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HEALTH CONSULTATION

FARIBAULT MUNICIPAL WELL FIELD
FARIBAULT, RICE COUNTY, MINNESOTA


I. BACKGROUND AND STATEMENT OF ISSUES

This Health Consultation documents cooperative investigations conducted on May 27, 1998 and June 30, 1998 by MDH, the MPCA, the Minnesota Department of Natural Resources (DNR), and city of Faribault staff. The purpose of the investigations was to help identify the source of contamination in the municipal well system.

The Faribault Municipal Well Field (site) is located in the city of Faribault and coversapproximately 20 acres (Figure 1). The well field consists of 6 wells, however, Municipal Well 2 isno longer used due to heavy sand pumpage (Figure 2). Wells 1 through 4 are located on the southside of the Cannon River and wells 5 and 6 are located on the north of the river. Trichloroethylene(TCE) has been detected in all wells except for Well 6 which was recently completed.

TCE contamination was first detected in the Faribault Municipal Well Field in October 1982although it is likely TCE existed in the municipal wells for an unknown period prior to October1982 (MDH 1997a). City drinking water has been maintained below federal and state health basedguidelines by blending water from multiple wells before distribution. No treatment is currentlybeing used to remove the TCE. The site is currently being addressed by the Minnesota PollutionControl Agency (MPCA) under the Minnesota State Superfund Program (Permanent List ofPriorities). In August 1997 the Minnesota Department of Health (MDH) completed a HealthConsultation on the Faribault Municipal Well Field summarizing site conditions and addressinglong term risks associated with the contamination in the municipal wells (MDH 1997b). Thisconsult recommended investigations continue to identify the source of the contamination in themunicipal wells and the a well search be conducted in an area near the municipal wells to identifyany private wells that might exist. The August 1997 Health Consultation should be referred to forfurther background information.

The most recent results for samples taken from the Faribault Municipal Water Distribution systemare reported below in Table 1. No volatile organic compounds (VOCs) were detected above thefederal regulatory values called Maximum Contaminant Levels (MCLs) or the Health Risk Limits(HRLs) established by MDH for drinking water. The HRLs are based solely on the health riskassociated with a contaminant. MCLs are based in part on health risks associated with acontaminant, but other factors like the cost and technical feasibility of treatment are also considered.

Table 1.

VOCs Detected in the City of Faribault's Water Supply that are thought to be Related Groundwater Contamination (March 1998)
ContaminantHRLMCLDetected Concentration (g/l)
cis-1,2-Dichloroethene70700.4
Trichloroethylene (TCE)3050.2

Results from Investigation Conducted on May 27, 1998

Municipal Well 4 has historically contained some of the highest TCE concentrations of the 6 wellsthat make up the municipal well field (Figure 1). This well was out of service for approximatelyone month prior to May 27, 1998 for pump maintenance and had not been used near its capacity ofapproximately 1300 gallons per minute (gpm) for an additional two months beforehand. Thehighest TCE concentrations have been detected in this well during periods of disuse. This suggeststhe well may be in the path of a plume under non-pumping conditions.

The depth of the TCE contamination plume is a significant unknown parameter. The best way todetermine the vertical profile of the contamination is through "nested" wells (monitoring wells opento different depth intervals in the aquifer). This option has not been conducted to date because it isexpensive ($50,000 to $200,000). A second option is to employ packers in existing wells. A packerarray consists of two inflatable "plugs" (the packers) that are deployed on a drop pipe with a pumpin the middle. The array is positioned and the packers inflated sealing off an interval of the well thatthe pump is then used to sample. Packers often fail in fractured and karstic media like the Prairie duChien aquifer because the fractures in the rock prevent the packers from getting a good hydraulicseal. In addition, packer testing is also expensive, ranging from $10,000 to $50,000 depending onthe scope of the job.

