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# Foreseeable Flooding Scenario of the Alsace Potash Mine and the StocaMine Underground Final Waste Storage Site (France): A Case Study

Philippe Gombert, Sylvain Favier

► To cite this version:

Philippe Gombert, Sylvain Favier. Foreseeable Flooding Scenario of the Alsace Potash Mine and the StocaMine Underground Final Waste Storage Site (France): A Case Study. Mine Water and the Environment, 2025, 10.1007/s10230-025-01049-w. ineris-05065142

HAL Id: ineris-05065142

https://ineris.hal.science/ineris-05065142v1

Submitted on 13 May 2025

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# Foreseeable Flooding Scenario of the Alsace Potash Mine and the StocaMine Underground Final Waste Storage Site (France): A Case Study

Philippe Gombert<fnref n="1" /> Ⓓ · Sylvain Favier<fnref n="1" /> Ⓓ

Received: 3 October 2024 / Accepted: 13 March 2025

© The Author(s) 2025

## Abstract

In 1999, a final waste storage site known as StocaMine was established at a depth of about 500 m within the former Alsace potash mines (France). The administrative abandonment procedure for this mine, which was initiated in 2010, provides for the permanent containment of the waste on site. The mining operator launched a large number of studies to assess the potential effect that the presence of this waste could have on current and future populations and environment. The main studies were aimed to describe the likely flooding scenario of the mining operations on the one hand, and of the storage site on the other. This flooding results from the inevitable percolation of water from the Alsace alluvial water table through the 15 abandoned mine shafts. After a few centuries, when the flooding ceases, the continued compaction of underground cavities under the weight of the overlying terrain may induce an effusion of potentially polluted brine at the base of the Alsace alluvial water table. A series of hydrogeological, geochemical, and geomechanical modelling exercises, mostly carried out by the French National Institute for Industrial Environment and Risks (Institut français de l'environnement industriel et des risques—Ineris) has thus been implemented to quantify this risk and to propose measures for managing it.

**Keywords** Closure · Salt · Pollution · Brine · Impact

## Introduction

The flooding of a salt or potash mine with fresh water can cause mass dissolution that can quickly lead to catastrophic events such as underground or surface subsidence or collapse, as well as uncontrolled releases of brine. This was the case in several salt or potash mines, such as those shown in Table 1.

In this context, the predictable flooding of the Alsace potash mines in France and the StocaMine underground final waste storage site, which was commissioned in 1999, required numerous studies that spanned over a decade. The main aim here was to assess the risk of long-term (over several centuries) release of potentially polluted brine into the

Alsace alluvial water table, which is currently being used for drinking, agricultural, and industrial water supplies.

## Presentation of the Alsace Potash Mines

### History

In 1869, a salt and potash deposit was discovered during drilling carried out near Mulhouse in Alsace (France). In 1904, a reconnaissance drilling campaign identified the Alsace potash basin, which was then mapped over an area of about 15 × 12 km wide between Mulhouse to the south and Guebwiller to the north. This covered 33 municipalities in the Haut-Rhin department (Fig. 1).

This extensive deposit was mined from 1910 by the German Alsace Potash Mining Company, which would later be nationalised by France in 1937 into the “Mines De Potasse d'Alsace” (MDPA). During a century of operation, 24 shafts ranging from 4.50 to 6.30 m in diameter were dug at depths varying from 466 to 1057 m. The operation reached its peak in 1948 with nearly 14,000 employees. The first shafts were

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<footnote>Sylvain Favier
sylvain.favier@ineris.fr</footnote>

<footnote>Philippe Gombert
philippe.gombert@ineris.fr</footnote>

<footnote n="1">¹ Ineris, Parc Technologique Alata,
60550 Verneuil-en-Halatte, France</footnote>
<table><caption>**Table 1** Some examples of the impact of uncontrolled flooding of salt or potash mines</caption><thead><tr><th>Country</th><th>Location</th><th>Date</th><th>Impacts of flooding</th><th></th></tr></thead><tbody><tr><td>Germany</td><td>Stassfurt</td><td>Beginning of twentieth century</td><td>Serious uncontrolled salt leaching, collapsing mined areas and further large-scale subsidence</td><td>Luo et al. (2012)</td></tr><tr><td>Poland</td><td>Wapno</td><td>1977</td><td>Catastrophic land subsidence</td><td>Zuber et al. (2000)</td></tr><tr><td>Poland</td><td>Inowroclaw</td><td>1907–1924</td><td>Number of land subsidences</td><td>Zuber et al. (2000)</td></tr><tr><td>USA</td><td>Jefferson Island (Louisiana)</td><td>1980</td><td>32 years after: apparition of a 37 acres sinkhole and evacuation of 359 residents</td><td>Yrle et al. (2015)</td></tr><tr><td>Canada</td><td>Patience Lake (Saskatchewan)</td><td>1975–1987</td><td>Closing of the mine due to uncontrollable flooding despite construction of underground grouting and bulkheads</td><td>Gendzwill and Martin (1996)</td></tr><tr><td>Romania</td><td>Ocna Dej</td><td>Late 1990s</td><td>Landslides, building subsidence, mine abandonment</td><td>Chitea et al. (2015)</td></tr><tr><td>Russia</td><td>Berezniki</td><td>2006</td><td>Huge sinkhole and land subsidence in the near city</td><td>Baryakh and Samodelkina (2017)</td></tr><tr><td>Ukraine</td><td>Solotvyno</td><td>2010</td><td>Dozens of sinkholes formed with diameters reaching 250 m, risk of river contamination</td><td>Stoeckl et al. (2020)</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 1** Map of the mined area (hatched) of the Alsace potash basin and access shafts (black points)—the black rectangle at the bottom left, between the shafts called Joseph and Else, marks the location of the StocaMine site (EMC 1996). Caption: thin dashes = mining concession limit, black line = extension limit of the potash layer, thick dashes = faults, hatched area = exploited area, circles = shafts</figcaption></figure>

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closed and backfilled starting in 1954, but new shafts were
still being sunk until 1972. Potash mining definitely ceased
in 2002.

