In-Situ Arsenic Remediation in Carson Valley, 

Douglas County, West-Central Nevada

Scientific Investigations Report 2010-5161

Prepared in cooperation with the Carson Water Subconservancy District and Douglas County

U.S. Department of the Interior

U.S. Geological Survey

In-Situ Arsenic Remediation in Carson Valley, 

Douglas County, West-Central Nevada

By Angela P. Paul, Douglas K. Maurer, Kenneth G. Stollenwerk, and Alan H. Welch

Prepared in cooperation with the Carson Water Subconservancy District and 

Douglas County

Scientific Investigations Report 2010-5161 

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Paul, A.P., Maurer, D.K., Stollenwerk, K.G., and Welch, A.H., 2010, In-situ arsenic remediation in Carson Valley, 

Douglas County, west-central Nevada: U.S. Geological Survey Scientific Investigations Report 2010-5161, 24 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Arsenic and Iron Interaction...............................................................................................................3

In-Situ Arsenic Remediation...............................................................................................................4

Purpose and Scope ..............................................................................................................................5

Hydrogeologic Setting...................................................................................................................................5

Phase I and II Approach and Experiments................................................................................................7

Phase I Field Experiment Design........................................................................................................7

Phase I Field Experiment Results..............................................................................................7

Phase II Laboratory Column Experiments ........................................................................................9

Laboratory Column Results ......................................................................................................10

Phase II Field Experiment Design ....................................................................................................12

First Phase II Field Experiment and Results ..........................................................................13

Second Phase II Field Experiment and Results ....................................................................16

Discussion of Phase I and II Experiments .....................................................................................19

Summary and Conclusions.........................................................................................................................19

Acknowledgments.......................................................................................................................................20

References Cited..........................................................................................................................................20
iv

Tables

Table 1. Concentrations of arsenic, arsenic speciation, iron, pH, and dissolved oxygen in

water from wells at or near the arsenic remediation site, Douglas County,

November 2005 .............................................................................. 5

Table 2. Source-water composition for laboratory column experiments using core

material from wells drilled as part of the Phase II, in-situ arsenic remediation

study, Douglas County, Nevada............................................................ 9

Table 3. Summary of treatment and concentrations of substances used during first

Phase II field experiment No. 1, in-situ remediation of arsenic remediation,

Douglas County, Nevada, June 23, 2008 ................................................ 15

Table 4. Volumes of groundwater through treatment system during first Phase II field

experiment No. 1, in-situ arsenic remediation, Douglas County, Nevada, June

23, 2008 ....................................................................................... 15

Table 5. Concentration of various arsenic species determined for samples collected

during first Phase II field experiment No. 1, in-situ arsenic remediation, Douglas

County, Nevada, June 23–24, 2008......................................................... 16

Figures

Figure 1. Map showing arsenic concentrations in selected wells and remediation test

well locations, Carson Valley, Douglas County, Nevada .............................. 2

Figure 2. Diagram showing lithology of core obtained November 2007 when drilling

Arsenic 2-inch (As-2) and Arsenic 6-inch (As-6) wells for Phase II, in-situ

arsenic remediation experiments, Douglas County, Nevada, June and October

2008 .......................................................................................... 6

Figure 3. Graphs showing concentrations of arsenic and iron and manganese withdrawn

from the EXP-2 shallow well after treatment with low, moderate, and high iron

concentrations at pH values ranging from 5.3 to 5.6 during Phase 1, in-situ

arsenic remediation experiments, Douglas County, Nevada, September 2006 ...... 8

Figure 4. Graphs showing concentrations of arsenic and iron and manganese withdrawn

from the EXP-2 deep well after treatment with low, moderate, and high iron

concentrations at pH values ranging from 5.3 to 6.0, during Phase I, in-situ

arsenic remediation experiments, Douglas County, Nevada, September 2006 ...... 8

Figure 5. Graph showing arsenic concentrations in leachate and source water from

column experiments, Douglas County, Nevada.......................................... 10

Figure 6. Graph showing pH of leachate from column experiments, Douglas County, 

Nevada ....................................................................................... 11

Figure 7. Graph showing manganese concentrations in leachate from column

experiments, Douglas County, Nevada ................................................... 12

Figure 8. Schematic diagrams of treatment and sampling ports for Phase II in-situ

arsenic remediation, Douglas County, Nevada .......................................... 14

Figure 9. Graphs showing measured pH and alkalinity during Phase II experiment No. 1,

in-situ arsenic remediation, Douglas County, Nevada, June 23, 2008 ............... 17

Figure 10. Graph showing remediation of arsenic during and after Phase II experiment No.

