MERCURY IN A MARINE RESIDENCE
MARINE, WASHINGTON COUNTY, MINNESOTA
The Minnesota Department of Health (MDH) was contacted in June 2000 by a resident of Marine,Washington County, Minnesota who was concerned about the possibility that her house had beencontaminated with mercury from four thermometers. The thermometers were accidentally brokenwhen they fell from a shelf in the hall closet and landed on a carpeted floor. Mercury that was visiblewas picked up and thrown in the trash. The carpet was then vacuumed. A couple of weeks latermore mercury was found in a tweezer case. This mercury was put into a plastic bag for disposal.
At the time of this contact, the Minnesota Pollution Control Agency (MPCA) was conducting pilotsurveillance of background levels of mercury in air in Minnesota. In conjunction with this effort,MDH and MPCA had identified other locations in the state where elevated levels of mercury vaporwere expected. These included a residence contaminated by mercury used to reclaim precious metalsfrom silverware and other industrial products, plots of land where biological solids from wastetreatment and fly ash from electrical generating facilities were applied, and Superfund sites withmercury contamination. To date, MPCA and MDH have sampled the contaminated residence andsamples of air above the land applications have been acquired. MDH has prepared a healthconsultation and a followup on the contaminated residence (MDH 1998; MDH 2001a).Additionally, there are data from another residence contaminated by a mercury-containing gasregulator in 2001. Information about this spill and cleanup is contained in another MDH document (MDH, 2001b).
On June 16, 2000, four thermometers were broken on the carpeted floor of a three-bedroom house inMarine. The house is located on a small rural plot of land situated just northeast of the Twin Cities.The owners raise their own goats and chickens, and have a good-sized garden. The house has about2000 finished square feet (MDH estimate). The bedrooms, living room, dining room, and kitchenare all on the main floor. In addition, the basement is partially finished. The main level, except forkitchen, bathrooms, and 1 hallway between the kitchen and the front door, was carpeted. A pregnantmother, father, 3 children aged from 2 to 12 years old, and their grandmother live in the house.
The thermometers were broken outside of a centrally located linen closet directly off the livingroom, in a hallway midway between the bedrooms and the kitchen. The contaminated carpet was inthe most highly trafficked area of the house. Therefore, the unintentional spreading of contamination was maximized.
On July 3, 2000, MDH and MPCA visited the residence. Sampling outside and inside the house wasconducted using an ambient air mercury vapor analyzer (Model 2537A Mercury Vapor Analyzer,Tekran Inc., Toronto, Canada: flowrate, 1.5 liter/minute (L/min); sample time, 5 min). Thedetection limit of this instrument is less than 1.0 nanogram per cubic meter (< 1.0 ng/m3). Windowsin the house were closed for 24 hours prior to the visit, and central air conditioning (recirculating)was in use. Weather was partly cloudy, warm (72.2 F) and humid (dew point 64.4).
Prior to sampling indoors, outdoor samples were taken next to the house. Samples were then takenin different areas in the house. Air vented from the air conditioning ductwork was also sampled. Aflux chamber was used to sample air immediately above the carpet in a number of locations in thehouse. Exhaust air from the vacuum cleaner used on the contaminated carpet was sampled outdoorswith the vacuum running. A summary of sample data acquired during this site visit is shown in Table 1.
|ng/m3||# of 5 min samples||% CV*|
|Front of linen closet |
(3' above floor)
|Master bedroom |
(3' above floor)
|A/C duct in MBR||728||3||0.6%|
|Flat (flux) chamber in front of linen closet||1997||3||9.9%|
|Flat (flux) chamber in hall||1012||2||13.2%|
|Flat (flux) chamber on living room floor||802||2||3.0%|
|Flat (flux) chamber on MBR floor||744||2||0.6%|
|Vacuum cleaner exhaust (outdoors)|
* Coefficient of variability: Standard deviation/Average concentration
The average mercury vapor concentration in the breathing zone (3 feet above the floor) was 811ng/m3. In the area near where the thermometers broke, the concentration was 802 ng/m3. Note, fromTable 1, that the mercury vapor concentrations in air conditioning ducts were 728 ng/m3. Air thatflowed through the flux chamber that was placed on the carpet where the thermometers broke had1997 ng/m3 mercury vapor. Discussion of the results appears in a later section of this document.
