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Analysis of the Human Exposure Pathway via Surface Water, Sediment, andBiota in Churchill County



In March 2001, the Nevada State Health Division (NSHD) requested that the Agency for ToxicSubstances and Disease Registry (ATSDR) and the National Center for Environmental Health(NCEH) evaluate environmental risk factors that might be linked to children who had developedleukemia in the area of Fallon, Churchill County, Nevada. ATSDR was asked to evaluatereleases of hazardous substances in the county and to provide an assessment of ways in whichpeople in the community could be exposed to the substances. NCEH was asked to develop andconduct a cross-sectional exposure assessment of selected substances using environmental(household) and biological specimens for families of the children and for a reference population [1].

ATSDR and NCEH developed a public health action plan to investigate ways in which peoplecould be exposed (i.e., environmental pathway analysis) [1]. Environmental pathway analysis isthe study of the movement of contaminants in environmental media (i.e., groundwater, air,surface water, sediment, soil, and biota) that is helpful in determining whether people could beexposed to hazardous substances. ATSDR divided its pathway analysis into segments accordingto environmental media and locations because of the large amount of work involved ChurchillCounty. The pathway analyses address the following specific elements:

  • JP-8 fuel pipeline
  • Fallon Naval Air Station
  • Churchill County air quality
  • Churchill County surface water, sediment, and biota
  • Churchill County soil and dust contaminants

This health consultation addresses the investigation of potential exposure to contaminatedsurface water, sediment, and biota in Churchill County. Other environmental media areaddressed in separate ATSDR documents. Appendix A is a listing of all ATSDR documentsconcerning Churchill County. In this health consultation, ATSDR evaluated availableinformation on chemicals found in the area's supply and drainage canals, as well as those foundin the area's rivers and reservoirs. ATSDR reviewed information collected by the US GeologicalSurvey, the US Fish and Wildlife Service, the US Environmental Protection Agency (EPA), andthe Nevada Division of Environmental Protection. ATSDR released a public comment version ofthis document on February 12, 2003. The comments and ATSDR's responses to them arepresented in Appendix F.


Environmental information for this health consultation is grouped into two categories anddiscussed separately in the following sections. The categories are surface water systemdescription and description of available environmental data (sediment, surface water, and biota).

  1. Surface water system description

There are four hydrographic basins (Humboldt River, Truckee River, Carson River, and CentralRegion) in Churchill County. The majority of Churchill County lies within the 4,000-square-mile Carson River Basin which encompasses the Fallon area. Portions of the watershed extendacross the state line into California. The watershed can be described by two integrated surfacewater features, Carson River in the Carson River Basin and the Newlands Irrigation Projectstructures.

Aquifer recharge comes primarily from snowmelt in the higher altitudes of the Sierra Nevadaand Pine Nut Mountain. The major stream in the basin is the Carson River, which flows about180 miles through five of Nevada's hydrographic areas (Carson Valley, Eagle Valley, DaytonValley, Churchill Valley and Lahontan Valley). The Lahontan Reservoir, which is about 18miles west of Fallon on the Carson River, is the only large storage reservoir in the Carson RiverBasin in Nevada. This reservoir stores surface water from the natural flow of the Carson River aswell as surface water diverted from the Truckee River by way of the Truckee Canal. From theLahontan Reservoir, Carson River flows northeast to its natural terminus, the Carson Sink, andsoutheast (via distributary channels and cannels) to the rest of the Carson Desert hydrographicarea which covers about 2,000 square miles in Churchill County. This area includes the city ofFallon, Fallon Naval Air Station, Stillwater, Fallon National Wildlife Refuge, Carson Lake,Stillwater Wildlife Management Area, Carson Sink, Stillwater National Wildlife Refuge, andNewlands Project irrigation area (Figure 1) [2-3].

The Newlands Project is a large-scale irrigation project started during the early 1900s by the USDepartment of the Interior's Bureau of Reclamation for agricultural irrigation in the lowerCarson River Basin near Fallon. The Newlands Project in Churchill County is shown in Figures2 and 3. Major water works in the project include four dams, six reservoirs, and 758.5 miles ofcanals and drains, as well as other water works [4-5].

Dams: Lahontan, Lake Tahoe, Carson River Diversion, and Derby Diversion

The Lahontan Dam, which is on the Carson River at the north end of the LahontanReservoir, was completed in 1915. The Lake Tahoe Dam, completed in 1913, regulatesthe outflow from Lake Tahoe into the Truckee River. The Carson River Diversion Dam,which is on the Carson River 5 miles downstream of the Lahontan Dam, diverts waterinto two main canals. The Derby Diversion Dam on the Truckee River diverts water intothe Truckee Canal.

Reservoirs: Lahontan, Shecker, S-Line, Old River, Harmon, and Stillwater Point

The Lahontan Reservoir, with a storage capacity of 314,000 acre feet, stores water fromthe natural flow of the Carson River along with water diverted from the Truckee River byway of the Truckee Canal. Five regulating reservoirs (Shecker, S-Line, Old River,Harmon, and Stillwater Point) further control flow through the extensive canal systemdescribed below and to the wetlands of Carson Lake and Stillwater.


The Newlands Project has built 758.5 miles of canals and drains. The Truckee Canal,which connects the Truckee River and the Carson River from Derby Diversion Dam toLahontan Dam, is 32 miles long. The length of the main Carson diversion canals (theT-Line and V-Line canals) is 69 miles. There are 312 miles of lateral canals, such as theA-Line, D-Line, L-Line, S-Line, Extension Canal, Diagonal Canal, Stillwater DiversionCanal, Swope Canal, and Paiute Diversion Canal. The drainage system contains about345 miles of deep, open-type drainage canals that return water to the Carson River,Carson Lake, and the wetlands of the Stillwater Wildlife Management Area and theStillwater National Wildlife Refuge.

Other Works

Other works of the Newlands Project include the Lahontan power plant, the V-Canalpower plant, and the power distribution system, which has 73 miles of transmission lines.

