Here's a photo from the report published in Medical Hypotheses:
1) They used carcasses of organic chickens.
2) The fourth sample consisted of tap water boiled with the same apparatus and for the same length of time, controlling for the possible concentration of lead by route of water evaporation.
3) The fact that the broth made with chicken bones contained more than 7 times as much lead as tap water, and the broth made with chicken cartilage and skin contained more than 9 times as much lead as tap water, clearly illustrates the effect of bioconcentration of contaminants. The higher you eat on the food chain, the more contaminants you consume.
4) Consider that if broth made from chicken skin and cartilage contain 10 times the concentration of lead found in tap water, the skin and cartilage themselves must have a much, much greater a lead concentration, because making broth released the lead from the chicken parts into a vat of water, diluting what occurred in the chicken parts.
Organic farming advocates would consider vegetables and animals grown on lead-contaminated soil "organically grown" so long as the producer followed organic protocols. Animals always concentrate contaminants found in the environment; the higher on the food chain the animal lives, the greater the concentration.
Pasture is not immune to lead contamination, as lead is an airborne contaminant. Combustion of fossil fuels and smelting metals releases lead into the atmosphere, where air currents can carry it to pastures.
A study in Hong Kong found soil and vegetation lead content was inversely proportional to the distance of the soil from motorways.  This suggests that chickens raised in urban areas will have higher lead contamination levels than chickens raised in remote rural areas.
Humans exposed to lead accumulate lead in their bones. One study found that bone lead content increased in Mexican men with age, with the result that "Bone, especially trabecular one, proved to be a significant endogenous lead source for blood and semen burdens in reproductive aged men."
A study in Boston found that "ingestion of lead-contaminated tap water is an important predictor of elevated bone lead levels later in life."  This study found an increase in bone lead in men who consumed more than one glass daily of tap water containing at least 50 mcg lead/liter, levels five times greater than found in the cartilage/skin broth and seven times higher than found in the bone-only broth in the Medical Hypotheses study.
Nevertheless, any increased dietary lead presents an increased risk of lead accumulation in your bones.
One study found an increased risk of hypertension associated with greater bone and blood lead levels.  Another study found lead exposure positively associated with left ventricular hypertrophy and that "halving of the population mean blood lead level would reduce myocardial infarctions by approximately 24,000 events per year and incidence of all cardiovascular disease by over 100,000" 
Another study found that "long term accumulation of lead is associated with an increased uric acid level in middle aged and elderly men."
One U.S. study found that low-level lead exposure increased the risk of kidney impairment.  A review of 23 studies found the same. 
In children, low level lead exposure is linked to intellectual deficits.  One study found that children's "IQ declined by 7.4 points as lifetime average blood lead concentrations increased from 1 to 10 μg per deciliter."
A senior health scientist with the National Toxicology Program seems not to agree with the promoters of bone broth, who think that its claimed benefits outweigh the risk presented by increasing lead intake:
"There does not appear to be a really safe level of lead exposure," said Andrew A. Rooney, a senior health scientist with the National Toxicology Program who coordinated the review of existing research. "The best course of action," he added, "is to eliminate all lead exposure from our environment." 
The CDC doesn't agree with the belief that low levels of lead exposure have no consequence or that we can correct the effects with zinc, calcium vitamin C, or any other nutrient:
"Protecting children from exposure to lead is important to lifelong good health. Even low levels of lead in blood have been shown to affect IQ, ability to pay attention, and academic achievement. And effects of lead exposure cannot be corrected." 
On a page detailing the adverse effects of lead exposure on children and adults, the CDC's Agency for Toxic Substances and Disease Registry states:
"Lead serves no useful purpose in the human body, but its presence in the body can lead to toxic effects, regardless of exposure pathway.To emphasize: "It is important to prevent all lead exposures."
"It must be emphasized that there may be no threshold for developmental effects on children.
- Lead toxicity can affect every organ system.
- On a molecular level, proposed mechanisms for toxicity involve fundamental biochemical processes. These include lead's ability to inhibit or mimic the actions of calcium (which can affect calcium-dependent or related processes) and to interact with proteins (including those with sulfhydryl, amine, phosphate and carboxyl groups) (ATSDR, 2005).
