FORT PECK INDIAN RESERVATION
POPLAR, ROOSEVELT COUNTY, MONTANA
On October 1, 2001, the Fort Peck Assiniboine and Sioux Tribes requested assistance from theAgency for Toxic Substances and Disease Registry (ATSDR) to evaluate their public healthconcerns associated with exposures to (1) shallow groundwater contaminants, including nitrateand volatile organic compounds (VOCs) and (2) naturally-occurring radioactive material(NORM) associated with oil field activities. This health consultation addresses public healthconcerns related to consumption of groundwater contaminated with nitrate. ATSDR will developother health consultations or reports to address VOCs in groundwater and NORM concerns forthe area.
The Fort Peck Reservation encompasses 2,093,318 acres in the northeast corner of Montana, asshown in Figure 1 (Appendix A) . The majority of the Reservation populance of 10,589 areNative American, representing various bands and pisions of the Assiniboine and Sioux IndianNations . The governing body for the Assiniboine and Sioux Tribes is the Tribal ExecutiveBoard, headquartered in Poplar, Montana.
The population within the reservation is concentrated in several cities along the Missouri Riverand along the highway transportation routes on the southern border of the Reservation. The citiesof Poplar, Wolf Point, Brockton, Fort Kipp, and Frazer have public water supply (PWS) systemsutilizing groundwater extraction wells to supply portions of the community with drinking water.For the PWS systems, groundwater quality parameters, including nitrate levels, are monitoredperiodically in accordance with the U.S. Environmental Protection Agency's (EPA) SafeDrinking Water Act requirements. Residents who are outside the PWS systems for these citiesand residents in remote locations across the Reservation use private groundwater wells for theirwater supply. According to the Water Resources Office for the Fort Peck Tribes, there are over3,000 private wells on the Reservation; an estimated 2,000 are currently active and used fordomestic or livestock purposes . Groundwater is the sole drinking water source for thepopulation of the Fort Peck Reservation .
ATSDR utilizes comparison values for screening chemical concentrations in environmentalmedia to identify contaminants that may require further evaluation. Chemical concentrations ator below the applicable comparison value may reasonably be considered safe. Concentrationsabove the comparison value are not necessarily expected to produce adverse health effects,however such values do indicate a need for further site-specific evaluation. Additionalinformation on ATSDR's comparison values is provided in Appendix B.
Applicable comparison values for nitrate in drinking water include the Fort Peck Tribes NumericWater Quality Standard (NWQS) of 10 mg/L, the EPA Maximum Contaminant Level (MCL) of10 mg/L, and ATSDR's Reference Media Evaluation Guide (RMEG) values of 20 mg/L (child)and 60 mg/L (adult) [5, 6]. The 10 mg/L nitrate value is utilized within this health consultationas the most conservative level for screening groundwater data.
Site-specific data for nitrate concentrations in groundwater wells within the Fort PeckReservation were available from the United States Geological Survey (USGS), Pioneer NaturalResources (a private oil company conducting business in the East Poplar Oil Field area), and theMontana Bureau of Mines and Geology (MBMG). A summary of the nitrate levels above 10mg/L for each data set are included in Table 1. Additional information for each of the data sets ispresented in the following Subsections.
Nitrate in groundwater values are commonly reported in milligrams per liter (mg/L) nitrate asnitrogen (N). For ease of reference in this text, values stated in mg/L are equivalent to 'mg/Lnitrate as N' unless otherwise noted. Typical laboratory analytical methods do not distinguishbetween nitrate and nitrite ions, therefore the sum of the two is often reported and interpreted toconsist primarily of nitrate due to its stability in the environment.
USGS Nitrate in Groundwater Data
The U.S. Geological Survey (USGS) conducted a study of nitrate in groundwater at the Fort Peck Indian Reservation from August 1994 through April 1996. A total of 112 wells were sampled for nitrate analysis; 82 (73%) of the wells were domestic wells and the remaining 30 were livestock wells or USGS test wells. Most of the wells were completed in one of three primary aquifers characterized for the region: the Flaxville Aquifer, Fort Union Aquifer, or Fox Hills-Lower Hell Creek Aquifer. The aquifers are listed in order of increasing depth, although the presence and/or thickness of each aquifer vary across the Reservation. The study wells ranged in depth from 18-440 feet below ground surface (bgs). The USGS nitrate study area covered approximately 720 square miles (22.5% of the Reservation) in the central and northern portions of the Reservation.
