U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3123

April 2015 Printed on recycled paper

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

Groundwater Quality in the Cascade Range and 

Modoc Plateau, 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 

Cascade Range and Modoc Plateau area constitutes one of the study units being evaluated.

The Cascade Range and Modoc Plateau Study Unit

The Cascade Range and Modoc Plateau (CAMP) study unit covers approximately 

15,000 square miles in northeastern California. The study unit was divided into six study 

areas on the basis of geologic features: Eastside Sacramento Valley, Honey Lake Valley, 

Cascade Range and Modoc Plateau Low-Use Basins, Quaternary Volcanic Areas, Shasta 

Valley and Mount Shasta Volcanic Area, and Tertiary Volcanic Areas (Shelton and others, 

2013; Fram and Shelton, 2015). 

The volcanic rocks of the CAMP study unit can be highly productive aquifers. 

Groundwater resides in porous zones between lava flows, fractures that formed during 

cooling of the lavas, lava tubes, and other features. These systems can be interconnected or 

isolated, resulting in variability in water levels, well yields, and water quality on local and 

regional scales. Sedimentary groundwater basins in the CAMP study unit primarily contain 

Quaternary-age alluvial fan, fluvial, and lake sediments (California Department of Water 

Resources, 2003). Aquifers composed of different materials can contain groundwater with 

different chemical compositions.

The primary aquifer system 

of the CAMP study unit is defined 

as those parts of the aquifer system 

tapped by wells and springs listed in 

the State of California’s database of 

public drinking-water supply sources. 

In the CAMP study unit, about 10 

percent of these sources are springs. 

Of those that are wells, most are 

drilled to depths of 140 to 400 feet, 

consist of solid casing or a seal from 

the land surface to a depth of about 50 

to 220 feet, and are open or have perforated casing below that depth. Water 

quality in the primary aquifer system 

can differ from that in the shallower 

and deeper parts of the aquifer system.

Average annual precipitation 

ranges from 10 to 20 inches over 

much of the area to over 80 inches at 

high elevations in Lassen Volcanic 

National Park and on Mount Shasta. The study unit includes three major hydrologic regions: 

the Klamath River watershed, the Sacramento River watershed, and closed drainage basins of 

the North Lahonton area. About 80 percent of the land in the study unit is natural, and much 

of that land is in national forests. About 16 percent of the land is used for agriculture, and 

about 4 percent is urbanized. 

Recharge to CAMP study unit aquifers is mainly from stream-channel infiltration, 

direct infiltration of precipitation and snow melt, flow through volcanic features, and in 

some areas, infiltration of irrigation water. Groundwater exits the aquifer system when it is 

pumped for irrigation or drinking-water supply, discharges into springs, streams and lakes, or 

evaporates from areas with a shallow depth to groundwater.

Study areas

Eastside Sacramento Valley

Honey Lake Valley

Cascade Range and Modoc Plateau 

 Low-Use Basins

Quaternary Volcanic Areas

Shasta Valley and Mount Shasta

 Volcanic Area

Tertiary Volcanic Areas

122° 121° 120° 42°

41°

40°

0 10 20 Kilometers

0 10 20 Miles

Overview of Water Quality

Inorganic

constituents

Organic

constituents

100

76

15

9

CONSTITUENT CONCENTRATIONS

High Moderate Low or not detected

Pie charts illustrate the percentage of the primary 

aquifer system, on an areal basis, with concentrations 

in the three specified categories.

GAMA’s Priority Basin Project 

evaluates the quality of untreated 

groundwater. However, for context, concentrations measured in groundwater are 

compared to benchmarks established for 

drinking-water quality, such as maximum 

contaminant levels (MCL). A concentration 

above a benchmark is defined as high. 

Benchmarks and definitions of 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 CAMP study 

unit, one or more inorganic constituents 

were present at high concentrations in about 

9 percent of the primary aquifer system 

and at moderate concentrations in about 15 

percent.

