Metal(loid)s - Small Arms Ranges

Metals and metalloids are deposited into small arms range soils from bullets, casings, and primers. The mobility potential depends on the individual metal(loid), its speciation, local pH, redox conditions and soil characteristics. This article discusses the speciation, mobility, and toxicity of metal(loid)s relevant to small arms ranges and briefly discusses management strategies to mitigate risk.

Related Article(s):

• Metals and Metalloids - Mobility in Groundwater
• Monitored Natural Attenuation (MNA) of Metal and Metalloids

Contributor(s): Dr. Amanda Barker

Key Resource(s):

• Army Small Arms Training Range Environmental Best Practices (BMPs) Manual
• Management of the Environmental Impact of Shooting Ranges, the Finnish Environment

Introduction

Military small arms training uses bullets made of metals and metalloids which can include lead, antimony, copper, nickel, zinc, chromium, tungsten, mercury, and arsenic. Some of these metal(loid)s are also key ingredients of small arms ammunition primers used to initiate bullet firing. Unlike organic compounds found in propellants and explosives, metal(loid)s do not degrade in the environment. As such, the pervasive use of metal(loid)s on military training grounds and shooting ranges has resulted in contamination of soils, sediments, surface water, and groundwater both local to specific sites and off-site. A simplified schematic showing potential pathways of metal(loid) release on shooting ranges is shown in Figure 1. Upon exposure to the atmosphere, initially zero-valent metal(loid)s are oxidized, form secondary mineral phases, and subsequently release oxidized species into soil solution. The formation of a weathering crust on the surface of fired bullets is shown in Figures 2 and 3. Mobilization of metal(loid)s in pore water and soil solution is controlled by a variety of factors, including pH, oxidation-reduction (redox) environment, presence of colloids and/or soil organic matter (SOM), soil type and hydraulic characteristics, temperature, and water infiltration.

Figure 1. Simplified schematic of shooting ranges with berm-style backstops that typically contain high loadings of metal(loid)s in the berm due to bullet fragmentation and also at the firing point as a result of residue and primer materials.

Figure 2. Chemical characterization of a 5.56mm corroding bullet that underwent 15 years of weathering in Alaskan soils using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Relative concentrations for iron (red), lead (green), and antimony (purple) are shown in the right x-ray map. Data was collected at the Advanced Instrumentation Laboratory (AIL), University of Alaska Fairbanks (UAF). Data collected by A.J. Barker

Figure 3. Optical images of a new 5.56mm bullet (left) and a 5.56mm bullet that weathered for 15 years (right). The tip of the bullet fragmented and the weathering crust can be hundreds of microns thick and comprised of secondary mineral phases. Photo by A.J. Barker.

Metal(loid)s

Lead (Pb)

Lead (Pb) and antimony alloy bullet cores are commonly used in small arms ammunition to provide the high density needed to penetrate surfaces. Pb is estimated to comprise approximately 90-99% of the bullet core with steel, copper, and zinc making up the majority of the remaining mass for the slug and casing. Pb is also used in the compounds lead styphnate and lead azide as primers for ammunition. As a result, Pb can be found on training ranges in soils/structures where the bullet fragments and also at the firing point. Concentrations of Pb in soils at shooting ranges can vary widely depending on the number of rounds fired at the site, but concentrations often range from 55 to 80,000 mg/kg. The mobility of Pb is highly dependent on local pH. Acidic conditions contribute to the mobilization of Pb species in the environment whereas basic conditions tend to promote precipitation and sorption reactions. The formation of secondary mineral phases is a common observation, including hydrocerussite [Pb₃(CO₃)₂(OH)₂], cerussite [PbCO₃], litharge and massicot [PbO polymorphs], but Pb also binds with sulfur and phosphorus depending on mineralogical conditions at a given site. Additionally, Pb has been known to bind with soil organic matter, clay minerals, and iron/aluminum/manganese - oxides. Pb is found in the near-surface environment in one oxidation state, Pb²⁺. Its toxicity is manifested primarily by effects to the central nervous system and brain function, but it can also cause damage to reproductive processes, auditory function, and vision.

