U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3143

April 2012

U.S. Geological Survey and the California State Water Resources Control Board

Groundwater Quality in the Tahoe and Martis 

Basins, California 

Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, the 

State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The 

Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s groundwater 

quality and increases public access to groundwater-quality information. The Tahoe and Martis Basins and surrounding watersheds constitute one of the study units being evaluated.

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Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Values are a percentage of the area of the primary aquifers

with concentrations in the three specified categories. Values

on pie chart may not equal 100 due to rounding of percentages.

67

20

13 98

1 1

The Tahoe-Martis Study Unit

The Tahoe-Martis study unit is approximately 460 square miles and includes the 

groundwater basins on the south, north, and west shores of Lake Tahoe, and the Martis Valley 

groundwater basin (California Department of Water Resources, 2003). The study unit was 

divided into three study areas based primarily on geography: the Tahoe study area composed 

of the three Tahoe Valley basins, the Martis study area, and the Hard Rock study area composed of the parts of the watersheds surrounding the basins (Fram and others, 2009).

The primary aquifers in the Tahoe study area consist of glacial outwash sediments (mixtures of sand, silt, clay, gravel, cobbles, and boulders), interbedded with lake sediments. The 

primary aquifers in the Martis study area are interbedded volcanic lavas, volcanic sediments, 

and glacial outwash sediments. In the Hard Rock study area, groundwater is present in fractured granitic rocks in the south and fractured volcanic rocks in the north. Aquifers composed 

of different materials commonly contain groundwater with different chemical compositions. 

The primary aquifers in the study unit are 

defined as those parts of the aquifers corresponding to the screened or open intervals of wells 

listed in the California Department of Public 

Health database. In the Tahoe study area, these 

wells typically are drilled to depths between 175 

and 375 feet, consist of solid casing from land 

surface to a depth of about 75 to 125 feet, and are 

screened or open below the solid casing. In the 

Martis study area, these wells typically are 200 

to 900 feet deep, and are screened or open below 

75 to 300 feet. Water quality in the shallower and 

deeper parts of the aquifer system may differ from 

that in the primary aquifers.The Hard Rock study 

area includes wells and developed springs.

The Tahoe-Martis study unit has warm, dry 

summers and cold, wet winters. Average annual 

precipitation ranges from 30 inches at Lake Tahoe 

to 80 inches in the surrounding mountains, and 

the majority of precipitation falls as snow. Land 

use in the study unit is approximately 88 percent 

(%) undeveloped (forests, grasslands, and bare 

rock), and 12% urban. The undeveloped lands are used mostly for recreation. The largest 

urban areas are the cities of South Lake Tahoe and Truckee.

Municipal and community water supply accounts for nearly all of the total water use in 

the study unit, with most of the remainder used for recreation, including landscape irrigation 

and snow-making. Groundwater provides nearly all of the water supply in the study unit, 

with limited use of surface water in some areas. Recharge to the groundwater flow system is 

mainly from mountain-front recharge at the margins of the basins, stream-channel infiltration, and direct infiltration of precipitation. Groundwater leaves the aquifer system when it is 

pumped for water supply or flows into streams and lakes.

Overview of Water Quality

GAMA’s Priority Basin Project evaluates the quality of untreated 

groundwater. However, for context, 

benchmarks established for drinkingwater quality are used for comparison. 

Benchmarks and definitions of high, 

moderate, and low concentrations are 

discussed in the inset box on page 3.

 Many inorganic constituents occur 

naturally in groundwater. The concentrations of the inorganic constituents can 

be affected by natural processes as well 

as by human activities. In the TahoeMartis study unit, one or more inorganic 

constituents were present at high concentrations in about 20% of the primary 

aquifers and at moderate concentrations 

in 13%.

 Human-made organic constituents 

are found in products used in the home, 

business, industry, and agriculture. 

Organic constituents can enter the environment through normal usage, spills, 

or improper disposal. In this study unit, 

one or more organic constituents were 

present at high concentrations in about 

1% of the primary aquifers and at moderate concentrations in about 1%.

