EXECUTIVE SUMMARY: LEAD MOBILITY AT SHOOTING RANGES
A variety of issues have been raised about potential environmental impacts of lead associated with shooting ranges, including the potential mobility of lead that may be transported from shooting ranges. Studies published to date indicate a general lack of lead mobility under most environmental conditions. Of the metals present in shot and bullets, lead is the dominant component, is the most likely driver of potential environmental risk at shooting ranges, and will be the focus of this assessment. Hence, this review of scientific literature was conducted to summarize the current understanding of the environmental mobility of lead in surface or ground water, sediment, and soil, and the factors controlling lead mobility.
Lead Chemistry and Mobility
When bullets strike an impact berm they behave in a number of ways, including penetrating, agglomerating, fragmenting, smearing, and ricocheting. Most of the mass of lead in impact berms exists as intact bullets and relatively large fragments. But it is the very small particles of lead and the lead compounds resulting from weathering of metallic lead that result in the most mobility. Furthermore, the continuous disturbance at some berms creates areas void of vegetation, resulting in erosion during rainstorms. The associated surface water runoff can then be transported to adjacent water bodies and can, under certain conditions result in considerable transport of soil containing lead particles. Larger lead shot particles are not typically subjected to such physical processes, but are exposed to atmospheric conditions that can result in transformation of metallic lead into more soluble forms.
The dissolution and mobility of lead derived from lead bullets and shot are ultimately dictated by a number of geochemical processes including oxidation/reduction, precipitation/dissolution, adsorption/desorption, and complexation/chelation. If metallic lead was inert in the environment, then it would not dissolve and would not be transported by any chemical means. However, metallic lead is not inert in the environment and can be oxidized to more mobile forms. It is these subsequent oxidation products that determine the mobility of lead at shooting ranges and not the metallic lead itself. The rate of oxidation and the type of resultant oxidation products are highly variable and site specific.
Once oxidized, lead can be precipitated in a variety of forms including hydroxides, sulfates, sulfides, carbonates, and phosphates. Each of these precipitates are soluble, controlled largely by the site-specific water chemistry to which they are exposed. The important factors that directly control solubility are pH, oxidation-reduction (redox) conditions, and the concentration of the components that determine solubility (the primary solubility controls). As these parameters are highly variable from one location to another, site-specific conditions determine how much lead can be solubilized. In general, lead is much more soluble under acidic (low pH) conditions than at neutral or alkaline (high pH) conditions, but this generality can be violated under a variety of situations. Some precipitates, especially phosphates and sulfides, are particularly effective at controlling lead solubility, often resulting in maintenance of very low lead concentrations in water. Additionally, factors controlling solubility can substantially reduce the bioavailability of lead in sediments and/or soils. The primary solubility control factors establish an upper limit on the amount of dissolved lead present in surface or ground water, and often keep lead concentrations below levels of concern.
Concentrations of dissolved lead in surface and ground water are also effectively controlled by adsorption processes. Lead can be adsorbed by a variety of materials including organic matter, iron and manganese oxyhydroxides, clays, carbonates, and sulfides. It is expected that lead mobility will be effectively controlled by adsorption under the majority of conditions found on shooting ranges. In general, neutral or slightly alkaline conditions will be expected to give rise to low mobility conditions and only acidic conditions will result in substantial mobility. However, there are several exceptions to this generality, as adsorption processes are extremely complex and highly dependent on site-specific conditions.
Complexation/chelation and transport of particulates that contain lead are complicating factors that may increase physical movement of lead. Particulate transport mechanisms may be effective in altering the distribution of lead over time but may not have a substantial effect on bioavailability.
From the relatively few lead mobility studies conducted at shooting ranges, pH appears to be an important factor. Surface water pH values of D 7.0 tend to indicate restricted lead mobility whereas pH values of D 6.5 tend to indicated enhanced lead mobility. Attempts to statistically correlate lead concentrations in water with pH, alkalinity, or shot density, however, have not been successful, likely due to complex interactions of several other environmental factors.
Management Options
If lead mobility needs to be controlled at a particular range, there are several potential options for effectively doing so. At some ranges, conditions will be such that control efforts will not be necessary. At other locations where control might be considered, site-specific conditions may favor some techniques over others. The mobility control techniques include recovery/recycling, control of storm water runoff, vegetative plantings, clay liners, lime addition, phosphate addition, addition of other natural or synthetic chemical additives, and soil capping. Recovery and recycling may be the most cost-effective approach, although a combination of techniques may be required in some situations to effectively control lead mobility.
Geochemical principles and experimental results demonstrated that lead is present in the environment in a variety of chemical forms. Each form of lead has unique properties that affect dissolution and mobility. The interplay of these various forms determines solubility and thus mobility, bioavailability and toxicity. Measuring the total quantity of lead present in environmental media without important parameters will yield an inaccurate picture of the dissolution and mobility of lead from a particular site. Because lead is readily precipitated and adsorbed, basing risk or cleanup decisions on total mass of lead alone will, in general, grossly overestimate risk, and result in unnecessary remediation efforts and unreasonable cleanup goals. This has occurred in the past and may occur in the future until more technically sound evaluative approaches are taken.
Analytical Considerations
A number of analytical procedures used to estimate lead mobility and bioavailability in environmental media can be misleading. The Toxicity Characteristic Leaching Procedure (TCLP) does not mimic realistic environmental conditions at shooting ranges, and is likely to overestimate lead mobility at most shooting ranges. Existing state leaching procedures may be more or less stringent than TCLP depending on the extraction procedure employed. Simple measurements of lead concentrations in soil and sediment do not provide an accurate assessment of lead bioavailability or mobility. The simplest and most reliable indicator of lead mobility in surface water in existing or historical ranges is analysis of the water body in question. If detailed evaluation of mobility in groundwater is necessary, appropriate mathematical models may be useful.
***For a complete copy of Lead Mobility at Shooting Ranges, please contact NSSF (e-mail: cswain@nssf.org ; phone 203-426-1320; or write 11 Mile Hill Road, Newtown, CT 06470 and ask for Catalog No. FD-1/708. The cost for this publication is $25 for non-members and no charge for NSSF/NASR members.