Salt traces in groundwater

In the north of Mulhouse, in the Haut-Rhin region of Alsace, the local groundwater contains salt residues from old potash mines. This salt has seeped into the aquifer through rainwater that infiltrated spoil heaps left over from mining operations.

Contamination levels and timeline

In 2011, more than 85 square kilometers of the lower aquifer had chloride levels exceeding 250 mg/l, which is the threshold for potable water. By 2022, this area had shrunk to about 24 square kilometers, a significant reduction. However, a 2016 study estimated that full rehabilitation of the aquifer would not be possible before 2050.

Origins of the salt pollution

The potash mines in the area operated from 1910 to 2002, extracting 567 million tons of raw salt. The ore contained potassium chloride, the desired material, and about 60% sodium chloride, or table salt. This excess salt was piled into spoil heaps on permeable ground, contributing to the contamination of the aquifer.

Evolution of salt disposal methods

From 1910 to 1933, spoil heaps were heavily concentrated, and by 1970, the practice had nearly stopped. Between 1934 and 1975, much of the salt was dissolved and discharged into the Rhine River. Today, about twenty spoil heaps remain in the area, with efforts focused on containing and removing the salt.

Efforts to clean the aquifer

Since 1975, various measures have been taken to clean the aquifer, including injection wells, brine conduits, and covering spoil heaps. These efforts have included pumping contaminated water through a 58 km pipeline to the Grand Canal of Alsace. By 1998, nearly 3.9 million tons of salt remained on the spoil heaps, with 58,500 tons of chlorides still entering the aquifer annually.

Progress and lingering concerns

A 2025 report from the BRGM shows that the area of the lower aquifer exceeding 250 mg/l of chlorides has decreased by 72% since 2011, and the upper aquifer by 57%. Despite these improvements, some water wells, like the one in Kingersheim, continue to show rising chloride levels, indicating that the problem is not entirely resolved.

Long-term outlook and challenges

The 2016 study suggests that the aquifer may not be fully rehabilitated before 2050, a timeline that is 39 years after the last major cleanup efforts in 2011. Although the state continues to monitor the spoil heaps and pumping network, past leaks in the brine conduit highlight the ongoing challenges in ensuring complete recovery of the aquifer.