Antimony

Antimony (Sb) is a hard, toxic, metalloid that is added to Pb in molten form to make the Pb-alloyed core of a bullet. Sb is thought to behave similarly in the environment to its group VB neighbor Arsenic (As), and, like As, it is toxic and a suspected carcinogen. Sb is added to bullets as a hardening agent because Pb is soft and would not have the same impact capacity without it. Bullet cores typically contain no more than 2% Sb and, correspondingly, concentrations in soils are typically lower than for Pb. Sb is also often found in the primers of small arms ammunition as Sb sulfide. Although Sb is present in bullets approximately two orders of magnitude less than Pb, in some aqueous systems Sb has been found to be more mobile than Pb. However, in more acidic soils Pb often exceeds measured Sb aqueous concentrations due to the high solubility of Pb at low pH values. The mobility of Sb on ranges is highly dependent on its speciation, which in near-surface environments is Sb(III) or Sb(V). The trivalent form has been shown to be less mobile than the pentavalent form, but the trivalent form is considerably more toxic, particularly Sb trioxide. Sb(V) is more mobile at higher pHs, while Sb(III) in solution is independent of pH. Certain environmental factors influence Sb speciation, including pH, redox potential, biological activity, and co-contamination. Sb oxidation is rapid, forming secondary products both in soils and within the weathering crust surrounding bullets. Iron (Fe) is a natural sink for Sb, forming primarily inner sphere sorption complexes (both corner and edge-sharing) with Fe(III) oxides found in soils.

Copper

Copper (Cu) is an essential nutrient for the body and has a relatively high maximum contaminant level (MCL) in drinking water set by the EPA (1.3 mg/kg) compared to other metal(loid)s that comprise bullets. However, toxicity from elevated Cu concentrations can arise and cause gastrointestinal distress (short-term) and liver and/or kidney damage (long-term). The outer casing and jacket of a bullet is comprised primarily of Cu. When a bullet is fired, the outer Cu casing remains at the firing point along with the primer and propellant residues while the Cu jacket impacts soil and either deforms or fragments. New 5.56-mm and 7.62-mm “green” bullets contain solid Cu cores in place of Pb/Sb alloys. Copper concentrations in shooting range soils typically range from 19 to upwards of 6,000 mg/kg. Similar to Pb, Cu mobility is highly dependent on pH, and it has been shown to easily complex with organic matter.

Nickel

Nickel (Ni) toxicity is typically a concern to humans due to its prevalence as an allergen and the ability of Ni to interfere with other essential metals in cellular processes. Trace amounts of Ni are found in the casing and jacket of a bullet, used to make the alloy with Cu. Ni is a biologically important micronutrient and an essential trace element for plants, microbes, animals, and humans. Ni concentrations in environmental systems tend to be conservative and uniformly distributed. In shooting ranges, typical concentrations of Ni are lower compared to the other metal(loid)s like Pb, Sb, Cu, and Zn. Ni tends to be relatively immobile in soil solution under neutral and alkaline conditions, but can become highly mobilized under acidic conditions (similar to the other cationic metals; Pb, Cu, and Zn). Similar to other metal(loid)s, key environmental processes that affect Ni cycling in geomedia include adsorption, (co)precipitation, and complexation.

Zinc

Zinc (Zn) is an essential micronutrient for sustained existence. Similar to Ni, trace amounts of Zn are found in the casing and jacket of a bullet, used to make the alloy with Cu. Zn has been found to become mobilized at acidic pHs, similar to other cationic metals (Pb, Cu, and Ni) and often associates with clay minerals, soil organic matter, and the roots and shoots of plants, if available .

Chromium

Chromium (Cr) in the +3 valence state (Cr(III)) is an essential micronutrient, but has been found to be toxic in large doses. Chromium in the +6 valence state (Cr(VI)) is classified as a human carcinogen. Depending on the quality of the source of metals used for the production of bullets, Cr may be found as an impurity in steel. One study on Cr presence at shooting range soils estimated the Cr content of steel to be up to 27%, which is a much higher estimate than other studies have reported. Typically, Cr concentrations in shooting range soils are reported comparable to background concentrations in local soils, indicating minimal environmental impact from training. Cr is primarily found in the near-surface environment as Cr(III) and Cr(VI). In its hexavalent oxidation state Cr is highly mobile, while Cr(III) is more likely to participate in precipitation and sorption reactions, and therefore mobility in aqueous environments and sediments is limited.

