INVESTIGATING GROUNDWATER TRANSPORT OF PARTICULATE LEAD SHOT USING ESEM/EDS

Lead contamination is an ever-present threat at shooting ranges, where lead concentrations commonly exceed 10 g/kg within the top 30 cm of soil. Lead projectiles typically form insoluble coatings of lead carbonates, oxides, or phosphates, and are assumed to remain stationary in the subsurface. Recent studies have shown, however, that projectile collisions can produce lead particles smaller than 5 microns in diameter, which might pass through aquifer pore throats and migrate with groundwater. The objective of this study is to evaluate the use of an Environmental Scanning Electron Microscope (ESEM) to determine if microscopic particles of lead can migrate through aquifer materials.

Lead-shot-contaminated soil was placed at the top end of a small (4.3-cm diameter by 15.4 cm long) column of medium-grained (0.5 mm) sand. Five pore volumes of water was passed through the column under a hydraulic gradient of 0.3. At the end of the experiment, the column was sliced into 1- to 10-mm sections, air-dried, then placed onto petrographic slides using epoxy. Samples were analyzed using an FEI QUANTA 400 ESEM with an Oxford INCA Energy 400 EDS. The instrument was programmed to image 30 randomly-located, 2-mm by 1.5-mm fields, then analyze all bright features by EDS. Some slides contained 4000 features within the 30 fields and required approximately 4 hours to scan.

After 5 PV, lead particles between 5 and 10 microns in diameter traveled a distance of 1.0 cm; equivalent to a velocity of approximately 3 m/day. Lead particles larger than 0.5 mm remained within the top 2 mm of the sand column, and lead particles 50-microns in diameter traveled 8 mm. Many lead particles included arsenic (14%) and antimony (35%), which are common toxic metals alloyed with lead shot. This study suggests that lead particulates have the potential to migrate through aquifer media, and ESEM serves as an effective method for evaluating the presence of particulate lead and other heavy metals in soil.