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, organic constituents were 

not present at high or moderate concentrations in the primary aquifer system.
RESULTS: Groundwater Quality in the Cascade Range and Modoc Plateau

 

 INORGANIC CONSTITUENTS

Trace 

elements

Uranium and

radioactive 

constituents

Nutrients

Total 

dissolved 

solids

Manganese 

and/or iron

Perchlorate

98

2

84

15 2

89

9

2

97

1 2

97

3 <1

80

14

6

SPECIAL-INTEREST CONSTITUENT

Inorganic Constituents with Human-Health Benchmarks

Trace elements are naturally present in the minerals of rocks and sediments 

and in the water that comes into contact with those materials. In the CAMP study 

unit, trace elements were detected at high concentrations in about 6 percent of 

the primary aquifer system and at moderate concentrations in about 14 percent. 

Arsenic, boron, molybdenum, and vanadium were the trace elements that most 

frequently occurred at high and moderate concentrations. 

 Radioactivity is the release of energy or energetic 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 

the decay of naturally occurring isotopes of uranium and thorium in minerals in 

aquifer materials. Radioactive constituents occurred at high levels in about 3 percent of the primary aquifer system and at moderate levels in less than 1 percent. 

Uranium was the constituent that most frequently occurred at high levels.

 Nutrients are naturally present at low concentrations in groundwater, and 

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. Nutrients were 

present at high concentrations in about 1 percent of the primary aquifers and at 

moderate concentrations in about 2 percent. Nitrate was the only nutrient detected 

at high concentrations.

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 can create nuisance problems, such as staining and scaling. 

The benchmarks used for these constituents were non-regulatory secondary maximum contaminant level benchmarks. 

 Total dissolved solids (TDS) concentration is a measure of the salinity of 

the groundwater, and all water naturally contains TDS as a result of the weathering and dissolution of minerals in rocks and sediments. The State of California 

has a recommended and an upper limit for TDS in drinking water. TDS concentrations were high (greater than the upper limit) in about 2 percent of the primary 

aquifer system and moderate (between the recommended and upper limits) in 

about 9 percent. 

 Anoxic conditions in groundwater (low amounts of dissolved oxygen) 

can result in release of naturally occurring manganese and iron from minerals into 

groundwater. Manganese or iron was present at high concentrations in about 15 

percent of the primary aquifer system and at moderate concentrations in about 2 

percent.

Special-Interest Constituent: 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 used in solid rocket fuel, fireworks, safety flares, 

and other products, is present in some fertilizers, and occurs naturally at low concentrations in groundwater. Perchlorate was not detected at high concentrations 

and was detected at moderate concentrations in about 2 percent of the primary 

aquifer system.
RESULTS: Groundwater Quality in the Cascade Range and Modoc Plateau

ORGANIC CONSTITUENTS

VOCs

Pesticides 100

100

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human-health 

benchmarks. VOCs and pesticides detected at these very 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 present in many household, commercial, industrial, and agricultural 

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

In the CAMP study unit, VOCs were not detected at high or moderate concentrations in 

the primary aquifer system. 

Pesticides with Human-Health Benchmarks

Pesticides, including herbicides, insecticides, 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 CAMP study unit, pesticides were not 

detected at high or moderate concentrations in the primary aquifer system. 

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) are used when available. Otherwise, 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) are used. 

High

Moderate

EX

Low or not detected

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

system with concentrations in the three specified categories.

PERCENTAGE OF STUDY AREA

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 and special-interest constituents, 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 concentrations include non-detections and values 

less than moderate concentrations. Methods for evaluating water quality are discussed by Fram and Shelton (2015).

Benchmark type and value for selected constituents.

