Farewell to the toxic sludge reservoir at the Mutiloa mine

The failure of the Aznalcóllar dam on April 25, 1998 marked a before and after in the control of toxic mining waste storage reservoirs in Spain. Soon after, in June 1998, The Basque government has publicly admitted that it has only seen a risk to human health in one of the 83 mining ponds. (includes ponds created in quarries or deteriorated industrial soils) that were inventoried in Euskadi. And now it seems the time has come to close this critical point and rehabilitate the more than five hectares it occupies in the heart of Goierri.

It referred to the mining reservoir of Troya, in Mutiloa, built to bury and store the toxic sludge generated in the extraction of lead and zinc from the subsoil of the Troya mine. A latent threat that has remained isolated and surrounded for more than three decades at the foot of the already closed Lurpe inert material landfill, two kilometers from Ormaiztegi and seven from Beasain.

Now, 30 years after the closure of the mine operated by the Exminesa company from the mid-1980s until 1992 In the municipality of Mutiloa, the owner of the land since 1996, Asfaltos Urretxu, presented a project for the rehabilitation of this enclave.

Accumulates 500,000 cubic meters of toxic waste from lead and zinc extraction until 1992 at the Troya mine

It is planned to empty the lagoon by pumping it, build a saturated roof to bury and isolate toxic waste and regenerate the area’s landscape and environment, where a two-kilometer side road and an artificial wetland will also be built to maintain the bird fauna.

The project was on public display from February 1st to mid-March in the Official Gazette of Gipuzkoa (BOG) and foresees that its closure and rehabilitation could cost more than 2.8 million euros and would take 24 months of work. Mutiloa was one of the last active mines in the Basque Country. In 1988 there were nine left.

Mutiloa dam3

The option chosen for the closure of the reservoir incorporates the criteria set out in the guide for the rehabilitation of abandoned mining waste facilities, published by the Ministry of Ecological Transition of the Government of Spain in 2019.

In this way, the risk of the dam breaking or overflowing is eliminated, a remote possibility, but one that has always been considered and would cause the release of up to 20% of the toxic mud downstream, which would have forced the evacuation of the dams, farms and houses located up to 800 meters away.

Mutiloa Dam2

The Threat of Breaking

According to studies, in the event of a dam failure, it would take between seven and eight months of work to remove the spilled toxic sludge, causing serious environmental damage to the surroundings and especially to the Oria River and its tributary Estanda.

In that bottomless pit that is the reservoir, almost 500,000 cubic meters (m3) of mining waste and a water table about 6 meters high (more than 160,000 m3) accumulate on top of this mud, according to the action. In total, a volume of 700,000 m3 contained by a dam over 30 meters high. The area occupies just over five hectares, about 54 thousand square meters.

The area will be landscaped and an artificial wetland and a two kilometer local road will be created.

To guarantee the stability of the toxic sludge, it must always be covered, isolated from external oxygen, and for that it was necessary to guarantee the flooding of the basin through the constant supply of water throughout all these years.

The installation situation is stable today. On the one hand, technicians ensure that the stored sludge does not leach polluting substances, or does so to a very limited extent. That is, according to these specialists, the waters of the lagoon “do not cause negative effects on the environment”.

Even so, the serious threat of a dam breaking is very great and this action aims to give stability to this area in the long term, without risk of rupture or overflow of the dam. In fact, this reservoir has gone through delicate moments in the last three decades.

rivers without fish

First, a study carried out between June 1992 and June 1993 (close to the closure of the mine) detected strong concentrations of free cyanides in surface waters, a few meters from the spillway and in the natural course of the Gesala stream.

Later, in the river system of the Ribeira de Estanda (where the Ribeira de Gesala flows) there were also considerable increases in various metals: above all cadmium, chromium, copper, lead and zinc; and also on the river Oria, where Estanda flows.

Drying the reservoir will take two and a half months of pumping water 24 hours a day

In fact, new data obtained in 1995 confirmed the contamination problems of the Gesala stream, where aquatic life, mainly the numerous small fish that inhabited it, was practically eliminated due to the high aluminum and zinc content and the precipitation of iron hydroxides.

The recovery of fish life occurred gradually after the creation of a lateral channel that flows into the dam and mixes with the resident water, purifying it, which produced a significant recovery in the Gesala and Estanda rivers. The metal concentration decreased in this way and the sludge accumulated in the channel bed gradually disappeared. through the avenues

Mutiloa dam

The Shadow of Aznalcollar

April 1998 was a before and an after. After the Aznalcóllar catastrophe and the dumping of five million cubic meters of toxic sludge in the Doñana natural park (the trial is scheduled for this year), control actions in this facility were intensified and a report made after an inspection in 1999, found that the situation in the Mutiloa reservoir was “unacceptable”, as the waters were only one meter from the top of the reservoir.

The Basque Energy Entity (EVE) also entrusted Idom Engenharia with the maintenance work from 2008 onwards. And in October 2011, an External Emergency Plan was drawn up which was finally approved on June 12, 2012.

The project went beyond the public exhibition period and expects the works to last 24 months

Also at the end of 2011, a monitoring system was installed that warned of any anomaly in the reservoir and controlled its levels. A system, however, that suffered damage, as it could have been verified in the last visit in December 2021, when checking the theft of the ferry spillway limnimetric control station and its respective datalogger.

The drafting of the closure and rehabilitation project for the reservoir was commissioned by Asfaltos Urretxu (owner of the enclave since 1996) to Idom engineering in December 2021. It is a document of more than 1,000 pages that details the action to be carried out outside.

garbage container

Construction of the dam was carried out in stages, between 1986 and 1992, depending on the rate of lead and zinc extraction at the Troya Mine. These facilities are built to accumulate the sludge generated after the extraction process, instead of dumping it directly into riverbeds. The dam is more than 30 meters high and has not been completely filled with silt, due to the premature closure of the mine.

ducks and herons

It describes that the artificial swamp seeks to minimize the damage that the depletion of the reservoir will cause to the region’s avifauna and he will also purify the waters of the region. Although it is not a protected area, this zone is currently home to several avifaunistic species associated with aquatic ecosystems, including common pits, real anads, common porrons, royal herons, somormujo lavanco, the common mosquito, the white lavandera, and the petirrojo , between others. None of them threatened.

The saturated deck, which will transport 67,853 cubic meters of gravel, another protective layer of 538,021 m3 and a vegetation cover of 21,855 m3, will make it possible to isolate toxic sludge from the surface and oxygen permanently and with guarantees.

The most delicate phase of the action will be emptying the dammed water. First, the height of the spillway will be lowered to evacuate the largest possible volume of water (51,224 m3) and then practically all the remaining water (112,260 m3) will be pumped.

If the results of the analysis of the pumped water are adequate, the pumping will be doubled to 24 liters per second, which would mean the extraction of 2,073 m3/day in 24 hours. A period of two and a half months is foreseen for drying.

It is likely, according to the project, that in the final phase of pumping, the water will have a high concentration of suspended solids or dissolved metals, which is why it will be necessary to decant and collect samples.

nearby cities

The dam is located next to the GI-3540 road, between Ormaiztegi and Gabiria, at an altitude of 230 meters above sea level. The closest population centers to the lagoon are Gabiria (428 inhabitants), at a distance of 1.1 km, but at a higher altitude. Ormaiztegi (1,320 inhabitants), located to the east, at a distance of 2.3 kilometers, following the course of the river Estanda. And Beasain (13,680 inhabitants), also to the east, at a distance of 7.8 km.