The potash was mainly mined by the caved longwall
mining method, which involves extracting the entire deposit
over a certain width without leaving any pillars, with
mechanical support being provided by rows of jacks. As the
operation progresses, these jacks are advanced to protect
the front of the workface, which causes the subsidence
or "caving" of the ground behind the workface. These
successive collapses reverberate to the surface in the form
of vast subsidence bowls that are several metres deep.

Some years before the planned closure of the mining
operation, an underground final waste storage site known
as StocaMine was dug in a salt bank located at a depth of ≈
500 m. StocaMine was opened in 1999 and was used to store
44,000 tonnes of waste before a fire broke out in one of the
blocks being filled in 2002, when operations were stopped.
Most (56%) of the waste stored was polluted soil and waste
containing mercury, chromium, arsenic, or plant protection
products; the remainder (44%) was incineration residues and
asbestos waste.

The MDPA wished to undertake administrative closure
of the mine, and thus the storage site, which would involve
either the extraction of the waste (its storage was designed to
be reversible), or its permanent containment. A large series
of studies was carried out to assess the risk of it polluting
the Alsace alluvial water table (the largest water table in
Europe) in a permanent waste containment scenario. Ineris,
the French National Institute for Industrial Environment and
Risks was chosen by the MDPA to study the mine evolution
scenario and quantify the expected effect of a possible leak of
potentially polluted brine in the future. A prefectural decree
dated 28 September 2023 was promulgated, authorising the
storage for an unlimited duration after implementation of the
risk control methods detailed below.

Given the predictable flooding timelines, which span
several centuries, and the associated uncertainties, the
reference scenario that was developed was based on a
so-called worst-case or security-based approach: at each
calculation stage, the most pessimistic assumptions were
considered in order to assess the maximum risk that the
presence of this waste storage could pose for the water
resource.

Location and Geometry of the Underground
Storage Site

In 1999, a few years before the planned cessation of the pot-
ash mining operation, the StocaMine site was put into ser-
vice. It is a storage facility for industrial waste dug between
500 and 550 m deep in a salt bank untouched by any mining

activity, located 23 m under the potash deposit of the Amé-
lie mine. It consists of a network of galleries 5.5 m wide
and 2.8 m high, forming a grid network measuring 225 m
on each side, which delineates 26 storage blocks (Fig. 2).
Ground support is provided by salt pillars measuring 20 m
on each side.

The storage site is accessed by the Joseph and Else shafts, which therefore remain the only open shafts in this extensive mining operation. The storage was designed to hold 320 kt of waste, but only 44 kt had been stored there by 2002, when an underground fire occurred in block 15, apparently caused by the storage of residues from a burnt-out plant protection products warehouse: due to the presence of asbestos, the big-bags received could not be opened for inspection prior to storage, and it is likely that exothermic chemical reactions occurred between the various products contained in some of these big bags. This fire, which lasted more than two months, led to the permanent cessation of storage activities at the end of 2002. The MDPA then undertook a procedure for the permanent closure of the site, with the option of permanent containment of the waste, ensuring the best possible control of residual risks. The main identified risk is linked to the following process: (i) percolation of subsurface water in the mine via abandoned shafts, (ii) potential solubilisation of all or part of the waste by the floodwater that has been transformed into brine through contact with salt and potash, and (iii) discharge of potentially polluted brine into the Alsace alluvial water table.

Nature and Quantity of the Waste Stored
at the StocaMine Site

The StocaMine site received and stored a total of 44 kt of
industrial waste of two distinct types (Hennebert 2011):

* 44% of so-called “class 0” waste, considered to be “particularly hazardous”, notably in the form of incineration ashes and asbestos waste;

* 56% of so-called “class 1” waste, considered to be “dangerous”, in the form of polluted earth, and of arsenic, mercury, galvanic, chromic and phytosanitary waste, quenching salts, industrial residues, and laboratory waste.

After receipt, this waste was labelled and stored in
specific cells or blocks, in two forms: big bags and metal
drums.
<figure><img src="image_6.png" /><figcaption>**Fig. 2** StocaMine underground storage plan (MDPA document) Caption: jour = surface (m asl), fond = bottom (m asl), voie centrale = central alley, bloc = block. P. = shaft</figcaption></figure>

**Geological and Hydrogeological Context**

The area is part of the Rhine Graben, whose fill is mainly represented by the thick Sannoisian salt series, which reaches a thickness of over 1,400 m in the Mulhouse region. It is locally overlain, except directly above the Joseph and Else shafts, by Stampian marls that can be up to 400 m thick, and, to the north of the Amélie mine, by Chattian sandstone and limestone measuring up to 300 m thick. Above these, there are 20 to 50 m of Plio-Quaternary alluvium.

