Fact Sheet 2014–3114

February 2015

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

Groundwater Quality in the Northern Coast Ranges 

Groundwater 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 (PBP) of the GAMA Program provides a comprehensive assessment of the 

State’s groundwater quality and increases public access to groundwater-quality information. The basins in 

the Coast Ranges of northern California constitute one of the study units being evaluated. 

The Northern Coast Ranges Study Unit

The Northern Coast Ranges (NOCO) study unit is 633 square miles and consists of 

35 groundwater basins and subbasins (California Department of Water Resources, 2003; Mathany 

and Belitz, 2015). These basins and subbasins were grouped into two study areas based primarily 

on locality. The groundwater basins and subbasins located inland, not adjacent to the Pacific 

Ocean, were aggregated into the Interior Basins (NOCO-IN) study area. The groundwater basins 

and subbasins adjacent to the Pacific Ocean were aggregated into the Coastal Basins (NOCO-CO) 

study area (Mathany and others, 2011). 

The primary aquifer system in the 

NOCO-IN study area occurs in alluvial basins 

made up of sand, silt, gravel, clay, and thin 

volcanic ash layers or lenses. Groundwater 

movement in the NOCO-IN study area follows 

the topography and direction of surfacewater features. In the NOCO-CO study area, 

groundwater is present in alluvial fan, floodplain, and terrace deposits. Groundwater movement in the NOCO-CO study area is from east 

to west towards the Pacific Ocean. 

The primary aquifer system in the study 

unit generally is defined as those parts of the 

aquifer system corresponding to the perforated 

intervals of sites listed in the California 

Department of Public Health (CDPH) database 

of public drinking-water supply sources. Well 

depths in the NOCO-IN study area ranged 

from 36 to 400 feet below land surface (ft bls), 

and depths to top-of-perforations ranged from 

15 to 148 ft bls. In the NOCO-CO study area, 

well depths ranged from 15 to 400 ft bls, and 

depths to top-of-perforations ranged from 10 to 

356 ft bls. Water quality in the primary aquifer 

system may differ from that in the shallower and 

deeper parts of the aquifer system. 

Average annual rainfall in the NOCO 

study unit ranges from 22 to 79 inches. In the 

NOCO-IN study area, the climate is classified as 

Mediterranean, with warm to hot, dry summers 

and cold, wet winters. In the NOCO-CO study area, the climate is influenced by the Pacific Ocean 

and is characterized by cool to mild summers and cold, wet winters. The study unit is drained by 

several rivers and their principal tributaries: the Eel, Russian, Mad, Navarro, Smith, Klamath, 

Noyo, and Big Rivers.

Land use in the study unit is about 60 percent (%) natural (mostly grassland and forest), 

29% agricultural, and 11% urban. The primary uses of agricultural lands are pasture, row crops, 

hay, vineyards, and timberlands. The largest urban areas are the cities of Crescent City, Arcata, 

Eureka, Fort Bragg, Willits, Ukiah, and Lakeport.

Recharge to the groundwater system is primarily from mixture of ambient sources, including 

direct percolation of precipitation and irrigation waters, infiltration of runoff from surrounding 

hills/areas, seepage from rivers and creeks, and subsurface inflow (from non-alluvial geologic 

units that bound the alluvial basins). The primary sources of discharge are evaporation, discharge 

to streams, and water pumped for municipal supply and irrigation.

U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper

Russian River

Navarro River

Big River

Noyo River

Smith River

Eel River

Klamath River

Ukiah

Wilits

Eureka Arcata

Covelo

Fortuna

Lakeport

Fort Bragg

Crescent City

SISKIYOU CO

TRINITY CO

HUMBOLDT CO

MENDOCINO CO

LAKE

CO

TEHAMA CO

SHASTA CO

GLENN CO

DEL

NORTE

CO

SONOMA CO

COLUSA CO

NAPA

CO

YOLO

CO

125° 124° 123°

41°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

NOCO-IN study area

NOCO-CO study area

Cities

Counties

Pacific Ocean

Overview of Water Quality

Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

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

with concentrations in the three specified categories. Values on

piechart may not equal 100 due to rounding of percentages.

<1

98

10

76

14

2

GAMA’s Priority Basin Project evaluates the quality of untreated groundwater. 

However, for context, groundwater quality 

was compared to benchmarks established 

for drinking-water quality. Benchmarks 

and definitions of high, moderate, and low

relative-concentrations are discussed in the 

inset box on page 3.

Many inorganic constituents occur 

naturally in groundwater. The concentrations 

of inorganic constituents can be affected by 

natural processes as well as by human activities. In the NOCO study unit, one or more 

inorganic constituents were present at high 

concentrations in about 10% of the primary 

aquifer system and at moderate concentrations in about 14% of the primary aquifer 

system.