For 110 of the wells sampled, nitrate concentrations ranged from <0.05 to 34 mg/L; 2 additionalwells (identified as livestock wells) exhibited nitrate concentrations of 82 and 83 mg/L .Analytical results for 51% of the 112 wells sampled had nitrate concentrations that equaled orexceeded the 10 mg/L screening level. For the 44 wells completed in the shallowest aquifer, 84%exceeded the 10 mg/L screening level for nitrate. Summary statistics for the wells and nitrateanalytical results from the USGS study are presented in Table 2.
USGS concluded that the dominant source of nitrate in groundwater for the study area is soil organic nitrogen from farmland that is managed using crop-fallow rotation techniques .Fertilizer did not appear to be a direct nitrate source for the area. Livestock wastes are a sourceof nitrate that may impact groundwater locally, particularly if nearby wells are constructedwithout an adequate surface seal and/or have been constructed at a particularly shallow depth. A relatively uniform spatial distribution of nitrate concentrations across the study area indicatedthat the nitrate loading is attributable to non-point sources . The USGS study conclusionswere based on extensive information that they collected on soils, land use, pore- andgroundwater chemistry, groundwater age, and stable isotope analyses.
Pioneer Natural Resources Nitrate in Groundwater Data
As part of an unrelated field investigation conducted at the East Poplar Oil Field Unit (EPU), located near the city of Poplar in the southeast quadrant of the Reservation, Pioneer Natural Resources collected and analyzed groundwater samples from 10 monitoring wells and 4 domestic wells . Nitrate was found at levels above 10 mg/L at one domestic well (20.9 mg/L nitrate) and one monitoring well (95.7 mg/L nitrate), as presented in Table 1.
Montana Bureau of Mines and Geology Nitrate Data
ATSDR obtained all water quality data available for wells within the Reservation from the Montana Bureau of Mines and Geology (MBMG) statewide monitoring network . For consistency with the USGS data, wells greater than 440 feet deep were excluded from the MBMG data set. The resulting data set consisted of 195 wells. Documented nitrate concentrations in groundwater ranged from 0.01 to 98.3 mg/L, with a calculated mean of 4.55 mg/L. As shown in Table 1, nitrate levels that exceeded the 10 mg/L screening level were found in 25 of the 195 wells evaluated.
Nitrate and nitrite, inorganic ions composed of nitrogen and oxygen, occur naturally in theenvironment as part of the nitrogen cycle. Nitrate is more prevalent than nitrite in theenvironment because it is more chemically stable. Nitrogen itself is an essential nutrient to plantsand animals. The nitrogen cycle includes the various forms that nitrogen takes as it is chemicallyand/or biologically transformed and used in the environment. Sources of nitrate and nitriteinclude natural organic human and animal wastes, nitrogen-based fertilizer, atmosphericnitrogen, and airborne nitrogen compounds from fuel combustion that are transformed anddeposited on land in the form of nitrate. Nitrate is very water-soluble and can be transported tothe groundwater by surface water infiltration. Factors that influence the nitrate concentration ingroundwater are land use, soil drainage, geology, and depth to groundwater . Agriculturalareas with well-drained soils and shallow groundwater aquifers are particularly susceptible toincreased nitrate concentrations .
Elevated nitrate levels in groundwater are present in various areas of the United States. As partof the National Water Quality Assessment (NAWQA) Program, USGS evaluated data from12,000 wells across the United States and found that nitrate concentrations in about 12 percent ofdomestic water wells in agricultural areas exceeded the 10 mg/L MCL for nitrate . As part ofthe study, USGS also determined that natural concentrations of nitrate in groundwater aregenerally less than 2 mg/L.
Based on available data, elevated nitrate levels (>10 mg/L) are present in groundwater used fordrinking water at the Fort Peck Reservation (Table 1). There are a number of environmentalfactors in the region that support an increased potential for elevated nitrate levels ingroundwater. The majority of the Fort Peck Reservation is overlain by glacial and/or alluvialdeposits and the water table aquifer is relatively shallow [7, 11]. Domestic wells in the area areoften installed in this shallow aquifer. Land use in the area is predominantly agricultural andrangeland; according to 1991 land use statistics, 43% of the reservation's land base was cropland and 55% was rangeland . Both land use activities can result in nitrogen loading on aregional and/or local level.
Nitrate is odorless and colorless, therefore the only way to know if a well is contaminated withnitrate is to collect a sample for analysis at a laboratory. Public water systems are tested andmonitored on a regular basis to ensure compliance with federal safe drinking water standards. Inrural areas where public water systems do not exist, the inpidual property owner is typicallyresponsible for testing and ensuring the quality of water from his/her well .