**Salt Series from the Sannoisian**

The upper part of this salt series is a marly formation of about 500 m thickness with evaporitic beds: gypsum, anhydrite, potash, and salt. It is subdivided into three units, the deepest being the “salt complex,” which encloses about 100 evaporitic beds, including:

* a 2 m thick potash layer called the “upper layer”, mined by the MDPA and located 30 m below the roof of the salt complex;

* a 6 m potash layer called the “lower layer”, mined by the MDPA and located 20–25 m below the preceding layer;

* a layer of rock salt located ≈ 20 m below the lower potash layer, i.e. ≈ 500 m deep or -230 m ASL. This is where the StocaMine underground storage site was dug.

**Geometry of the "Host Block"**

The StocaMine storage site is situated in an impermeable “host block,” corresponding to the Sannoisian salt series, which is naturally free of groundwater and isolated from water flows. In the horizontal plane, it is indeed isolated first to the west and south, by the boundary of the deposit or the erosion of the evaporitic formations, then to the east by a submeridional flexure nearly 300 m wide, as shown in Fig. 1 (between Richwiller and Pulversheim), with a displacement of ≈ 150 m. It did not show any water inflow when it was cut

<figure><img src="image_7.png" /></figure>
through and thus isolates the "western sector" (within which StocaMine was excavated) from the rest of the potash basin. Finally, the host block is isolated to the north, by a zone of unmined ore or barrier pillar, 20 to 50 m thick, supposed to isolate the Amélie mine from the rest of the western sector. Most authors, however, consider this unmined ore zone to be fractured, thereby exhibiting some permeability (CESAME 2006, 2008; Combes and Ledoux 1990; Gombert 2010; IBEWA 2012; MICA 2004).

In the vertical plane, the host block corresponds to the upper salt zone, which is separated from the nearest water table levels at its base by ≈ 1000 m of mainly marl formations ranging from the Triassic to the Jurassic (down to the first water table located ≈ 1500 m deep), and at its summit by ≈ 200 m of marl, anhydrite and Tertiary salt which also form the wall of the overlying Alsace alluvial water table (Fig. 3).

## Alsace Alluvial Water Table

Above the host block, the alluvium of the Alsace water table has a double structure (Fig. 4). In the horizontal plane, the alluvial fans of the rivers from the Vosges, ≈ 25 m thick, are found to the west, while to the east is located the area of Rhenish alluvium, which measures 20 m thick near Mulhouse. These two alluvial domains are separated by the Ill flood plain, a Vosges river whose course is here subparallel to that of the Rhine, with an interstratification of Vosges and Rhine alluvium over a width of 2–3 km (Fig. 4a).

In the vertical plane, from the surface, there is a first layer of very permeable recent alluvium (a few metres thick), a second layer of moderately to poorly permeable older clayey-sandy alluvium (about 10 m in thickness), and a third layer of basal alluvium, with varying permeability depending on the area considered (thickness from 0 to ≈ 10 m). The basal layer is often poorly permeable in the Rhine region, but it is more permeable in the Vosges region where it is made up of east-west oriented paleo-channels filled with coarse sands and pebbles. Moreover, directly above StocaMine, layers 2 and 3 are sometimes separated by a few metres of very poorly permeable clay intercalations (Fig. 4b).

The flow direction of the Alsace alluvial water table is known due to piezometric data and especially to the monitoring of chloride plumes induced by the percolation of water through salt slag heaps deposited on the surface during mining operations. This veritable tracing revealed two "salt plumes" over 20 km in length (Fig. 5), with the eastern salt plume originating from the Thann slag heaps and passing through the Joseph-Else and Amélie slag heaps. This allows us to visualise the flow direction of a potential brine leak from the StocaMine site. At the foot of the Vosges, ground water flows towards the east and then, from Mulhouse, to the north-northeast, that is, in

the direction of regional flow of the Alsace alluvial water table.

## Mechanism of Mine Cavity Flooding

To date, flooding of the mine has already begun due to percolation of water from the Alsace alluvial water table through the plugs of the abandoned mine shafts. This phenomenon began as soon as the first shafts were dug in 1907 but was offset by the pumping of percolating water until they were closed and backfilled between 1954 and 2008 (with the exception of the Joseph and Else shafts, which remained open for a few years to maintain the site). This has been confirmed by deep piezometric measurements that revealed the presence of water at ≈ 978 m deep downstream of StocaMine and at ≈ 905 m deep at the lowest point in the western sector (CESAME 2022). This water is rising slowly at a rate between 0.4 and 1.4 m·year⁻¹ and gradually filling the residual mining cavities (Gombert 2023). However, the cost of these piezometers is high because of their depth and it is not possible to install enough of them to properly monitor the rise in groundwater levels. The measurements only concern the deepest workings, which are not well connected with the rest of the mine, but which show the first arrivals of water. This is why it is not possible to define a precise duration for flooding the entire mine.

The state of these cavities also changes over time due to two opposing effects (Gombert 2011): creep, which tends to close them, and dissolution by flooding water, which tends to increase their volume. Given the high solubility of salt and potash, a rapid dissolution in the percolating water and the instantaneous formation of brine has been considered at the time scale of flooding. In addition, this brine induces a confining pressure that slows down the salt creep once the cavities are flooded. The final duration of mine flooding, which spans several centuries, is therefore the result of a complex interplay between the flooding rate and these three parameters (creep, dissolution, and pressure) affecting the closure or increase in volume of the cavities.