  1. Available environmental data description

The US Geological Survey (USGS) collects water, sediment, and animal tissue samples fromacross the nation and analyzes the samples for their chemical, physical, and biologicalproperties. ATSDR obtained Churchill County water quality data from the USGS NationalWater Information System for the period from 1956 through 1999. This information comes fromdifferent national programs, such as the Drinking Water Program, the National Irrigation WaterQuality Program, the National Stream Quality Accounting Network, and the National Water-Quality Assessment Program. Water quality data are collected for contaminants, such as volatileorganic compounds (VOCs), metals, pesticides, and nutrients [6].

EPA maintains surface-water-quality information for water that is collected throughout thecountry by federal, state, and local agencies; Indian tribes; volunteer groups; academicinstitutions; and others. ATSDR retrieved surface water data for Churchill County from theSTORET legacy data center, which stores data supplied to EPA before 1999. Sampling dataprovided by STORET included the date each sample was collected, the location and type ofenvironmental medium, sampling and analytical methods, and quality-control checks [7].

The National Pollutant Discharge Elimination System Permit Program (NPDES) is designed tocontrol water pollution by regulating point sources that discharge pollutants into waters of theUnited States. The Nevada Division of Environmental Protection issued NPDES permits to 30facilities (Table 1) in Churchill County, including the City of Fallon, the Fallon Naval AirStation, and some small dairies [8]. ATSDR reviewed available discharge monitoring reportsthat provided discharge locations and volume, sampling locations, and analytical parameters(e.g., metals, nutrients, and microbiological agents). Since the early 1970s, the USGS, US Fishand Wildlife Service, EPA, the Nevada Division of Environmental Protection, and the Universityof Nevada have conducted various studies to determine the extent of surface water and sedimentcontamination and levels of contaminants in fish, ducks, and other biota materials in the Carson River Basin and in the area of the Newlands Irrigation Project. ATSDR reviewed the following available documents:

  • Total mercury in water, sediment, and selected aquatic organisms, Carson, River,Nevada [9]
  • Pharmacokinetics of methyl mercury bioaccumulation in carp [10]
  • Total mercury in sediment, water, and fishes in the Carson River drainage, West-central Nevada [11]
  • Detailed study of irrigation drainage in and near wildlife management areas, west central Nevada [12]
  • Mercury in fish collected from the Indian Lakes system, Stillwater Wildlife ManagementArea [13]
  • Mercury characterization in Lahontan Valley wetlands [14]
  • Draft Ecological Risk Assessment, Carson River mercury site [15]
  • Field screening of water quality, bottom sediment, and biota associated with irrigation in and near the Indian Lakes area, Stillwater National Wildlife Management Area, Churchill County, west central Nevada [29]
  • Sources and impacts of irrigation drainwater contaminants in arid wetlands [30]
  • Toxicity of striped bass (morone saxatilis) and daphnia magna [31]
  • Stillwater National Wildlife Refuge wetland contaminant monitoring [33]
  • Reconnaissance investigation of water quality, bottom sediment, and biota associatedwith irrigation drainage in and near Stillwater Wildlife management area, ChurchillCounty, Nevada [34]
  • Physical, chemical, and biological data for detailed study of irrigation drainage in and near Stillwater, Fernley, and Humboldt Wildlife Management Areas and Carson Lake,west-central Nevada [35]


ATSDR provides public health advice on the basis of a review of toxicological literature, acomparison of levels of environmental contaminants detected at a site to published comparisonvalues, an evaluation of potential exposure pathways and duration of exposure, and thecharacteristics of the exposed population. Whether a person will be harmed by exposure tohazardous substances depends upon several factors, including the type and amount of thecontaminant; the manner in which the person was exposed; the length of time the person wasexposed; the amount of the contaminant absorbed by the body; site-specific conditions; geneticfactors; and individual lifestyle factors.

ATSDR uses different comparison values (chemical-specific, health-based standards andguidelines) derived by various state and federal government agencies to screen contaminants andto identify those that could require further evaluation of their potential to cause adverse healtheffects. Although concentrations at or below the relevant comparison values might reasonably beconsidered safe, concentrations above these values will not necessarily cause harm. ATSDR usessite-specific exposure scenarios and performs an in-depth evaluation for substances detected atconcentrations above the screening values.

ATSDR used the following comparison values for this health consultation: the environmentalmedia evaluation guides (EMEGs), reference dose media evaluation guides (RMEGs), cancerrisk evaluation guides (CREGs), minimal risk levels (MRLs), the EPA drinking water maximumcontaminant levels (MCLs), EPA Region 9 preliminary remediation goals (PRGs), and the USFood and Drug Administration action levels for poisonous or deleterious substances in human food and animal feed.


ATSDR reviewed and evaluated the available environmental data for sediment, surface water,biota, flooding issues, and for environmental data limitations and gaps. These data aresummarized and discussed in the following sections. Table 2 shows the dates and sources of theenvironmental data.

Sediment Data

From 1973 through 1999, USGS collected sediment samples at 88 sampling locations throughoutChurchill County. Samples were taken from Carson River, Carson Lake, and Carson Sink, aswell as Newlands Irrigation Project areas, such as Lahontan Reservoir, Truckee Canal, and themain and lateral delivery canals and drains. Samples were analyzed for VOCs, semi-volatileorganic compounds (SVOCs), metals, pesticides and related compounds, as well as nutrients. Ofthe 137 substances analyzed, 82 (60%) different substances were detected. None of the sampleanalyses indicated the presence of VOCs in sediment. ATSDR reviewed the detected chemicalsafter grouping them into categories of metals, pesticides and related compounds, and SVOCs.Table 3 provides a summary of the chemicals detected in sediment.


In all samples collected in the Fallon area (that is, all samples taken at or downstream of theLahontan Reservoir), 37 metals were detected. With the exception of arsenic and mercury,the levels of the detected metals were below their applicable comparison values. Therefore, only arsenic and mercury will be discussed further.

Arsenic is a naturally occurring element with an average concentration of 2-5 parts permillion (ppm) in the earth's crust. Arsenic is therefore present at low levels in soil, water,food, and air. Because of natural factors of geology and climate, levels of arsenic in soil andother surface materials in Nevada are elevated compared to the national average [16]. Themaximum level found for arsenic in Churchill County is 100 ppm.

