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=HoltraChem mercury spill (Orrington, Maine)= The HoltraChem Manufacturing Plant in Orrington Maine was a chlor/alkali producer that operated from 1967-2000. (Miller 2019)

History
Situated in Orrington Maine on the East bank of the Penobscot (see Figure 1), Under its initial operation from 1967-1970 the plant dumped Hg contaminated brine sludge directly into the river. Average losses of Hg were estimated to be 107 pounds of Hg, 19 of which was lost through brine effluent, but an unknown amount of this was recycled back into the plant. No testing on the amount of Hg in the effluent was conducted until 1970 where it was found that the plant dumped 1.5 to 2.5 pounds of Hg into the Penobscot daily. These tests prompted the construction of a detention pond. (Carter 2002) This remained the status quo until 1986 when the EPA filled administrative action against the current owner, Hanlin, that called for testing and cleanup of contaminated soil. In the 1991 settlement, Mallinckrodt agreed to partially fund the mercury contamination study. In 1994, during the ongoing testing, HoltraChem purchased the plant from Hanlin, assuming the responsibility from previous suits. (Carter 2002) The chemical plant continued to operate until 2000 when 2 environmental protection groups (Natural Resources Defense Council and the Maine People’s Alliance) filed suit against HoltraChem and the parent company, Mallinckrodt. NRDC and MPA alleged that Mallinckrodt caused “imminent and substantial endangerment to health and the environment” by discharging mercury into the Penobscot River. (Carter 2002) The court ordered a 3-phase study to assess the health of the river and suggest methods for remediation. (Miller 2019) Phase I of the study concluded in 2008 and found there was reasonable evidence available to conduct a thorough investigation of the Penobscot river/estuary. Phase II began later that year, and the study was concluded by 2013. Phase II of the study found that there were large deposits of inorganic Hg from the HoltraChem plant present in sediments of the Penobscot River and Estuary. A massive pool of mercury contaminated sediment of approximately 320,000 tons was found to be trapped due to reverse acting undercurrents pushing sediments back up the river. It was estimated that it will take 106 to 390 years for certain areas for the mercury to reach ecologically negligible levels. It was also discovered that bacteria were converting the metal into methylmercury, a far more toxic and bio-accumulative form of mercury. (Amec Foster Wheeler employees 2020 Jan 24) This led to the closure of benthic fishing South of Verona Island in 2014 and an extension of the exclusion zone in 2016. (see Figure 2) Phase III of the study was an engineering investigation of possible remediation methods. The study concluded in 2018. Phase III of the study recommended dredging, capping, sediment amendments, sediment trapping, Confined Aquatic Disposal (CAD) cells and/or monitored natural recovery. (Amec Foster Wheeler employees 2020 Jan 24) More information about these methods can be found later in this paper under Future Outlook. The most recent information available as of the writing of this paper was the settlement reached between MPA/NRDC and Mallinckrodt in the 2021. Mallinckrodt agreed to pay between $187 million and $267 million to remediate and monitor Hg pollution in the Penobscot. This also included funding for other beneficial environmental projects in the region. (Marks 2021 Mar 19)

Source of Pollution
Part of the second phase of the Holtrachem study (2013) investigated the pathways of Hg contamination as well as other possible point sources of pollution. (Turner 2013) The major use of mercury in chemical production at the HoltraChem plant was the Castner-Kellner process. Similar chlor-alkali production plants are the largest sources of Hg pollution in Europe and North America. (Beiras 2018)

The Castner-Kellner process, more commonly known as the mercury cell process, was patented in 1894. (Castner 1984 Oct 30) This chemical process allowed the production of highly in demand industrial chemicals (chlorine gas, hydrogen gas, and caustic soda) from saturated saltwater (brine). This version of the chlor/alkali process uses mercury as the cathode of the reaction so that the sodium is easily separated from the brine. The chlorine reacts at the anode of the cell to form cholorine gas which is vented out of the top of the tank. The mercury amalgamates with the sodium sending the product to an alternate chamber where the sodium displaces hydrogen ions in the water. This causes the reduction of aquatic hydrogen into hydrogen gas and has a byproduct of sodium hydroxide, more commonly known in industry as caustic soda. (Eurochlor 2019) For a visual depiction of this process, see Figure 4.