1, in-situ arsenic remediation, Douglas County, Nevada, June 23–26, 2008 ......... 17

Figure 11. Graphs showing groundwater flow through activated carbon filters and air-lift

rates during the second Phase II field experiment, in-situ arsenic remediation,

Douglas County, Nevada, October 16, 2008 ............................................. 18
v

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

square foot (ft2) 0.09290 square meter (m2)

Volume

gallon (gal) 3.785 liter (L)

cubic foot (ft3) 7.48 gallon (gal)

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Mass

pound, avoirdupois (lb) 2.205 kilogram (kg)

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

pound per cubic foot (lb/ft3) 0.01302 grams per milliliter (g/mL)

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 

To convert grams per milliliter (gm/mL) to milligrams per milliliter, multiply by 1,000.

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.
vi

Abbreviations and Acronyms

Abbreviation 

or acronym

Meaning

HFO hydrous ferric oxide

ka kiloannum; unit of time equal to one thousand years

MCL maximum contaminant level

SAP South Airport Production well

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
In-Situ Arsenic Remediation in Carson Valley, 

Douglas County, West-Central Nevada 

By Angela P. Paul, Douglas K. Maurer, Kenneth G. Stollenwerk, and Alan H. Welch

Abstract

Conventional arsenic remediation strategies primarily 

involve above-ground treatment that include costs involved 

in the disposal of sludge material. The primary advantages of 

in-situ remediation are that building and maintaining a large 

treatment facility are not necessary and that costs associated 

with the disposal of sludge are eliminated. A two-phase study 

was implemented to address the feasibility of in-situ arsenic 

remediation in Douglas County, Nevada.

Arsenic concentrations in groundwater within Douglas 

County range from 1 to 85 micrograms per liter. The primary 

arsenic species in groundwater at greater than 250 ft from land 

surface is arsenite; however, in the upper 150 ft of the aquifer 

arsenate predominates. Where arsenite is the primary form 

of arsenic, the oxidation of arsenite to arsenate is necessary. 

The results of the first phase of this investigation indicated 

that arsenic concentrations can be remediated to below the 

drinking-water standard using aeration, chlorination, iron, and 

pH adjustment. Arsenic concentrations were remediated to less 

than 10 micrograms per liter in groundwater from the shallow 

and deep aquifer when iron concentrations of 3–6 milligrams 

per liter and pH adjustments to less than 6 were used. Because 

of the rapid depletion of dissolved oxygen, the secondary 

drinking-water standards for iron (300 micrograms per liter) 

and manganese (100 micrograms per liter) were exceeded 

during treatment. Treatment was more effective in the shallow 

well as indicated by a greater recovery of water meeting the 

arsenic standard. 

Laboratory and field tests were included in the second 

phase of this study. Laboratory column experiments using 

aquifer material indicated the treatment process followed 

during the first phase of this study will continue to work, 

without exceeding secondary drinking-water standards, 

provided that groundwater was pre-aerated and an adequate 

number of pore volumes treated. During the 147-day 

laboratory experiment, no decrease in flow through the column 

was observed. The primary mechanism of arsenic removal is 

through coprecipitation with iron oxide.

Calculations based on the results of the column 

experiments and assuming 10 and 30 percent 

porosity indicated that treatment of approximately 

237,000–714,000 gallons of water would be required in 

order to remediate arsenic concentrations to less than 10 

micrograms per liter. During the first second-phase field 

experiment, effective injection of treated groundwater back 

into the aquifer was prevented due to clogging likely caused 

by entrained gases and the fine texture (sand, clay, and gravel) 

of the aquifer sediments. Because of the overflow of treated 

water from the injection wells, only 3,760 gallons of treated 

water were injected. Immediately upon terminating this first 

experiment, no arsenic remediation was apparent. However, 

approximately 24 hours after terminating the experiment 

arsenic concentrations in groundwater collected from one 

of the injection wells showed a decrease from about 30 to 

15 micrograms per liter, indicating that some remediation 

had taken place. In agreement with the laboratory-column 

experiments, pre-aeration prevented the exceedence of the 

secondary drinking-water standards for iron and manganese. 

Because of complications associated with system hydraulics, 

no additional experiments were performed.

Introduction

In January 2001, the U.S. Environmental Protection 

Agency (USEPA) lowered the maximum contaminant 

level (MCL) for drinking water from 50 to 10 μg/L (U.S. 