The method used to investigate the vertical interval of TCE contamination in Well 4 is considerablycheaper than either a packer test or the construction of nested wells, but has been used successfullyto investigate contamination in multi-aquifer wells. A 4-inch submersible pump supplied by anMPCA contractor was set within the well at approximately 20 feet below the water table level (28feet below the surface). This pump discharged approximately 120 gpm while a video logged thewell. The submersible video camera is lowered into the well casing with a light source to illuminatethe area being filmed. The pump induces upward flow in the well; simultaneously the video cameraobserves the vertical intervals which appear to provide the water to the well.

These vertical intervals were identified by observing the paths of iron bacteria flocs in the well. Ironbacteria are essentially harmless and ubiquitous in wells. The orange-red flocs are formed from bio-slime and fibers produced by the bacteria and are suspended or easily dislodged from the walls of thewell by the movement of the video camera. These flocs have a slightly negative buoyancy and aterminal velocity of about 3 feet per minute in water, depending on their size and composition.

Observation of the flocs showed that most of the water that was pumped appeared to enter the wellfrom the bottom of the steel well casing at 47 feet to about 100 feet deep. The video camera showedthat the Prairie du Chien dolomite in this interval was somewhat fractured and contained some smallcave like features but these did not contribute sufficiently large quantities of water to disturb thepaths of the flocs.

In the interval between 100 and 200 feet there appeared to be substantially less contribution of waterto the well (a decreasing rate upward movement of the flocs). In this interval the flocs appeared toalso have a clockwise rotation in the well. One explanation for this rotation is that it is consistentwith the rotation of the impellers of the submersible pump. Similar vortexes have been observed inpipe flow. Between 150 and 200 feet flocs near the well wall could be observed falling while thosenear the center were rising. This is consistent with the expected velocity profile in a pipe where thevelocity is lowest at the walls due to frictional drag.

At about 200 feet deep, there was little if any upward movement of the flocs suggesting that themaximum velocity in the center of the well was less than about 3 feet per minute. Given theborehole diameter of about 23 inches this yields an estimate of no more than 10 gpm or less than10% of the overall pumping rate. Given that the well is about 400 feet deep, this means that about90% of the water yield come from the upper portion of the well.

As part of this investigation, the DNR ran a gamma log of Well 4. This device records variations inthe natural gamma radiation from the geologic materials. Higher gamma radiation is associatedwith clay minerals found in shales and other fine grained aquitard horizons. This log showed thatthere are several zones of shale in the Prairie du Chien dolomite at approximately 140, 180 and 320feet (Figure 3). Some of these zones may provide sufficient retardation of vertical groundwatermovement to separate flow in the upper and lower portions of the aquifer.

Conductivity and temperature, were collected at various depths using a device constructed byMPCA and MDH staff. Neither of these parameter varied significantly with depth (Attachment 1).

The final task was the collection of vertically stratified groundwater samples. This wasaccomplished by re-installing the pump and purging the well at 120 gpm for 1 hour. Although thiswas sufficient time for the removal of one well volume of water, most of the water was likelyremoved from the upper 100 to 200 feet given the observed flow velocities. Hence, samples takenbelow 100 feet likely contained some un-purged water. This water likely contains some residualTCE from passive mixing in the well prior to the purging process. The samples were taken at 60,85, 120, 150, 200, 260, and 322+ feet. The samples were collected using Kemmerer sampler. Thisdevice consists of a tube with spring loaded closures at each end. The sampler is lowered into thewell on a rope and a small weight attached to the is released from the surface which activates thetube closure device. After retrieval, the water from the sampling port is poured into 40 ml vials andkept on ice before analysis. VOCs results did not vary significantly with depth (Attachment 2).

The conductivity, temperature, and VOCs results suggests two possibilities: first, that there is notmuch variation in these parameters in the various vertical horizons that contribute water to the well;second that a complex hydrogeological situation exist involving the hydraulic head, the nearby riversystem and/or the hydraulic conductivity. This complex situation may have made it impossible toidentify the portion of the aquifer(s) where the contamination is entering the well.