Truckee

Truckee River

Upper

River

LAKE

TAHOE 

Martis study area

Hard Rock

study area

Tahoe study area

Watershed boundary

0 4 8 Miles

0 8 Kilometers 4

Nevada

County

Placer

County

El Dorado

County

Alpine

County

NEVADA

CALIFORNIA

Truckee

South Lake

Tahoe
 

 

RESULTS: Groundwater Quality in the Tahoe-Martis Study Unit

Inorganic Constituents with Human-Health Benchmarks

Trace and minor elements are naturally present in the minerals in rocks and 

soils, and in the water that comes into contact with those materials. In the TahoeMartis study unit, trace elements were present at high concentrations in about 19% 

of the primary aquifers, and in moderate concentrations in about 4%. Arsenic was 

the trace element that most frequently occurred at high and moderate concentrations. Three trace elements with non-regulatory health-based benchmarks, boron, 

molybdenum, and strontium, also were detected at high concentrations.

Radioactivity is the emission of energy or particles during spontaneous decay 

of unstable atoms. Humans are exposed to small amounts of natural radioactivity 

every day. Most of the radioactivity in groundwater comes from decay of naturally 

occurring uranium and thorium in minerals in the rocks or sediments of the aquifers. Radioactive constituents occurred at high levels in about 3% of the primary 

aquifers, and at moderate levels in about 12%. Gross alpha particle and radon-222 

activities were the radioactive constituents that most frequently occurred at high 

and moderate levels. 

Nutrients, such as nitrogen, are naturally present at low concentrations in 

groundwater. High and moderate concentrations generally occur as a result of 

human activities. Common sources of nutrients include fertilizer applied to crops 

and landscaping, seepage from septic systems, and human and animal waste. In the 

Tahoe-Martis study unit, nutrients were not detected at high or moderate concentrations in the primary aquifers.

Inorganic Constituents with Non-Health Benchmarks

(Not included in water-quality overview charts shown on the front page) 

Some constituents affect the aesthetic properties of water, such as taste, color, 

and odor, or may create nuisance problems, such as staining and scaling. The State 

of California has a recommended and an upper limit for total dissolved solids 

(TDS). All water naturally contains TDS as a result of the weathering and dissolution of minerals in soils and rocks. Iron and manganese are naturally occurring 

constituents that commonly occur together in groundwater. Anoxic conditions in 

groundwater (low amounts of dissolved oxygen) may result in release of manganese 

and iron from minerals into groundwater. 

In the Tahoe-Martis study unit, TDS was present at high concentrations 

(greater than the upper limit) in about 8% of the primary aquifers, and at low concentrations (less than the recommended limit) in about 92% of the primary aquifers. 

Manganese, with or without iron, was present at high concentrations in about 14% 

of the primary aquifers. 

Perchlorate 

(Not included in water-quality overview charts shown on the front page)

Perchlorate is an inorganic constituent that has been regulated in California 

drinking water since 2007. It is an ingredient in rocket fuel, fireworks, safety flares, 

and other products, may be present in some fertilizers, and occurs naturally at low 

concentrations in groundwater. Perchlorate was not detected in the primary aquifers. 

Trace 

elements

Total 

dissolved 

solids

Radioactive

constituents

Nutrients

Manganese

Perchlorate

INORGANIC CONSTITUENTS

INORGANIC CONSTITUENTS SPECIAL-INTEREST CONSTITUENTS

77

19

4

3

85

12

92

100

100

86

14

8
Solvents

Pesticides

Other volatile 

organic 

compounds

ORGANIC CONSTITUENTS

1 <1

98

100

100

RESULTS: Groundwater Quality in the Tahoe-Martis Study Unit

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect the presence 

of low concentrations of volatile organic compounds (VOCs) and pesticides, far below 

human-health benchmarks. VOCs and pesticides detected at these low concentrations 

can be used to help trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs are in many household, commercial, industrial, and agricultural products, 

and are characterized by their tendency to volatilize (evaporate) into the air. 