During the July 3rd visit, MDH and MPCA recommended that an area of carpet, including the areaof the spill, be removed and discarded. It was also recommended that the house be aired out as muchas possible during the summer months. Although it is not clear that a physician would treat elevatedmercury in any resident, MDH suggested during a follow-up telephone conversation that analysis ofbiological samples, such as blood or urine, could demonstrate if mercury was present in the bodies ofresidents at elevated levels.
At the request of the residents, MDH sent them a letter on July 10th summarizing the analyticalfindings and recommending removal of a segment of the carpet. The text of this letter is attached(Attachment #1: (MDH 2000)).
On July 13th MDH was notified that the pregnant resident and her 2-year-old daughter had normal blood levels of mercury (below 5 µg/L).
A large section of contaminated carpet was removed from the hallway on July 5, 2000. (Theresident was able to pay for replacement of the carpet through his homeowner's policy.) The barefloor was cleaned and then vacuumed. The house was closed and not ventilated overnight. On July6th the house was revisited by MPCA, and air in the house was sampled. MPCA staff reports ambientair concentrations of mercury vapor had decreased to about 110 ng/m3 (MPCA, 2001).
Fourteen months following the incident (August 15, 2001), MDH revisited this house to determineif residual ambient air mercury levels in the house had decreased further. A Lumex RA-915+Mercury Vapor Analyzer (OhioLumex, Cleveland, OH) was used to survey the house. The Lumexhas an air flowrate of 20 L/min, and a sampling time of 10 seconds, with the mean of 3-10 secondsamples typically reported as a single data point. The detection limit of this instrument is about 2ng/m3 and MDH uses a reporting limit of 10 ng/m3 due to analytical variability. Ambient airconcentrations throughout the house and basement were all below 10 ng/m3 (30 second means, n=9). The house was ventilated to the outside during this sampling.
From the time of the accident on June 16, 2000 until the day the contaminated carpet was removed from the residence, it is expected that mercury concentrations in ambient air inside the house decreased steadily. The maximum level of exposure is not known. Since mercury concentrations found in ambient air in different rooms of the house and concentrations measured in a central air duct were similar, it is assumed that the air in the house is effectively mixed. Mercury levels throughout the house on July 3rd were approximately 800 ng/m3.
It is interesting to note that the mercury concentration of the exhaust air of the vacuum cleaner used during the initial cleanup was 640 - 730 µg/m3 for an extended period of time (entire 15 minute outdoor test). Removal of the vacuum bag resulted in some decrease in the mercury concentration in the exhaust, but even after running the vacuum for more than an hour, the exhaust air concentration could be increased to over 450 ng/m3 by shaking the vacuum cleaner. MDH recommends against the use of vacuum cleaners for mercury cleanup because they can aerosolize elemental mercury, causing a large amount to disperse throughout a building and increasing the contaminated area. These data demonstrate that it is difficult to clean a once contaminated vacuum.
Sampling data acquired during the followup visit on July 6th demonstrate a significant reduction inthe mercury concentration in ambient air in the house. This reduction has been associated with theremoval of approximately 65 square feet of carpet in the area of the spill. Mercury concentrationsfound during this visit were about 110 ng/m3 (C. Herbrandson, Minnesota Department of Health,personal communication, 20001)).
During both visits to the house in 2000, it was recommended that the house be aired out as much aspossible for the duration of the summer. The resident noted that they generally liked to keepwindows and doors open, so this recommendation was easy to follow.
Since doors and windows were closed prior to and during the July 2000 sampling events and openduring the August 2001 sampling, these data are not directly comparable. Typically, the house isopen during the summer; therefore, data from the August 2001 sampling may reflect actualsummertime exposures. On the other hand, conditions during the July 2000 sampling were moresimilar to those during the rest of the year.
Mercury exposure and toxicology
The California Office of Environmental Health Hazard Assessment (OEHHA) has adopted an acute Reference Exposure Level (REL) for mercury vapor based on developmental effects in the offspring of exposed rats (CA OEHHA 2001). Central nervous system effects in pups were noted following exposure of dams to 1.8 mg/m3 for 1 or 3 hours/day during gestation. A cumulative uncertainty factor of 1000 is attached to this REL because it is based on a lowest observable adverse effect concentration (LOAEC: 10X), the primary study was an animal study (10X), and human response to all chemicals is variable (10X). The OEHHA acute REL for mercury vapor is 1,800 ng/m3, with a critical endpoint of reproductive or developmental effects (CA OEHHA 2001). Exposure to mercury for one hour at the acute REL should not result in any significant adverse health effects.
