Groundwater carries dissolved organic and inorganic compounds that can bind to the bone matrix.

Microbial activity produces organic acids and other compounds that become incorporated into the bone.

Root action introduces plant-derived carbon.

None of these additions is the original carbon of the person — they are the carbon of the burial environment, ranging in age from very old to very recent.

If a batch of ancient bone is radiocarbon dated without careful removal of these contaminants, the result will reflect a mixture of ages — the original biological carbon of the individual plus the accumulated environmental carbon.

Depending on the nature and age of the contaminants, this can make the sample appear either older or younger than it actually is.