Analysis of Long-term Research on Mercury Content in the Soils in the Immediate Surroundings of the Southern Ring Road of Krakow

According to literature, intensive use of roads increases the level of soil pollution with individual metals and its extent depends on the type of soil, depth of sampling, and decreases with distance from the road. Supposition of possible soil contamination with mercury in the immediate vicinity of transportation routes results from reported mercury content in crude oil and the observed phenomenon of co-occurrence of mercury and oil-derived substances in soil. This work presents and analyses the results of research on mercury content in the surface layer of soil taken from the immediate vicinity of the southern Krakow ring road. Observations were conducted from 2003–2012 which allowed to show variability of mercury content in the soil over many years. The studies indicated varied distribution of mercury content in the soil, however, mercury content is generally at the same level on both sides of the road. Points with limited exposure to the impact of traffic, i.e. locations elevated above the road and those outside the acoustic screens were excluded from the analysis. Taking into account only the points in heavy traffic impact, slow accumulation of mercury in the top layers of the soil at a depth of 0.4–0.6 m was observed. This was particularly evident in the section of the road from Balice to Swoszowice, which has been located in an area of intense traffic for about 20 years. Average mercury content for all points ranged from 0.025 mg/kg in 2003 to 0.041 mg/kg in 2012. Averaged mercury content in soils from all points collected from other parts of the ring road also showed an upward trend and ranged from 0.026 mg/kg in 2003 to 0.036 mg/kg in 2012. Mercury content in soils collected at points with limited exposure to pollution and at a point with short exposure indicate no fluctuation over the years. Its content is low and is generally from 0.01 to 0.02 mg/kg in the areas that are not in the vicinity of the traffic. Studies of samples from a slightly greater depth of 0.8–1.0 m do not show any characteristic trend in mercury content in the soil. At this stage, long-term studies of soil in the vicinity of Krakow southern ring road do not allow to formulate conclusions regarding the estimation of the migration rate of mercury compounds in the aeration zone profile. They rather suggest the possibility of slow accumulation in the top layers of soil or very slow migration into the layers. In view of the entire research, it may be said that there are hints pointing to a very slow increase in the mercury content in the areas of long-term impact of heavy traffic. Interpretation of the research results, however, is definitely ambiguous. The observed values may be due to either the impact of heavy traffic, to other industrial factors or to natural heterogeneity of soil