The preferred management option for ash flows from industrial combustion of solid fuels is utilization rather than landfilling. Due to the high mineral content of kukersite oil shale (45-47 mass%) about 6-7 million tons of oil shale ash (OSA) are produced in Estonia annually. OSA is used currently in the production of cement and concrete, however the share of reuse is still small: up to 3% a year. To raise the percentage of OSA usage and promote OSA as a valuable construction material, Eesti Energia AS with the support of European Financial Instrument LIFE + has organised the pilot road construction with OSA. OSA was used as a binder in mass-stabilisation of peat in Simuna-Vaiatu pilot section constructed in the years 2013-2014. Use of OSA in civil engineering always raises questions about the environmental safety of such activities. The ash is rendered as a hazardous waste according to Estonian legislation due to highly alkaline reaction in contact with water.
The pilot road section was followed by post-environmental monitoring in 2014-2015. The monitoring program involved surface water and soil sampling campaigns. Samples were analyzed for selected constituents and parameters of environmental importance.
OSA contains major matrix elements such as Ca (around 50%), Si (ca 22%), Al (ca 5%), and Fe together with significant amount of minor elements, e.g. Mg, K, P, S and negligible amounts of trace elements. Leaching of hazardous compounds to surrounding aqueous environment is regarded as the main concern.
According to the post-environmental monitoring, the content of trace elements in soil samples was below national environmental quality limits during the follow-up monitoring and the road construction has not affected the soil quality when compared with the background data.
The road construction has not affected the natural level of pH or electric conductivity of surrounding surface water. The content of priority hazardous metals, Hg, Cd and Pb was below limit of detection in all water samples. Amount of nickel was found close to detection limits, however it was similar to the natural background level.
Among hazardous substances the following anions and elements were detected: fluoride, arsenic, barium, copper and zinc. Only the content of barium was found above Estonian national environmental quality standard (0.1 mg/L) in two samples (á 0.14 mg/L). At the same time the natural level of barium in inland water of Estonia is 0.02-0.22 mg/L. Content of other trace elements such as Cu, Zn, Sb, V and Cr in surface water samples was below or close to limit of quantification.
There were seasonal fluctuations in mean content of sulfates in water samples and the total content of sulfates decreased during the follow up monitoring program. At the same time the annual average content of sulfates has risen in some sampling points. The annual average content of sulfates in surface water of the pilot section was 15-62 mg/L, which is not harmful to aqueous environment. Natural level of sulfate in Estonian surface water is 20-50 mg/L and safe content to ecosystem is below 80 mg/L.
OSAMAT project post-environmental campaign has shown that OSA is safe to use in road construction. It is recommended to follow the content of sulfates and barium in nearby surface water during road construction in future monitoring campaigns.
Monitoring and long term performance, both technical and environmental