The maximum concentration measured in the house was 2166 ng/m3 in a flux chamber over the spotwhere the thermometers broke. This concentration is about 120% of the acute REL. While thisexceedance is minimal, MDH is typically concerned about any exceedance of acute criteria. This isespecially true when sensitive individuals may be exposed and if the exposure may be more than 1hour in time. The maximum concentration measured in the breathing zone (3 feet above the floor)was 837 ng/m3, or about ½ of the acute REL.
The studies on which the acute REL are based exposed pregnant rats to mercury for 1 or 3 hours oneach of 8 days. Since it is not possible to determine from available data whether the effects onoffspring were caused by the total exposure or a single exposure at an opportune time, the acuteREL is based on a single 1 hour exposure concentration. Therefore, this REL maybe protective evenif exposures are extended over durations greater than 1 hour.
The EPA Integrated Risk Information System database (IRIS) (EPA IRIS 2001) specifies a Reference Concentration (RfC) for chronic exposure to mercury of 300 ng/m3. An EPA RfC is a level of exposure that is judged to be safe for an entire lifetime. This RfC is based on studies of exposures associated health effects for employees in chlor-alkali plants. A threshold type response is presumed, given the standard model used by EPA for non-carcinogens. The critical effects seen were hand tremors, increases in memory disturbances, and slight subjective and objective evidence of autonomic dysfunction. The lowest observable adverse effects concentration (LOAEC) in most occupational studies is about 25 micrograms per cubic meter (µg/m3) (EPA IRIS 2001). Blood mercury concentrations of affected workers averaged 10-12 µg/L. Adjusted to a 24 hour, 7 day per week exposure, the LOAECadj is 9.0 µg/m3. An uncertainty factor of 30 was applied to the LOAECadj to reduce the RfC to a level which is assumed to be associated with no adverse effects. This uncertainty factor includes an uncertainty factor of 10 for human variability in sensitivity, and a factor of 3 for lack of studies on the reproductive and developmental effects of elemental mercury.
Occupational exposures for the primary studies used to develop the RfC took place over an average of 13 to 16 years. It is unlikely that the exposure of residents of the Marine house to elevated levels of mercury occurred for longer than 1 month. However, it should be noted that adverse health effects can result from shorter exposures as demonstrated in another study which provided supporting evidence for the current RfC. In this study dentists who were exposed to mercury vapors for an average of 5.5 years had blood mercury levels of 9.8 µg/L (calculated average exposure; 23,000 ng/m3; 40 hours per week). The calculated LOAECadj for this study was 6000 ng/m3 (Ngim et al. 1992).
Three children, their father, pregnant mother, and grandmother live in this house. Maternalelemental mercury exposure may result in some fetal exposure. However, the endogenousconversion of elemental mercury into mercuric or ionic mercury may inhibit its ability to traversethe placental barrier into the fetus (EPA 1997). This hypothesis is based on a direct comparison offetal blood concentrations to maternal blood following exposure to typical amounts of elementalmercury (Larsson and Sagulin 1990). But these blood levels may not be similar or comparablemeasures of the total body burden to the fetus and mother, respectively. Additionally, the fetus maybe more sensitive to mercury than an adult. Definitive studies on the reproductive anddevelopmental effects of elemental mercury have not been conducted; however, because of the wellknown developmental effects of mercury as methylmercury on the fetus, exposures of pregnantwomen should be minimized. In one methylmercury study, a calculated maternal blood level of 44µg/l total mercury was associated with a 10 % incidence of developmental abnormalities (Marsh etal. 1987 as cited in EPA IRIS 2001).
Cases of children and families being affected by mercury vapor from small mercury spills have beenreported. Specifically, a report of acrodynia (pink disease: pinkness and swelling of extremities,often accompanied by severe pain in joints) in 3 children exposed to mercury from a singlethermometer is noted (von Muhlendahl, 1990). Another report with no known spill source cited acontaminated vacuum cleaner as a source of recurring contamination of a house. Three children wholived in the house suffered difficulty in walking and sadness, nephrotic symptoms, or muscleweakness. Blood mercury from 2 of the children (only samples taken), decreased over 3 ½ monthsfrom 48 and 74 µg/L to 16 and 15 µg/L. Their parents, who also had elevated blood mercury, werewithout symptoms. The parents' blood mercury levels were 12 and 13 µg/L. Three and one-halfmonths later their levels had decreased to 6.6 and 7.4 µg/L. While all family members underwentchelation treatments, the timing and the number of treatments are unclear (Bonhomme et al. 1996).
