Abstract Conceptual models of the fracture networks in shale were evaluated at a site contaminated with chlorinated solvents. Prior borehole testing in eight holes under open hole ambient and pumping conditions identified 14 flow zones (140 m bedrock interval) with zero to five zones per hole. Cross-hole testing showed only a few cross-connections between transmissive fractures. The initial conceptual model thus featured a sparse fracture network with few dominant fractures. Detailed profiles (hydraulic head, rock core volatile organic compounds, groundwater volatile organic compounds from packer and multi-level sampling, cross-hole multi-level monitoring of permanganate injections) were collected from several holes and indicated a well-connected fracture network with many hydraulically active fractures not influenced by open hole cross-connection. This contrasting conceptual model contained numerous well-connected horizontal and vertical fractures that allowed chlorinated solvents to penetrate the upper 50–60 m of bedrock as dense non-aqueous phase liquids, followed by diffusion-driven mass transfer from fractures into the porous rock matrix, such that nearly all the contaminant mass resided as dissolved and sorbed phases, measurable in rock core without cross-contamination during drilling. The difference in the two conceptual models has important implications for source zone and plume attenuation.

Abstract This paper reports a long-term field investigation of a fractured dolostone aquifer that was penetrated by a dense non-aqueous phase liquid. High-resolution source zone characterization shows the evolution of deep penetration to the back-diffusion conditions seen at the present day. Metolachlor, a common herbicide, was released into the overburden overlying a fractured dolostone aquifer within a short time window (1978–81). In 2000, the plume front arrived at a municipal supply well located 930 m down-gradient, increasing to a maximum concentration of 2 μg l −1 . Groundwater monitoring with high-resolution, depth-discrete multi-level sampling systems since 1992 shows a clearly delineated bedrock plume. Numerous rock core samples show metolachlor in the low-permeability rock matrix at the bottom of the aquifer. The mass distribution and bedrock hydraulic head pattern strongly suggest that metolachlor entered the bedrock as a free-phase dense non-aqueous phase liquid penetrating to the aquifer bottom, preferentially accumulating in some horizontal fractures, dissolving quickly as a result of the rapid groundwater flow and then diffusing into the rock matrix, where back-diffusion sustains a dilute, persistent and stable plume. Strong plume retardation by matrix diffusion and sorption has greatly mitigated the impact on water quality in the down-gradient supply well, allowing for its continued use, while back-diffusion and degradation maintain a persistent, dilute plume managed by appropriate monitoring.

The results of seven case studies by the author, colleagues, and graduate students performed at contaminated sites indicate that monitoring wells and piezometers installed in karstic carbonate aquifers often yield unreliable data. These devices more often gave misleading than useful information concerning aquifer properties, groundwater flow, and contaminant movement. These findings are in accord with the highly anisotropic and heterogeneous nature of these aquifers. The following cautions are provided when monitoring wells are to be used in karstic carbonate regions. (1) Monitoring wells may be unreliable in detecting contaminant releases. (2) A monitoring well that detects a contaminant is unlikely to provide valid data regarding the quantity of the release or the velocity and direction of the contaminant movement. (3) Water levels measured in wells often give erroneous indications of groundwater flow direction. (4) Well water levels and chemical parameters taken at random or traditional quarterly calendar intervals give little insight into the fluctuations that may actually occur in the well. (5) Head fluctuations in wells in response to nearby pumping or injection do not necessarily indicate flow connections. (6) Traditional well tests in carbonate aquifers typically do not sense the most important elements of the permeability structure. (7) Virtually every well in a carbonate aquifer is influenced by a unique suite of permeability and recharge elements. In spite of their manifest shortcomings in carbonate aquifers, monitoring wells are specified by law in virtually every case where contaminants may be or have been released. Unfortunately, these wells are usually placed using criteria appropriate for granular aquifers. Alternative and more appropriate means of aquifer assessment and monitoring in these aquifers are available, including wells augmented with tracer investigations and the use of springs and other access points to the conduit elements of the porosity system.