The Redding–Red Bluff Shallow Aquifer Study Unit

The Redding–Red Bluff

study unit covers approximately 

1,200 square miles in Shasta and 

Tehama Counties, California, 

at the northern end of the 

Sacramento Valley. The study 

unit covers groundwater basins 

in the Redding area and the 

northern Sacramento Valley. The

Sacramento River flows through 

the study area. 

Groundwater aquifers 

within the regional study area are 

composed of marine, continental, 

and volcanic alluvial sediments 

derived from the surrounding 

mountain ranges: The Cascade

Range to the east, the Klamath

Mountains to the north, and 

the Coast Ranges to the west. 

The study unit is dominated by 

natural land use (60 percent), 

with urban use more common in the Redding study area (36 percent) and agricultural 

land use more common in the Red Bluff study area (20 percent).

This study was designed to provide a statistically representative assessment of the 

quality of groundwater resources used for domestic drinking water in the Redding–

Red Bluff study unit. A total of 50 wells were sampled between December 2018 and 

April 2019 (Shelton and others, 2020). Domestic wells in the study unit typically are 

drilled to depths of 80–338 feet (10th–90th percentiles; Shelton and others, 2020), 

which are shallower (p<0.001) than the depths of public-supply wells in the same area 

(typically 115–450 feet deep; Bennett and others, 2011). Water levels in domestic wells 

in the study unit typically are 15–163 feet below land surface (10th–90th percentiles; 

Shelton and others, 2020).

Previous investigations of public supply wells in the study area found relatively 

low concentrations of inorganic and volatile organic compounds compared to state and 

national benchmarks, except for arsenic (4.6 percent; Bennett and others, 2011). A State

Water Resources Control Board GAMA survey of domestic wells in Tehama County

(223 wells), which includes the Red Bluff study area, reported arsenic concentrations 

above the benchmark (see page 3) in 13 percent of wells, primarily in the southeast part 

of the study area (California State Water Resources Control Board, 2009). However, 

these wells were not spatially distributed and do not represent aquifer-scale portions as 

described herein.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2020–3025

April 2020 ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20203025

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

Groundwater Quality in the Redding–Red Bluff Shallow Aquifer

Study Unit of the Northern Sacramento Valley, 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. Private domestic and small system drinking water wells in the 

Redding–Red Bluff study unit primarily draw from shallow aquifers which are the target for this assessment.

Overview of Water Quality

GAMA’s Priority Basin Project

evaluates the quality of untreated 

groundwater. For the purpose of this study, 

concentrations measured in groundwater 

are compared to benchmarks established for 

drinking water quality, such as maximum 

contaminant levels (MCL). In this study, a 

concentration above a benchmark is defined 

as high. Benchmarks and definitions of 

moderate and low concentrations are 

discussed under “Methods for Evaluating

Groundwater Quality.”

Many inorganic constituents 

naturally occur in groundwater, and the 

concentrations of the inorganic constituents 

can be affected by natural processes 

as well as by human activities. In the 

Redding–Red Bluff study unit, one or more 

inorganic constituents were present at high 

concentrations in about 4 percent of the 

groundwater resources used for domestic 

drinking water.

Human-made organic constituents 

are found in products used in the home, 

business, industry, and agriculture and 

can enter the environment through normal 

usage, spills, or improper disposal. 

Organic constituents were not present at 

high concentrations in the groundwater 

resources used for domestic drinking water 

in the Redding–Red Bluff study unit.

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122°30’0”W 122°0’0”W

40°30’0”N

40°0’0”N

Redding

Red Bluff

SHASTA CO

TEHAMA CO

C o a s t R a n g e s

C a s c a d e R a n g e

K

l a

m

a

t

h

M

o

u

n

t

a

i n s

Base map source credit note. List base sources, scales.