Exposure to mercury at Marine residence
Mercury concentrations in ambient air on the third of July were measured to be 2½ times the EPA chronic RfC and ½ of the OEHHA acute REL throughout the house. Exposure to this concentration of mercury vapor is of concern to MDH.
The maximum concentration of 2,166 ng/m3 was recorded in air from a flux chamber placed on thecarpet where the thermometers broke. While an individual, especially a small child, may have beenexposed to this level of contamination for short periods of time, it is doubtful that these exposureslasted minutes or hours. Therefore, the best estimate of a maximum 1 hour exposure is between 800and 900 ng/m3 in the breathing zone. Windows and doors at the house were kept closed on July 3 atthe request of MPCA so that ambient air measurements could record potential maximum exposurelevels. Typically, windows and doors at this house are kept open in the summer. Therefore, mercuryconcentrations in air were probably significantly lower than measured concentrations for much ofthe period prior to the removal of the carpet. Concentrations may have been higher immediately afterthe spill. However, unless the source is removed, mercury in air would not be expected to havedecreased substantially before the carpet source was removed.
The mercury vapor concentration in the Marine house on July 3 was about 750 ng/m3. This concentration is 2 ½ times the chronic RfC.. While these occupational exposures resulted in blood mercury levels of 10 - 12 µg/L, the 2 Marine residents tested show blood mercury levels of less than 5 µg/L. This could be due to one or more of the following:
- Exposure to measured levels of mercury in air for less than 1 month may be associated with normal levels of blood mercury.
- The tested individuals may eliminate mercury relatively efficiently.
- Blood mercury may not be a sensitive indicator of relatively small, yet still significant exposures to elemental mercury.
- Blood mercury concentrations were reported to MDH orally and may not have been properly recorded or reported.
Chronic exposure to an average of 5,500 ng/m3 elemental mercury for 5.5 years has been associated with an average blood mercury level of 9.8 µg/L (n=98) (Ngim et al. 1992). Using a linear correlation based on this ratio, 750 ng/m3 inhalation exposure for an extended period may be associated with a blood mercury level of 1.3 µg/L. This blood mercury concentration is considered to be in the 'normal' range. Blood mercury levels in similarly exposed individuals will be different (inter-individual variability). Furthermore, as exposure and blood mercury levels decrease, the percent variability would be expected to increase. Therefore, low but yet significant exposures could be masked by inter-individual variability. In addition, methylmercury exposure which occurs mainly from consumption of fish, appears to correlate better than elemental mercury exposure with whole blood mercury concentrations (Sandborgh-Englund et al. 1998; Sandborgh-Englund et al. 2001). Therefore, fish consumption may mask significant elemental mercury exposures in humans. These difficulties of associating small environmental exposures to specific biomarkers and subtle health effects is typical of human epidemiological data.
Mercury exposure in other Minnesota residences
MPCA measured ambient air concentrations in other houses in the Twin Cities in July and October 2000. These data are listed in Table #2. These data were acquired with the Tekran mercury vapor analyzer. The Tekran mercury vapor analyzer, used at the Marine house in 2000, is believed to be accurate at concentrations below 1 ng/m3. All mercury vapor concentrations measured at the Marine house in 2001 were less than the reporting limit for the Lumex of 10 ng/m3, and thus comparable to data from uncontaminated houses.
|House Built||Average Mercury Concentration||% CV|
EPA RfCs and OEHHA RELs are developed to be protective of sensitive subpopulations, includingchildren. Children and fetuses have been shown to be sensitive to some mercuric compounds.Increased sensitivity to elemental mercury has not been demonstrated but is likely if elementalmercury exposures result in increases in mercury biomarkers that are similar to those observed aftermethylmercury exposures.
In developing the chronic RfC for mercury, an uncertainty factor of 10 was applied to the LOAECcalculated from the primary studies to protect sensitive individuals, and another uncertainty factor of3 was added to counterbalance the lack of a significant database on reproductive and developmentaleffects. As a result, the RfC is equal to the adjusted LOAEC divided by 30 (EPA IRIS 2001).