Environmental Protection Agency, 2001a, 2001b). Because of 

this change, many areas in the United States that previously 

met the drinking-water standard now must remove arsenic 

from the water and water purveyors are searching for new 

cost-effective methods. Arsenic is commonly associated with 

volcanic and alluvial/lacustrine sedimentary deposits and is 

one of the most common contaminants in groundwater in the 

western United States (Welch and others, 1988; Walker and 

others, 2008). Arsenic concentrations in the northeastern part 

of Carson Valley, Douglas County, Nevada, are greater than 

the new arsenic standard (fig. 1). Douglas County and the 

Carson Water Subconservancy District are considering the 

development of a well field in the valley near the airport to 

treat the water to remove arsenic. Other methods, as well as 

those considered conventional, for the removal of arsenic are 

being considered, including below-ground (in-situ) treatment. 
2 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

Figure 1. Arsenic (A) concentrations in selected wells and (B) remediation test well locations, Carson Valley, 

Douglas County, Nevada.

tac09-4161_fig01a

A.

Arsenic, in micrograms per liter

1 - 10

11 - 25

26 - 85

Hydrographic area boundary

Gardnerville Minden

CARSON

East Fork

Carson River

Genoa

RANGE

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

MindenTahoe

Airport

SAP#1 EXP-2

CARSON CITY

DOUGLAS COUNTY

West Fork

119°50' 119°45' 119°40'

39°5'

39°

38°55'

38°50'

EXPLANATION

0 1 2 3 4 5 Miles

0 1 2 3 4 5 Kilometers

Area of

map

Base from ESRI ArcGIS Online Map service

http://services.arcgisonline.com/ArcGIS/services: 

World_Imagery, 2009; and U.S. Geological Survey

digital data, 1:100,000-scale, 1978-2009.

Universal Transverse Mercator projection, Zone 11

North American Datum of 1983

NEVADA

CALIFORNIA
Introduction 3

Conventional methods for treating drinking-water, 

including point-of-use by reverse osmosis (Walker and others, 

2008), above-ground iron or alumina coprecipitation, or both 

(Cadena and Kirk, 1995; Kartinen and Martin, 1995), and ionexchange (Kartinen and Martin, 1995), have been investigated 

for effectiveness in removing arsenic. A potentially costeffective alternative to removing arsenic using traditional 

above-ground treatments is lowering arsenic concentrations 

within the aquifer by a process called “in-situ” removal. In this 

process, metals are retained in the aquifer as a coprecipitate 

with metal oxides or sulfides (Rott, 1990; Mettler and others, 

2001; Druhan and others, 2008). Arsenic retention is enhanced 

by increasing the adsorption capacity of the aquifer sediments 

by adding iron, adjusting pH, or both. 

Arsenic and Iron Interaction

The geochemical cycling of arsenic is largely controlled 

by sorption reactions associated with the presence of 

aluminum, iron, and manganese oxides (Anderson and others, 

1976; Oscarson and others, 1981; De Vitre and others, 1991). 

In natural systems, adsorption and coprecipitation often are 

indistinguishable and, as a result, these processes often are 

used interchangeably (Drever, 1988). Generally, the same 

geochemical conditions that optimize the adsorption of arsenic 

onto iron oxide (pH below 8, adequate dissolved oxygen, and 

predominance of arsenate) optimize their coprecipitation. In 

areas enriched in arsenic, high concentrations of dissolved 

arsenic often are associated with groundwater with alkaline 

pH because the adsorption of arsenic to metal oxides decreases 

with increasing pH (Edwards, 1994). 

Figure 1. Continued.

tac09-4161_fig01b

B.

EXP-2

EXP-1

SAP#1

USGS

 AG

As-6

As-2

119°45'18" 119°45'16" 119°45'14" 119°45'12" 119°45'10" 119°45'8" 119°45'6" 119°45'4" 119°45'2" 119°45' 119°44'58" 119°44'56"

38°59'30"

38°59'28"

38°59'26"

38°59'24"

38°59'22"

119°45'18" 119°45'16" 119°45'14" 119°45'12" 119°45'10" 119°45'8" 119°45'6" 119°45'4" 119°45'2" 119°45' 119°44'58" 119°44'56"

38°59'30"

38°59'28"

38°59'26"

38°59'24"

38°59'22"

Base from ESRI ArcGIS Online Map service

http://services.arcgisonline.com/ArcGIS/services: 

World_Imagery, 2009.

Universal Transverse Mercator projection, Zone 11

North American Datum of 1983

0 200 400 600 800 Feet

0 50 100 150 200 Meters

0 200 400 600 800 Feet

0 50 100 150 200 Meters

EXP-1

SAP#1

USGS

 AG

As-6

EXPLANATION

Monitoring

Production

Monitoring/nested and used in experiments

Remediation test

Wells
4 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

In natural waters, inorganic arsenic typically occurs 

as two species: arsenite (As(III)) or arsenate (As(V)). 

