The geochemistry of mercury in sedimentary rocks and soils in Pennsylvania

Abstract

The mercury contents of 11 sandstone, 11 shale and 6 limestone samples from Pennsylvania average 7, 23 and 9 ppb Hg, respectively, which is lower than the values for sedimentary rocks reported in the literature. The differences may arise because many of the reported high values are from regions characterized by more mineralization and volcanism than is present in central Pennsylvania. The lowest values found for shale and sandstone in Pennsylvania (0.4 and 0.5 ppb Hg, respectively) are lower by an order of magnitude than the lowest previously reported values. The mercury content of sedimentary rocks varies markedly due to the effects of volcanism, organic material and sulfur in reducing environments, iron and manganese oxides in oxidizing environments, diagenesis, hydrothermal processes, and the thermal history of the rock. Soils in Pennsylvania have much greater amounts of mercury than their parent rocks even after taking into account possible residual concentration, suggesting that mercury is added to the soils from an outside source. Rain is the major source of mercury absorbed by the soil. A portion of the absorbed mercury returns to the atmosphere, establishing a rain-soil-atmosphere mercury cycle. The general enrichment of mercury in soils and sediments compared to rocks is supported by the observation that the mercury content of rain is greater than freshwater. The much higher values of mercury in unconsolidated sediments compared to sedimentary rocks suggest that mercury is lost during diagenesis. Man's contribution of mercury to the surface environment is nearly equal to the natural contribution. Industrial loss contributes more than 65 per cent of man's total, and the contribution of fossil fuel consumption is small, although it may be locally important. The implications of mercury loss and absorption by soils may be an important factor in concentrating mercury in crops and other living matter, especially near industrialized areas.