Tungsten

Exposure to high tungsten (W) concentrations has been linked to altered thyroid function, heart disease, and potentially childhood leukemia and lung cancer. Historically, W and nylon have been used as an alternative for Pb in bullets. The tungsten-nylon bullets were termed ‘green bullets’, but did not stay in production for longer than a decade as subsequent research eventually revealed W to be both toxic and highly mobile. Concentrations of W in shooting range soils where green bullets have been spent ranged from below detection to greater than 2,000 mg/kg. W has been shown to be relatively mobile in the environment, with its mobility dependent on speciation. However, there is evidence to suggest W mobility decreases as soil ages, as long as the source is limited. W metal is not stable in the environment and instead primarily exists as W(VI) and prefers to form a variety of polyatomic anions (like WO₄^2- and H₂W₁₂O₄₀^6-, etc.) and polyoxometalates in solution.

Mercury

A compound containing mercury (Hg) (mercury fulminate, Hg(CNO)₂) was previously used as a primer for ammunition for approximately 100 years before it was widely replaced by lead azide, silver azide, or lead styphnate which have greater stability. Despite the minimal to negligible use in recent history, there is some evidence to suggest that legacy shooting ranges may have persistent Hg contamination in soil. Additionally, marine environments near legacy military training grounds may contain elevated concentrations of Hg from the dumping of ammunition after World War II, including the Baltic Sea. Similar to other primer metal(loid)s, Hg in firing range soils should primarily be located near the firing point instead of traveling with the bullet. An old Swiss shooting range was found to have slightly elevated total Hg concentrations above background values (44-315 μg/kg) with maximum values ranging from 906 to 1,517 μg/kg in soils at the firing point, with concentrations decreasing with increasing distance from the firing point. While the concentrations of Hg found at shooting ranges are significantly lower than concentrations reported for Pb and Sb, it still remains a health and environmental hazard due to its harmful effects and toxicity even at low concentrations.

Arsenic

Figure 4. Soil berms with high amounts of soil organic matter and clay minerals as shown in this picture tend to retain concentrations of metal(loid)s as a result of large surface area and active binding sites. Image shows constructed shooting range berms in Delta Junction, Alaska on the Donnelly Training Area. Photo by A.J. Barker.

Arsenic (As) is a highly toxic and carcinogenic metalloid that affects nearly all organ systems. Arsenic has been found at shooting ranges to a much lesser extent than other metal(loid)s, as it is associated with Pb as an impurity. While As is a well-known contaminant in other systems, at shooting ranges it is often not measured or measured at concentrations close to background concentrations in nearby soils. Arsenic exhibits multiple oxidation states in the near-surface environment, but is most often found as arsenite (As(III)) and/or arsenate (As(V)). Major environmental factors that affect As mobility are pH, redox condition, and soil type.

Management Practices

Range design is an important factor in containing metal(loid)s and preventing their migration into the subsurface. Target berms may be comprised of a natural hillside or slope, or constructed with off-site source material that is shaped into an artificial barrier (as shown in Figure 4). Ensuring berm construction includes a berm face that is steep enough to prevent bullet skipping, but stable enough to minimize soil erosion, can minimize contaminated sediment losses. Siting the berm with respect to hydrologic pathways to waterbodies, aquifers, and potential ecological receptors is crucial. Additionally, it is important to decide what type of soil will comprise the berm or hillside (i.e. sand, organic-rich material, etc.) and determining soil texture and major mineral and element fractions. These soil composition variables will affect the overall speciation and mobility of the target contaminant species in bullets (Pb, Sb, etc.). Bullet catchers (see Additional Resources) are designed to catch bullets and contain bullet-related metal(loid)s for later collection and disposal. These systems are designed for long-term use on range. The bullet-related metal(loid)s are stored internally in a protective liner to minimize soil and water interactions with the fired bullet rounds.

Additionally, soil amendments such as phosphate and lime can alter soil pH to inhibit mobility of some metals (mainly cationic metals like Pb, Cu, Ni, and Zn), and other amendments such as Fe/Mn hydro(oxides) can facilitate sorption reactions with metal(loid)s, such as Sb and As. Although costly, spent bullets and large bullet fragments can also be removed from soils.

See Also

• Environmental Bullet Catcher