[Benchmark types: California, State Water Resources Control Board Division of Drinking Water; Federal, U.S. Environmental Protection Agency; HAL, lifetime 

health advisory; MCL, maximum contaminant level; NL, notification level; SMCL, secondary maximum contaminant level. Abbreviations: ppb, parts per billion or 

micrograms per liter (μg/L); ppm, parts per million or milligrams per liter (mg/L)]

Constituent

Benchmark

Constituent

Benchmark

Type Value Type Value

Arsenic Federal MCL 10 ppb Nitrate, as nitrate Federal MCL 45 ppm

Boron California NL 1 ppm Total dissolved solids 

(TDS)

California SMCL 1,000 ppm

Molybdenum Federal HAL 40 ppb Iron California SMCL 300 ppb

Vanadium California NL 50 ppb Manganese California SMCL 50 ppb

Uranium Federal MCL 30 ppb Perchlorate California MCL 6 ppb
Factors that Affect Groundwater Quality

Inorganic constituents with health-based benchmarks were present at high or moderate concentrations in 24 percent of the primary aquifer system of the CAMP study unit; 

however, the proportions ranged from a low of 9 percent in the Eastside Sacramento 

Valley study area to a high of 64 percent in the Honey Lake Valley study area. In addition, 

the individual constituents present at high or moderate concentrations were not the same 

in the six study areas. These differences in water quality reflect differences among the 

six study areas in the composition of the rocks and sediments that compose the aquifers, 

the distribution of developed land uses, the depths of wells, and other factors (Fram and 

Shelton, 2015).

 Arsenic, boron, molybdenum, vanadium, uranium, and nitrate were all present at 

high or moderate concentrations in the Honey Lake Valley study area. Conditions in the 

Honey Lake Valley study area that contribute to elevated concentrations of trace elements 

in groundwater include mixing between groundwater and hydrothermal fluids; interaction 

between groundwater and granitic rocks of the Sierra Nevada, or sediments derived from 

those granitic rocks; interaction between groundwater and ancient lakebed sediments; 

and presence of ancient groundwater with low concentrations of dissolved oxygen (Mayo 

and others, 2010; Fram and Shelton, 2015). Elevated nitrate concentrations are associated 

with the combination of shallow wells, modern groundwater, and presence of agricultural 

activity.

Honey

Lake

Valley

Shasta Valley

and

volcanic area

Eastside

Sacramento

Valley

Quaternary

volcanic

areas

Tertiary

volcanic

areas

Cascade-Modoc

Low-Use

basins

Percentage of primary aquifer system

0

10

20

30

40

9

91

22

75

3

11

89

1

86

13

22

69

9

26

36 38

Pie diagrams show proportions of the primary aquifer system in each study area with high,

 xxx moderate, or low concentrations of inorganic constituents with human-health 

benchmarks

Arsenic

Boron

Molybdenum

Vanadium

Uranium

Nitrate

CONSTITUENT

High

Moderate

CONCENTRATION

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 

may differ from water quality in shallower and deeper parts of the aquifers. GAMA’s Domestic Well Project 

assesses water quality in the shallower 

parts of the aquifer system. Ongoing 

PBP 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 the protection of human health and for aesthetic 

concerns for drinking water. 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 water quality, 

hydrologic, geographic, and other data 

to help assess the factors that affect 

water quality. In the CAMP study 

unit, data were collected by the PBP 

in 2010 and from the State of California database for 2008–2010. 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 Program 

may be obtained from:

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

6000 J Street, Placer Hall

Sacramento, CA 95819-6129

Telephone number: (916) 278-3000

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

By Miranda S. Fram and Jennifer L. Shelton

SELECTED REFERENCES

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

Water Resources Bulletin 118, 246 p. 

Fram, M.S., and Shelton, J.L., 2015, Status and understanding of groundwater quality in the Cascade 

Range and Modoc Plateau study unit, 2010: California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2014-5238, 131 p. http://pubs.usgs.gov/sir/2014/5238/. 

Mayo, A.L., Henderson, R.M., Tingey, David, and Webber, William, 2010, Chemical evolution of shallow playa groundwater in response to post-pluvial isostatic rebound, Honey Lake Basin, CaliforniaNevada, USA: Hydrogeology Journal, v. 8, p. 725-747.

Shelton, J.L., Fram, M.S., and Belitz, K., 2013, Groundwater-quality data in the Cascade Range and 

Modoc Plateau study unit, 2010: Results from the California GAMA Program: U.S. Geological Survey Data Series 688, 124 p. http://pubs.usgs.gov/ds/688/.

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

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143123