Human Health Effects
Nitrate is naturally-occurring in the environment as well as in some of the foods in a typical human diet. Cauliflower, spinach, collard greens, broccoli, and root vegetables such as beets and carrots have naturally greater nitrate contents than other plant foods. While vegetables seldom are a source of acute toxicity, they account for more than 70% of the nitrate in a typical human diet. The remainder of the nitrate in a typical diet comes from drinking water (approximately 21%) and from meat and meat products (approximately 6%) in which sodium nitrate is used as a preservative and color-enhancer .
In humans, ingested nitrate can be converted into nitrite by bacteria in the gastrointestinal system(i.e., stomach, large intestine). The nitrite metabolite is highly reactive and can interact withhemoglobin in the bloodstream, converting it to a form (methemoglobin) that cannot carryoxygen. In adults and children, the normal level of methemoglobin in the blood is 1-2% of thetotal hemoglobin [13, 14]. Enzymes in the body regulate methemoglobin levels by converting itback to hemoglobin. Approximately 60% to 70% of an ingested nitrate dose is metabolized inthe body or excreted in urine within the first 24 hours. Half-lives of parent nitrate compounds areusually less than 1 hour; half-lives of metabolites range from 1 hour to 8 hours .
Excess exposure to nitrate can produce methemoglobinemia, a condition where methemoglobinlevels in the blood become high enough to create oxygen deficiencies in the body.Methemoglobin concentrations of 10-20% in the blood may create symptoms of cyanosis (bluishor slate-gray skin color) and labored breathing. Headache, dizziness, nausea, vomiting, diarrhea,and irregular heartbeat are common symptoms associated with methemoglobin concentrations of20-40% in the blood. As methemoglobin levels increase, shock, convulsions, or coma may result[13,15].
Mild nitrate toxicity may not require treatment other than avoiding ingestion or inhalation ofnitrate-containing substances. Depending on the severity of the case, methemoglobinemia maybe treated by administering oxygen, administering methylene blue, or through a bloodtransfusion [13,15].
The conventional approach of boiling water to destroy microorganisms is not a safe practicewhen nitrate contamination is suspected; evaporation actually increases the nitrate concentrationin water.
Health Effects at Fort Peck
The Chief Medical Officer at the Indian Health Service clinic in Poplar, Montana reported that no known cases of methemoglobinemia have been documented .
Children's Health Considerations
ATSDR recognizes that infants and children might be more vulnerable than adults to exposure in communities faced with environmental contamination. Children can be especially vulnerable to chemical exposures because they have a relatively low body weight and their body systems are still developing. Because children depend completely on adults for risk identification and management decisions, ATSDR is committed to evaluating their special interests.
Nitrate found in groundwater is of special concern for young infants because fetal hemoglobin ismore readily oxidized to methemoglobin and the gastrointestinal tract in infants tends to have anaturally high pH that promotes the nitrate to nitrite conversion . Methemoglobinemia isoften called "blue-baby syndrome" because of the symptomatic blueish skin color that resultsfrom low oxygen levels in the blood. Infants less than 6 months old, and especially those underfour months of age, are particularly susceptible to methemoglobinemia as a result of ingestingwater containing elevated levels of nitrate. For infants, the major source of nitrate exposure isnitrate-contaminated drinking water used to dilute formula [13, 17].
The Wisconsin Department of Health and Family Services reported that two independent casesof infant methoglobinemia that involved formula prepared from well water containing 22.9 and27.4 mg/L nitrate . A survey conducted by the American Public Health Associationevaluated 214 cases of methemoglobinemia in infants for which nitrate data in water wereavailable. No methemoglobinemia cases were identified for infants that consumed watercontaining less than 10 mg/L nitrate; 209 cases (98% of the total) were associated with infantsexposed to water containing greater than 20 mg/L nitrate .
Other Susceptible Human Populations
Pregnant women may be more susceptible to developing acquired methemoglobinemia because of naturally increased levels of methemoglobin around the 30th week of pregnancy . Other sensitive populations include adults with methemoglobin reductase enzyme deficiencies or an abnormal hemoglobin molecule (hemoglobin M disease) [13, 14]. Diseases such as achlorhydria or atrophic gastritis, which increase the pH of the gastric fluids, may also promote conversion of nitrate to nitrite within the body and result in an increased risk of acquired methemoglobinemia .