Arsenic was found in 72 sediment samples evaluated for Churchill County, including 22samples from the Fallon area. The highest concentration of arsenic detected was 680 ppm;the average concentration was 81.1 ppm. The ATSDR chronic EMEG for a child is 20 ppm;for an adult, it is 200 ppm. Those concentrations are considered to be safe levels for peopleexposed to arsenic for more than 1 year on a daily basis [17]. However, the most likely waythat people could be exposed to arsenic in sediment is through occasional ingestion ofcontaminated sediment or through infrequent dermal contact with contaminated sedimentwhile working or participating in recreational activities. Infrequent exposure to sedimentcontaining arsenic is not likely to result in any adverse health effects.

Mercury also occurs naturally in several forms and at very low levels. For the Carson Riverdrainage basin of west central Nevada, the background mercury concentration in sedimentand soil ranges from 0.1 ppm to 1.6 ppm [18]. In the late 1800s, mercury was imported to thearea to be used in gold and silver mining operations. Numerous mining operations along theCarson River Gorge area discharged an estimated 7,500 tons of mercury, primarily in theform of mercury-contaminated tailings, into the Carson River drainage basin [18]. Theaverage concentration of mercury detected in sediment samples was 1.77 ppm. The highestconcentration of 34 ppm was detected at a sampling station on the Carson River near FortChurchill in 1997 (table 3). It appears that (1) the mercury sediment concentrations increasewith the distance downstream from the sites of early mining operations and (2) levels ofmercury and other metals in sediment samples have increased over time [17-20]. The mostlikely exposure to mercury in sediment is occasional ingestion or infrequent dermal contactwith contaminated sediment by people who are working or participating in recreationalactivities. Infrequent exposure is not likely to cause any adverse health effects. However,mercury in sediment can enter and accumulate in the food chain (for example, in fish andother wildlife). Therefore, human consumption of mercury-contaminated fish or other foodscould result in exposure. The biota (e.g., plants, animals) exposure pathway is discussed later in this document.

Pesticides and related compounds

The sediment samples were analyzed for 38 pesticides and related compounds. Of these, 14(37%) were detected at very low concentrations (11 in the Fallon area). All of the detectedchemicals were at concentrations below their applicable comparison values.

Semi-volatile organic compounds

Semi-volatile organic compounds (SVOCs) such as phenols, phthalate esters, and polycyclicaromatic hydrocarbons (PAHs) were analyzed in nine sediment samples and found atconcentrations below their applicable comparison values (Table 3). Thirty-one chemicalswere detected at very low concentrations (nine in the Fallon area). The most frequentlydetected SVOCs were 2, 6-dimethylnaphalene (maximum concentration of 0.12 ppm); di-(2-ethylhexyl) phthalate (maximum concentration of 0.55 ppm); diethyl phthalate(maximum concentration of 0.02 ppm); and di-n-butyl phthalate (maximum concentration of0.07 ppm).

Surface Water Data

From 1956 through 1999, USGS and EPA collected surface water samples at 198 samplinglocations throughout Churchill County, including the Carson River, Carson Lake, and CarsonSink, as well as Newlands Irrigation Project areas, such as the Lahontan Reservoir, TruckeeCanal, and the main and lateral delivery canals and drains. Samples were analyzed for pesticidesand related compounds, metals, and other inorganic substances, VOCs, and SVOCs. Of the 284substances analyzed, 79 (28%) were detected. Table 4 provides a summary of chemicals detectedin surface water. ATSDR also reviewed and evaluated documents [2-4, 22-23] related to waterquality in the Churchill County area and the National Pollutant Discharge Elimination System Permit Program.

Pesticides and related compounds

Twenty six pesticides and related compounds were detected at very low concentrations (11 inthe Fallon area). Only two chemicals, p, p'-dichlorodiphenyldichloroethylene (p, p'-DDE),average concentration of 152 parts per billion (ppb), and gamma-hexachlorocyclohexane(lindane), average concentration of 0.10 ppb, were found at concentrations above theirrespective comparison values (table 4).

Metals and other inorganic substances

More than 1,000 surface water samples were analyzed for metals and metalloids. Thirty-onemetals were detected in most of the surface water samples (28 in the Fallon area). Fivemetals exceeded their respective comparison values. Those metals were (1) aluminum, withan average concentration of 10,200 ppb; (2) arsenic, with an average concentration of 54.5ppb; (3) beryllium, with an average concentration of 0.80 ppb; (4) lead, with an averageconcentration of 12.1 ppb; and (5) mercury, with an average concentration of 297 ppb.

Only one recent study containing limited information was found on tungsten in EasternSierra Nevada rivers (Truckee, Walker, and Carson Rivers). Surface water samples werecollected at 12 locations along the Carson River from the East Fork to the LahontanReservoir. Trace elements analyzed for this study were boron, molybdenum, tungsten, andvanadium. Study results indicated that (1) Carson River and the other two rivers haverelatively high concentrations of the selected trace elements, including tungsten; (2) theupper reaches of the Carson River have higher levels of tungsten than the lower reaches; and(3) relatively high concentrations of the selected trace elements reflect a combination ofprocesses, such as weathering of rocks with high concentrations of those elements.

Nineteen other inorganic substances, including nitrate, nitrite, cyanide, phosphate, andsulfate, were detected in many of the surface water samples (25 in the Fallon area). All of thedetected chemicals were found at concentrations below their applicable comparison values.

Volatile organic compounds and semi-volatile organic compounds

Only one SVOC, di (2-ethylhexyl) phthalate, with a maximum concentration of 5 ppb, wasfound in one surface water sample. Trans-1, 3-dichloropropene and methyl isobutyl ketonewere two VOCs detected with concentrations of 5 ppb and 10 ppb, respectively. All were atconcentrations below their applicable comparison values.

All comparison values used to evaluate the surface water data are those established fordrinking and tap water. These comparison values are based on conservative assumptions anddefault exposure factors, such as water ingestion rates (2 liters per day for adults and 1 literper day for children); exposure frequency (350 days per year for residents); and exposureduration (30 or 70 years for adults, 6 years for children). For the site-specific exposurescenarios, surface water in Churchill County was classified as Class C water. Class C wateris "located in areas of moderate-to-urban human habitation, where industrial development ispresent in moderate amounts, agricultural practices are intensive and where the watershed isconsiderably altered by man's activity. The beneficial uses of class C water are municipal ordomestic supply, or both, following complete treatment, irrigation, watering of livestock,aquatic life, propagation of wildlife, recreation involving contact with the water, recreationnot involving contact with the water, and industrial supply" [21].