Results from Investigation Conducted on June 30, 1998

The uniformity of the VOCs, conductivity, and temperature results from the May 27th investigationdid not help identify what aquifer much less the aquifer interval from which the TCE is entering thewell. MPCA and MDH hydrogeologist assessed the available data and determined a higherpumping rate may be able to determine what aquifer or portion of an aquifer from which thecontaminants are entering. To determine if this is correct, a second investigation was conducted onMunicipal Well 4 on June 30, 1998. The investigation was similar to the one conducted on May27, 1998 only the municipal well pump with a pumping rate of approximately 1300 gpm was usedalong with the smaller pump (120 gpm) used in May.

The results of this investigation are reported in Table 2. Low TCE concentrations were found to inthe upper Prairie du Chein. Higher concentrations were detected in the mid-range and lowerportions of the Prairie du Chein as well as the Jordan (Figure 4). The video log from theinvestigation conducted in May 1998 indicates a low flow rate in the Jordan and lower Prairie duChien. This information, combined with other assumptions made about the hydraulic head leadMPCA and MDH hydrogeologists to conclude contaminants were most likely entering the well fromthe mid-range area of the Prairie du Chein at an approximate depth of 150 feet.


II. DISCUSSION

The detections of contaminants in Well 4 represent a potential health concern. The latest compositesample taken in June 1998 from Well 4 is 13 g/l. The MCL for this contaminant is 5 g/l. Inaddition, there is evidence that TCE contamination is degrading into other contaminants of healthconcern, including vinyl chloride (see Table 2 and Attachment 1). Vinyl chloride was detected inWell 4 but at concentrations below the reporting limit of 0.5 g/l. The report limit is typically 10times greater than the detection limit and is a number that the laboratory can report with a highdegree of confidence. MDH laboratory staff conducted this analysis and stated that vinyl chloridedetected in Well 4 was likely within the range of 0.1 to 0.4 g/l (MDH 1998b). The HRL for vinylchloride is currently 0.2 g/l and the MCL is 2 g/l. Therefore, water from Well 4 may be abovethe HRL for vinyl chloride. However, it is important to note that this well water is mixed with waterfrom other municipal wells before distribution, lowering contaminant concentration in the city'sdrinking water supply. In addition, the health risk associated with vinyl chloride is currently beingreassessed by the U.S. EPA. The outcome of this reassessment may result in an increase of thecurrent HRL for vinyl chloride to between 0.5 and 1 g/l (MDH 1998c).

Determining what interval contaminants are entering Well 4 is an important question from a healthperspective. Data collected from the May and June 1998 investigations of Well 4 do not indicateTCE contamination in Well 4 is originating from the Jordan. However, these investigations do noteliminate this possibility either. Contamination of the Jordan is considered to be a more seriouspublic health concern. TCE contamination in the Jordan suggests a large contamination sourceand/or a source that could be located a great distance from the municipal wells.

Fortunately, the results from the May and June investigations suggest the TCE contaminants areentering from the mid-range of the Prairie du Chein. However, due to variability and unknowninformation regarding the hydrogeology of the area, there is still uncertainty regarding where TCE isentering the well.

If the contamination is coming from the mid-range of the Prairie du Chein, it indicates the sourcesarea is likely to be in Faribault area. Working on this assumption, MPCA staff are currentlyconducting further groundwater investigations relatively near the municipal wells to help identifythe source. This data will be analyzed by both MDH and MPCA hydrogeologist when if becomesavailable.

Another issue of public health concern includes the potential for individuals to be exposed togroundwater contaminants via private wells. While progress is being made in identifying the sourceof the groundwater contamination, the source still is unknown and private wells are used in theFaribault area. To partially address this problem, the 1997 MDH health consultation recommendedthat a well search be conducted in the area near the Faribault Municipal Wells (MDH 1997b). Local authorities are aware of this recommendation and are currently considering options on how toexecute a well search (MDH 1998d). To fully address this issue it will, at a minimum, be necessaryto identify the source area and the extent of the contamination before this potential exposurepathway can be eliminated.