Livestock Health Effects
Livestock are also susceptible to nitrate poisoning. Generally, livestock drinking water containing less than 100 mg/L nitrate should be safe provided the animals are fed a balanced diet that is not high in nitrate . The Idaho Department of Environmental Quality (DEQ) further elaborates that if livestock feed contains more than 1,000 parts per million (ppm) nitrate, problems can occur with drinking water concentrations of 20-40 mg/L nitrate . The Idaho DEQ proposes that if well water contains 40-100 mg/L nitrate, livestock feed should be low in nitrate, well-balanced, and fortified with vitamin A.
Within some areas of the Fort Peck Reservation, elevated nitrate concentrations in groundwaterare a public health hazard for young infants, pregnant women, and people with medicalconditions that affect hemoglobin or methemoglobin levels in their bloodstream. This conclusionis based on the following:
- The aquifers that serve as the sole drinking water source for the Fort Peck Reservationare relatively shallow and vulnerable to nitrate loading from a variety of non-point sources,including soil organic nitrogen, fertilizers, and livestock and septic system wastes.
- Groundwater data collected from wells within the Fort Peck Indian Reservation indicatenitrate levels that are above 10 mg/L for a significant number of the sampled wells (Table 1).
- A portion of the residents in the Fort Peck area that rely on private wells for their watersupply may be exposed to nitrate levels that would be expected to cause adverse health effects,particularly in sensitive populations such as pregnant women and young infants.
The issue of elevated nitrate concentrations in groundwater is one that many states, includingnearby Idaho and Nebraska, are currently monitoring and attempting to manage. In general, themost important activities from a public health perspective are to identify specific wells or areaswith high nitrate concentrations and to promote safe practices (e.g., limiting drinking waterintake from the contaminated source) among sensitive populations. Specific recommendationsinclude the following:
- Identify specific wells or areas with high nitrate concentrations using USGS study resultsand water quality data from the MBMG [7, 9]. Test private wells within the Fort PeckReservation regularly for nitrate levels. USEPA Region 8, which includes Montana, recommendsthat well users test their water every one to three years for both total nitrate and biologicalcontent .
- Prevent pregnant women and infants less than six months old from drinking well waterthat contains nitrate at concentrations greater than 10 mg/L. Do not prepare baby formula withwell water that contains nitrate at concentrations greater than 10 mg/L. Children should limittheir daily intake of well water if it contains nitrate concentrations in excess of 10 mg/L nitrate.Adults should limit their intake of well water if it contains nitrate concentrations in excess of 60mg/L nitrate.
- Conduct community health education and promote awareness among medical personnelthat may encounter patients with exposures to nitrate in groundwater.
- Ensure that domestic wells are installed with an adequate surface seal and are located atleast 100 feet away from livestock areas and septic fields to prevent localized contamination dueto infiltration of high nitrate-containing wastes. Generally, deeper wells that are constructed witha proper surface seal are less susceptible to nitrate contamination. In some areas of thereservation (e.g., the East Poplar Oil Field Unit), deeper well installations may not be prudentbecause of groundwater contaminant concerns associated with oil field activities.
Barbara Anderson, PE
Environmental Health Scientist
Exposure Investigations and Consultations Branch
pision of Health Assessment and Consultation
Office of Regional Operations, Region VIII
Agency for Toxic Substances and Disease Registry
Environmental Health Specialist
Office of Tribal Affairs
Agency for Toxic Substances and Disease Registry
Exposure Investigations and Consultations Branch
pision of Health Assessment and Consultation
John Abraham, PhD, MPH
Exposure Investigations and Consultations Branch
pision of Health Assessment and Consultation
- Indian Health Service (IHS). History of the Fort Peck Reservation. Rockville Maryland:Indian Health Service. Accessed April 2002. Available at http://www.ftpeck.ihs.gov/fpsu-history.asp
- Bureau of the Census. 2000 Census Population. Washington: U.S. Department of Commerce.
- Agency for Toxic Substances and Disease Registry. ATSDR record of activity fortelephone communication with Water Resources Office for the Fort Peck Tribes. Atlanta,Georgia. April 26, 2002.
- Agency for Toxic Substances and Disease Registry. ATSDR record of activity fortelephone communication with the Director of Environmental Affairs for the Fort Peck Tribes.Atlanta, Georgia. April 18, 2002.
- Fort Peck Tribes Office of Environmental Protection. Fort Peck Tribes Numeric WaterQuality Standards. Poplar, Montana. August 14, 2001.
- Agency for Toxic Substances and Disease Registry. Substance Comparison Values May2002. Department of Health and Human Services.