## Origin of the Flooding Water

Currently, 13 of the 15 shafts have been abandoned and are sealed with 50-m thick layers of fly ash, resting on a gravel layer. This type of backfilling was carried out to withstand high mechanical pressures and prevent the collapse of shaft columns, but it does not ensure total water-tightness; the mine is therefore destined to gradually flood (Gombert 2010). Thus, when the Amélie 1, Amélie 2, and Max shafts were no longer used but still accessible at their base, the cumulative water percolation was ≈ 0.9 m³·h⁻¹ or 8000 m³·year⁻¹ (MICA 2004). In 1981 and 1985, isotopic analyses of these waters
<figure><img src="image_8.png" /><figcaption>**Fig. 3** Simplified stratigraphic map and hydrogeological characteristics of the formations directly above the StocaMine site (MICA 2004)</figcaption></figure>

showed tritium levels ranging from 8 to 38 TU, confirming the presence of recent surface waters alongside some pockets of fossil water (MICA 2004).

## Estimation of the Flooding Rate

In 2010, during the preparation of the mine's administrative

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 4** a Geomorphological map of the Rhine plain between Mulhouse and Colmar and b West–East profile according to the BRGM water table model (Noyer and Elsass 2006)</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 5** Mapping of the salt plumes of the Alsace plain in 2005 (Elssas 2006)</figcaption></figure>

closure file, the flooding rate had to be estimated in the absence of sufficient on-site measurements. The goal was to forecast the future evolution of the site and to propose a scenario for the release of potentially polluted brine. Three hypotheses were proposed to estimate this flooding rate through the sealed shafts.

The first hypothesis is based on the equivalent permeability of the ash plugs on the inner shaft surface, estimated at $6 \cdot 10^{-7} \text{ m} \cdot \text{s}^{-1}$ by Ineris (2002). It assumes that the water percolates through the inner surfaces of shafts whose casing and cementing remain intact. Extrapolated to the 15 shafts in the western sector, this hypothesis suggests a total flooding rate of $\approx 22,000 \text{ m}^3 \cdot \text{year}^{-1}$.

The second hypothesis is based on the equivalent transmissivity of the sealed shafts, estimated at $2 \cdot 10^{-6} \text{ m}^2 \cdot \text{s}^{-1}$ (Gombert 2011). It assumes an increase over time of water flow at the outer surface of the shafts because of (i) the progressive impermeability of the fly ash plugs caused by their swelling which forces the water to bypass them via the outer shaft surface, (ii) the deterioration of the casing and

masonry elements of the shafts, and (iii) the enlargement of the water pathways around the outer shaft surface through dissolution. This hypothesis is also consistent with the following two observations:

* the fact that the water flow from certain sealed shafts corresponds to that observed prior to their closure.

* the chemical analysis of water samples from the last accessible mine shafts in 2010 did not reveal substantial concentrations of tracer elements in the flow at the inner shaft surface (the passage through the fly ash plugs should lead to an enrichment of the water with certain characteristic trace metal elements).

Extrapolated to the 15 shafts of the western sector, this second hypothesis suggests a total flooding rate of about $58,000 \text{ m}^3 \cdot \text{year}^{-1}$.

The third hypothesis considers the discharge rate measured at the base of the shafts during their operational phase. When combined for the 15 shafts in the western

<figure><img src="image_12.png" /></figure>
sector, this discharge reached a maximum of ≈ 99,000 m³·year⁻¹.

When considering the long-term future of the mine, the first hypothesis is no longer valid as it does not account for the gradual deterioration of the shafts. To build a safety reference scenario in terms of environmental risks, a worst-case hypothesis was therefore chosen that maximises the volume and thus the brine discharge rate. To achieve this, the flooding rate must be high so that the residual mining cavities are flooded before they are completely closed by the natural convergence of surrounding ground. To quantify this hypothesis, the maximum value of the flow rate calculated for each mine shaft according to hypotheses 2 or 3 was selected. Extrapolated to the 15 shafts of the western sector, the total flooding rate considered slightly exceeds the discharge flow rate during operation, reaching ≈ 105,000 m³·year⁻¹, which is the basis for the reference scenario presented below.

## The Volume of Mining Cavities

In total, the operation will have produced 200 Mm³ of mining cavities, divided into two main types in function to their differing hydraulic and mechanical behaviour: longwall mines (95.5%) and their access galleries (4.5%). The latter represent a known volume of ≈ 9 Mm³. On the other hand, the longwall mining operations created cavities with extensive lateral dimensions which, in the absence of supporting pillars, collapse rapidly, causing large surface subsidence. As a result, only a certain percentage of the cavities created by the mining remain after this caving and are thus available for flooding. Since these cavities are no longer accessible, their residual volume was estimated by Laouafa (2010) at 20% of the volume generated during operation, based on precise measurements of surface subsidence, whereas the operator estimated it at 10% (MDPA 2015). The reference scenario adopted by Ineris therefore assumes a residual volume of caved longwalls of ≈ 40 Mm³. This is a worst-case approach as it results in a larger flooding volume, likely to subsequently result in a higher discharge rate and volume of brine effusion.

These two types of cavities also differ in their mechanism and rate of convergence under the effect of the weight of the overlying terrain: salt creep for the galleries, and compaction of collapsed ground for the caved longwalls. At the average depth of storage, geomechanical modelling (Laouafa 2010) and field measurements have estimated this convergence rate at 0.9% per year for the galleries and 0.1% per year for the caved longwalls. According to the Norton rheological model (Ghoreychi 1991), this convergence rate increases with the 4th power of the stress deviator, i.e. the difference between the internal confining stress and the external lithostatic stress. As a result, the

<figure><img src="image_13.png" /><figcaption>**Fig. 6** Modelling of the closure time of the galleries as a function of depth (Laouafa 2010)</figcaption></figure>

convergence rate decreases with flooding, which increases the confining pressure, but it increases with depth, which increases the lithostatic pressure. This last case is illustrated by Fig. 6 with, for instance, a rate of convergence of the galleries multiplied by 16 when their depth is doubled; therefore, a faster closure.

