Topic Area: 8b. Groundwater/Surface Water Interactions

Formatted Title:

Understanding the Spreading of a Complex Multi-Component Contamination to the North Sea from a Danish Megasite

Background/Objectives:

The Kaergaard Plantation megasite on the western coast of Denmark represents one of the most difficult remediation challenges in Scandinavia. Disposal of an estimated 280,000 metric tons of pharmaceutical wastes into six waste pits from 1956 to 1973 resulted in the development of a complex mixture of contaminants in soil and groundwater, including sulfonamides, barbiturates, aniline, pyridine, chlorinated solvents (chloroethenes), fuel hydrocarbons, mercury, cyanide, lithium and other compounds. Although the waste pits were excavated down to the water table, residual dense non-aqueous phase liquid (DNAPL) beneath the water table continues to dissolve and impact underlying groundwater. Contaminants in groundwater migrate and discharge into the beach and the North Sea, and public health concerns have prompted the closing of a 1.4-kilometer section of beach at the site. Although remediation efforts employing in situ chemical oxidation (ISCO) and enhanced reductive dechlorination (ERD) have significantly reduced the mass of contaminants in the waste pits, it is essential to establish a set of stop criteria for each waste pit corresponding to acceptable human health exposure criteria for groundwater contact at the downgradient beach area and swimming at the North Sea before remediation and monitoring activities can be terminated in the source areas. To achieve this, groundwater fate and transport and dilution models have been developed. Validation of these models have raised additional questions, particularly regarding inconsistencies with existing theories of saltwater intrusion and challenges in tracking pharmaceutical contaminants.

Approach/Activities:

The site’s geology consists of four distinct units formed during a geological marine regression. The uppermost layer consists of aeolian sands, followed by coarser near-marine sands, silty marine sands with lower permeability, and an impermeable marine clay at the base. The primary compounds of concern, particularly pharmaceuticals and chlorinated solvents, exhibit varying physicochemical properties and concentrations in each of the four most significant waste pits. Chlorinated solvents are predominantly detected in the upper layer, while pharmaceuticals are more concentrated in the deeper layers. According to the literature on saltwater intrusion, groundwater being lighter (less saline) should emerge near the coast and discharge at the foreshore; however, this phenomenon has not been observed for the pharmaceuticals. A comprehensive investigation of the anticipated discharge area at the beach, near-shore porewater and seawater, and the transport pathways through the dunes has been conducted. This investigation utilized various methodologies, including depth-specific water sampling, MiHpt, slug tests, groundwater and seawater samples collected up to 600 m offshore.

Thea Jensen

Affiliation

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Understanding the Spreading of a Complex Multi-Component Contamination to the North Sea from a Danish Megasite