by 0.05-m wide flow cell was packed with three layers and five lenses consisting of four different sands. All lenses and layers had horizontal interfaces, except the lowest interface, which was pointed down in the middle. Groundwater flow was imposed by manipulating the water levels in two head chambers. Over 500 ml of dyed TCE was allowed to infiltrate at a constant rate into the porous medium from a narrow source located on the surface. A dual-energy gamma radiation system was used to determine TCE saturations at 1059 locations. Fluid samples were collected from 20 sampling ports to determine dissolved TCE concentrations. The TCE migrated downwards in the form of several relatively narrow (3--8 mm) fingers. Visual observations and measured TCE saturations indicated that the spilled TCE accumulated on top of, but did not penetrate into, fine-grained sand lenses and layers but that some TCE infiltrated into medium-grained sand lenses. This behavior is a result of the different nonwetting--fluid entry and permeability values of the sands. Most of the TCE finally pooled on top of a fine-grained sand layer located in the bottom part of the flow cell. A multifluid code (STOMP f subsurface transport over multiple phases), accounting for TCE entrapment, was used to simulate the movement of liquid TCE. Using independently obtained hydraulic parameter values, the code was able to qualitatively predict the observed behavior at the interfaces of the lenses and sand layers. Simulation results suggest that most of the liquid TCE at the lowest interface was in free, continuous form, while most of the other TCE remaining in the flow cell was entrapped and discontinuous. A simple pool dissolution model was used to predict observed dissolved TCE concentrations. Results show that the measured concentrations could only be predicted with unrealistically high transverse dispersivity values. The observed TCE concentrations are a result of a combination of entrapped and pool dissolution.