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The Agency for Toxic Substances and Diseases Registry (ATSDR) was requested by the U.S. Environmental Protection Agency (EPA) Region VIII to determine if the provision of public water service would be a prudent public health action for residents currently using water from private wells at residences located to the north of the East Helena Smelter (ASARCO) site arsenic groundwater plume in East Helena, Montana [1].

According to recent sampling, a known arsenic groundwater plume has recently migrated off-site of a historical smelter operation toward a residential area. The plume was previously contained in the shallow aquifer, but has now reached the intermediate aquifer, from which nearby residences draw their well water. Sampling of these residential wells shows that the plume has not yet reached the residential area; however, EPA is concerned that use of the wells will inadvertently draw in any contaminants. A groundwater sample from the intermediate aquifer, taken 3 blocks (about ¼ mile) up-gradient from the residential wells, detected arsenic at 4900 parts per billion (ppb). Residents of approximately 20 individual homes currently use private residential wells for drinking water and/or irrigation. Municipal water is available to the residents, but they are not connected to the main water line. EPA will require that the residential wells be monitored on a regular basis. However, if the plume reaches the residential wells, EPA is concerned that residents will potentially be exposed to contaminated water during a period of 60-90 days (the amount of time needed to complete routine water sampling and analysis).


Residential water wells drawing water from the shallow aquifer have already shown evidence of contamination from the plume-arsenic has been detected at levels of 18-80 ppb. As of November 2001, arsenic had not been detected at levels above the 5 ppb detection limit in any of the residential wells that draw water from the intermediate aquifer [2]. However, the residential wells are located 3 blocks down-gradient from an intermediate aquifer well, which was recently sampled and found to have a high concentration of arsenic (4900 ppb). When the same location was sampled 10 years ago, no arsenic was detected [2]. It is not known when the intermediate aquifer became contaminated with arsenic or how fast the arsenic plume is moving.

The arsenic plume potentially could reach the residential wells for the following reasons:

  • these wells are down-gradient from the plume,
  • these wells are currently only ¼-mile from the edge of the plume,
  • recent evidence indicates that the plume is moving toward the residential wells,
  • the plume has reached the intermediate aquifer,
  • residents' use of intermediate-aquifer residential wells may draw the contaminants to their wells.

In order to detect the edge of the plume, continuous sampling would have to take place. However, the required lag time for completing water sampling and analysis means that there is a potential for exposure to residents of up to 90 days.

ATSDR's Minimal Risk Level (MRL) (a level at which no health effects are expected to occur) for acute exposure (<14 days) to arsenic is 0.005 mg/kg/day. If the plume reaches the residential wells, the estimated dose of arsenic through ingestion of well water is 0.14 mg/kg/day for adults and 0.50 mg/kg/day for children(1). These estimated doses for adults and children, exposed to 4900 ppb of arsenic in their drinking water, are 100 times higher than the MRL. The lowest level of arsenic associated with adverse health effects is 0.05 mg/kg/day. At these levels, acute exposure has resulted in adverse health effects (e.g., gastrointestinal, neurological, and cardiovascular effects) [3].

The time frame of 60-90 days, which is required for sampling and detecting arsenic in private wells, is not protective of public health. Adverse health effects may result from exposure to 4900 ppb arsenic in water from an exposure period as short as 14 days.


The data that ATSDR received from EPA showed that some residents use their wells for irrigation. Arsenic is relatively immobile in agricultural soils and usually remains in upper soil layers [3]. There is some evidence that plants can accumulate arsenic through root uptake or aerial deposition on the leaves; however, the potential for either to occur is dependent on numerous factors, such as soil characteristics [3]. The potential exposure pathways that could occur from using well water for irrigation, if the water becomes contaminated with arsenic, are:

  1. Ingestion of food contaminated with arsenic resulting from irrigation with well water,
  2. Incidental ingestion of arsenic in soil deposited through irrigation,
  3. Dermal contact with irrigation water.

Each of the potential exposure pathways needs further evaluation with more site-specific data for long-term exposure and for sensitive subpopulations (e.g., children). Such an evaluation is outside the scope of this limited health consultation.

Question to ATSDR: Is switching from residential wells to the municipal water service a prudent public health action?

Yes, ATSDR determined that providing municipal water service to the residents currently using private wells, who live north of the arsenic plume, would be a protective and prudent public health action.


  1. ATSDR concludes that there is a potential future public health hazard from arsenic via ingestion of well water.

  2. ATSDR cannot determine, without further evaluation, whether a public health hazard might occur from future use of well water for irrigation.


  1. Provide municipal water service to the residents using well water who live north of the arsenic plume for domestic water use.

  2. Evaluate the exposure pathways from using irrigation water with site-specific data.


Jennifer A. Noack, MPH
Environmental Health Scientist
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation

Ken Orloff, PhD
Senior Toxicologist
Exposure Investigations Section
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation

Reviewer of Report:

Don Joe
Section Chief
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry


  1. Agency for Toxic Substances and Disease Registry. Strike Team Request Form from Dan Strausbaugh, ATSDR. January 30, 2002.

  2. Jennifer Noack, Agency for Toxic Substances and Disease Registry, personal communication with Susan Zazzali, EPA, concerning the private well water data. February 5, 2002.

  3. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Arsenic. Atlanta: U.S. Department of Health and Human Services; 1998.

1. Doses are based on: default drinking water ingestion rates of 2 liters/day for adults and 1 liter/day for children; exposure frequency of 365 days/year; body weights of 70 kg for adults and 10 kg for children; and concentration of arsenic at 4900 ppb.

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