"While the immediate health effect of concern in children is typically neurological, it is important to remember that childhood lead poisoning can lead to health effects later in life including renal effects, hypertension, reproductive problems, and developmental problems with their offspring (see below). The sections below describe specific physiologic effects associated with major organ systems and functions."
- The practicing health care provider can distinguish overt clinical symptoms and health effects that come with high exposure levels on an individual basis.
- However, lack of overt symptoms does not mean “no lead poisoning.”
- Lower levels of exposure have been shown to have many subtle health effects.
- Some researchers have suggested that lead continues to contribute significantly to socio-behavioral problems such as juvenile delinquency and violent crime (Needleman 2002, Nevin 2000).
- It is important to prevent all lead exposures.
Which nutrients protect against lead exposure?
Regarding the effect of nutrients on lead absorption and excretion, the CDC has these remarks :
Iron: "Although iron may help prevent lead absorption in animals, studies of the association between iron deficiency and BLLs in children have produced inconsistent results. There is little evidence that iron promotes a clinically important increase in lead excretion."
Vitamin C: "In summary, although there is fairly strong evidence to support giving vitamin C to adults with EBLLs, there is insufficient evidence to recommend for or against vitamin C supplementation for children with EBLLs."
Calcium: "The results of both animal studies and human laboratory studies provide good evidence that dietary calcium competitively inhibits lead absorption."
Zinc: "Some evidence from animal studies suggests that high levels of dietary zinc inhibit the absorption and retention of lead in animals (56). However, in one small clinical study in which zinc was given with and without vitamin C to lead-exposed workers, the zinc had no demonstrable effect on their BLLs (36). As with calcium, we do not recommend adding zinc supplements to the diet of children with EBLLs."
Other nutrients: "Many other factors have been evaluated as mediators of lead absorption and excretion in adults or animals. These factors include vitamins (thiamin, pyridoxine, vitamin D), minerals (phosphorus), dietary chelators (phytatic acid, alginates, oral EDTA), and frequency of meals. These were not included in this review because of a lack of evidence to determine their efficacy in children."
Of these nutrients, only two get CDC endorsement for countering lead exposure: vitamin C and calcium. Plants and sun exposure provide all of the listed nutrients without the concentration of lead found in bone broth. In fact, phytatic acid and alginates occur primarily in plants (grains and legumes, and sea vegetables, respectively), not animal products.
Lead in plant foods
According to the University of Minnesota Extension Service:
"In general, plants do not absorb or accumulate lead. However, in soils testing high in lead, it is possible for some lead to be taken up. Studies have shown that lead does not readily accumulate in the fruiting parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries, apples). Higher concentrations are more likely to be found in leafy vegetables (e.g., lettuce) and on the surface of root crops (e.g., carrots)."According to Cornell University Cooperative Extension:
"Lead in soil is not particularly mobile; that is, it isn't easily taken up by plant roots. However, if there is sufficient lead in the soil, it can be absorbed by plant roots and leaves of vegetables but is largely excluded from the fruiting parts of these plants (e.g. tomatoes, corn, beans, squash, eggplant, pepper). We found that by knowing the amount of lead in a particular soil it is difficult to predict whether vegetables grown in that site will contain corresponding amounts of lead. This is due to several factors. By far the most important factor was the amount of organic matter in the soil. Lettuce plants grown in soils with low levels of organic matter took up much more lead than those grown on soils with high levels (greater than 25%). With soils of very high organic matter content (40-50% or greater), no lead uptake was found even if the lead present in the soil was as high as 3,000 ppm."
If you want to minimize the potential effect of lead on your health or that of your children, I see three reasonable steps:
1) Reduce lead exposure by removing unnecessary sources of lead from your diet and environment.
2) Eat diet based on fruits, legumes, nuts, seeds, and vegetables.
3) Get sun exposure.
I don't know about my readers, but given the choice between two sources of calcium, iron, and other minerals, one that contains 10 times more lead than tap water, and one that contains virtually none, I will choose the latter. I feel fairly confident that I have lower levels of lead exposure and accumulation eating a plant-based diet rich in all essential nutrients than I would eating a diet rich in all essential nutrients but also including significant lead sources like bone broth. If you want to reduce your lead exposure or body lead levels, why would you ingest a non-essential item that increases your lead exposure?