Solvents are used for a number of purposes, including manufacturing and cleaning. 

In the Tahoe-Martis study unit, solvents were present at high concentrations in about 

1% of the primary aquifers. The solvent detected at high concentrations was tetrachloroethylene (PCE), which mainly was used in dry-cleaning businesses. Solvents were 

present at moderate concentrations in about 1% of the primary aquifers, and at low 

concentrations (or not detected) in about 98%. 

Other VOCs include trihalomethanes, gasoline additives and oxygenates, refrigerants, and organic synthesis reagents. Trihalomethanes form during disinfection of water 

supplies, and may enter groundwater by the infiltration of landscape irrigation water, or 

leakage from distribution lines. Gasoline additives and oxygenates increase the efficiency of fuel combustion. Other VOCs were not detected at high or moderate concentrations in the primary aquifers. Trihalomethanes and gasoline oxygenates were detected 

at low concentrations in the primary aquifers.

Pesticides with Human-Health Benchmarks

Pesticides, including herbicides, insecticides, fungicides, and fumigants, are 

applied to crops, gardens, lawns, around buildings, and along roads to help control 

unwanted vegetation (weeds), insects, fungi, and other pests. In the Tahoe-Martis study 

unit, pesticides were not detected at high or moderate concentrations in the primary 

aquifers. Herbicides were occasionally detected at low concentrations. 

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

 GAMA’s Priority Basin Project uses benchmarks established for drinking water to 

provide context for evaluating the quality of untreated groundwater. After withdrawal, 

groundwater may be disinfected, filtered, mixed, and exposed to the atmosphere before 

being delivered to consumers. Federal and California regulatory benchmarks for protecting human health (Maximum Contaminant Level, MCL) were used when available. 

Nonregulatory benchmarks for protecting aesthetic properties, such as taste and odor 

(Secondary Maximum Contaminant Level, SMCL), and nonregulatory benchmarks for 

protecting human health (Notification Level, NL, and Lifetime Health Advisory, HAL) 

were used when Federal or California regulatory benchmarks were not available.

High, moderate, and low concentrations are defined relative to benchmarks

Concentrations are considered high if they are greater than a benchmark. For inorganic constituents, concentrations are moderate if they are greater than one-half of a benchmark. For organic constituents and perchlorate, concentrations are moderate if they are 

greater than one-tenth of a benchmark; this lower threshold was used because organic constituents are generally less prevalent and 

have smaller concentrations relative to benchmarks than inorganic constituents. Low values include nondetections and values less than 

moderate concentrations. Methods for evaluating water quality are discussed in Fram and Belitz (2012).

High concentrations

Moderate concentrations

EX

Low concentrations

AMPLE Values are a percentage of the area of the primary aquifers

with concentrations in the three specified categories. Values

on pie chart may not equal 100 due to rounding of percentages.

CONSTITUENT CONCENTRATIONS
ARSENIC, IN MICROGRAMS PER LITER

100

10

1

0.1

pH, IN STANDARD UNITS

6 7 8 9

Aquifer rock type

granitic rock

sedimentary

volcanic rock

anoxic oxic

RelativeConcentration

High

Moderate

Low

EXPLANATION

Factors that Affect Groundwater Quality

In the Tahoe-Martis study unit, arsenic was the constituent that most frequently 

occurred at high concentrations. About 18% of the primary aquifers had arsenic concentrations greater than the human-health regulatory benchmark Federal MCL) of 10 μg/L 

(micrograms per liter). Natural sources of arsenic to groundwater include dissolution of 

arsenic-bearing sulfide minerals, desorption of arsenic from the surfaces of manganeseor iron-oxide minerals (or dissolution of those oxide minerals), and mixing with geothermal waters (Welch and others, 2000).