Base modified from U.S. Geological Survey, ESRI, and 

other Federal and State digital data, various scales

122°30’0”W 122°0’0”W

40°30’0”N

40°0’0”N

0 5 10 MILES

0 5 10 KILOMETERS

Sacramento River

Red Bluff study-area cells

Redding study-area cells

! GAMA grid well

EXPLANATION

16

80

4

Inorganic 

constituents

(percent)

100

Organic 

constituents

(percent)

High Moderate Low or not detected

CONSTITUENT CONCENTRATIONS

Values indicate percentages of the area of the groundwater resources used for domestic drinking water 

with concentrations in the specified categories.
RESULTS: Groundwater Quality in Shallow Aquifers in the Western Mojave Desert, California

INORGANIC CONSTITUENTS Inorganic Constituents with Human-Health Benchmarks

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

that compose groundwater aquifers and dissolve into groundwater that comes into 

contact with those materials. About 2 percent of the groundwater resources used for 

domestic drinking water in the Redding–Red Bluff Shallow Aquifer study unit had 

high concentrations of one or more trace elements and about 10 percent had moderate 

concentrations. Two elements were present at concentrations above benchmarks: 

strontium and barium; and three elements were present at moderate concentrations 

(half the benchmark): arsenic, boron, and chromium.

Radioactivity is the release of energy or energetic particles during spontaneous 

decay of unstable atoms. Most of the radioactivity in groundwater comes from 

the decay of isotopes of uranium and thorium in aquifer materials. In 2 percent 

of domestic wells from the study unit, gross alpha particle activities were above 

the benchmark.

Nutrients, including nitrate, are naturally present at low concentrations in 

groundwater, and high concentrations generally result from human activities. 

Common sources of nutrients include fertilizer applied to crops and landscaping, 

seepage from septic systems, and human and animal waste. Nitrate was present at 

moderate concentrations in about 4 percent of the groundwater resources used for 

domestic drinking water in the study unit.

Hexavalent Chromium

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

Hexavalent chromium Cr(VI) is a species of the trace metal chromium that may 

be toxic to humans. Hexavalent chromium can be naturally present in groundwater 

interacting with sediments derived from ultramafic rocks (or serpentinites) or released 

from anthropogenic uses such as stainless-steel manufacturing or other industrial 

processes. The drinking-water benchmark used for hexavalent chromium comparisons 

in this study is the USGS health-based screening level of 20 parts per billion (ppb), 

because the California MCL of 10 ppb was withdrawn in 2017 (California State Water 

Resources Control Board, 2017).

Hexavalent chromium was present at high concentrations above the benchmark 

in 4 percent of groundwater resources used for domestic drinking water in the 

Redding–Red Bluff study unit and moderate concentrations above half the benchmark 

in 8 percent of groundwater resources. 

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 an indicator of groundwater 

salinity, and all water naturally contains TDS as a result of interactions with rocks and 

sediments such as the weathering and dissolution of minerals. The State of California 

has a recommended and an upper limit for TDS in drinking water. Total dissolved 

solids concentrations were high (greater than the upper limit) in about 2 percent of the 

groundwater resources used for domestic drinking water in the Redding–Red Bluff 

study unit.

Anoxic conditions, defined as having a dissolved oxygen concentration less 

than 0.5 parts per million (ppm), can result in release of manganese, iron, and other 

associated trace elements from minerals into groundwater (McMahon and Chapelle, 

2007). Manganese or iron was present at concentrations above benchmarks set for 

aesthetic concerns in about 4 percent of the groundwater resources used for domestic 

drinking water.

2

88

10

Trace 

elements

(percent)

98

2

Uranium and 

radioactivity

(percent)

96

4

Nitrate

(percent)

88

4

Hexavalent

chromium

(percent)

8

2

4

94

Manganese 

or iron

(percent)

98

2

Total 

dissolved 

solids

(percent)
RESULTS: Groundwater Quality in Shallow Aquifers in the Western Mojave Desert, California

Benchmark type and value for selected constituents.

[Benchmark types: EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; USGS, U.S. Geological Survey; HBSL, health-based screening 

level; HAL, lifetime health advisory; CA, California State Water Resources Control Board Division of Drinking Water; SMCL, secondary maximum contaminant level. 

Abbreviations: ppb, parts per billion; ppm, parts per million; pCi/L, picocuries per liter; TT, treatment technique; >, greater than]

Constituent Benchmark Constituent Benchmark

Type Value Type Value

Arsenic EPA-MCL 10 ppb Iron CA-SMCL 300 ppb

Nitrate, as nitrogen EPA-MCL 10 ppm Manganese CA-SMCL 50 ppb

Hexavalent chromium 

(non-cancer and cancer)

USGS-HBSL 20 ppb and 4 ppb Total dissolved solids (TDS) 