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 

the NOCO study unit, organic constituents 

were present at high concentrations in less 

than 1% of the primary aquifer system. One 

or more organic constituents were present at 

moderate concentrations in about 2% of the 

primary aquifer system.
 

 

RESULTS: Groundwater Quality in the Northern Coast Ranges Study Unit

Trace

elements 

Nutrients

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

10

9

81

Manganese

or iron

52

40

9

2

98

96

4

Total

dissolved

93 solids 

7

Uranium and

radioactive

100 constituents

Inorganic Constituents with Health-Based Benchmarks

Trace elements and major and minor ions are naturally present in the minerals 

in rocks and soils and in the water that comes into contact with those materials. In 

the NOCO study unit, trace elements were detected at high concentrations in about 

10% of the primary aquifer system. Boron and arsenic were the trace elements that 

most frequently occurred at high concentrations. Barium also was detected at a high 

concentration, but in less than 2% of the primary aquifer system. 

Nutrients, such as nitrate and nitrite, are naturally present at low concentrations 

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

human activities. Common sources of nutrients, aside from soils, include fertilizers 

applied to crops and landscaping, seepage from septic systems, and human and animal waste. Nutrients were not present at high concentrations in the primary aquifer 

system. Nitrate was present at moderate concentrations in about 4% of the primary 

aquifer system.

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 naturally occurring isotopes of uranium and thorium in minerals in the sediments of the aquifer. Uranium and other radioactive constituents were not present at 

high or moderate levels in the primary aquifer system.

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. Anoxic 

conditions in groundwater (very low amounts of dissolved oxygen) may result in the 

release of naturally occurring elements, such as iron and manganese, from minerals 

and into groundwater. In the NOCO study unit, iron and manganese (one or both) 

were present at high concentrations in about 40% of the primary aquifer system and 

at moderate concentrations in about 9%. 

The State of California has a recommended and an upper limit for total dissolved solids (TDS) in drinking water. All water naturally contains TDS as a result 

of the weathering and dissolution of minerals in soils. TDS was not present at high 

concentrations (above the upper limit) but was present at moderate concentrations 

(between the recommended and upper limit) in about 7% of the primary aquifer 

system.

Constituent of Special Interest: 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 fuels, fireworks, and other 

products, may be present in some fertilizers, and occurs naturally at low concentrations in groundwater. In the NOCO study unit, perchlorate was present at moderate 

concentrations in about 2% of the primary aquifer system. 
RESULTS: Groundwater Quality in the Northern Coast Ranges Study Unit

ORGANIC CONSTITUENTS

Pesticides

100

Solvents

100

Trihalomethanes

98

2

Gasoline

components 100

<1

Organic

synthesis

reagents 98

2

Organic Constituents

The GAMA-PBP uses laboratory methods that can detect volatile organic 

compounds (VOCs) and pesticides and pesticide degradates at low concentrations 

far below human-health benchmarks. VOCs and pesticides and pesticide degradates 

detected at these very low concentrations can be used to 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. In the 

NOCO study unit, VOCs were detected at high concentrations in less than 1% of the 

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

aquifer system.

Gasoline components include gasoline hydrocarbons and oxygenates. The 

gasoline oxygenate methyl tert-butyl ether (MTBE) was detected at high relativeconcentrations in less than 1% of the primary aquifer system. 

Trihalomethanes can form during municipal water disinfection and may enter 

groundwater by the infiltration of landscape irrigation water. Trihalomethanes were 

present at moderate concentrations in about 2% of the primary aquifer system.

Organic synthesis reagents are compounds used in the manufacture or preparation of organic compounds and are commonly used to make polyvinyl chloride (PVC) 

pipes. The organic synthesis reagent vinyl chloride was detected at moderate concentrations in about 2% of the primary aquifer system.

Solvents were not detected at either high or moderate concentrations in the primary aquifer system.

Pesticides with Human-Health Benchmarks

Pesticides and pesticide degradates, 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 

NOCO study unit, pesticides and pesticide degradates were not detected at high or 

moderate concentrations in the primary aquifer system. Herbicides were occasionally 

detected at low concentrations in the primary aquifer system. 

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

The GAMA-PBP 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. Values are a percentage of the area of the primary aquifer system

with concentrations in the three specified categories. Values

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

High concentrations

Moderate concentrations

Low concentrations

or not detected

EX

AMPLE

CONSTITUENT CONCENTRATIONS

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 generally are 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 Mathany and Belitz (2015). 
Factors that Affect Groundwater Quality

In the NOCO study unit, boron was the constituent that most frequently occurred at 

high concentrations. About 7% of the primary aquifer system had boron concentrations 

greater than the nonregulatory 

California notification level 

of 1,000 micrograms per liter. 

Boron is a naturally occurring 

element with high solubility 

in water. Natural sources 

include igneous rocks, evaporite minerals, seawater, and 

geothermal waters.