The most likely human exposure is occasional ingestion and/or infrequent dermal contactwith contaminated surface water by persons participating in recreational activities or work.Infrequent exposure to the contaminated surface water is not likely to result in any adversehealth effects. Some metals, such as mercury, in surface water can enter and accumulate inthe food chain (for example, fish and other wildlife). Therefore, human consumption ofcontaminated fish or other wildlife could result in exposure. The biota pathway is discussedlater in this document.

Water quality documents

ATSDR also reviewed documents related to water quality in the Carson River Basin, theCarson Desert agricultural area, and in the irrigation-drain systems of the NewlandsIrrigation Project [2-4, 22-23]. Findings from these studies include the following:

  • The primary sources of nutrients, synthetic organic compounds, and trace elementsare urban activities. Concentrations of most constituents vary with season andstreamflow.
  • Agricultural areas have contributed pesticides, nutrients, trace elements, anddissolved solids to the water resource through irrigation drainage.
  • Historical mining activities and natural factors of geology and climate caused highconcentrations of dissolved solids, arsenic, mercury, and other elements in this area.
  • The average concentration of nutrients in the Carson River was lower than thenational median. However, nutrients were enriched in the Carson River downstreamfrom the Carson Valley and Carson Desert.
  • Pesticides were present at low levels in agricultural areas, but the levels were lowerthan the national median.

National Pollutant Discharge Elimination System Permit Program

The Nevada Division of Environmental Protection (NDEP) has issued National PollutantDischarge Elimination System permits to 30 facilities in Churchill County, including the Cityof Fallon, the Fallon Naval Air Station, and some small dairies. ATSDR reviewed selecteddischarge-monitoring reports, which contain information on discharge locations and volume,sampling locations, and analytical parameters (e.g., metals, nutrients, and microbiologicalagents).

The City of Fallon and Kennametal, Inc., are the largest point sources of wastewater in theFallon area. The Fallon municipal wastewater treatment plant discharges into the New RiverDrain. Samples taken from three locations upstream of the drain were analyzed for flow, pH,total dissolved solids, total suspended solids, percentage of suspended solids removal,biochemical oxygen demand, percentage of biochemical oxygen demand removal, totalnitrogen, total phosphorus, coliform bacteria, and free and residual chlorine. No significantlevels of tested chemicals were observed from the available discharge monitoring reports.Kennametal discharges waste water to groundwater via approved fields consisting of double-lined evaporation ponds. The discharge volume is 10,750 gallons per day. Chemicals testedat the main septic system include iron, magnesium, manganese, copper, zinc, fluoride,calcium, sodium, beryllium, arsenic, chloride, silica, boron, sulfate, nitrate, and totaldissolved solids. No significant levels of tested chemicals were observed from the available discharge-monitoring reports.

Biota Data

Mercury is the primary contaminant of concern for bioaccumulation and biomagnification in thefood chain for Churchill County. Since the early 1970s, several studies have been conducted onfish, ducks, algae, and other wildlife species in the Carson River Basin and the area of theNewlands Irrigation Project. Samples of more than 36 species of wildlife were analyzed formercury concentrations in different tissues. Wildlife samples included 18 species of fish, fivespecies of duck and two non-duck bird species, four species of benthic invertebrates, and otherbiota (e.g., algae, corixid, detritus (bottom sediment containing mostly organic debris), lizards,and zooplankton.

Mercury concentrations in fish tissues ranged from less than 0.02 ppm to 11 ppm, wet weight.Mercury concentrations in 112 composite drift samples (containing algae, submerged vascularplants, detritus, corixids, brine flies, and other organic materials) ranged from less than 0.04 ppmto 97.8 ppm, dry weight. For the five species of duck, mercury concentrations ranged from lessthan 0.5 ppm to 38.9 ppm, wet weight. The average mercury concentrations in duck muscle andliver were 5.9 ppm and 17.8 ppm, respectively. Other major findings of biota studies aresummarized in the following paragraphs [9-15, 29-31].

  • Mercury was derived from historical mining activities located upstream of the CarsonRiver and transported during high streamflow (flooding events) before the constructionof the Lahontan Reservoir in 1915. Most of the mercury deposited in Carson Lake, theStillwater Wildlife Management Area, the mouth of the Carson River in Carson Sink, andalong the floodplain of the river. Mercury remains biologically available and can bebioaccumulated within the area.
  • The probable mercury contamination pathway in biota appears to be as follows: fluvialsediment transport to bottom sediment; from bottom sediment to bottom-dwelling biota(invertebrates, algae, plants, and forage fish); and from the biota to waterfowl.
  • Most of the total mercury in corixids, a representative aquatic invertebrate, existed asmethyl mercury, the form of mercury that can accumulate in the food chain. In swallowstested for mercury contamination, more than 90% of the total mercury detected existed asmethyl mercury.
  • Bioaccumulation factors for methyl mercury from sediment to corixids ranged from lessthan 100 to more than 2,000 times in wetlands of the Stillwater National Wildlife Refugeand the Fallon National Wildlife Refuge.
  • Within irrigation drains, mercury is being bioaccumulated in plants and plant detritus andbiomagnified in vertebrates by factors up to 10,000 times the concentrations measured inassociated drainwater.
  • The irrigation system, especially drainage canals, has redistributed mercury throughout the Lahontan Valley.

Table 5 provides a summary of all available biota data for total mercury concentrations in Churchill County.

Although most of the mercury found in the environment is in the form of metallic mercury andinorganic mercury compounds, microorganisms convert inorganic mercury to methyl mercury.Methyl mercury can accumulate in the food chain and is also most easily absorbed by the humanbody. Inorganic mercury does not accumulate in the food chain to any extent [24]. Therefore,health-related comparison values used in this document are for methyl mercury only.

