Abstract Investigations of geology, hydrogeology, and ground water chemistry in the aquifer downgradient from Sjoelund Landfill, Denmark, formed the basis for an evaluation of natural attenuation as a remediation technology for phenoxy acid herbicides at the site. Concentrations of phenoxy acids were up to 65 μg/L in the ground water, primarily 4‐chlor‐2‐methylphenoxypropionic acid (MCPP) and 2,4‐dichlorophenoxypropionic acid (dichlorprop). Mass removal of the phenoxy acids was shown within 50 to 100 m of the landfill by calculation of contaminant fluxes passing transects at three distances. There was accordance between increasing oxygen concentrations and decreasing phenoxy acid concentrations with distance from the landfill, indicating that aerobic degradation was a major mass removal process. Presence of high concentrations of putative anaerobic phenoxy acid metabolites suggested that anaerobic degradation was also occurring. Laboratory degradation experiments using sediment and ground water from the aquifer supported aerobic and anaerobic degradability of MCPP at the site. It was concluded that natural attenuation may be applicable as a remedy for the phenoxy acids at the Sjoelund Landfill site, although uncertainties related to calculations of chloride and phenoxy acid fluxes at a complex site and identification of specific in situ indicators were encountered. Thus, there is a pronounced need for development and broader experience with evaluation tools for natural attenuation of phenoxy acids, such as specific metabolites, changes in enantiomeric fractions, compound‐specific stable carbon isotope ratios, or microbial fingerprints.
Abstract Mapping high permeability sand occurrences in clayey till is fundamental for protecting the underlying drinking water resources. Crosshole ground penetrating radar (GPR) amplitude data have the potential to differentiate between sand and clay, and can provide 2D subsurface models with a decimeter‐scale resolution. We develop a probabilistic straight‐ray‐based inversion scheme, where we account for the forward modeling error arising from choosing a straight‐ray forward solver. The forward modeling error is described by a Gaussian probability distribution and included in the total noise model by addition of covariance models. Due to the linear formulation, we are able to decouple the inversion of traveltime and amplitude data and obtain results fast. We evaluate the approach through a synthetic study, where synthetic traveltime and amplitude data are inverted to obtain slowness and attenuation tomograms using several noise model scenarios. We find that accounting for the forward modeling error is fundamental to successfully obtain tomograms without artifacts. This is especially the case for inversion of amplitude data since the structure of the noise model for the forward modeling error is significantly different from the other data error models. Overall, inversion of field data confirms the results from the synthetic study; however, amplitude inversion performs slightly better than traveltime inversion. We are able to characterize a 0.4–0.6 m thick sand layer as well as internal variations in the clayey till matching observed geological information from borehole logs and excavation.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Accounting for modeling errors in linear inversion of crosshole ground-penetrating radar amplitude data: detecting sand in clayey tillAuthorsBoletteBadsberg JenseniDThomas MejerHanseniDLarsNielseniDKnud SkouCorduaNinaTuxenAikateriniTsitonakiMajken CarolineLoomsSee all authors Bolette Badsberg JenseniDCorresponding Author• Submitting AuthorDepartment of Geosciences and Natural Resource Management, University of CopenhageniDhttps://orcid.org/0000-0003-4378-1209view email addressThe email was not providedcopy email addressThomas Mejer HanseniDAarhus UniversityiDhttps://orcid.org/0000-0003-4529-0112view email addressThe email was not providedcopy email addressLars NielseniDDepartment of Geosciences and Natural Resource Management, University of CopenhageniDhttps://orcid.org/0000-0003-2024-2288view email addressThe email was not providedcopy email addressKnud Skou CorduaDepartment of Geosciences and Natural Resource ManagementUniversity of Copenhagenview email addressThe email was not providedcopy email addressNina TuxenCapital Region of Denmark, Center for Regional Developmentview email addressThe email was not providedcopy email addressAikaterini TsitonakiWSP Denmark A/Sview email addressThe email was not providedcopy email addressMajken Caroline LoomsDepartment of Geosciences and Natural Resource Management, University of Copenhagenview email addressThe email was not providedcopy email address
