Effects of Slopes, Rainfall Intensity and Grass Cover on Runoff Loss of Mercury from Floodplain Soil in Oak Ridge TN: A Laboratory Pilot Study

Mercury is recognized as a global pollutant. A large amount of elemental mercury was used in the Manhattan Project in Oak Ridge, TN and Hg is still present in the water-shed surrounding the Y-12 facility and East Fork Poplar Creek. Soil erosion during rainfall-runoff events result in nonpoint source Hg transport from floodplain soils to aquatic ecosystems. This study investigated transport mechanisms of Hg through surface runoff processes from floodplain soils in Oak Ridge, TN into water with a simulated rainfall laboratory pilot scale experiment. The experiments simulated rainfall intensity (50–140 mm/h), the ground grass coverage (5 %-85 %), the soil moisture conditions and the slopes (7.5–16.32 degrees) producing soil runoff erosion processes. Results show that high grass coverage, decreased landscape slope and low rainfall resulted in the decreased Hg concentrations in sediments transported through runoff from the floodplain soil. Hg loss was strongly correlated with and controlled by soil loss. The order of environmental factors on Hg loss was the slope > rainfall intensity > grass cover. As grass cover increased from 5 % to 85 %, Hg in sediment decreased from 124 to 72.3 µg/kg and Hg in penetrating water decreased from 4.7 to 2.8 µg/kg on wet soil condition at the slope 12.560. Hg loss and soil loss significantly increased with the increase of the soil slope. As the slope increased from 7.550 to 12.560, Hg loss increased from 364 to 2905 mg/m2 in the dry soil condition with 5 % grass cover. Meanwhile Hg loss and soil loss significantly decreased with the increase of grass cover. The Hg concentration in sediment increased with increasing initial soil moisture from Dry Soil, to Wet Soil and Very Wet Soil. The roots of grasses stabilized soil Hg and the Hg content in soil rhizosphere were significantly higher than that in non-rhizosphere. This study indicates that rainfall intensity, ground grass coverages, landscape slopes and initial soil moisture content strongly controlled Hg transport from the floodplain ecosystem and its flux into stream water. Mercury associated with sediment was the major Hg input into stream waters from floodplain soils in Oak Ridge, TN. Proper onsite management, such as increasing ground coverages with local low lying grasses and decreasing landscape slopes may significantly reduce potential Hg transport into local stream water in Oak Ridge TN.