On the basis of a 1985 Nevada Department of Conservation and natural Resources study results[11] and the Food and Drug Administration's action level for methyl mercury in the edibleportion of fish and duck (1 ppm, wet weight), the Nevada State Health Division issued areahealth advisories for the consumption of fish and duck. The duck advisory was for consumptionof northern shoveler (Anas clypeata) harvested in the area of Carson Lake. The fish advisory wasfor all fish caught from Dayton to the Lahontan Dam of the Carson River and from all waters inthe Lahontan Valley area. The following chart provides additional information on the healthadvisories.

PollutantSpeciesArea AffectedYearIssuedType IssuedStatus
MercuryCarp/blackfishLahontan Reservoir1982CFBRescinded
MercuryAll fishLahontan Reservoir1986RCGP/RCSPRescinded
MercuryDuck (shoveler)Carson Lake1989RCGP/RCSPUnknown
MercuryAll fishCarson River1993NCGP/NCSPActive
MercuryAll fishLahontan Valley1994NCGP/NCSPActive

Blackfish: Sacramento blackfish
CFB: commercial fishing ban
RCGP: restricted consumption advisory for the general population
RCSP: restricted consumption advisory for sensitive subpopulations
NCGP: no consumption advisory for the general population
NCSP: no consumption advisory for sensitive subpopulations
Carson River: Dayton to the Lahontan Dam

Fish can be an important part of a balanced diet for general population. It is a good source ofhigh quality protein and other nutrients and low in fat. However, fish contaminated with highlevels of methyl mercury may be harmful to human health if eaten regularly. ATSDR establisheda minimum risk level (MRL) of 0.0003 milligrams per kilogram per day for chronic exposure(i.e., more than 365 days) to methyl mercury. MRLs are estimates of daily human exposure tomercury that are likely to be without an appreciable risk of adverse health effects. The chronicMRL for methyl mercury is based on a no-observed-adverse-effect level (NOAEL) forneurodevelopment effects in humans, with an uncertainty (or safety) factor of 4.5 (3 for humanvariability and 1.5 modifying factor of the selected study). The subtle neurodevelopmentendpoints evaluated include children's cognitive ability, expressive and receptive languageability, social and adaptive behavior. It should be noted that the MRL is based on a study inwhich the population consumes more fish than that of the U.S. population and the NOAEL wascalculated from the mercury in maternal hair of the most sensitive groups (pregnant women andtheir developing fetuses) [24].

Because fish and duck consumption rates for Churchill County are not available for thisevaluation, ATSDR used the EPA intake recommendations for recreational freshwater anglersand other extremely conservative assumptions (Appendix C) to calculate the estimated minimalconcentration of total mercury that could result in exceeding the respective MRL [25]. Anaverage mercury concentration in fish of 2.6 ppm would result in an exposure exceeding theMRL for an adult and a concentration of 0.75 ppm would result in an exposure exceeding theMRL for a child. The most current data indicated that the mercury concentration was in excessof the 2.6 ppm in five species of fish (channel catfish, white catfish, white bass, large mouthbass, and walleye) and two species of duck (shoveler and green-winged teal). Therefore, frequentand prolonged human consumption of mercury-contaminated fish and duck could lead to dosagethat approach reported NOAEL (0.0013 milligrams per kilogram per day) in human studies.Since the primary concern is the in utero exposure of the developing fetus, the potential levels ofexposure are sufficiently of concern to consider this exposure pathway a public health hazard. Itshould be noted that this is a very conservative approach and exposures are likely to be less. Forexample, the following site-specific exposure scenarios could significantly reduce exposure tomethyl mercury:

  1. ATSDR used the average fish intake for recreational freshwater anglers in the aboveevaluation. However, the estimated mean fish intake for the general population is lowerthan that for recreational freshwater anglers.

  2. Posted health advisories in the Lahontan Valley area could minimize area residents' intake of fish and duck.

  3. The total mercury concentrations are the sum of inorganic and organic mercurycompounds. The true methyl mercury concentrations are expected to be lower than thoseused for the risk evaluation.

  4. The absorption and bioavailability of methyl mercury in food might be affected by other dietary components, such as dietary fiber, phytate, and selenium.

Please refer to Appendix C for intake assumptions and dose calculations.

ATSDR contacted the local office of the US Fish and Wildlife Service, Bureau of Indian Affairs,Indian Health Services, and the Fallon Paiute Tribe and confirmed that tribe-specific fish andwildlife consumption rates are not available. However, in general, fish and wildlife consumptionrates for Native Americans are higher than that for the general population [25]. Therefore, tribemembers might have a higher exposure from consumption of mercury-contaminated fish andwildlife.

While biomagnification in the aquatic food chain is well studied, data are limited for thepotential biomagnification or bioconcentration of mercury and other metals in the terrestrial foodchain (i.e., local crops and livestock, such as alfalfa, melons, cattle, and milk) [24]. The CarsonRiver is the source of drinking water for some local livestock, and it is the source of water forirrigating local crops and pastures. Consumption of mercury-contaminated, locally produceddairy products, meat, fruits, and vegetables might represent another route of human exposure tomercury and other contaminants.

There are limited biota data on other trace metals in the Churchill County area. In addition tomercury, analyzed trace metals include aluminum, arsenic, boron, fluoride, selenium, uranium,and zinc. Selenium is another metal having significant biomagnification in the aquatic foodchain. However, concentrations of selenium in edible portions of selected waterfowl sampleswere below the level of human health concern [12]. Arsenic can accumulate in some fish andshellfish; however, most arsenic in fish tissues is in an organic form (arsenobetaine or "fisharsenic"), which does not appear to be harmful to humans and is excreted rapidly in urine [17].


There are community concerns about a major flood that occurred in 1997, in the Carson Valley.Community members want to know whether the flood could have some link with the occurrenceof acute lymphocytic leukemia among members of the Fallon community. The movement ofwater is a significant force in redistributing chemicals in the environment. Water and sedimentfrom floods can be points of environmental exposure because pollutants can travel dissolved inwater or attached to particles that move with water. Particles in water come mostly from thebottom of the river or lake or from runoff discharged into the waters.