(upper and recommended) CA-MCL 1,000 ppm and 500 ppm

Strontium EPA-HAL 4,000 ppb Perchlorate CA-MCL 6 ppb

Barium EPA-MCL 1,000 ppb Enterococci EPA-TT No bacteria

Boron EPA-HAL 6,000 ppb Escherichia coli (E. coli) EPA-MCL Repeat detection at a site

Chromium CA-MCL 50 ppb

Adjusted gross alpha 

particle activity

EPA-MCL 15 pCi/L Total coliforms EPA-TT >5 percent of samples with 

detections per month

ORGANIC CONSTITUENTS

VOCs

(percent) 100

Pesticides

(percent) 100

Total coliforms

Enterococci

88

8 4

Microbial 

indicators

(percent)

Organic Constituents with Human-Health Benchmarks

The Priority Basin Project uses laboratory methods that can detect low concentrations 

of volatile organic compounds (VOCs) and pesticides that are far below human-health

benchmarks. The VOCs and pesticides detected at these very low concentrations can be used to 

help trace the movement of water from the landscape into the aquifer system.

Many household, commercial, and industrial products, including solvents, gasoline 

components, and refrigerants, contain VOCs. Pesticides, including herbicides, insecticides, and 

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

unwanted vegetation, insects, fungi, and other pests. There were no VOCs or pesticides found at 

high or moderate concentrations. Volatile organic compounds were detected at low concentrations 

in 16 percent and pesticides were detected in 4 percent of domestic wells. The most common 

VOCs detected were disinfection by-products and the most common pesticides were atrazine 

and simazine.

Microbial Indicator Constituents

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

Microbial indicator constituents are used to evaluate the potential for fecal contamination of 

water sources. Total coliforms and Enterococci are naturally present in soils and digestive tracts 

of animals. In the Redding–Red Bluff Shallow Aquifer study unit, total coliforms were present in 

8 percent of the domestic drinking water wells. Enterococci was present in 4 percent of the wells 

but not in the wells with total coliforms present. Escherichia coli (E. coli), analyzed as part of the 

total coliform measurements, were not detected. The pie diagram for microbial constituents uses 

different colors because the benchmarks for microbial constituents specify repeat sampling to 

confirm detections, which was not done in this study.

Moderate: Concentrations are less than 

the benchmark, but greater than 

one-half (inorganic constituents) or 

one-tenth (organic constituents) of the 

benchmark.

High: Concentrations are considered 

high if they are greater than the 

selected benchmark.

Low: Concentrations are less than moderate, or 

the constituent was not detected.

Methods for Evaluating Groundwater Quality

GAMA’s Priority Basin Project uses benchmarks established 

for drinking water to provide context for evaluating the quality of 

groundwater. The quality of drinking water can differ from the quality of 

groundwater because of contact with household plumbing, exposure to 

the atmosphere, or water treatment. The U.S. Environmental Protection

Agency (EPA) and California State Water Resources Control Board

Division of Drinking Water (CA) regulatory benchmarks set for the 

protection of human health (maximum contaminant level, MCL) were 

used when available. Otherwise, non-regulatory benchmarks set for the 

protection of aesthetic properties, such as taste and odor (secondary 

maximum contaminant level, SMCL) and nonregulatory benchmarks 

set for the protection of human health (health-based screening levels, 

HBSL; response levels, RL; notification levels, NL; and lifetime health 

advisory levels, HAL) were used (Shelton and others, 2020). Water

quality in private domestic wells is not regulated in California.

Pie diagrams are used to summarize groundwater-quality

results. The pie slices represent the percentage of the 

domestic water wells with high, moderate, and low 

concentrations of a constituent. Methods for calculating 

the percentages shown in the pie diagrams are discussed 

by Bennett and others (2011).
Comparison of Water Quality Between North and South Sacramento

Valley Shallow Aquifer Study Units

Aquifer proportions 

(percentages of wells) 

with moderate with 

high concentrations in 

domestic drinking-water

wells were lower in 

the Redding–Red

Bluff study unit in the 

northern Sacramento

Valley for some 

groundwater constituents 

with water-quality

benchmarks compared 

to domestic wells in the 

southern Sacramento Valley (Bennett, 2019).

