Underground disposal of residual solutions in the Devladovskoe uranium ore deposit

22:33 22.08.11

Devladovskoe deposit of uranium ore is located in the Sophia area of Dnepropetrovsk region. It was discovered and developed in 1955-1957.

The mine was developed in 1962-1983 by using underground acid leaching method. ISL mine consisted of three complexes: production unit, transport complex and processing facility.
Working solutions with a concentration of sulfuric acid from a few grams per liter to 50 g per liter with the addition of nitric acid and ammonium nitrogen solutions were used. These solutions were injected through a special well at a depth of 70 meters (buchaksky groundwater reservoir).

After in-situ leaching of uranium, slurry in the form of the acid solution was pumped off other wells. Then it was concentrated, filtered and fed to a further extraction of uranium. Acids were added in a stock solution remained after the extraction in order to obtain the required concentration of working solution. After that, it was re-injected through wells into groundwater reservoir.

Over 2 thousand wells spaced 15-25 meters apart in parallel rows were used. The distance between the rows was 40-60 m. Total area is 2350 thousand m2. In the course of commercial production about 50 million m3 of product solution were used.

According to the specialists’ research, upon completion of exploitation of this deposits, there are more than 6 million m3 (7.8 million tons) of spent acid-radioactive wastes left in the subsurface horizons at a depth of 70 meters. The main pollution agents of the exhausted solution are radionuclides of natural origin: uranium-238, radium-226, thorium-230, polonium-210, lead-210, as well as sulphate and nitrate ions.

Upon completing the operation of Devladovskoe field, land restoration of working area was carried out. Recovery of subsurface water state in the area of ​​development was not carried out, since it was not required by legislation in force at that time.

To monitor the state and movement of contaminated subsurface water, a special monitoring network of 97 wells was established. In addition, pits and ponds in Dolgaya gully are used for monitoring observations.

The impact of abandoned Devladovskoe uranium ore deposit on the environment

Air pollution

Development of uranium deposit by underground leaching eliminates organized sources of dust release, containing radionuclides, as well as manifold reduces release of radioactive substances into the atmosphere when compared with traditional methods - quarrying or mining.

An area source of radon release was the production wells. When pumping with airlift, there was a small aerosol cloud of sputtered particles of solution and gaseous products, including radon near the wells. Upon its release into the atmosphere, the cloud quickly dissipated, and short-lived radionuclides decayed rapidly. At present, a possible source of radioactive contamination in the flowback area can be a dust release originated from contaminated and non-reclaimed soil.

Soil pollution

In applying the ISL technology, soil pollution was only localized in the areas of emergency pipe breaches. Studies have shown that the radioactive contamination penetrates into a shallow depth due to the neutralization of acidic solutions with loam carbonates and binding both sulfates and radionuclides in the gypsum.

Upon mine development, the site was reclaimed by soil removing to a depth of 50 cm and replaced with clean soil. The area of ​​land to be reclaimed was 10% of the land allotment, including zones along the development wells and pipelines.

At present the radiation situation on the surface does not exceed the standard indicators: the total alpha activity in the arable layer of 25 cm does not exceed 7400Bk/kg. The intensity of equivalent dose of external gamma-radiation at a height of 1m above the ground does not exceed 60 RAM.

Surface water contamination

Surface water on the site of deposit are presented by a cascade of ponds in Dolgaya gully, which is the left tributary the Saksagan River. Indicators of surface water contamination by radionuclides in the ponds of the eastern area of the site and 2 km westward of the boundaries of the buchaksky horizon dissemination and do not exceed  standard ones. In the ponds in the western area of the site, surface water is contaminated by nucleotides, but this information is not reliable and should be investigated further.

Currently there are no data on surface watercourse, namely, the Saksagan River and the Kamensk River, which are respectively the regions of discharge and feed of buchaksky groundwater reservoir, in the interstream area of which the ISL area is situated. These rivers should be included in the monitoring system at the former of ISL Devladovo site.

Ground water contamination
Ground water contamination in the area Devladovskoe deposit is of a halo character and covers all the groundwater reservoirs. The site has an area of observation of about 15 km2, which is located in the watershed of Saksagan River and Kamensk River interstream area. Over-the-surface flow from ISL area is directed towards the Saksagan River. Within the ISL area the following groundwater reservoirs are developed: quaternary, sarmatian, buchaksky, a complex of Precambrian crystalline rocks and their crust of weathering.

At the time of completion of operation (1984), a halo of contaminated water covered the full area of ​​ uranium deposits blowback, stretching 4.9 km from the north-east to south-west with a width of 700-1500 m. Downstream the groundwater it has spread 950 m from the south-western border of waste ore deposit.

In 1987-1991, a halo of residual ISL solutions of radionuclides mainly copies the pattern of that in 1984. The differences available, such as the 200 meter shift of south-western boundary to the center, can be attributed to the subsoil self-purification processes.

In 1998, self-purification processes became more visible. The boundaries of the halo shrank in the north-east and then in the lower south-western area.

Almost all groundwater reservoirs under observation have concentration of pollutants, including radionuclides, which exceeds permissible standards. The reason for this is not only natural features of the area of observation, but also ISL deposit, since the concentrations of some pollutants— sulphates, total salts, and radionuclides are above background concentrations, which are typical for this area.

Buchaksky groundwater reservoir is the most contaminated one, as it encroaches uranium deposits. In addition, it was used as an engineering reservoir. Chemical and radionuclide composition of buchaksky groundwater reservoir waters like the waters of other horizons were not suitable for domestic purposes even before the mining development.

Application of ISL technology led to a deterioration of the chemical composition of groundwater and further contamination by sulfates, nitrates, radionuclides.

The content of sulfate ions ranges from 1772 to 23 950 mg/l, which 1,3-17,5 times exceeds the background content; the content of nitrate ions ranges from 50 to 1859 mg / l, while the background one is from 3 to 40 mg/l. Total salt content within the halo varies from 4206 to 36 013 mg/l, which 1,2-10,4 times exceed the background one.

Cumulative contamination of the buchaksky horizon water 1.8-50.68 times exceeds the background. Uranium, lead-210 and polonium-210 made the largest contribution to the pollution of the horizon. Uranium concentrations ranges from 14,76 to 442,8 Bq/l, lead-210 – from 0,51 to 14,87 Bq/l, polonium-210— from 0,21 to 0,95 Bq/l.

Thus, despite the horizons slow self-purification processes, halo pollution abatement, abandoned Devladovskoe uranium ore deposit remains a potentially dangerous source of underground water contamination of different horizons and surface water reservoirs.

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