Water moving at a higher velocity than normal (as in flooding) has a greater capacity to carrymore particles, suspend particles that were previously at the bottom of the river or lake, erodeshorelines, and potentially move greater amounts of pollutants. Smaller particles need lowerwater velocities than larger particles to stay suspended. On a per-mass basis, more pollutantsadsorb onto smaller particulates because of their greater surface area.When water enters a slow-moving area, such as a lake or a pooled area of a stream, the larger particles tend to settle out.Stagnant areas, such as areas of receding floodwater, can receive sediment deposits in a widerange of particle sizes, including smaller particles. Deposited sediment could contain pollutants,and people could be exposed to them through direct contact. Furthermore, these pollutants couldvolatilize into air or move into the groundwater where, again, people could be exposed to themthrough other exposure routes (e.g., inhalation and ingestion).

For dissolved pollutants, complex relationships exist between pollutant concentrations in thewater and the total amount of pollutants moving downstream. During flooding events, dissolvedpollutants could occur at lower concentrations because dilution in the greater amount of water,although the total amount of the pollutant mass moving downstream could be the same.However, greater amounts of water might cause the pollution source to increase its contributionto water.

Flooding is part of the natural hydrologic cycle and hydrography in a watershed. In Fallon, thehydrography has been significantly altered by the Newlands Irrigation Project, a large-scaleirrigation project started during the early 1900s by the Bureau of Reclamation for agriculturalirrigation in the lower Carson River Basin near Fallon. The project infrastructure consists ofdams, canals, drainage ditches, and reservoirs. The Truckee-Carson Irrigation District operatesthe Newlands Project and its infrastructure. As a normal course of operation, agricultural fieldsin the Fallon vicinity are irrigated with a surface/gravity type of irrigation system. This methodcould be described as a form of controlled flooding. These controlled flooding activities occur atleast annually on most active agricultural fields in the Fallon area. A true flood, however, is notcontrolled by humans; flooding occurs when surface water spills over the banks and shorelinesof rivers and lakes, inundating buildings and land. Both types of flooding are important in thetransport of pollutants in the environment. Because of community concerns, this section willfocus on flooding from nonirrigation activities.

To evaluate this flooding as a potential point of exposure, ATSDR conducted the following activities:

  • Reviewed USGS stream gaging data
  • Interviewed resident of the community
  • Reviewed floodplain maps
  • Reviewed newspaper articles
  • Reviewed historical satellite images

USGS stream gaging station data

USGS routinely monitors stream flow at 10 stream gaging stations in Churchill County from 2stations on the Carson River. Gage station number 10312150 (Figure 4) is located in the CarsonRiver below the Lahontan Reservoir. It is the most appropriate gaging station for evaluatingstream flow in the Carson River and in the irrigation canals that run through Fallon. Communitymembers had specified concerns about the 1997 flood, and ATSDR also noted even greater peakflow in June 1983, June 1986, June 1995, and May 1996. Figure 5 provides additionalinformation about the station's annual peak stream flow and the highest values for gage flow ratefrom 1980 through 2000. Records about the flow events that were higher than that of the January1997 flood are shown in the following table.

Records of Gage Station 10312150, Carson River, Below Lahontan Reservoir
Record DateGage Height


Flow rate

(cubic feet per second)


From this data, ATSDR decided to evaluate the 1997 event and the three other flow events (i.e.,June 1983, June 1995, May 1996) that had peak gage heights greater than that in 1997.


ATSDR staff discussed the community's concerns about flooding with representatives fromTruckee Carson Irrigation District, City of Fallon, Churchill County Mosquito AbatementDistrict, the Nevada State Department of Health, and one Fallon resident. The generalinformation provided by the state and local government officials is that a large amount offlooding occurred upstream of the Lahontan Reservoir and in Carson City and Reno, Nevada, in1997. However, the 1997 flooding that occurred downstream of the Lahontan Reservoir wascontrolled by the diversion of water from the Carson River to the irrigation and drainage canalsand to regulatory reservoirs (including the Harmon, S-Line, and Sheckler Reservoirs). Water wasalso allowed to spill out from the Carson River Diversion Dam to the north and from theSheckler Reservoir to the south. The water eventually drained to Carson Lake and the StillwaterWildlife Management Area. Farmers were requested to flood their fields at this time. Minimalproperty damage occurred in 1997, except for damage reported in the Bafford Lane areanortheast of Fallon.

The resident interviewed by ATSDR staff lives on property that borders the South Branch of theCarson River. Because of the irrigation system, the South Branch is now a drain and is notconnected to the Carson River. The front part of the resident's property, which includes thehouse, is located in the 500-year flood boundary, and the back portion of the property is locatedin the 100-year flood boundary. The house and front yard are elevated above the backyard by agentle slope that dips about 5 feet toward the river. The resident remembers that during the 1983flood, the water rose almost to the top of the slope but did not flood the house. In the 1995 flood,the backyard and the backyards of the adjoining properties were covered with a thick blanket ofsilt. Although water levels also rose during the high water events in 1996 and 1997, they werenot as high as those in 1983 or 1995.

Floodplain Maps

ATSDR digitized flood maps produced by the Federal Emergency Management Agency (seeAppendix C for a list of the maps) and overlaid them on maps showing the property parcels ofthe case houses (i.e., houses associated with children diagnosed with leukemia). Two of thosehouse property parcels are within the 100-year flood zone, but at its edge, and one house islocated within, but at the of the 100- to 500-year flood zone (Figure 6).

Newspaper Articles

ATSDR reviewed historical newspaper articles from local community newspapers dating back to1983. The articles reviewed were from the Lahontan Valley News and the Fallon EagleStandard.

Most articles indicated that the biggest flood occurred in 1983, and the critical area affected byfloods was the Bafford Lane area of Fallon. The 1997 flood had minimal impact in the Fallonarea. For example, during the 1983 event, about 14-25 properties were affected; in the 1995event only one property was reported to be affected; no properties were reported as affected inthe 1996 and 1997 events. The articles also indicated that after the 1983 flood, farmers wererequested to release water into their fields during periods of high water.

Historical Satellite Images

ATSDR reviewed satellite images acquired from the US Landsat Program for 1983, 1993, 1995,1996, and 1997 to determine whether this data could confirm the information from the oneresident's interview and the historical newspaper articles. The dates of those images were chosento be as close as possible after the peak discharge dates. The 1993 image is considered a baselow flow event (854 cubic square feet) [27].Two images were obtained for 1997 because theJanuary 7, 1997, image, the day of the peak flow, only covered half of the Fallon area. TheJanuary 16, 1997, image covered the entire Fallon area, but it was taken 9 days after the peakflow. Appendix D provides technical information for the Landsat images.