By Miranda S. Fram and Kenneth Belitz

SELECTED REFERENCES

California Department of Water Resources, 2003, California’s groundwater: California Department of 

Water Resources Bulletin 118, 246 p. http://www.water.ca.gov/groundwater/bulletin118/update2003.

cfm.

 Fram, M.S., Munday, Cathy, and Belitz, Kenneth, 2009, Groundwater quality data for the Tahoe-Martis 

study unit, 2007—Results from the California GAMA Program: U.S. Geological Survey Data Series 

432, 87 p. (Also available at http://pubs.usgs.gov/ds/432/.)

Fram, M.S., and Belitz, Kenneth, 2012, Status and understanding of groundwater quality in the TahoeMartis, Central Sierra, and Southern Sierra study units,2006–2007—California GAMA Program 

Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2011-5216, 222 p. 

(Also available at http://pubs.usgs.gov/sir/2011/5216.)

Smedley, P.L., and Kinniburgh, D.G., 2002, A review of the source, behavior, and distribution of arsenic 

in natural waters: Applied Geochemistry, v. 17, p. 517–568.

Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 2000, Arsenic in ground water of the 

United States—occurrence and geochemistry: Ground Water, v. 38, no. 4, p. 589–604.

Priority Basin Assessments 

GAMA’s Priority Basin Project 

(PBP) assesses water quality in that 

part of the aquifer system used for 

drinking water, primarily public supply. 

Water quality in the primary aquifers, 

assessed by the PBP, may differ from 

that in the deeper parts of the aquifer, 

or from the shallower parts, which are 

being assessed by GAMA’s Domestic 

Well Project. Ongoing assessments 

are being conducted in more than 120 

basins throughout California.

The PBP assessments are based 

on a comparison of constituent concentrations in untreated groundwater with 

benchmarks established for protection of human health and for aesthetic 

concerns. The PBP does not evaluate 

the quality of drinking water delivered 

to consumers.

The PBP uses two scientific 

approaches for assessing groundwater quality. The first approach uses a 

network of wells to statistically assess 

the status of groundwater quality. The 

second approach combines waterquality, hydrologic, geographic, and 

other data to help assess the factors 

that affect water quality. In the TahoeMartis study unit, data were collected 

by the PBP in 2007, and from the 

CDPH database for 2004–2007. The 

PBP includes chemical analyses generally not available as part of regulatory 

compliance monitoring, including 

measurements at concentrations much 

lower than human-health benchmarks, 

and measurement of constituents that 

can be used to trace the sources and 

movement of groundwater.

For more information

Technical reports and hydrologic 

data collected for the GAMA PBP Program may be obtained from:

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

4165 Spruance Road, Suite 200

San Diego, CA 92101

Telephone number: (619) 225-6100

WEB: http://ca.water.usgs.gov/gama

GAMA Program Unit

State Water Resources Control Board

Division of Water Quality

PO Box 2231, Sacramento, CA 95812

Telephone number: (916) 341-5779

WEB: http://www.waterboards.ca.gov/gama

In the Tahoe-Martis study unit, elevated arsenic concentrations likely are caused 

by two different processes (Fram and Belitz, 2012). In aquifers composed of sediments 

or volcanic rocks, high and moderate arsenic concentrations were found in groundwater 

that was oxic (high dissolved oxygen concentration) and alkaline (pH values greater 

than about 8). The elevated arsenic concentration in oxic, alkaline groundwater likely is 

due to desorption of arsenic from the surfaces of manganese- and iron-oxide minerals 

(Smedley and Kinniburgh, 2002). Oxic, alkaline conditions increase arsenic solubility 

in groundwater by inhibiting arsenic from adhering to mineral surfaces (sorption). In 

aquifers composed of granitic and volcanic rocks, high arsenic concentrations also were 

found in anoxic (low dissolved oxygen concentration) groundwater with low pH values. 

Dissolution of manganese- and iron-oxide minerals under anoxic conditions likely 

results in release of arsenic associated with these minerals.