Evaluation of chloride and pesticide transport in a fractured clayey till using large undisturbed columns and numerical modeling

Abstract

Abstract. Saturated groundwater flow and tracer experiments using fluorescent dye, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large-diameter (0.5 m) undisturbed columns of fractured clayey till. Hydraulic conductivity of the columns ranged from 10 -s m/s in the shallowest column (1 rn depth) to 10 -7 m/s in the deepest column (4 rn depth) and were similar to field-measured values for these deposits. Results of the tracer experiments are consistent with a conceptual model of advective transport along the fractures combined with diffusion into the finegrained matrix between the fractures. Arrival of the chloride tracer in the effluent was highly retarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic conductivity. The herbicides were more strongly retarded than the chloride at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorption of the herbicides to fracture walls and the matrix solids. The columns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (and the few root holes in the case of the shallowest sample) were active transport pathways, with the dye appearing mainly on the fracture surfaces and as a "rim" in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbicides had also moved into the matrix, apparently by diffusion. Simulations of solute transport with a discrete fracture flow/matrix diffusion model showed that the simulations could be "fit" to the data if all of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primary fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using the measured total porosity as the effective porosity. The simulations could likely be fit with a smaller value of effective porosity, but this would limit applicability to field situations because fitted effective porosity is expected to change with physical scale and residence time of the solute in the soil.

Investigation of the Contamination of Fractured Formations by Non-Newtonian Oil Pollutants

“…Potential remedial actions include: (i) dewatering of the contaminated zone at high pumping rates and removal of the volatile NAPLs through soil vapor extraction (Goldford et al, 1994); (ii) surfactant-enhanced NAPL dissolution and mobilization (Pennell et al, 1994); (iii) in situ bioremediation (Madsen et al, 1993); (iv) steam injection (Keyes et al, 1994). However, the rational design and installation of an appropriate remediation scheme on a contaminated fractured site requires information about the distribution of liquid pollutants throughout the subsurface (Jorgensen et al, 1998). Even though a great deal of work has been focused on the development of very sophisticated numerical codes (Schwartz and Smith, 1988;Jorgensen et al, 1998;Vogel et al, 2000) which forecast the contaminant spreading in fractured reservoirs and are used as tools in risk assessment procedures, there is a lack of fundamental knowledge concerning the¯ow physics in single fractures of complex geometry (Zimmerman and Bodvarsson, 1996).…”

Removal kinetics of organic compounds and sum parameters under field conditions for managed aquifer recharge

“…Within the NASRI project 7 retardation factors were determined during column experiments (Jekel, 2006, Licht et al, 2005. Referenced values for comparable conditions (sandy aquifer, low sedimentary organic carbon) are used for further 6 parameters (Braids, 2001, Broholm et al, 2001, Buss et al, 2006, Grischek et al, 1997, Heberer and Jekel, 2006, Jørgensen et al 1998, Mersmann et al, 2003, Scheytt et al, 2006, Stuyfzand et al, 2007, Tuxen et al, 2000. Eight retardation values are applied based on similarity considerations regarding the substances' structural characteristics (Table 2).…”

On the scope and management of pesticide pollution of Swedish groundwater resources: The Scanian example

“…Likely, fracture flow increases pollution vulnerability via an overall reduction in both travel time and natural remediationpotential relative to matrix flow, thereby allowing for comparatively rapid transfer of infiltrating water and pesticides. This proposal is theoretically valid (Freeze and Cherry 1979) and well-established regarding fracture and macropore flow, and associated pollutant transport in e.g., clay tills (Jorgensen et al 1998;Jarvis 2007). However, few studies comparing pesticide pollution in aquifers dominated by matrix flow to those dominated by fracture flow have been able to support such a hypothesis (Neil et al 1989;Barbash and Resek 1996).…”