Because of the relative low resolution of the images and the difficulty in distinguishingvegetation from vegetation covered with shallow water, ATSDR was only able to makequalitative statements about the images. From this review, ATSDR identified that the reservoirsand lakes were much fuller during the flood events than during the low flow periods.

The Carson River appears to be the most full in 1983 and much less full in 1995, 1996, and1997. The 1995 event appears to have been the second most full event, followed by 1996, and1997. The 1983 image indicates that the Carson River formed small "pools" (an indication offlooding) along the river's course north of Fallon. Because of the satellite image resolution, themain canals can barely be distinguished in the images, and the secondary canals and drainscannot be distinguished. In the 1995 and later images, canals were seen as one or two pixelswide, with each pixel equal to 30 meters. The 1983 image had a resolution of 60 meters, andtherefore the canals in the 1983 image were barely discernable.

From the review, ATSDR determined that the satellite images showed that none of the casehouses appeared to have been inundated with water. However, the images showed that water in pools was potentially present near several of the case houses.

  • A house for one individual in the case group is in a subdivision about 125 feet from thebranch of the V-Line canal that runs along the adjacent road and about 650 feet from theSouth Branch of the Carson River. In the 1995 and 1996 satellite images, a pool of waterapparently formed near the river about 650 feet from the house. This same case familylived in another location at a different time in a more rural area next to fallow and farmedland. In the 1996 satellite image, a pool of water apparently formed about 800 feet fromthis second house.

  • A house for one individual in the case group is located about 1,000 feet south of theCarson River and about 700 feet from the V-Line Canal. The house was built in 1995. Inthe 1983 satellite image, the field between the house and the river appears to be coveredwith water (as close as 275 feet to the house). The land occupied by the house does notappear to be flooded. No flooding was evident in the other Landsat images for the otheryears.

  • A house for one individual in the case group is located about 190 feet from the V-Linecanal and about 1,000 feet from the Upper West Side Drain. The house was built in 1945.The 1995 satellite image might document some flooding from the Upper West SideDrain. A dark area in a false color composite (Landsat bands 4, 3, and 2) of the satelliteimage might indicate that water or saturated soils extended from the Upper West SideDrain toward the house. The degree of darkness indicates the depth of water or degree ofsoil saturation or lack of vegetation. The area around the house is slightly darker than thenearby drier areas, possibly indicating that some of the soils around the house might bewetter. ATSDR cannot discern the degree of wetness. Free standing water is probably notpresent adjacent to the house but this conclusion is not certain.

  • A house for one individual in the case group is located about 140 feet from a branch ofthe A-Line Canal and about 440 feet from the Upper West Side Drain. Pooling of waterin an adjacent field and into the drain might have occurred in 1996 (approximately 500feet from the house). There also might be some wet soils or recently harvested cropslocated in a farm field about 80 feet from the house. The house was constructed in 1995.

  • A house for one case is located approximately 140 feet from the L-Line canal and the"DIX" drain. The property lot might have a low spot that collects water during theseflooding events or it might be somehow connected to the L-Line. Images from 1995,1996, and 1997 indicate that water might have been present approximately 75 to 200 feetfrom the house.

The locations of the case and control houses in this review were based on addresses matched tothe addresses in the Churchill County parcel map, global positioning system locations collectedby USGS, 1994 digital aerial photography obtained from the Bureau of Reclamation, and houseaddress matching using the US Census Bureau digital street maps [28].

In summary, the 1997 flood had minimal impact in the Fallon area. The previous floods in 1983,1995, and 1996 were more significant. The 1983 flood had the most impact in Fallon, followedby the 1995 flood. The satellite image from one case house during the 1995 flood may have had"wet" soils located near it but probably not free standing water. The Bafford Lane area northeastof Fallon was the most affected area during flooding. No house in this area was occupied by anyindividuals in the case group.

Persistent environmental contaminants, such as mercury and other contaminants from upstream(e.g., the EPA Superfund site, the Carson River mercury site) are transported downstream by theCarson River and associated canals and drains. During historical flooding events, larger amountsof contaminants are probably transported and ultimately deposited in Carson Lake, the StillwaterWildlife Management Area, and the braided canals of the Carson River near and in the CarsonSink, the floodplain of the river, and farm fields open to irrigation at the time. For example,during the flood year 1997 (January to September), about 10,000 pounds of total mercury movedpast a sampling site near Fort Churchill; and about 2,000 pounds of total mercury wasdischarged from the Lahontan reservoir [32]. Farmers who flooded their fields during the high-water events and areas of minor flooding, as reported by the interviewed resident, received moresuspended sediments from the water. Overall, the irrigation system and the drainage canals haveredistributed mercury throughout Lahontan Valley.

The flooding events in Fallon are not likely to be direct exposure points, based on the location ofthe houses. However, ATSDR cannot determine the past activities of area residents and,therefore, cannot determine whether they would have come into contact with sediment outsidetheir houses. Furthermore, pollutants in the sediment could volatilize into the air (for example,mercury) or move into the groundwater (for example, arsenic). ATSDR addressed the air andgroundwater pathways in separate documents.

Environmental data limitation and data gaps

Available environmental data for this evaluation are subject to limitations affecting the interpretations made from the data.

  • Environmental samples reviewed were collected for purposes not related to public health.For example, these data were collected in area hydrology summaries, general waterquality assessments, and compliance monitoring for federal and state regulation.

  • The involvement of multiple agencies (USGS, EPA, state and local governments, and themunicipal water treatment plant) might result in great differences in field collectiontechniques, sample preservation methods, sampling frequency, and laboratory analyticalmethods. These differences make comparison of data sets difficult.

  • Environmental samples were taken over a long period of time (1956-1999). Detectionmethod and technology changes and improvements over time could result inoverestimating or underestimating the true value of data.

  • Samples were not analyzed for all possible contaminants; only limited analytes wereselected. For example, of 190 pesticides used in Nevada, only 68 were analyzed for in apesticide study. Only one sample was analyzed for acrolein (Magnacide®), a herbicideused for vegetation control in irrigation canals.