The acute REL for mercury is based on a developmental LOAEC from an animal study. A factor of10 is used to account for uncertainties associated with using a LOAEC instead of a NOAEC, andadditional factors of 10 were used for inter-species and intra-species variability. As a result the acuteREL contains a cumulative uncertainty factor of 1000 (CA OEHHA 2001).
MDH believes that these criteria are adequate to protect the developing fetus or child, but that there may be risk if these criteria are exceeded.
Four mercury thermometers were accidentally broken on a carpeted floor of a single family house onJune 16, 2000. The resulting spill was cleaned up and then vacuumed. The five residents included apregnant woman and three children under age 12. Seventeen days later, ambient air was sampled inthe house and air conditioning ductwork. Air above the contaminated carpet and exhaust air fromthe vacuum cleaner were also sampled.
Mercury concentrations in air throughout the house (and in the vacuum exhaust) was 2 ½ times thechronic EPA RfC and ½ of the acute OEHHA REL for elemental mercury. Mercury concentrationdirectly above the contaminated carpet was 7 times the RfC and about 20 % greater than the acuteREL. Blood mercury was analyzed for 2 individuals and was in the 'normal' range.
Following removal of the contaminated section of carpet and further aeration of the house, ambientair concentrations of mercury were reduced to about 1/3 of the RfC.
The mercury spill at this house created a health hazard. Had no cleanup been conducted, subtleadverse health effects may have occurred in residents over time. Upon recommendation from MDH,the contaminated carpet was removed from the house. This removed the source of contaminationand there is no remaining apparent contamination. Additionally, MDH recommendations to ventilatethe house thoroughly, to dispose of the vacuum cleaner used during the cleanup, and to consult witha family physician, were followed. As such, this mercury spill event is categorized as posing no public health hazard.
While MDH has no current recommendations for this specific site, activities and ambient airsampling results described in this document reinforce general MDH recommendations on thecleanup of mercury spills.
- Protect yourself. Before beginning to cleanup a mercury spill:
- open windows,
- change into old clothes that can be discarded if mercury spills on them,
- remove all jewelry,
- wear gloves, and
- wash thoroughly after cleanup.
Twin Cities metro area (651) 649-5451
Statewide (800) 422-0798
The following MDH recommendations were also followed at this site:
- Remove contaminated carpet from the house.
- Ventilate the house.
- Dispose of contaminated vacuum cleaner.
- Consult personal physician and test 1 - 2 of the residents for biomarkers of exposure.
Carl Herbrandson, Ph.D.
Site Assessment and Consultation Unit
Environmental Surveillance and Assessment Section
Minnesota Department of Health
Bonhomme C, Gladyszaczak-Kholer J, Cadou A, Ilef D, Kadi Z. 1996. Mercury poisoning by vacuum-cleaner aerosol [letter]. Lancet 347:8994 p115.
California Office of Environmental Health Hazard Assessment. 2001. Reference Exposure Levels. Officeof Environmental Health Hazard Assessment. [online] [cited 2001 Jan 22] Available from: URL: http://www.oehha.org/air/hot_spots/index.html.
Environmental Protection Agency. 1997. Mercury study report to Congress, vol V: Health effects ofmercury and mercury compounds. Office of Air Quality Planning and Standards and Office of Researchand Development, Washington: US Environmental Protection Agency.
Environmental Protection Agency. 2001. Integrated Risk Information System. Office of Health andEnvironmental Assessment, Environmental Criteria Office, EPA IRIS. Available from: URL:http://www.epa.gov/iris/.
Larsson K S, Sagulin G B. 1990. Placental transfer of mercury from amalgam [letter]. Lancet 336:8725p1251.
Marsh D O, Clarkson T W, Cox C, Myers G J, Amin-Zaki L, Al-Tikriti S. 1987. Fetal methylmercurypoisoning. Relationship between concentration in single strands of maternal hair and child effects. ArchNeurol 44:10 p1017-22 (as cited in EPA IRIS, 2001).
Minnesota Department of Health. 1998. Health Consultation: Chemically contaminated southMinneapolis residence. Minneapolis, Hennepin County, Minnesota. June 11, 1999.
Minnesota Department of Health. 2000. Letter to resident concerning contamination from brokenthermometers. Herbrandson, C., Minnesota Department of Health, July 2000.