As the residual cavities are flooded, their rate of convergence gradually decreases to reach a limiting value when the brine reaches the base of the Alsace alluvial water table. The hydrostatic pressure of this brine, of density 1.2, will then be about 6 MPa at the storage depth, corresponding to ≈ half of the lithostatic stress which is here ≈ 12 MPa. The stress deviator will thus be divided by 2 and the creep rate by 16. It will thus be reduced to ≈ 0.01% per year for the mined workfaces and 0.05% per year for the galleries.

To this should be added the cavities newly formed by dissolution following the flooding of the mine by fresh groundwater from the Alsace alluvial water table, which has a high dissolution potential. Lourdel (2004) and CESAME (2006) have shown that the volume of the additional cavities thus created reaches 24% of the volume of the water introduced, which amounts to ≈ 25,200 m³·year⁻¹ in the reference scenario. Under the effect of ground convergence, these newly formed cavities will then evolve at the same rate as the galleries or wall mining to which they are related.

Ultimately, the equation that governs the evolution of the volume of potentially floodable mining cavities is given below at time step t (in years):
$$V_R(t) = V_R(t-1) - [(V_{DG} + V_D * P_G) * C_{DG} + (V_{DW} + V_D * (1-P_G)) * C_{DW} + (V_{FG} + Q * P_G) * C_{FG} + (V_{FW} + Q * (1-P_G)) * C_{FW}] + V_D$$

where $V_R(t)$ is the residual volume of mining cavities (m³) at time $t$, $V_{DG}$ is the residual volume of dry galleries (m³) and $C_{DG}$ their convergence rate (% per year), $V_D$ is the volume of cavities created by dissolution from incoming water (m³), $V_{DW}$ is the residual volume of dry workings (m³) and $C_{DW}$ their convergence rate (% per year), $V_{FG}$ is the cumulative volume of flooded galleries (m³) and $C_{FG}$ their convergence rate (% per year), $V_{FW}$ is the cumulative volume of flooded workings (m³) and $C_{FW}$ their convergence rate (% per year), $Q$ is the flooding rate (m³·year⁻¹) and $P_G$ is the proportion of galleries within the mining cavities.

In order to estimate $V_R(t_0)$, that is the initial volume of potentially floodable residual mining cavities at time $t_0$ of the study (arbitrarily set at 2010), it is necessary to consider both the volume of cavities created at the start of operation and their foreseeable evolution over time. This latter depends on the age of digging operations (the first dating from 1910) and their depth (up to more than 910 m). This volume was thus estimated at 33.3 Mm³, or only 16.7% of the mining cavities initially created by the mining operation (Table 2).

## Calculation of the Duration of Flooding

The shaft bases were pumped of water, which was sent to the surface. This process ended when the shafts were backfilled with fly ash plugs, i.e. it was at this point that the flooding of the mining district began. This backfilling took place from 1956 to 2008, except for the Joseph and Else shafts, which are still open to provide access to the storage site. As of 2010, flooding had therefore already begun 2–54 years ago depending on the shaft considered. Thus, by multiplying the percolation flow of each shaft—according to the worst-case hypothesis—by the time elapsed since its backfilling, a theoretical flooding volume of the western sector of 2.1 Mm³ was obtained in 2010, the start date of the site flooding study.

Ultimately, the total flooding time will result from a complex compromise between:

<table><caption>**Table 2** Estimated volumes of initially created mine cavities and residual cavities in 2010 (Mm³)</caption><thead><tr><td>Volume of the cavities</td><td>Initial state</td><td>Year 0 (2010)</td></tr></thead><tbody><tr><td>Access galleries</td><td>9</td><td>0.6</td></tr><tr><td>Wall mining (workings)</td><td>191</td><td>32.7</td></tr><tr><td>Total</td><td>200</td><td>33.3</td></tr></tbody></table>

* the flooding volume which depends on the percolation flow rate at the inner and outer shaft surfaces of the shafts, considering the time elapsed since they were backfilled;

* the volume of the residual mining cavities (longwalls and galleries) and those newly formed by dissolution; and

* the gradual closure of these mining cavities through convergence (creep in the galleries, compaction of the longwalls), depending on the depth, age, and flooding rate of the mining cavities.

A residual flooding duration $D_{RF}$ has been calculated at each time step $t$ (year) as the ratio of the residual volume of the mining cavities at that time $V_R$ to the flooding rate $Q$:

$$D_{RF}(t) = \frac{V_R(t)}{Q}$$

As a result of the gradual decrease in the remaining volume to be flooded, this duration also decreases over time and is zero when flooding has ceased. Considering the hypotheses adopted by various authors, flooding will end within a time frame of 150 years (CESAME 2008) to 800 years (MICA 2004). In the reference scenario considered by Ineris, the total duration of flooding was estimated at 300 years, i.e. a theoretical end in 2310. This period is divided into ≈ 230 years to reach the waste at the storage site if no risk control measures are implemented, and ≈ 70 more years to fill the overlying mining cavities and spill at the base of the Alsace alluvial water table (Gombert 2010).

