Jørn Kristian Pedersen

Distribution of Redox-Sensitive Groundwater Quality Parameters Downgradient of a Landfill (Grindsted, Denmark)

The leachate plume stretching 300 m downgradient from the Grindsted Landfill (Denmark) has been characterized in terms of redox-sensitive groundwater quality parameters along two longitudinal transects (285 samples). Variations in the levels of methane, sulfide, iron(II), manganese(II), ammonium, dinitrogen oxide, nitrite, nitrate, and oxygen in the groundwater samples indicate that methane production, sulfate reduction, iron reduction, manganese reduction, and nitrate reduction take place in the plume. Adjacent to the landfill, methanogenic and sulfate-reducing zones were identified, while aerobic environments were identified furthest away from the landfill. In between, different redox environments, including apparent transition zones, were identified in a sequence in accordance with the thermodynamic principles. The redox zones are believed to constitute an important chemical framework for the attenuation processes in the plume

Correlation of nitrate profiles with groundwater and sediment characteristics in a shallow sandy aquifer

Abstract Sediment profiles of a shallow sandy aquifer in an agricultural area of Denmark can be characterized by a new term called total reduction capacity (TRC), quantifying the total amount of reduced compounds in the aquifer material (ferrous iron, reduced sulfur compounds, solid organic carbon). A distinct increase of TRC was observed at and below the oxidation-reduction front; in this study the front was a zone ∼ 30 cm wide. Oxygen, nitrate and sulfate were present above the oxidation-reduction front but disappeared at the front. Detailed porewater profiles indicate that denitrification also takes place above the front, supposedly in anaerobic microenvironments not evident in the water samples from the small wells. The TRC profiles showed that some reduction capacity is still present above the oxidation-reduction front and is apparently able to support denitrification.

Characterization of an old municipal landfill (Grindsted, Denmark) as a groundwater pollution source: landfill hydrology and leachate migration

The migration of leachate from an old municipal landfill (Grindsted, Denmark) was investigated by intensive mapping of groundwater potentials and groundwater quality at the downstream borders of the landfill and beneath the landfill. A groundwater mound controlling the migration of the leachate into the aquifer was observed beneath the landfill, especially after the wet season The lateral spreading in the leachate plume was significantly increased due to the water table mound present and the seasonal variations of the mound. Also, a significant vertical transport of leachate in the aquifer was observed below the landfill. Detailed informa tion about the spatial and seasonal changes in groundwater flow directions and location of intensive leaching is needed in order to design a cost-effective delineation of the plume in the downstream aquifer and to establish appropriate remedia tion.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 1. Experimental setup, tracer movement, and fate of aromatic and chlorinated compounds

A continuous, natural-gradient field injection experiment, involving 18 xenobiotic compounds and bromide as tracers, was performed in the anaerobic region of a leachate plume downgradient from the Grindsted Landfill, Denmark. The injection lasted for 195 days, and within this period a continuous cloud was established. Over a period of 924 days the cloud movement was monitored in approximately 70 discrete sampling points in the central part of the cloud, and the spatial distribution was described by seven cloud snapshots involving 400–700 sampling points. The bromide cloud movement closely followed the varying flow direction predicted by the water table measurements. Moment analysis showed decreasing tracer flow velocities and reduced capture of bromide mass with time, which may be explained by varying flow conditions (direction, hydraulic gradient) and the heterogeneous geological conditions in the sandy aquifer. Naphthalene, having the highest log Kow value, was the most retarded compound, with a retardation of less than 10%. Therefore sorption was not considered to be a significant attenuation process for any of the compounds studied. Transformation under iron-reducing conditions was observed for toluene, o-xylene, TeCM, 1,1,1-TCA, PCE, and TCE, while transformation of benzene and napthalene was not detected in the aquifer within the time frame of this study. First-order transformation rates were in the range of 0.028–0.039 d−1 and 0.0014–0.0028 d−1 for the aromatic compounds toluene and o-xylene, respectively. The rates for the chlorinated aliphatic compounds, tetrachloromethane, 1,1,1- trichloroethane, tetrachloroethylene, and trichloroethylene, were >0.7 d−1, 0.0044–0.0054 d−1, 0.0012–0.0038 d−1, and 0.0003–0.001 d−1, respectively. Long lag periods and slow transformation rates were observed for some of the compounds, suggesting that lack of transformation reported in the literature may be attributable to short experimental periods in those studies.

Risk-based prioritization of ground water threatening point sources at catchment and regional scales

Contaminated sites threaten ground water resources all over the world. The available resources for investigation and remediation are limited compared to the scope of the problem, so prioritization is crucial to ensure that resources are allocated to the sites posing the greatest risk. A flexible framework has been developed to enable a systematic and transparent risk assessment and prioritization of contaminant point sources, considering the local, catchment, or regional scales (Danish EPA, 2011, 2012). The framework has been tested in several catchments in Denmark with different challenges and needs, and two of these are presented. Based on the lessons learned, the Danish EPA has prepared a handbook to guide the user through the steps in a risk-based prioritization (Danish EPA, 2012). It provides guidance on prioritization both in an administratively defined area such as a Danish Region, and within the bounds of a specified ground water catchment. The handbook presents several approaches in order to prevent the prioritization from foundering because of a lack of data or an inappropriate level of complexity. The developed prioritization tools, possible graphical presentation and use of the results are presented using the case studies as examples. The methodology was developed by a broad industry group including the Danish EPA, the Danish Regions, the Danish Nature Agency, the Technical University of Denmark, and consultants - and the framework has been widely accepted by the professional community in Denmark. The concepts are quite general and can be applied in other countries facing similar challenges.

Redox Buffering Capacity of Aquifer Sediment

The redox potential of an aquifer is a controlling factor for the chemical and microbial transformation of organic and inorganic contaminants. Although the groundwater may contain reduced (S2-, Fe2+, Mn2+, NH 4 + ) and oxidized (SO 4 2- , NO 3 - , NO 2 - , O2) species, the redox buffering capacity is on a volume basis primarily related to the aquifer sediment. The oxidation capacity of an aquifer, e.g. a shallow aerobic aquifer, will act as a buffer against the migration of reduced leachate from a point source rich in organic matter or other reduced compounds and as such limit the migration of the strongly reduced plume. The reduction capacity of an aquifer, e.g. a polluted aquifer, will act as a buffer against oxidized species added in order to increase the aquifer redox potential as part of a remedial action. Reduced species will consume oxidizing agents entering the aquifer until the available reduction capacity is depleted.