In an open-pit gold mine, shortly before midnight a vehicle driver detected a leak on the enclosure of a settling basin for ore processing effluents, as well as a dense cracking pattern at the level of the dyke crest. A bypass channel heading towards the open-pit mine was built as an emergency measure in order to confine 1.3 million m³ of liquid. A total of 2.9 million m³ of effluent containing 25 to 30 mg/l of cyanide reached the OMAI creek, 7 km upstream of the confluence with the ESSEQUIBO River, which was used by the 18,000 residents of the downstream city of Bartica for its fishing resources and water supply. 400 fish were found dead in the OMAI, but none downstream of the confluence. The high flow rates of these watercourses helped dilute the effluent. Maximum cyanide contents of 0.15 mg/l in the OMAI and 0.07 mg/l in the ESSEQUIBO were recorded (in France, the water quality limit for human consumption is set at 0.05 mg/l). The mine was closed. Tanks filled with drinking water were shipped to the site by the Guyana government, which also called upon the international community for aid. An expert from the World Health Organisation (WHO) was dispatched to the scene. Since cyanide is not a bioaccumulative substance, any major health risks appeared to be averted. The OMAI creek was cleaned and the mine remained closed for 6 months, to allow for the broken dam to be secured and to build a new dam upstream.
On 24 August, the leak was plugged, and the operator issued a press release indicating that cyanide content levels observed on 23 August had returned to below 0.03 mg/l.
The retention basin placed in service in 1993 had been built in compliance with Canadian standards of the time. Its design had been approved by the Commonwealth Secretariat after submission by the Guyana government. When the basin broke, the volume of effluent stored was 8 times greater than the maximum specified in the environmental impact study, which served as the mine’s sole operating plan.
The type of break observed was unusual: the dam did not fail, yet its clayey core was cracked throughout, allowing the entire liquid phase to flow out and leaving just a very small solid fraction. An assessment committee assembled by the government noted flaws in terms of both design and execution:
- The steel culvert used to channel the natural flow during dam construction had been backfilled without adopting any special measures to prevent flows running along its outer surface. This culvert acted like a drain crossing the dam and in turn compromised its strength.
- The materials put in place downstream of the dam’s watertight core did not comply the filter design criterion: the rock filling was too coarse to prevent the sand filter from migrating due to the effect of internal flows. The core was thus subjected to internal erosion, causing a material loss estimated at 2% by mass.
- The rock filling had been covered up downstream of a substantial deposit of low permeability solid and clayey mining tailings, without any structural utility.
Erosion along the construction culvert caused transverse penetration by the retained liquid then accumulating in the rocky filling between dam core and impermeable clayey tailings downstream. In the presence of water, the sand filter underlying the core was dispersed, in turn creating a cavity. Without any support, the core was exposed to tensile stress and cracked.
The replacement dam was built according to a different method, using just clayey materials without any rock filling.
Neighbouring Venezuela, claiming its sovereignty over the 159,000-km² Essequibo territory, criticised the mining and forestry concession management practices adopted by the Guyana government.
