I watched a program on the History channel a while back on the Civil War. They dug up some lead & inspected the soil. They found it had leached out to a distance of 1/4 of an inch!
I did a little research on lead leaching online & found a study by Virginia Tech.
Highlight from the article:
There were 20 million metric tons of lead bullets fired in the United States in the 20th century. Is that lead having an environmental impact?
Not at or near the U.S. Forest Service firing range near Blacksburg, Va., according to research by Virginia Tech geological scientists.
"We were invited by the U.S. Forest Service to look at the shooting range in the National Forest near Blacksburg."
The researchers' survey found 11 metric tons of shot in the shotgun range and 12 metric tons of lead bullets in the rifle range. "These ranges are 10 years old. Most of the lead shot has accumulated on about four or five acres. Some shots have been into the woods, which cover hundreds of acres," Rimstidt said.
Professor James Craig, now retired, and Rimstidt looked first at lead corrosion and whether lead is leaching into the water table or streams. "Lead metal is unstable when it is in contact with air and water. It corrodes and forms hydrocerrussite, the white coating seen on old bullets in museums. That slows corrosion," Rimstidt said.
However some lead escapes, he said. "But we learned that it is absorbed in the top few inches of soil and does not migrate beyond that," Rimstidt said. "Lead is not very mobile. It does not wash away in surface or ground water."
I think the deteriorating rubber from the tires will cause more contamination than the bullets!