## Brine Effusion Mechanism at the End of Flooding

### Foreseeable Scenario

A residual mining reservoir will thus remain at the end of the flooding, on which the convergence will continue to act. This pressure will cause a rise of brine within the shafts that will counteract the percolation of surface water that has flooded the mining cavities, thereby becoming the main driver of brine effusion towards the Alsace alluvial water table. By that time, the galleries will have already been mechanically closed, and the remaining mining reservoir will only involve the wall mining, with a volume estimated at ≈ 29 Mm³ in the reference scenario.

Considering a residual compaction rate of 0.01% per year, it is thus necessary to expect an average brine effusion rate of ≈ 2,900 m³·year⁻¹, distributed among the 15 shafts in the western sector, in proportion to their previously calculated percolation rate. This value corresponds to a flow rate of ≈ 0.5 m³·day⁻¹ per shaft.
Since the brine will rise mainly via the outer surface of the sealed shafts, its effusion is expected at the point where the shaft casings come into contact with the Alsace alluvial water table, specifically at the level of the basal alluvium. Given the substantial density of this saturated brine (density of 1.2 for a salinity of 350 g·L⁻¹) and the geometry of the basal alluvium (paleochannels), it should circulate mainly at the base of this water table.

## Effusion in the Absence of Risk Control Measures

In the absence of risk control measures preventing the brine from entering and leaving the storage site, the brine could be polluted by leaching all or part of the waste. About 90% of the brine from the bottom of the mine could potentially reach the storage site and percolate through the stored waste before flooding the overlying residual mining cavities; the remaining 10% will have bypassed the storage site via lateral galleries or passed through a storage area free of waste (such as blocks 16, 25, and 26).

The residual volume of these mining cavities overlying the storage site has been estimated at 6.8 Mm³ at the end of flooding, of which ≈ 6.1 Mm³ would be flooded with polluted brine. The expected effusion rate of this polluted brine will therefore be on the order of 610 m³·year⁻¹ (Gombert 2010). CESAME (2006, 2008), MICA (2004), and Gombert (2010) consider that only the 5 Amélie mine shafts will be affected by these effusions: these are the Joseph, Else, Amélie 1, Amélie 2 and Max shafts. They are in fact both the highest in the western sector and the closest to the storage site, from which they are only 0.3–1.5 km away. The other shafts are between 5 and 10 km downstream of the storage site.

Distributed over these five shafts, this flow rate will ultimately amount to ≈ 0.3 m³·day⁻¹ per shaft at the start of the effusion process. This flow rate, however, will depend on the rate of compaction, which is likely to decrease slowly in the long term, but which will only be cancelled out when the residual cavities are completely closed. The order of magnitude for this period has been estimated at 1,000 years

(Gombert 2011). This is the period during which the risk control measures must remain effective in order to prevent pollution of the expelled brine.

## Presentation of the Risk Control Measures

The main risk control measures envisaged to reduce this long-term impact are presented in Table 3. They involve (AnteaGroup 2023):

* a measure undertaken prior to containment, requested by the state shareholder, involving the removal of mercury-containing waste, which was the main potential source of impact on the groundwater. This operation was carried out from 2014 to 2017 with the removal of 2,400 tonnes of mercury and phytosanitary waste, representing 95% of the mercury present;

* measures aimed primarily at slowing the penetration of brine into the storage site, as well as its possible exit, for ≈ 1000 years, i.e. until the mine is completely closed by compaction;

* additional measures to plug the boreholes reaching the storage, and backfill its galleries and unused blocks.

If these various measures prove effective, all residual mining cavities, in particular those located above the StocaMine site, will therefore be flooded with clean brine, unpolluted by any interaction with the stored waste.

The proposed sealing barriers, which constitute the risk main control measure, are complex structures designed as follows (Fig. 7):

* the galleries are over-excavated by 0.6 to 2 m over a stretch of 12 m to eliminate the excavation damaged zone or EDZ (area damaged by mining extraction) where the salt is potentially more permeable;

* a 3-m-long abutment made of shotcrete mixed with brine is supported on the lean concrete backfill on the internal side of the storage;

<table><caption>**Table 3** Proposed risk control measures and desired effect (from Gombert 2011)</caption><thead><tr><td>Desired effect</td><td>Proposed risk control measures</td></tr></thead><tbody><tr><td>Reduce the impact of the most relevant pollutant</td><td>Removing of 95% of mercury waste (already done)</td></tr><tr><td>Prevent the brine from leaching the waste in the storage site, at least during the phase when the storage site and the overlying residual mining cavities are flooding</td><td>System facilitating the bypass of the storage site (peripheral drainage) allowing brine to reach the overlying residual mining cavities without crossing the storage site + plugging the boreholes</td></tr><tr><td>Contain in the interior of the storage site any brine that may have managed to penetrate it, until the mining cavities have been completely closed by compaction</td><td>Water-tight sealing barriers preventing brine from entering and leaving the storage site for 1,000 years</td></tr><tr><td>Reduce the volume of interstitial cavities within the blocks containing waste in order to limit brine-waste contact</td><td>Backfill the interstitial cavities with an inert substance (lean concrete)</td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>Fig. 7 Cross-section of a sealing barrier (from AnteaGroup 2023)</figcaption></figure>

* a very low permeability 6-m-long core is made of concrete mixed with brine in order to achieve the chosen waterproofing criterion (permeability < $10^{-18}$ m², or ≈ $10^{-11}$ m·s⁻¹);

* a second 3-m-long abutment of shotcrete mixed with brine to isolate the core on the external side of the storage;

* a compacted salt embankment 10–20 m long applied on the outside in order to saturate any fresh water inflows from the Joseph and Else shafts after their closure.