The nutrient nitrate was found at moderate concentrations in about 14 percent of 

the southern Sacramento Valley domestic wells but only about 4 percent of the northern 

Sacramento Valley domestic wells; the higher nitrate in the south likely is related 

to higher agriculture land use in the study unit (Bennett and others, 2011). Arsenic

occurred at moderate or high concentrations in about 36 percent of the domestic wells 

in the southern Sacramento Valley but was only present at moderate concentrations 

(4 percent) in the domestic wells in the northern Sacramento Valley. Manganese was at 

moderate or high concentrations in about 22 percent of the southern Sacramento Valley

domestic wells, more frequently than for the northern Sacramento Valley (6 percent), 

which reflects widespread low oxygen (anoxic) in the sampled wells in the south. The

more frequent anoxia observed in the southern Sacramento Valley wells is a likely 

mechanism for higher and more widespread arsenic concentrations (Bennett and 

others, 2011).

Moderate or high hexavalent chromium was found in about 12 percent of domestic 

drinking water wells in both shallow aquifer study areas. Hexavalent chromium in 

groundwater can be derived from natural sources through water-rock interactions but 

also has been linked to industrial releases in some areas. It is most commonly found in 

oxygenated (oxic) groundwaters since hexavalent chromium is more soluble under these 

conditions (California State Water Resources Control Board, 2017).

By Jennifer S. Harkness and Jennifer L. Shelton

Nitrate Arsenic Manganese Chromium-6 0

5

10

15

20

25

30

35

Aquifer proportion (percentage)

EXPLANATION

Concentration

Moderate

High

North Sacramento 

Valley Study Unit

South Sacramento 

Valley Study Unit

Proiority Basin Assessments

GAMA’s Priority Basin Project

(PBP) assesses water quality in 

groundwater resources used for 

drinking water supply. This study in the 

Northern Sacramento Valley focused on 

groundwater resources used for domestic 

drinking water. Domestic wells typically 

tap shallower parts of aquifer systems 

than public-supply wells, and water 

quality can vary with depth in aquifer 

systems. Ongoing assessments are being 

carried out in more than 120 basins 

and areas outside of basins throughout 

California. The PBP assessments 

compare constituent concentrations in 

untreated groundwater with benchmarks 

established for the protection of human 

health and for aesthetic concerns. The

PBP does not evaluate the quality of 

drinking water that arrives at the tap 

following filtration or treatment.

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 Redding–Red Bluff 

Shallow Aquifer study unit, data were 

collected by the PBP in 2018–2019. The 

PBP includes chemical analyses generally 

not available as part of regulatory 

compliance monitoring, including 

measurements at concentrations lower 

than human-health benchmarks and 

measurements of constituents that can be 

used to trace the sources and movement 

of groundwater.

References Cited

Bennett, G.L.,V, 2019, Groundwater quality in the Sacramento Metropolitan shallow 

aquifer, California: U.S. Geological Survey Open-File Report 2019–1047, 4 p., 

https://doi.org/10.3133/ofr20191047. 

Bennett, G.L.,V, Fram, M.S., and Belitz, K., 2011, Status of groundwater quality in the 

southern, middle, and northern Sacramento Valley study units, 2005–08—California

GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations

Report 2011–5002, 120 p., https://doi.org/10.3133/sir20115002. 

California State Water Resources Control Board, 2009, Domestic well project 

groundwater quality data report Tehama County focus area: State Water

Resources Control Board Division of Water Quality GAMA Program, 28 p., 

https://www.waterboards.ca.gov/gama/docs/tehama/tehama_summary_report.pdf.

California State Water Resources Control Board, 2017, Groundwater information sheet— 

Hexavalent chromium: State Water Resources Control Board Division of Water Quality

GAMA Program, 8 p., https://www.waterboards.ca.gov/water_issues/programs/gama/

docs/coc_hexchromcr6.pdf.

McMahon, P.B., and Chapelle, F.H., 2007, Redox processes and water quality 

of selected principal aquifer systems: Ground Water, v. 46, no. 2, p. 259–271, 

https://doi.org/10.1111/j.1745-6584.2007.00385.x.

Shelton, J.L., Bennett, G.L., Watson, E., and Johnson, T.D., 2020, Groundwater-quality

data in the Redding–Red Bluff shallow aquifer study unit, 2018–2019—Results from 

the California GAMA Priority Basin Project: U.S. Geological Survey data release, 

https://doi.org/10.5066/P9ZKAH3O.

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

Telephone number: (916) 278-3000

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

GAMA Program Unit Chief

State Water Resources Control Board

Division of Water Quality

P.O. Box 2231, Sacramento, CA 95812

Telephone number: (916) 341-5779

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