Finally it is worth noting that if the TCE in Well 4 is coming from the mid-range portion of thePrairie du Chien, extending the casing depth below this interval may substantially decrease the TCEconcentrations detected in Well 4. However, this method may not work for Well 4 since thefractured bedrock that makes up the Prairie du Chien may cause unknown but direct conduits fromthe TCE contaminated interval to the remaining open interval of the well once the casing isextended. The effectiveness of this method can only be determined after it has been executed.

Agency for Toxic Substance and Disease Registry (ATSDR) Child Health Initiative

ATSDR's Child Health Initiative recognizes that the unique vulnerabilities of infants and childrenmake them of special concern to communities faced with contamination of their water, soil, air, orfood. Children are at greater risk than adults from certain kinds of exposures to hazardoussubstances. They are more likely to be exposed because they play outdoors and they often bring foodinto contaminated areas. They are shorter than adults, which means they breathe dust, soil, andheavy vapors close to the ground. The developing body systems of children could conceivablysustain permanent damage if toxic exposures occur during critical growth stages.

Children's health concerns were not specifically addressed here because the purpose of this healthconsultation is to review the hydrogeological data to help identify the source of contamination and the possibility of private well contamination. Future evaluations that involve potential exposures at levels of health concern will include a special emphasis on children's health.


III. CONCLUSIONS

  • The VOC contamination of the Faribault Municipal Well Field poses no apparent publichealth hazard because the city's drinking water has been maintained within health guidelinesby blending water from multiple wells before distribution. Private wells pose anindeterminate public health hazard since a well search has not yet been completed. The siteis currently being addressed by the MPCA under the Minnesota State Superfund Program(Permanent List of Priorities). The purpose of this Health Consultation is to document acooperative effort conducted on May 27, 1998 and June 30, 1998 by MDH, MPCA, DNRand city of Faribault staff to identify the source of contamination in the municipal well system.

  • Identifying the portions of the aquifer(s) that are contaminated is important because it willprovide helpful information for identifying the source(s) of the TCE contamination. TheJune 1998 VOC results collected throughout the vertical extent of the well suggest TCEcontamination detected in Well 4 is entering the well from the mid-range portion of thePrairie du Chien aquifer at approximately 150 feet. This suggest the contamination source is relatively near the municipal well fields. Therefore the MPCA is currently conducting groundwater investigations at potential source areas near the municipal wells.

  • While past investigations of Municipal Well 4 suggest contamination is coming from themid-range portion of the Prairie du Chein, the possibility that the contamination is resultingfrom the Jordan Aquifer can not be entirely dismissed. If contamination is originating fromthe Jordan it indicates a more significant public health concern, since the contamination would likely be more be of greater magnitude in terms of the area contaminated and the concentrations of contaminants within the groundwater.

  • Private wells exist in the Faribault area and represent a potential exposure pathway at least until the source of contamination and its vertical and horizontal extent have been identified. Local authorities are currently discussing ways of completing a well search recommended in the August 1997 health consult for this site. This well search would eliminate much of the uncertainty surrounding this exposure pathway.

  • If the TCE in Well 4 is coming from the mid-range portion of the Prairie du Chien,extending the casing depth below the interval of the Prairie du Chien where TCE is entering the well may substantially decrease the TCE concentrations detected in Well 4.

IV. RECOMMENDATIONS

  • Based upon the results of the June 30, 1998 investigation of Well 4, MDH concurs with MPCA actions to conduct further groundwater investigations at suspected sources near the municipal wells.

  • Depending on the results of future investigations, the city of Faribault should consider extending the casing below the contaminated portion of the aquifer.

  • MDH will provide further technical assistance and/or community outreach activitiesregarding health issues associated with the contamination of the Faribault Municipal Wells when appropriate and requested by the MPCA, the city of Faribault, or others.

V. REFERENCES

Minnesota Department of Health (MDH). Phone Conversation with Dick Clark, MDH, Public Water Supply Unit. (Tel: 612-215-0747). 1997a.

MDH. Faribault Municipal Well Field Health Consultation. Faribault, Rice County, Minnesota. 1997b.

MDH. Public Water Supply Database. Faribault Municipal Water Supply (Distribution System). 1998a.