## Effusion in the Presence of Risk Control Measures

If the risk control measures presented above are implemented, the waste held in the storage site will, in theory, not adversely affect the brine quality. However, in the worst-case approach, the possibility of a low and slow flow of polluted brine within the storage site was still considered, due to the combined effects of ongoing compaction, ageing of the risk control measures, and the non-zero permeability of salt, particularly in the new EDZ around the over-excavated galleries (Gombert 2011).

The only porosity available for the brine within the storage site, with a volume of 22,000 m³, corresponds to the interstitial voids between the barrels or large bags and the porosity of the backfill grout that would be used. Given the block filling methods and the intrinsic porosity of the waste, this available porosity has been estimated at ≈ 31%, i.e. a volume of 7000 m³ (Gombert 2011). Under the effect of salt creep, which also compacts the waste, this available porosity should slowly decrease over time. However, in the

absence of data on the evolution of the mechanical resistance of this material, and within the framework of a worst-case approach, this mechanism was not considered, and the available porosity was assumed to be constant over time. On the other hand, continued compaction could expel some of the polluted brine that may have entered the storage, with a flow rate estimated at ≈ 0.7 m³·year⁻¹, based on the rate of closure of the flooded workings. In the reference scenario, this polluted brine will therefore be diluted in the flow of clean brine that will come out through the five shafts of the Amélie mine.

# Risk of Pollution of the Alsace Alluvial Water Table

## Calculation of the Source Term

The source term corresponds to the concentration of pollutants in the brine that has leached the waste. Their leachability was calculated by liquid-solid geochemical equilibrium in order to arrive at equilibrium concentrations. This approach was carried out by means of modelling with the Visual MINTEQ calculation software, using the MINTEQ3.0 thermodynamic database (Hennebert 2011). Table 3 shows the result of equilibrium simulations for brine at 35 °C (the mean temperature of the StocaMine site) and for the following two scenarios: in the absence of risk control measures (volume = 6.1 Mm³, equilibrium pH = 8.8), and in the presence of such measures (volume = 7,000 m³, equilibrium pH = 11.8).
<table><caption>**Table 4** Maximum concentrations at equilibrium of the main pollutants in the source term in $g \cdot L^{-1}$ (from Hennebert 2011)</caption><thead><tr><th>Element</th><th>Without risk control measures</th><th>With risk control measures</th></tr></thead><tbody><tr><td>Antimony</td><td>1.34 $10^{-2}$</td><td>1.89 $10^{-1}$</td></tr><tr><td>Mercury</td><td>1.20 $10^{-2}$</td><td>7.52</td></tr><tr><td>Arsenic</td><td>1.00 $10^{-2}$</td><td>1.54 $10^{-2}$</td></tr><tr><td>Chrome</td><td>9.10 $10^{-3}$</td><td>7.07</td></tr><tr><td>Cadmium</td><td>1.50 $10^{-3}$</td><td>1.54 $10^{-3}$</td></tr><tr><td>Cyanides</td><td>8.30 $10^{-4}$</td><td>6.80 $10^{-1}$</td></tr><tr><td>Barium</td><td>1.06 $10^{-6}$</td><td>2.30 $10^{-7}$</td></tr><tr><td>Lead</td><td>4.35 $10^{-7}$</td><td>4.53 $10^{-7}$</td></tr><tr><td>Nickel</td><td>3.05 $10^{-7}$</td><td>3.05 $10^{-7}$</td></tr></tbody></table>

<table><caption>**Table 5** Selected quality criteria (in $\mu g \cdot L^{-1}$) considering data on the local control environment and the regulatory quality criteria (from Quiot 2012)</caption><thead><tr><th>Element</th><th>Local control environment</th><th>Regulatory quality limit (DWW)</th></tr></thead><tbody><tr><td>Antimony</td><td>Unknown</td><td>5</td></tr><tr><td>Mercury</td><td>0.2</td><td>1</td></tr><tr><td>Arsenic</td><td>&lt;10</td><td>10</td></tr><tr><td>Chrome</td><td>7</td><td>50</td></tr><tr><td>Cadmium</td><td>0.7</td><td>5</td></tr><tr><td>Cyanides</td><td>&lt;100</td><td>50</td></tr><tr><td>Barium</td><td>Unknown</td><td>700</td></tr><tr><td>Lead</td><td>13</td><td>10</td></tr><tr><td>Nickel</td><td>3.7</td><td>20</td></tr></tbody></table>

Equilibrium thermodynamics predict that the dissolved concentrations of each element is fixed by the solubility product of the least soluble solid phase: if this threshold is not reached, the element in question remains in solution. Moreover, the high concentrations of anions here (notably, chlorides from the host rock and cyanides from the waste), affects the speciation in solution and, in particular, the complexation of some metals, which can increase their solubility. Thus, certain metal ions will be able to form complexes or pairs of stable ions with $Cl^{-}$ and $CN^{-}$ ions, and thus find themselves in solution beyond their theoretical equilibrium concentrations: this is particularly the case for antimony, mercury, and chromium (Table 4).

