Soil chemical and microbiological properties mapping at an abandoned small-arms firing range in Oak Ridge, TN

Background/Question/Methods

Lead (Pb) contaminated soils represent severe environmental problems as well as a significant hazard to human health.  Prolonged exposure to high levels of toxic metals exerts considerable selective pressure on the soil microbial community and alters soil and ecosystem health.  The objective of this study is to understand how the total level of Pb contamination has affected the soil chemistry and fungal community at an abandoned small-arms firing range in east Tennessee. 92 soil samples were collected in a spatially explicit sampling design across two ecosystem types, grassland and forested, surrounding the backstop at the abandoned firing range.  Soils were analyzed via qPCR for total fungi and bacteria and fungal:bacterial ratios were calculated.  Soil pH, total Pb, extractable Pb, total C and N, nitrate N, organic matter, phosphate, sulfate, and  water content were all determined across the site and site maps, or interpolations, were created using ArcGIS 10.1.

Results/Conclusions

Soil Total Pb concentrations values range from below detection to well over 9,000 ppm or mg Pb kg^-1 dry soil.  Highest values are observed to the northwest of the firing backstop and geographically coincide with an elevated earthen berm behind the backstop.  However, high Total and extractable PB was also detected at sampling locations of lower elevation along the first row both to the north and south of the backstop.  Fungal:Bacterial ratios ranged from below 1.0 to over 6.3, and displayed a patchy distribution.  However several soils with high fungal abundance occur transecting the forested area in a line straight south.  Carbon:Nitrogen ratios were roughly 10.0 across most of the site, but ranged up to over 45.0 within the forested area. 

Pb appears to be moving from behind the backstop to lower elevations and towards the local water source  which could pose a hazard to downstream biota.  Based on previous work at this site which revealed fungal species including Phanerochaete flavido-alba present and capable of Pb-mineral dissolution, the transect of high fungal numbers which directly corresponds with the direction of Pb movement, could be a contributing factor to Pb movement across the site.  The highest levels of Pb also correspond to areas of carbon accumulation, potentially indicating decreased overall soil microbial activities due to Pb toxicity.  Further examination of the soil fungal community for specific genera and functions implicated by previous work in Pb-mineral dissolution and movement is currently being explored.