MDH. Phone Conversation with Paul Swedenborg, MDH. July 14, 1998b.

MDH. E-Mail from Helen Goeden, MPCA to Rita Messing, MDH. 1998c.

MDH. Phone Conversation with Mary Ho, Rice County Community Health Service Director (tel: 507-332-5911). September 1998d.


PREPARERS OF THE REPORT

Rich Soule, MDH
Hydrogeologist
Tel: (651) 215-0917

Mark Staba, MDH
Health Assessor
Tel: (651) 215-0913


ATSDR Regional Representative

Louise Fabinski
Office of Regional Operations, Region V
Office of the Assistant Administrator


ATSDR Technical Project Officer

William Greim
Division of Health Assessment and Consultation
Superfund Site Assessment Branch


CERTIFICATION

The Faribault Municipal Well Field Health Consultation was prepared by the MinnesotaDepartment of Health under a cooperative agreement with the Agency for Toxic Substance andDisease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the health consultation was initiated.

William Greim
Technical Project Officer, SPS, SSAB, DHAC


The Division of Health Assessment and Consultation, ATSDR, has reviewed this health consultation and concurs with its findings.

Chief, SPS, SSAB, DHAC, ATSDR


FIGURES

Faribault Municipal Wells Site and the City of Faribault
Figure 1. Faribault Municipal Wells Site and the City of Faribault

Approximate Locations of Municipal Wells
Figure 2. Approximate Locations of Municipal Wells

Municipal Well #4 Well Log
Figure 3. Municipal Well #4 Well Log

TCE Detections at Various Depths in Well 4 (June 1998)
Figure 4. TCE Detections at Various Depths in Well 4 (June 1998)


Table 2.

Contaminants detected in Municipal Well #4 in June 1998
Depth Sampled Contaminants Detected (g/l) HRLs (g/l)
70 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Trichloroethene
Vinyl Chloride
0.4
<0.5*
22
1.5
5.5
<0.5*
70
6
70
100
30
0.2
86 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Trichloroethene
Vinyl Chloride
0.7
<0.5*
20
1.3
8.9
<0.5*

70
6
70
100
30
0.2

91 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
1.4
<0.5*
20
1.5
<0.2*
12
<0.5*
70
6
70
100
7
30
0.2
123 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
2.3
0.6
17
1.9
0.2
16
<0.5*
70
6
70
100
7
30
0.2
136 feet

1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride

2.4
0.6
18
2.1
0.2
17
<0.5*

70
6
70
100
7
30
.2

145 feet 1,1-Dichloroethane
1,1,-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
2.3
0.6
18
2.0
0.2
16
<0.5*
70
6
70
100
7
30
0.2
160 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
2.5
0.6
19
2.3
0.2
17
<0.5*
70
6
70
100
7
30
0.2
223 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
2.0
0.5
17
1.8
<0.2*
14
<0.5*
70
6
70
100
7
30
0.2
330 feet 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
2.3
0.6
19
2.0
<0.2*
16
<0.5*
70
6
70
100
7
30
0.2
Composite 1,1-Dichloroethane
1,1-Dichloroethene
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
Tetrachloroethene
Trichloroethene
Vinyl Chloride
1.7
0.5
16
1.4
0.2
13
<0.5*
70
6
70
100
7
30
0.2


ATTACHMENT 1: TEMPERATURE AND CONDUCTIVITY DATA COLLECTED AT VARIOUS DEPTHS FROM WELL 4 (MAY 1998)

Temperature Specific Conductance Profile


ATTACHMENT 2: VOC DATA COLLECTED AT VARIOUS DEPTHS FROM WELL 4 (MAY 1998)

Attachment 2 was not available in electronic format for conversion to HTML at the time of preparation of this document. To obtain a hard copy of the document, please contact:

Agency for Toxic Substances and Disease Registry
Division of Health Assessment and Consultation
Attn: Chief, Program Evaluation, Records, and Information Services Branch E-56
1600 Clifton Road NE, Atlanta, Georgia 30333


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