## Definition of Vulnerabilities

The existing vulnerabilities that could be affected by the effusion of potentially polluted brine, are drinking water wells (DWW), industrial or agricultural water supply wells, as well as private wells. The drinking water wells primarily

draw from layer 2 of the intermediate alluvium, while the industrial and agricultural wells, being less deep, tap into layer 1 of the upper alluvium or layer 2 of the intermediate alluvium. No catchment has been identified in the study zone within layer 3 of the basal alluviums, a deep and discontinuous layer where the potentially polluted brine effusions should occur.

The nature of the observed uses, which are assumed to continue in the future, determine the choice of quality criteria to be adopted within the framework of the national risk management policy (Table 5). In the case of DWW use, the regulatory criteria are specified by the French Order of January 11, 2007, relating to the limits and quality standards for raw water and water intended for human consumption. Regarding the natural environment, the approach that applies is that of the local control environment, which characterises the initial state of the water table prior to the establishment of the storage site. This is based on the analyses of water table carried out in the study area as part of the process of assessing the initial state of the site and then its regulatory monitoring since 1998.

## Expected Impact on the Alsace Alluvial Water Table

The source term considered for the following was calculated for the presence of risk control measures. Pollutant concentrations were diluted in the clean brine stream before being injected in the hydrogeological model for managing the Alsace alluvial water table developed by BRGM (Noyer and Elsass 2006). This model splits the alluvial fill into 3 layers and uses a nested grid with square cells of 500 m on each side, refined to 125 m directly above the potash basin where the storage site is located (see Fig. 4b). The approach is based on simulating the transport of a conservative tracer, in this case chloride ions, with the assumption that metal pollutants will behave similarly. This represents a new worst-case hypothesis because it disregards processes likely to reduce the concentration of certain reactive pollutants (for instance, adsorption or degradation).

Within the model, the expected mass flux of chlorides is injected into layer 3 of the basal alluvium, from which it disperses, both vertically (into the overlying layers 1 and 2) and horizontally (downstream) within the freshwater flow of the Alsace alluvial water table. The concept of a conservative tracer then enables the concentrations of each studied pollutant to be calculated by using a specific correction factor (Quiot 2012). The maximum expected concentrations pertain to all of layer 3 in the model (basal alluvium), notably $0.007 \mu g \cdot L^{-1}$ for mercury, for which the plume is illustrated in Fig. 8. Moreover, regardless of the pollutant considered, the dilution factor is $\approx 4$ with layer 2 (intermediate alluvium) and 14 with layer 1 (upper alluvium).
<figure><img src="image_15.png" /><figcaption>**Fig. 8** Map of the expected mercury plume in the basal alluvium of the Alsace water table, for an outflow over 1000 years in the presence of risk control measures (from Hennebert and Quiot 2013)</figcaption></figure>

However, these are average concentrations within the 125 m grid cells that contain each shaft, which introduces a dilution artefact. To take this into account, a refinement of each of these grids was carried out with a 5 m step. It thus seems that the mercury plume could locally exceed the quality criteria over a distance of 10–105 m downstream from each Amélie mine shaft, and over a width of 6 to 50 m, particularly at the Joseph and Amélie 2 mine shafts (Fig. 9).

In the presence of risk management measures, the maximum mercury concentration would, at all points in the model except immediately downstream of the 5 Amélie mine shafts, be below the local control environmental quality criteria prior to the installation of the StocaMine

site (0.2 µg·L⁻¹) and below the French regulatory standards for drinking water (1 µg·L⁻¹).

## Conclusion

The StocaMine final waste storage site, located about 500 m deep within the former Amélie potash mine, is expected to gradually flood over several centuries according to the following scenario:

* freshwater from the Alsace alluvial water table enters the mine, mainly through the outer surface of the sealed shafts;
<figure><img src="image_16.png" /><figcaption>**Fig. 9** Mercury pollution plumes immediately downstream of the Joseph and Amélie 2 mine shafts at 1000 years in layer 3 of the model and in the presence of risk control measures (from Hennebert and Quiot 2013)</figcaption></figure>

* this water transforms into brine after contacting the salt and potash, floods the residual mining cavities, and gradually rises to the higher levels where the storage site is located.

If no risk management measures are applied, the brine reaching the storage site may percolate through the waste and dissolve some of it. On the other hand, if effective measures are implemented (sealing barriers, bypass galleries, backfilling of interstitial voids in the storage, etc.), the brine should not come into contact with the waste and should remain "clean".

After three centuries under the continued compaction of the residual cavities, clean brine might flow into the base of the Alsace alluvial water table after passing through the five Amélie mine shafts but, in the event of a failure of risk control measures occurring after 1000 years, pollutants could potentially mix with this brine.

Nevertheless, at any point in the model, except immediately downstream of the five shafts of the Amélie mine, the maximum mercury concentration (which was the main potential source of impact on the groundwater) will be lower than that of the quality criteria adopted for current, and presumed future, uses of the groundwater.

The succession of analysis stages and the choice of hypotheses adopted to develop this scenario constitute an operational process for predicting the rate of flooding of old mining cavities within evaporitic terrain. This can serve as a guide for other industrialists or consulting firms facing similar challenges.

This scenario is the outcome of specific studies conducted on this site by Ineris over more than a decade. It combines various areas of expertise (mining, hydrogeological, geomechanical, geochemical, seismic, toxicological

and ecotoxicological) to form the unique know-how and integrative expertise of this institute.

**Acknowledgements** We thank the successive members of the "Mines De Potasse d'Alsace" (MDPA), for the quality of the information they provided and for the many fruitful discussions that took place during this long period of study.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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