Mercury is a naturally-occurring chemical element. In general, there are three types of mercury:
- Elemental mercury (the type in older-style thermometers)
- Inorganic mercury compounds (that occur in natural deposits, mostly as cinnabar or mercuric sulfide)
- Organic mercury (commonly occurring as methyl mercury ions)
Methyl mercury is a type of organic mercury that concerns scientists due to its toxicity. Methyl mercury is created through a process called methylation, where inorganic mercury becomes organic or methyl mercury often by naturally-occurring bacteria that live in soil and river sediments. This particular form of mercury, methyl mercury, which is available to organisms is what poses a potential risk to human health and the environment.
Methyl mercury poses a risk related to brain and nerve development in fetuses, embryos, infants, toddlers, and children. This is why consumption advisories are more restrictive for young children and women who are pregnant or nursing. The most common source for methyl mercury exposure to humans comes from eating fish and shellfish. When a person eats fish or shellfish (or other food) that have unacceptably high methyl mercury concentrations, the methyl mercury is nearly completely absorbed through the digestive process. Once ingested, methyl mercury stays in the body for several months.
The half-life of methyl mercury in the human body has been observed to be within the range of 29 to 60 days, with an average of 42 days. Methyl mercury leaves slowly over a period of several months, mostly as inorganic mercury in feces.
Methyl mercury bioaccumulates and biomagnifies.
- Bioaccumulation - if an animal eats food that is contaminated with mercury, then the mercury is absorbed into its body and attaches to its tissues.
- Biomagnification - in general, mercury concentrations increase up the food chain. A species which is high up the food chain - meaning it’s a predator that eats other predators, like tuna or shark - will generally have more methyl mercury compared to a species that is lower on the food chain - meaning a forager that eats plants and lower level organisms, like flounder and halibut.
In addition, mercury can be mobile and does not degrade over time.
- Mobility - it is possible for mercury in sediment to be remobilized by tides and currents, and transported into areas where it is converted to methyl mercury and then bioaccumulated and biomagnified.
- Non-degradability - mercury is a chemical element that does not break down over time. The only pathways for mercury to leave the Penobscot River system are:
- by burial of sediments or by deposition on the marsh plains,
- loss of sediments to the open ocean via tidal exchange, or
- escape to the atmosphere as gaseous elemental mercury as a result of bacterial metabolism.
It has been almost 50 years since the HoltraChem plant’s initial mercury discharges into the Penobscot River. It might be expected that all of the mercury that was discharged into the Penobscot River during the late 1960s would no longer be in the system, yet this is not the case. This has been a puzzle for scientists and has resulted in extensive studies. Refer to the Phase II Study findings under Previous studies.
One possible explanation for the slow recovery is the physical characteristics of the Penobscot River and Estuary. Mercury moves through the Penobscot River by attaching to extremely small sediment particles and organic carbon. Mercury bound to sediment moves downstream as part of the natural transport of sediment. Over time, the sediment is deposited on the River bed in layers where, generally, the higher concentrations are found at depth, and less contaminated sediment is closer to the surface. However, Upper Penobscot Bay is part of a tidal zone where fresh and salt water meet. The heavier salt water flows upstream as a wedge trapping sediment at the bottom of the River, and this salt wedge restricts the sediments from being flushed out to the ocean with the lighter fresh water. Seasonal variations that result in spring melt waters and runoff cause fluctuations in the locations of sediment, but the movement of the sediment is inhibited by the seasonally varying salt water wedge, which essentially traps the sediment in the Estuary.