- Nimick, D.A. and Thamke, J.N. Extent, Magnitude, and Sources of Nitrate in theFlaxville and Underlying Aquifers, Fort Peck Indian Reservation, Northeastern Montana. U.S. Geological Survey Water Resources Investigations Report 98-4079. 1998.
- Pioneer Natural Resources. Field Investigation Biere Well Evaluation, Poplar, Montana.Prepared by CH2MHILL. August 2000.
- Montana Groundwater Information Center (GWIC). Water quality data set received May8, 2002 from Tom Patton, Montana Bureau of Mines and Geology (MBMG). Butte, Montana.2002.
- Mueller, D.K. and Helsel, D.R. Nutrients in the Nation's Waters - Too Much of a GoodThing?. U.S. Geological Survey Circular 1136. 1996.
- Thamke, J.N., Craigg, S.D., and Mendes, T.M. Hydrologic Data for the East Poplar OilField, Fort Peck Indian Reservation, Northeastern Montana. U.S. Geological Survey Open FileReport 95-749. January 1996.
- United States Environmental Protection Agency Region 8. Nitrates In Groundwater -Answers to Frequently Asked Questions. Accessed April 2002. Available athttp://www.epa.gov/Region8/water/dwhome/wycon/dwregs/dwnitrate.html
- Agency for Toxic Substances and Disease Registry. Case Studies in EnvironmentalMedicine: Nitrate/Nitrite Toxicity. Atlanta: U.S. Department of Health and Human Services;October 1991.
- United States Environmental Protection Agency. "Integrated Risk Information System ( IRIS)" database. Accessed May 2002. Available at http://www.epa.gov/iris
- Vanderbilt University Medical Center. Methemoglobinemia (1998). Accessed May 2002.Available at PERLINK http://www.mc.vanderbilt.edu/peds/pidl/hemeonc/metheme.htm
- Agency for Toxic Substances and Disease Registry. ATSDR record of activity fortelephone communication with a Clinician at the Poplar Indian Health Service Clinic. Atlanta,Georgia. May 16, 2002.
- Kross B.C., Ayebo A.D., and Fuortes L.J. Methemoglobinemia: nitrate toxicity in ruralAmerica. American Family Physician 46(1):183-8. July 1992.
- Knobeloch L, Salna B, Hogan A, Postle J, Anderson H. Blue babies and nitrate-contaminated well water. Environmental Health Perspectives. 108(7):675-8. July 2000.
- Stoltenow, C. and Lardy, G. Nitrate Poisoning of Livestock. North Dakota StateUniversity Extension Service. September 1998. Available athttp://www.ext.nodak.edu/extpubs/ansci/livestoc/v839w.htm
- West, D. Nitrates in Groundwater: A Continuing Issue for Idaho Citizens. Boise, Idaho:Idaho Department of Environmental Quality. 2001.
|No. of Wells Sampled||Range of Well Depths (feet)||Range of Nitrate Detection (mg/L)||Mean Nitrate Concentration (mg/L)||Nitrate Screening Value(1) (mg/L)||Number of Wells that Exceed the Screening Value (10 mg/L)||% of Wells that Exceed the Screening Value|
|USGS Data Set(2) |
Flaxville (Shallow) Wells
Domestic Wells Only
18 - 440
<0.05 - 83
0.19 - 82
<0.05 - 32
|PNR Data Set (3)||14||30 - 89||<0.05 - 95.7||9.62||10||2||14.3%|
|MBMG Data Set (4)||195||18 - 290||0.01 - 98.3||4.55||10||25||12.8%|
mg/L = milligrams per liter
1 Corresponds to the Fort Peck Tribes Numeric Water Quality Standard (NWQS) and USEPA Maximum Contaminant Level (MCL) for nitrate.
2 United States Geological Survey (USGS) Data Set: Nimick, D.A. and Thamke, J.N. 1998. Extent, Magnitude, and Sources of Nitrate in the Flaxville and Underlying Aquifers, Fort Peck Indian Reservation, Northeastern Montana. U.S. Geological Survey Water Resources Investigations Report 98-4079.