  • Limited samples were taken from the Fallon area where the majority of Churchill Countyresidents live. For example, very few sediment samples were taken within a 5-mile radiusof the center of Fallon.

  • Information for biota exposure pathways was limited to selected chemicals (for example,mercury and selenium) and wildlife species (mainly fish and ducks). There is insufficientdata on the consumption rate of fish, duck, and other wildlife species as well as theconsumption rate and contamination of locally produced dairy products, meat, fruits, andvegetables.


On the basis of the available information, ATSDR concludes the following:

Frequent and prolonged human consumption of mercury-contaminated fish and duck could leadto dosage that approach reported NOAEL in human studies. While the primary concern is the inutero exposure of the developing fetus, the potential levels of exposure are sufficiently ofconcern to consider this exposure pathway a public health hazard. Total mercury concentrationsof five species of fish (channel catfish, white catfish, white bass, large mouth bass, and walleye)and two species of duck (shoveler and green-winged teal) exceeded the estimated concentrationof 2.6 ppm that could result in an exposure exceeding the chronic methyl mercury MRL.

Infrequent exposure to the contaminated sediment is not likely to result in any adverse healtheffects. Thirty-seven metals, 14 pesticides, and 31 semi-volatile organic compounds weredetected in sediment samples. All of the detected chemicals, with the exception of arsenic andmercury, were at concentrations below their applicable comparison values.

Infrequent exposure to the surface water is not likely to result in any adverse health effects.Twenty-six pesticides and related compounds, 50 metals and other inorganics, two volatileorganic compounds, and one semi-volatile organic compound were detected in surface watersamples. All of the chemicals detected in the surface water samples, with the exception of fivemetals (aluminum, arsenic, beryllium, lead, and mercury) and two pesticides ( p,p'-DDE andlindane), were present at concentrations below their applicable comparison values.

The 1997 flood had minimal impact in the Fallon area. The flooding events in Fallon are notlikely to be direct exposure points based on the location of the houses of individuals in the case and control groups.


  • Minimize the intake of mercury-contaminated fish and duck through comprehensiveapproaches, such as health education, surveillance of fish and wildlife consumption, and a widedistribution of information related to the health advisories issued by the Nevada State Health Division on human consumption of fish and duck in the Lahontan Valley area.

  • Although occasional exposure to the contaminants in surface water and sediment in theCarson River, reservoirs, lakes, and canals is not expected to result in adverse health effects,concerned persons could reduce the potential for exposure by cleansing their skin and washing clothing following contact with surface water and sediment.


  1. Agency for Toxic Substances and Disease Registry. Investigation of childhood leukemiain Fallon, Nevada, draft public health action plan. Atlanta: US Department of Health andHuman Services; August 17, 2001.

  2. US Geological Survey. Water quality assessment in the Las Vegas valley area and the Carson and Truckee river basins, Nevada and California–Nutrients, Pesticides, and Suspended Sediment, October 1969-April 1990. Carson City (NV): US Department of the Interior; 1997.

  3. US Geological Survey. Hydrogeology and potential effects of changes in water use,Carson Desert agricultural area, Churchill County, Nevada. Carson City (NV): USDepartment of the Interior; 1996.

  4. US Geological Survey. Concentrations, loads, and yields of potentially toxic constituentsin irrigation drain system, Newlands Project Area, Carson Desert, Nevada, November1994-October 1995. Carson City (NV): US Department of the Interior; 1997.

  5. Bureau of Reclamation. Factual data on the Newlands Project. Carson City (NV): USDepartment of the Interior; August 1990.

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  10. G.C.A. Ekechukwu. Pharmacokinetics of methyl mercury bioaccumulation incarp, Cyprinus Carpio Linnaeus. Unpublished PhD dissertation No.1049, University ofNevada. 1976.

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  13. US Fish and Wildlife Service. Mercury in fish collected from the Indian Lakessystem, Stillwater Wildlife Management Area, Churchill County, Nevada. Carson City(NV): US Department of the Interior; 1992.

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  27. WM Keck Earth Sciences and Mining Research Information Center. Availablefrom URL: (accessed June 2002).

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  29. US Geological Survey. Field screening of water quality, bottom sediment, andbiota associated with irrigation in and near the Indian Lakes area, Stillwater NationalWildlife Management Area, Churchill County, west central Nevada, 1995. Carson City(NV): US Department of the Interior; 1998.

  30. Lemly AD, Finger SE, Nelson MK. Sources and impacts of irrigation drainwatercontaminants in arid wetlands. Environmental Toxicology and Chemistry1993(12)2265-79

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  34. US Geological Survey. Reconnaissance investigation of water quality, bottom sediment, and biota associated with irrigation drainage in and near Stillwater Wildlife management area, Churchill County, Nevada, 1986–87. U.S. Geological Survey Water-Resources Investigations Report 89-4105. Carson City (NV): US Department of the Interior; 1990.

  35. US Geological Survey. Physical, chemical, and biological data for detailed studyof irrigation drainage in and near Stillwater, Fernley, and Humboldt WildlifeManagement Areas and Carson Lake, west-central Nevada, 1987-89. Carson City (NV):US Department of the Interior; 1991.


Jane Zhu
Consultation Section
Exposure Investigation and Consultation Branch
Division of Health Assessment and Consultation

Brian M. Kaplan
Environmental Health Scientist
Consultation Section
Exposure Investigation and Consultation Branch
Division of Health Assessment and Consultation

Gail Scogin
Environmental Health Scientist
Exposure Investigation Section
Exposure Investigation and Consultation Branch
Division of Health Assessment and Consultation

Technical Assistance

Steve Martin
FFIMS Data Manager
Information Resources Management Branch
Office of Program Operations and Management


John E. Abraham, PhD
Chief, Exposure Investigation and Consultation Branch
Division of Health Assessment and Consultation

Susan Moore
Chief, Consultation Section
Exposure Investigation and Consultation Branch
Division of Health Assessment and Consultation

W. Allen Robison, PhD
Superfund Site Assessment Branch
Division of Health Assessment and Consultation

Wendy Kaye, PhD
Chief, Epidemiology and Surveillance Branch
Division of Health Studies

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