On the ground in the environment, metallic lead from ammunition slowly reacts with oxygen, forming a thin oxide layer of “tarnish” that envelopes the lead and protects it from further oxidation. As a result, the lead can remain on the ground in the environment for a very long period of time in an essentially inert state. The alkaline nature of most soils, particularly in the Western United States, protects the lead and renders the metallic lead quite resistant to oxidation when exposed to water. In addition, any small particles of lead that may occur are quickly bound by the clay layers of the soil which prevents the lead from migrating. Thus, bullets on the ground in the environment, such as resulting from hunting activities, are essentially inert and not a cause for concern.
The solubility of lead is an important factor in determining the relative bioavailability of lead that has been ingested by wildlife. Fortunately, wildlife does not ingest spent lead bullets that are lying on the ground. Although studies have shown that about 3% of doves may ingest a few lead shot pellets incidental to feeding, other studies have indicated that such incidentally ingested pellets are rapidly passed out of the digestive tract without resulting in poisoning.
Even in experiments where lead is deliberately fed repeatedly to wildlife with food, it has been shown that it is very difficult to poison the animals. This lack of bioavailablity of metallic lead is attributed to the combination of metallic lead’s insolubility as well as the buffering of the stomach acid with food and the rapid passage of the food bolus out of the stomach into the neutral intestinal tract.
In a publication directly on point, researchers attempted to poison turkey vultures with lead shot by continuously feeding six turkey vultures large amounts of lead shot. When the birds excreted the shot, the researchers cleaned and re-administered the shot to the birds, with the goal of determining the amount of time necessary for a turkey vulture to succumb to lead poisoning. After 211 days of being continuously fed significant amounts of metallic lead in their food, four of the six turkey vultures in the study showed few signs of lead poisoning. Only when very large amounts of lead were continuously administered for 143 days were the researchers able to induce a fatal lead poisoning in two of the six turkey vultures in the experiment. This experiment clearly shows how difficult it is for wildlife to be poisoned by food containing lead ammunition.
Thus, metallic lead (insoluble lead), such as used in ammunition, has a minimal bioavailability in wildlife.
In stark contrast, many forms of industrial compounded lead, such as the lead in paint chips or environment contaminated with legacy leaded gasoline or leaded pesticides are quite soluble in the digestive tract of wildlife. This ready bioavailability constitutes a substantial environmental threat.
In addition to direct ingestion of industrial lead (soluble lead) contaminated items in the environment, lead poisoned livestock can concentrate a form of highly soluble, toxic lead compounds in the internal organs of their body. When carcasses of such lead poisoned livestock are ingested by scavenger wildlife, such as the California condor, these organs can provide very high levels soluble lead to the wildlife, resulting in poisoning.