3 Pioneer Natural Resources (PNR). August 2000. Field Investigation Biere Well Evaluation, Poplar, Montana. Prepared by CH2MHILL
4 Montana Bureau of Mines and Geology (MBMG) Groundwater Information Center Database. Received May 8, 2002 from Tom Patton, Hydrogeologist, MBMG.
|Number of Wells||Mean Well Depth (feet)||Range of Well Depth (feet)||Number of Samples||Mean Nitrate-Nitrite Concentration(2) (mg/L)||Range of Nitrate-Nitrite Concentrations(2) (mg/L)|
|Flaxville Aquifer||58||45||18 - 95||44||20||0.19 - 82|
|Fort Union Aquifer||41||108||40 - 303||24||7.3||<0.5 - 27|
|Fox Hills-Lower Hell Creek Aquifer||45||148||65 - 440||44||9||<0.5 - 83|
mg/L = milligrams per liter
1 Table Modified from Nimick, D.A. and Thamke, J.N. 1998. Extent, Magnitude, and Sources of Nitrate in the Flaxville and Underlying Aquifers, Fort Peck Indian Reservation, Northeastern Montana. U.S. Geological Survey Water Resources Investigations Report 98-4079.
2 Nitrate values are mg/L nitrate as nitrogen.
ATSDR comparison values (CVs) are media-specific concentrations considered safe underdefault exposure scenario assumptions. They are used as screening values for the identification ofcontaminants (site-specific substances) that require further evaluation to determine the potentialfor adverse health effects.
Generally, a chemical is selected for further evaluation because its maximum concentration in air,water, or soil at the site exceeds one of ATSDR's comparison values. However, it cannot beemphasized strongly enough that comparison values are not thresholds of toxicity. Whileconcentrations at or below the relevant comparison value may reasonably be considered safe, itdoes not automatically follow that any environmental concentration that exceeds a comparisonvalue would be expected to produce adverse health effects. Indeed, the whole purpose behindhighly conservative, health-based standards and guidelines is to enable health professionals torecognize and resolve potential public health problems before they become actual health hazards.The probability that adverse health outcomes will actually occur as a result of exposure toenvironmental contaminants depends on site specific conditions and individual lifestyle andgenetic factors that affect the route, magnitude, and duration of actual exposure, and not onenvironmental concentrations alone.
Screening values based on non-cancer effects are obtained by dividing NOAELs ( no-observed-adverse-effect levels) or LOAELs (lowest-observed-adverse-effect levels) determined in animal or (less often) human studies by cumulative modifying factors that typically range from 10 to 1,000 or more.
Listed and described below are the comparison values that ATSDR uses to select chemicals for further evaluation for this health consultation, along with the abbreviations for the most common units of measure.
EMEG Environmental Media Evaluation Guides RMEG Reference Dose Media Evaluation Guides MCL Maximum Contaminant Level ppm Parts Per Million, e.g., mg/L ppb Parts Per Billion, e.g., µg/L mg Milligram (0.001 grams) µg Microgram (0.000001 grams) L Liter
Environmental Media Evaluation Guides (EMEGs) are concentrations of a contaminant inwater, soil, or air that are unlikely to be associated with any appreciable risk of deleterious non-cancer effects over a specified duration of exposure. EMEGs are derived from ATSDR minimalrisk levels by factoring in default body weights and ingestion rates. Separate EMEGs arecomputed for acute (14 days), intermediate (15-364 days), and chronic (365 days) exposures.
Reference Dose Media Evaluation Guides (RMEGs) are concentrations of a contaminant inwater, soil, or air that are unlikely to be associated with any appreciable risk of deleterious non-cancer effects over a specified duration of exposure. RMEGs are derived from EPAs ReferenceDose or Reference Concentration, and are for chronic exposures.
Lowest-observed-adverse-effect levels are the lowest exposure level of a chemical in a study, orgroup of studies, that produces statistically or biologically significant increase in frequency orseverity of adverse health effects between the exposed population and its appropriate control.
Maximum Contaminant Levels (MCLs) represent contaminant concentrations in drinking waterthat EPA deems protective of public health (considering the availability and economics of watertreatment technology) over a lifetime (70 years) at an exposure rate of 2 liters of water per day.
National Primary Drinking Water Regulation (NPDWR or primary standard) is a legally-enforceable standard that applies to public water systems. Primary standards protect drinkingwater quality by limiting the levels of specific contaminants that can adversely affect publichealth and known or anticipated to occur in water. They take the form of MCLs or TreatmentTechniques.
National Secondary Drinking Water Regulation (NSDWR or secondary standard) is a non-enforceable guideline regarding contaminants that may cause cosmetic effects ( such as skin ortooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water.
No-observed-adverse-effect level is the dose of a chemical at which there were no statistically orbiologically significant increases in frequency or severity of adverse health effects seen betweenthe exposed population and its appropriate control. Effects may be produced at this dose, but theyare not considered to be adverse.