A current report shows: The Landgraben is contaminated with pollutants. Removing them could contaminate the groundwater.
A clean-up of the polluted Landgraben between Darmstadt and Trebur is currently out of the question. This is the result of a study presented by Environment Minister Priska Hinz (Greens) on Wednesday.
Almost four years ago, the case of an organic farmer from the Groß-Gerau district caused a stir: Because he irrigated his spinach with groundwater in the dry summer of 2018, it was contaminated with the pesticide Dikegulac and the farmer was not allowed to sell the vegetables. At that time, the Landgraben also came into focus: this river, designed to drain Darmstadt, has a connection to the groundwater. After a study in 2019 showed that there was hardly any dikegulac in the watercourse, the study now presented was commissioned in 2020 – this time on the pollution situation and rehabilitation options of the Landgraben.
Poison in the lower layers
The report by the planning office Koenzen Wasser und Landschaft comes to the conclusion that the pollution with pollutants in the sediment of the Landgraben has decreased since 2003. The deeper layers contain toxic substances such as lead, arsenic, mercury and cyanide. “The deposited pollutants essentially come from the discharge of sewage in the past,” said the ministry. Substance inputs via sewage treatment plants have decreased over the years. According to expert reports, for example, the carcinogenic polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) exceed the limit values. The PAH values show a “slightly increasing tendency” at the Trebur bridge, for example. The cause is unknown, they say. Overall, the water quality has gone from “poor to unsatisfactory, i.e. one step better,” said Michael Denk, head of the water management department at the ministry, the FR.
The Landgraben
At low tide the water in the Landgraben consists almost exclusively of treated sewage that is discharged from sewage treatment plants.
The loads measured today , especially in the sediment of the Landgraben, result mainly from the discharge of sewage in the past. According to the Ministry, substance inputs via sewage treatment plants have decreased over the years as wastewater treatment has been successively improved.
The current report which was created against the background of the question of the necessity and feasibility of the remediation of the Landgraben sediments, evaluates more than 20 reports on the pollution situation and the remediation possibilities of the Landgraben, which have been drawn up since 1990, as well as current measurement data.cka
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In addition, the pollutants should be well bound in the bottom sediment. As long as these are not removed, they would stay there and the sediment also protects the groundwater from the ingress of pollutants from the wastewater.
“Renovation of the Landgraben harbors two dangers,” said Minister Hinz. On the one hand, the wastewater would get directly into the groundwater because of the soil sediment removed. On the other hand, the pollutants could be released from the bottom sediment and thus also get into the surface water and the groundwater. “In both cases, the groundwater quality for drinking water production would be impaired.”
Therefore, the state government will follow the recommendation of the report and refrain from rehabilitating the Landgraben at the current time.
However, the Environment Minister announced that the intention was to push ahead with improving the water quality. This includes not only the reduction of phosphorus and nitrogen through less fertilization on the fields, but also the expansion of the fourth cleaning stage of the sewage treatment plants.
However, it will take another four to five years before the plants in Darmstadt, Weiterstadt and Büttelborn are expanded. The fourth cleaning stage of the Merck sewage treatment plant is already under construction. With the fourth cleaning stage, trace substances, such as medicines, can be filtered out.
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