Factors controlling the distribution and transport of trichloroethene in a sand aquifer: Hydrogeology and results of an in situ transport experiment

Groundwater underlying the Perth metropolitan area was found to be contaminated by extensive plumes of trichloroethene (TCE), ammonium and sulphate, and regions of high dissolved organic carbon concentration. At the study site, approximately 600 m downgradient of the presumed source area, depth-specific sampling in the vertical plane across the plumes showed TCE concentrations up to 1300 μg L−1, and ammonium and sulphate concentrations up to 90 and 470 mg L−1, respectively. The plumes were approximately 100-150 m wide and between 3-9 m thick. No significant degradation of TCE was observed. Also, depth- specific sampling suggests that separate TCE plumes are present within the shallow aquifer, which may be indicative of multiple TCE sources. Depth-specific sampling enabled a better definition of the plumes which is important for possible risk analysis or remediation. To determine the transport characteristics of the TCE plume, a natural gradient field transport experiment using deuterium labelled TCE (TCEdl) and bromide was carried out over a displacement distance of 13 m. TCEdl was stable and non- retarded. No in situ transport experiment making use of deuterated TCE has been reported previously. Also, no previous field experiment has shown TCE to move conservatively. Modelling of the field transport experiment indicated a longitudinal dispersivity of 0.005 m which is the lowest longitudinal dispersivity ever reported for the scale of the experiment (i.e. for a vertical interval of 0.8 m and a horizontal distance of 13 m). Although not measured, the vertical transverse dispersivity is expected to be less than the longitudinal dispersivity. The aquifer at the site, therefore, is characterised by exceptionally little heterogeneity. No aquifer, however, is homogeneous, and the tracer experiment revealed the aquifer to be hydraulic conductivity stratified. The data suggest that the longitudinal dispersivity, estimated from depth integrated concentrations, will increase with both the vertical and the horizontal scale and is not expected to approach a constant value. Hydrogeological investigations indicated that horizontal transverse spreading of the plume was negligible since annual variations in the flow direction (±3°) were minor. Therefore, any significant transverse spreading of the plumes must result from disturbances of the mean advective flow (e.g. due to localised recharge or extensive pumping). These effects are site-specific, preventing the use of generalised dispersion coefficients. Together with the results of the transport experiment, these data indicate that TCE within the acquifer moves as a conservative compound, is highly mobile, and that high concentrations will be preserved to substantial distances downgradient of the source areas.