Anders G. Christensen

Unsaturated zone leaching models for assessing risk to groundwater of contaminated sites

Risk assessments of sites contaminated with organic contaminants are typically conducted using models that ignore gas phase transport in the unsaturated zone. Here a general approach to developing analytical solutions to multiphase transport is presented. The approach is based on a combined gas and aqueous phase contaminant transport equation. The equation has the same general form as the standard advection-diffusion equation for which many analytical solutions have been derived. Four new analytical solutions are developed using this approach: a three-dimensional solution accounting for infiltration, lateral gas diffusion, sorption and degradation; a simple one-dimensional screening model, and two one-dimensional radial gas diffusion models for use in simulating volatile organic contaminant diffusion in unsaturated soils with an impermeable cover. The models show that both degradation and diffusion are important mechanisms for attenuation of contaminant concentrations at the water table. Finally, model results are compared with field data to illustrate the applicability of the solutions in risk assessment.

ZVI-Clay remediation of a chlorinated solvent source zone, Skuldelev, Denmark: 1. Site description and contaminant source mass reduction

Field investigations on the effects of ZVI-Clay soil mixing were conducted at a small DNAPL source zone with PCE as the parent compound. In a one-year monitoring program, soil samples were collected at three horizontal sampling planes (2.5, 5.0 and 7.5m bgs.). PCE was found to have a pseudo first-order degradation half-life of 47days resulting in more than 99% depletion of the source mass after one year. The main degradation product was ethene, while only low concentrations of the primarily biotic sequential degradation products (cDCE, VC) were detected. The soil mixing resulted in more homogeneous vertical conditions, while the horizontal homogenization was very limited. Iron was delivered in the full targeted depth with an average iron enrichment of 3.1%, and an average decline in the oxidation-reduction potential of more than 500mV. Due to the applied top-down addition of ZVI, the iron content decreased from 4.6% to 2.1% on average over a depth of 5m; hence, there is a potential for optimization of the delivery method. Most in situ technologies are limited by subsurface heterogeneities, whereby the successful dispersion of geological units and contaminants holds great promise for remediation of DNAPL source zones with ZVI-Clay soil mixing.

Characterization of chlorinated solvent contamination in limestone using innovative FLUTe® technologies in combination with other methods in a line of evidence approach.

Characterization of dense non-aqueous phase liquid (DNAPL) source zones in limestone aquifers/bedrock is essential to develop accurate site-specific conceptual models and perform risk assessment. Here innovative field methods were combined to improve determination of source zone architecture, hydrogeology and contaminant distribution. The FACT™ is a new technology and it was applied and tested at a contaminated site with a limestone aquifer, together with a number of existing methods including wire-line coring with core subsampling, FLUTe® transmissivity profiling and multilevel water sampling. Laboratory sorption studies were combined with a model of contaminant uptake on the FACT™ for data interpretation. Limestone aquifers were found particularly difficult to sample with existing methods because of core loss, particularly from soft zones in contact with chert beds. Water FLUTe™ multilevel groundwater sampling (under two flow conditions) and FACT™ sampling and analysis combined with FLUTe® transmissivity profiling and modeling were used to provide a line of evidence for the presence of DNAPL, dissolved and sorbed phase contamination in the limestone fractures and matrix. The combined methods were able to provide detailed vertical profiles of DNAPL and contaminant distributions, water flows and fracture zones in the aquifer and are therefore a powerful tool for site investigation. For the limestone aquifer the results indicate horizontal spreading in the upper crushed zone, vertical migration through fractures in the bryozoan limestone down to about 16-18m depth with some horizontal migrations along horizontal fractures within the limestone. Documentation of the DNAPL source in the limestone aquifer was significantly improved by the use of FACT™ and Water FLUTe™ data.

Assessment of a landfill methane emission screening method using an unmanned aerial vehicle mounted thermal infrared camera – A field study

An unmanned aerial vehicle (UAV)-mounted thermal infrared (TIR) cameras ability to delineate landfill gas (LFG) emission hotspots was evaluated in a field test at two Danish landfills (Hedeland landfill and Audebo landfill). At both sites, a test area of 100 m2 was established and divided into about 100 measuring points. The relationship between LFG emissions and soil surface temperatures were investigated through four to five measuring campaigns, in order to cover different atmospheric conditions along with increasing, decreasing and stable barometric pressure. For each measuring campaign, a TIR image of the test area was obtained followed by the measurement of methane (CH4) and carbon dioxide (CO2) emissions at each measuring point, using a static flux chamber. At the same time, soil temperatures measured on the surface, at 5 cm and 10 cm depths, were registered. At the Hedeland landfill, no relationship was found between LFG emissions and surface temperatures. In addition, CH4 emissions were very limited, on average 0.92-4.52 g CH4 m-2 d-1, and only measureable on the two days with decreasing barometric pressure. TIR images from Hedeland did not show any significant temperature differences in the test area. At the Audebo landfill, an area with slightly higher surface temperatures was found in the TIR images, and the same pattern with slightly higher temperatures was found at a depth of 10 cm. The main LFG emissions were found in the area with the higher surface temperatures. LFG emissions at Audebo were influenced significantly by changes in barometric pressure, and the average CH4 emissions varied between 111 g m-2 d-1 and 314 g m-2 d-1, depending on whether the barometric pressure gradient had increased or decreased, respectively. The temperature differences observed in the TIR images from both landfills were limited to between 0.7 °C and 1.2 °C. The minimum observable CH4 emission for the TIR camera to identify an emission hotspot was 150 g CH4 m-2 d-1 from an area of more than 1 m2.