Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4d. Large, Dilute and Commingled Plume Case Studies

Formatted Title:

Flow of a Complex Large-Scale Plume, Grindsted Factory Stream 1,200 meters Downstream

Background/Objectives:

Activities at the former Grindsted Factory since 1924 have led to contamination of the groundwater with pharmaceutical substances, primarily sulfonamides and barbiturates, as well as chlorinated ethenes and BTEX. The flow of contamination to Grindsted Stream is controlled by the geological and hydrogeological conditions. The plume sinks to a depth of approximately 80 m below ground level (b.g.) in the multilayered sandy aquifer before it discharges to the stream. Investigations has been initiated and the results will be used in a remediation program to ensure that the contamination flux to the stream complies with regulatory requirements.

The spatial distribution of the three main groups of contaminants varies in the aquifer and maximum concentrations for each contaminant type are found in different parts of the aquifer.

A deeper understanding of the flow and contamination transport conditions between the factory site and the stream is necessary for targeting remediation efforts.

Approach/Activities:

Surface water from Grindsted Stream was sampled at 12 measuring stations in the period 2021 to 2023. Forty-seven MiHPT probes were conducted along the stream with semi-quantitatively estimated concentration values of chlorinated ethenes in autumn 2021. The results of the explorations were used to place 26 monitoring wells along the stream each containing three to six screens. From the drilling sites along the stream and drilling sites in the town, water was sampled and analyzed from approximately 125 screens.

The drilling work was carried out in two phases in 2022. Three rounds of synchronous water level measurements have been performed in the period of August 2021 to October 2022 to determine the hydraulic gradient and direction of flow in the area. In addition, slug tests have been carried out in all new and some existing wells to determine the hydraulic conductivity.

Based on concentration measurements and estimated flow conditions, a conceptual model has been developed for the distribution of the contamination plume between the factory site and the stream.

Results/Lessons Learned:

The results show that the chlorinated ethene is located both in the surface near aquifer and deep in the lower secondary aquifer at the factory site. This indicates that there are still high concentrations close to/on the factory site and that there will be a flux of contamination into the stream for several years, probably >30 years. Below the town, the plume is located medium to deep in the lower secondary aquifer, and near the stream the plume mitigates towards the surface near aquifer before it discharges to the stream.

Overall, the investigations revealed that the highest concentrations of the plume differ spatially for the three main groups of substances. The chlorinated ethene are detected over great depth, probably because PCE and TCE are originally spread over the depth as DNAPL. Due to the origin of the pharmaceutical compounds, they are distributed in other parts of the plume. The location of the discharge areas along the stream of the various contaminant groups, is a crucial input for the preparation of a remediation program.

Cecilie Ottosen

Affiliation

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(Group 1, Poster Board #80) Flow of a Complex Large-Scale Plume, Grindsted Factory Stream 1,200 meters Downstream