Contaminated Brine
Hg losses were not monitored until 1970, but it is estimated from HoltraChem’s production and values from similar plants, that losses ranged from 4.4 to 10.6 Kg Hg/day. After the construction of the detention pond, the only measurement taken from 1970-1987 showed 0.1Kg Hg/day being drained into the Penobscot though the actual average of this period is assumed to be 0.075 Kg Hg/day. Losses continued to drop, but runoff of the pond from storms meant that even after the plant’s closure in 2000, Hg losses remained just below 10g/day. (Turner 2013) All values taken or estimated from this portion of the study did not include groundwater contamination or air pollution. A summary of estimated levels of Hg from Phase II of the HoltraChem study can be found above in Figure 5. As can be seen by the red cumulative mass line, the majority of HoltraChem’s pollution occurred in the first 3 years of its operation though its operation till 2003 still makes up a significant portion of the Hg lost. (Turner 2013)

Groundwater
Groundwater on the site of the HoltraChem plant was pumped, tested, then treated from 2005 to 2007. After the initial 6 months of pumping, the concentration of Hg stabilized at around 300 ng/L which is above the EPA’s Maximum Contaminant Level of 200 ng/L. (US EPA 2015 Sep 9) (Turner 2013) This suggests that the groundwater surrounding the plant is contaminated and will not be useable without drainage or treatment.

Air Pollution
It is unknown how much Hg was lost in the early years of the plant but it is estimated that approximately, 90lbs of Hg/day was unaccounted for in the effluent leaving the plant. A possible route of this major loss was through vaporization of the mercury in the hot industrial environment. There was mention of using Hg levels above the Veazie dam as a possible metric of Hg deposited by air due to the Northern blowing prevailing winds over Orrington. Mercury deposition would have been found in multiple directions from the plant and Hg levels above the Veazie dam were found to have Hg levels similar to control streams, so this comparison was not used to determine air Hg losses. (Carter 2015)