All of the high concentrations of boron and most of 

the moderate concentrations 

occurred in the NOCO-IN 

study area. High and moderate 

concentrations of boron were 

associated with elevated 

groundwater temperatures, 

groundwater chemistry characteristics similar to those of geothermal waters, and proximity 

to known geothermal areas, 

suggesting that a likely source 

of the boron was groundwater 

interaction with hydrothermal 

systems in the NOCO-IN study 

area (Mathany and Belitz, 

2015). Previous investigations 

in the Geysers–Clear Lake 

area (and surrounding areas) 

by the U.S. Geological Survey 

and others have identified 

elevated water temperatures, 

high concentrations of boron, 

arsenic, and lithium, and 

elevated helium isotope ratios 

as the most common chemical 

signatures of hydrothermal 

system interaction (DonnellyNolan and others, 1993; Goff 

and others, 1993; Peters, 1993; 

Forrest and others, 2013).

SELECTED REFERENCES

California Department of Water Resources, 2003, California’s groundwater: California Department of Water Resources Bulletin 118, 

246 p., accessed August 2, 2011, at http://www.water.ca.gov/groundwater/bulletin118/update2003.cfm.

Donnelly-Nolan, J.M., Burns, M.G., Goff, F.E., Peters, E.K., and Thompson, J.M., 1993, The Geysers-Clear Lake area, California—

Thermal waters, mineralization, volcanism, and geothermal potential: Economic Geology, v. 88, no. 2, p. 301–316.

Forrest, M.J., Kulongoski, J.T., Edwards, M.S., Farrar, C.D., Belitz, Kenneth, and Norris, R.D., 2013, Hydrothermal contamination 

of public supply wells in Napa and Sonoma Valleys, California: Applied Geochemistry, v. 33, p. 25–40.

Goff, F., Adams, A.I., Trujillo, P.E., Counce, D., and Mansfield, J., 1993, Geochemistry of thermal/mineral waters in the Clear Lake 

region, California, and implications of hot dry rock geothermal development: Los Alamos, New Mexico, U.S. Department of 

Energy, Los Alamos National Laboratory, Technical Report LA-12510-HDR, 23 p., accessed September 26, 2014, at http://www.

osti.gov/energycitations/servlets/purl/7090069-IdLmoI/native/7090069.pdf.

Mathany, T.M., and Belitz, Kenneth, 2015, Status and understanding of groundwater quality in the Northern Coast Ranges study unit, 

2009—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2014–5215, 86 p. (Also 

available at http://pubs.usgs.gov/sir/2014/5215/.)

Mathany, T.M., Dawson, B.J., Shelton, J.L., and Belitz, Kenneth, 2011, Groundwater-quality data in the Northern Coast Ranges 

study unit, 2009—Results from the California GAMA Program: U.S. Geological Survey Data Series 609, 92 p. (Also available at 

http://pubs.usgs.gov/ds/609/.)

Peters, K.E., 1993, D-18O enriched waters of the Coast Range Mountains, northern California—Connate and ore-forming 

fluids: Geochimica et Cosmochimica Acta, v. 57, no. 5, p. 1093−1104. (Also available at http://dx.doi.org/10.1016/0016-

7037(93)90043-V.)

GeysersClear Lake

area

123°30’ 123° 122°30’

39°

30’

39°

Geothermal well

EXPLANATION

NOCO-IN study area

USGS-GAMA grid site

Low Moderate High

Hot spring 

Relative-concentrations of boron

 

0 5 10 20 MILES

0 10 20 5 KILOMETERS

Clear

Lake

Boron concentration, in micrograms per liter

Field water temperature, in degrees Celsius

10 100 1,000 10,000

25

20

15

10

NOCO-CO

NOCO-IN

Priority Basin Assessments 

The GAMA-PBP assesses water 

quality in that part of the aquifer system used for drinking water, primarily 

public supply. Water quality in shallow 

and deep parts may differ from that in 

the primary aquifer system assessed by 

the GAMA-PBP. GAMA’s Domestic 

Well Project assesses water quality 

in the shallower parts of the aquifer 

system. Ongoing assessments are being 

conducted in more than 120 basins 

throughout California. 

The GAMA-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. The PBP does not 

evaluate the quality of drinking water 

delivered to consumers. 

The GAMA-PBP uses two scientific approaches for assessing groundwater quality. The first approach uses 

a network of sites 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 NOCO 

study unit, data were collected by 

the GAMA-PBP in 2009 and from 

the CDPH database for 2006–2009. 

The GAMA-PBP includes chemical 

analyses not generally 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

Telephone number: (916) 278-3100

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

GAMA Program Unit Chief

State Water Resources Control Board

Division of Water Quality

PO Box 2231, Sacramento, CA 95812

Telephone number: (916) 341-5779

By Timothy M. Mathany and Kenneth Belitz

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

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143114