Under more oxidizing conditions As(V) is the primary 

arsenic species whereas under reducing conditions As(III) 

predominates (Ferguson and Gavis, 1972).1 Generally, As(III) 

is more mobile in aqueous systems than As(V) (Masscheleyn 

and others, 1991; Chiu and Hering, 2000; Nordstrom and 

Archer, 2003), mostly because of the neutral charge of this 

species at pH ranges typically encountered in the natural 

environment (Ferguson and Gavis, 1972). In systems with 

a pH near 7, the negatively-charged As(V) readily adsorbs 

to positively-charged metal oxides, such as aluminum and 

iron. The initial adsorption of arsenic onto iron oxides is 

relatively fast, typically within a few minutes to hours (Fuller 

and others, 1993; Raven and others, 1998); however, arsenic 

adsorption can continue during prolonged exposure to the 

oxide (Fuller and others, 1993). Under similar environmental 

conditions, As(III) has limited affinity for metal oxides. When 

coprecipitating arsenic with iron oxide, therefore, converting 

As(III) to As(V) is necessary to enhance the reaction (Rott and 

Friedle, 1999). In alkaline waters, As(V) generally becomes 

increasingly mobile as a result of the repulsion between the 

negatively charged metal oxide surfaces and arsenic anion 

(Anderson and others, 1976). Theoretically, at pH ranges less 

than 8.0, the surface charge of iron oxide is positive, which 

enhances the association between the arsenate anion and oxide 

surface (Raven and others, 1998; Chui and Hering, 2000; Zeng 

and others, 2008).

In-Situ Arsenic Remediation

Although a relatively new treatment in the United 

States, in-situ treatment for the removal of arsenic, iron, or 

manganese, or combination thereof, has been successfully 

applied to municipal-supply wells for several years in some 

parts of Europe (Rott, 1990; Mettler and others, 2001). In the 

United States, in-situ remediation of other trace-elements, 

such as chromium, is gaining regulatory acceptance (Grosse 

and others, 2000). In-situ remediation is generally thought 

to be less expensive than above-ground treatment because 

of the relatively lower costs associated with operation and 

maintenance of in-situ methods (Grosse and others, 2000; 

Water Research Commission South Africa, 2004). Costs 

associated with the in-situ removal of plume-related chromium 

using techniques similar to those used in this study varied 

depending on site-specific considerations and ranged from 

about $250,000 to $1,677,800 (Brown and others, 1998; Rouse 

and others, 1999; Grosse and others, 2000). Although sitespecific considerations increased the general costs associated 

with the above-ground arsenic treatment plant in Fallon, 

Nevada, the cost for building this facility was $19 million 

(White, 2009). Rott and Friedle (1999) achieved arsenic 

removal efficiencies greater than 10, meaning for each volume 

of treated water injected, 10 volumes of water with arsenic 

concentrations less than 10 μg/L were withdrawn. Samples 

collected during an in-situ treatment of arsenic in a fractured 

basalt aquifer in Churchill County, Nevada, by reduction of 

pH and addition of iron to increase the adsorption capacity of 

the aquifer material, showed a 50 percent reduction in arsenic 

concentration (Welch and others, 2003).

Commonly expressed concerns associated with in-situ 

remediation practices include precipitation of iron and 

arsenic that may decrease aquifer permeability, or increase 

concentrations of arsenic or other trace elements to values 

greater than before remediation was started. However, 

published research has shown that during in-situ remediation, 

iron oxide does not accumulate near the well-screen, but 

moves out into the aquifer (Appelo and others, 1999; Mettler 

and others, 2001; Mettler, 2002; Appelo and deVet, 2003). 

Additionally, the mass of iron and arsenic removed are small 

compared with the mass of the aquifer material likely to be 

affected. For example, the aquifer within 200 ft surrounding 

a typical well in northeast Carson Valley contains about 2.4 

× 109 kg of sediment2 (Welch and others, 2008). The amount 

of iron that would be added from the injection of water for a 

period of 100 years would be about 0.1 percent of this mass.3 

In comparison to the mass of the aquifer material, the small 

amount of additional mass resulting from the coprecipitation 

of arsenic and iron near a production well make the potential 

of significantly lowering well production unlikely (Mettler and 

others, 2001).

The amount of arsenic introduced to the same mass of 

aquifer material would be much less than the amount of iron 

added. If removal of 33 μg/L of arsenic were complete, then 

the amount of arsenic retained in the aquifer would result 

in a two-fold increase of the arsenic concentration in the 

sediment.4 Most of the arsenic is expected to be associated 

with the iron oxide phase, which becomes less susceptible to 

reductive dissolution over time because of recrystallization 

(Jones and others, 2000; Zobrist and others, 2000; Mettler and 

others, 2001; Ford, 2002; Violante and others, 2007). 