Fate and Transport
Penobscot Estuary Over the course of its operation the HoltraChem plant released 6-12 tons of mercury into the Penobscot River. Of this mercury emitted it is estimated that 9.3 tons remains trapped in both sedentary and mobile sediments in the Penobscot Estuary. Most of this contamination remains in Fort Point Cove and Penobscot Bay. (Rudd 2013) This area of high concentration (see Figure 6) led to the closure of fisheries above Wilsons Point in 2014. (Dolan 2014 Apr 3) This area along with the 2016 extension were made permanent by the Maine legislature in November of 2016. See Figure 1 for a map of the restricted area. (Amec Foster Wheeler employees 2020 Jan 24)	A problem that is somewhat unique to the Penobscot river is the high mobility of Hg. Due to its history as the lumber capitol of the world during the 1800s, the Penobscot river has over 3 million tons of sawdust deposits; some up to 30 feet think. This deposit of course, light “sediment” allows for the mass movement of MeHg requiring the continued monitoring of the contaminated regions. (Dr. Merritt 2022) Methylmercury Different forms of mercury have different solubilities, bioavailabilities, and toxicities. While all forms of Hg are toxic, methyl mercury (CH3Hg+ or MeHg) is especially dangerous due to its lipid-like structure. This allows methyl mercury to bypass barriers within the body such as the blood-brain barrier or placental barrier. (Beiras 2018) For this reason, “MeHg is about 10 times more bioaccumulable and toxic than inorganic Hg.” (Beiras 2018) The mercury used at the HoltraChem plant was elemental mercury (see Castner-Kellner process), however methylmercury was discovered in multiple sediment sites in the Penobscot at toxic levels. (Turner 2013) This is because bacteria in the sediment catalyze a reaction known as methylation where dissolved organic carbon is attached to oxidized mercury. (Beiras 2018) The Penobscot River chemistry is very favorable to these kinds of reactions. There is also very little sediment flowing downstream meaning that once formed, methylmercury doesn’t get buried very quickly. (Dr. Merritt 2022) Methylmercury makes up 2-5% of total Hg on average in the Penobscot river depending on location, though for some hot spots of microbial activity (such as Mendall Marsh) methyl mercury is up to 22% of total Hg. (Rudd 2013) Health Effects Toxicity Most health effects of metallic mercury (Hg) come from inhalation of mercury vapor from broken devices that use elemental mercury, such as thermometers. High does acute exposure or chronic exposure can cause: tremors, emotional changes, insomnia, weakness, headaches, loss senses, changes in nerve response, and poor cognitive function. Particularly high exposure can cause kidney failure. (US EPA 2015 Sep 3) (World Health Organization 2017 Mar 31) While metallic mercury can be dangerous, it is not the major source of concern for the Penobscot Estuary. Methylmercury is a powerful neurotoxin that is especially dangerous to developing brains such as infants. Because of its affinity to lipids, MeHg can bypass the blood-brain, and placental barrier. Affinity to lipids also makes washing the pollutant out of an organism difficult. This means that organisms will have higher concentrations of MeHg than their surroundings. This effect repeats for each trophic level of an ecosystem in a process known as biomagnification. MeHg concentrations are magnified by about 4x for a predator when compared to their prey. (Beiras 2018) Methylmercury in the Penobscot is the byproduct of the large source of elemental mercury from the HoltraChem plant and large amounts of organic material, especially in the surface waters of marshes. (Rudd 2013) Toxic effects of MeHg include: loss of peripheral vision, numbness in hands, feet, or mouth, lack of coordination, impairment in speech, hearing, and/or muscle function. Infants and children and children are more susceptible to MeHg poising and exposure can lead to mental and neurological disabilities. (US EPA 2015 Sep 3) The “Immediately Dangerous to Life or Health” concentration of MeHg is 2 mg Hg/m^3. (CDC 2018 Nov 2) Carcinogen Neither the World Health Organization (World Health Organization 2017 Mar 31) nor the Environmental Protection Agency (US EPA 2015 Sep 3) claim that mercury or methylmercury cause increased rates of cancer. The EPA does acknowledge that high doses of Hg in rodents did cause an increase in tumors, but that there is not sufficient data/evidence to tie mercury exposure to cancer in humans. (US EPA 2015 Sep 3) Studies from outside WHO and EPA jurisdiction, such as China, have conducted studies suggesting a link between Hg and oxidative DNA damage. (Chen 2005) (Xu 2017) Future Outlook Mercury is no longer required in chlor/alkali processes. Improved membranes/diaphragms can separate the caustic soda from the brine without an amalgam. (Bommaraju and O’Brien 2015 Aug) The United States has only 5 remaining mercury cell plants as of 2008, Japan closed their last mercury cell operations in 2003, and western Europe has committed to close or convert all remaining chlor/alkali plants by 2020. (UNEP Chemicals 2004 Jul) As for the Hg already deposited in the Penobscot; In a 2021 court settlement, Mallinckrodt agreed to pay at least $187 million to remediate mercury in the Penobscot estuary. Of this money: (Marks 2021 Mar 19) (Mallinckrodt US LLC et al. 2021) •	 >$50 million will go towards capping 130 acres of intertidal sediments on affected beaches in Orrington •	>$70 million will go towards the removal of contaminated sediment and sawdust trapped in the Penobscot Bay •	$30 million will go towards remediation of the Orland River and the channel east of Verona Island. •	$20 million to beneficial environmental and community projects in the region affected by contamination •	>$10 million will go towards the long-term monitoring (30-45 years) of Hg concentrations in sediment, water, and wildlife though this length of time may vary depending on results. •	And $7 million will go towards project management In addition, Mallinckrodt agreed to pay up to $80 million more if current litigation is unable to hit targets for remediation. (Marks 2021 Mar 19) (Mallinckrodt US LLC et al. 2021)