Minnesota Department of Health. 2001a. Health Consultation: Chemically contaminated southMinneapolis residence. Minneapolis, Hennepin County, Minnesota. Nov 2001.
Minnesota Department of Health. 2001b. Health Consultation: Mercury from a gas regulator spill. St.Paul, Ramsey County, Minnesota. Nov 2001.
Ngim C H, Foo S C, Boey K W, Jeyaratnam J. 1992 Chronic neurobehavioural effects of elementalmercury in dentists. Br J Ind Med 49:11 p782-90.
Sandborgh-Englund G, Ask K, Belfrage E, Ekstrand J. 2001. Mercury exposure in utero and duringinfancy. J Toxicol Environ Health A 63:5 p317-20.
Sandborgh-Englund G, Elinder C G, Johanson G, Lind B, Skare I, Ekstrand J. 1998. The absorption,blood levels, and excretion of mercury after a single dose of mercury vapor in humans. Toxicol ApplPharmacol 150:1 p146-53.
von Muhlendahl K E. 1990. Intoxication from mercury spilled on carpets [letter]. Lancet 336:8730 p1578.
This Mercury in a Marine Residence Health Consultation was prepared by the Minnesota Department ofHealth under a cooperative agreement with the Agency for Toxic Substances and Disease Registry(ATSDR). It is in accordance with approved methodology and procedures existing at the time the health consultation was begun.
Alan W. Yarbrough
Technical Project Officer, SPS, SSAB, DHAC, ATSDR
The Division of Health Assessment and Consultation, ATSDR, has reviewed this public health consultation and concurs with the findings.
Chief, Superfund Site Assessment Branch, DHAC, ATSDR
July 10, 2000
Dear Mr. and Mrs. xxxxxxxx:
This letter is written in response to your request for a report on the mercury contamination found in your house.
It is my understanding that on or about June 16, 2000 four mercury thermometers were accidentally broken on the carpet outside of the linen closet in your house. The carpet is in a high traffic area of the house on the corner of the living room, between the hall to the kitchen and the hall to the bedrooms. At the time of the accident the carpet was cleaned to remove visible mercury and then vacuumed.
On or about June 28, 2000 additional mercury was found in a case containing tweezers and Ms. xxxxxxxx called the Minnesota Department of Health for help in evaluating the contamination and to inquire about the possibility of the mercury affecting the health of the family. In response to this request, Dr. Edward Swain, Harold Weigner, both from the Minnesota Pollution Control Agency (MPCA), and I took many air samples from your house on July 3, 2000. These samples confirmed that the carpet in the vicinity of the closet was significantly contaminated with mercury. Further, the air throughout the house, as well as the vacuum cleaner were also significantly contaminated. The concentration of air throughout the house was about 2.5 times the US EPA Reference Concentration (RfC) of 300 nanograms/cubic meter (ng/m3). The RfC is the concentration in air which is considered safe for long-term exposure. Concentrations of mercury in air outside your house were less than 5 ng/m3. Typical concentrations in uncontaminated houses are around 5 to 10 ng/m3.
I would recommend that you either hire someone experienced in cleaning up contaminated sites to remove contaminated carpet, or that you carefully remove it yourself. If you chose to remove it yourself try not to disturb the carpet: bag it in a plastic bag with minimal shaking. Use your best judgement when collecting the dirt that remains under the rug. Sticky tape can collect some mercury droplets. Another way to minimize the volatilization of mercury is to dust the area with a mercury amalgamating 'spill kit'. These kits should be obtainable through the MPCA (651/296-6300). Follow the directions in the kit to remove the amalgam from the floor. Ventilate your house during cleanup and as much as possible throughout the next few months to maximize the exhausting of mercury vapor to the outside.
The area of carpet that you remove should minimally include an area a few feet to either side of the closet. It may be prudent to remove all the carpet in both hallways and perhaps some in the living room. Unfortunately, it is not possible to determine exactly how much of the carpet will need to be removed in order to lower the mercury concentration in air to safe levels. Furthermore, air testing for mercury is very expensive and, therefore, may make it uneconomical to test the effectiveness of your chosen remedy. MDH would recommend that it is better to err on the side of being overly protective.
If I can be of further assistance, please let me know.
Carl Herbrandson, Ph.D.
Site Assessment and Consultation Unit
cc: Edward Swain, Ph.D.