1

 For a definition of what constitutes an “oxidizing” or “reducing” environment the reader is directed to the following references: Hem, 1985 (p. 20-22); 

Drever, 1988 (p. 281-304); Langmuir, 1997 (p. 403-430).

2

 Assuming (1) a density of 2.6 gm/cm3, approximately the density of quartz and feldspar, (2) a porosity of 0.3, and (3) an aquifer thickness of about 360 ft, as 

indicated by drilling logs for selected production wells in northeastern Douglas County. 

3

 Assuming (1) a flow rate of 1,800 gal/min and (2) an iron concentration of 7.5 mg/L.

4

 Assuming an arsenic content of the pre-treatment sediment is equal to the geometric mean for the United States of 5.2 mg/kg for surficial sediments 

(Shacklette and Boerngen, 1984).
Hydrogeologic Setting 5

Purpose and Scope

This report, prepared in cooperation with the Carson 

Water Subconservancy District and Douglas County, presents 

the results of a two-phase experiment evaluating the potential 

for in-situ remediation to reduce arsenic concentrations in 

groundwater from Douglas County, Nevada. This evaluation 

incorporates laboratory column and field experiments 

summarizing lithologic data from wells drilled and results of 

field and laboratory experiments during November 2005 to 

October 2008. 

Hydrogeologic Setting

Carson Valley is in northwestern Nevada east of the 

Sierra Nevada Mountains. The alluvial sediments of the 

Carson Valley are composed of weathered material from 

plutonic, mafic and intermediate volcanic, meta-volcanic, 

and metasedimentary rocks (Welch, 1994; Welch and others, 

2008). Mineralogic analysis of aquifer material using X-ray 

diffraction showed the aquifer sediments were composed of 

granodiorite, as indicated by varying amounts of quartz and 

plagioclase (John McCormack, Nevada Bureau of Mines and 

Geology, written commun., May 2008). The aquifer materials 

consisted of interbedded layers of gravel and coarse sand, clay 

and gravel, and clay ranging in thickness from 10 to 40 ft. The 

lithology associated with the screened interval at about 290–

335 ft below land surface consisted of sand, clay, and gravel 

(fig. 2). The estimated age of the sediments in the aquifer 

near the wells used in these experiments is about 200–500 ka 

(Welch and others, 2008). 

From 1971 to 2000, valley floor precipitation averaged 

about 7.9 in. (Maurer and Halford, 2004). As the Carson River 

flows through the Carson Valley, its flow is diverted through 

natural and man-made channels for flood irrigation of pasture 

grasses and alfalfa. Annual streamflow loss through the valley 

from 1990 to 2000 was estimated at 75,000 acre-ft/yr (Maurer 

and others, 2008). Most streamflow losses from the Carson 

River are consumed by irrigated crops and maintain a shallow 

water depth of less than 5 ft below land surface within the 

valley (Maurer and others, 2008; Welch and others, 2008). 

Although the valley is irrigated for alfalfa, vegetation near the 

in-situ remediation experiments is mostly native rabbitbrush 

and sage.

Generally, arsenic concentrations near the Minden-Tahoe 

Airport ranged from about 28 to 38 μg/L except in samples 

collected from a shallow (25 ft deep) well, USGS AG, which 

contained about 85 μg/L (fig. 1B, table 1). The predominant 

arsenic species is As(V) in wells screened in the upper 150 ft 

of the aquifer, indicating conditions oxidizing to arsenic, 

whereas the predominant arsenic species is As(III) in wells 

screened below about 300 ft, indicating reducing conditions. 

The predominant arsenic species from the SAP#1 well is 

As(III), although the well is screened from 150 to 405 ft, 

suggesting that most production is from the lower part of the 

screened interval. Detailed descriptions of sediments made 

during well installation describe color changes generally from 

brown to olive green, gray, and black at depths greater than 

about 150 ft (ECO:LOGIC Engineering, 2003). These changes 

in color were additional evidence of oxidizing conditions at 

depths above about 150 ft and reducing conditions below 

150 ft. Iron concentrations in water samples are less than 

detection (less than 6 μg/L) for all wells, except the 25-ft 

USGS AG well (12 μg/L), and groundwater contained 

dissolved-oxygen concentrations less than 1 mg/L.

Table 1. Concentrations of arsenic, arsenic speciation, iron, pH, and dissolved oxygen in water from wells at or near the arsenic 

remediation site, Douglas County, November 2005.

[Abbreviations: SAP, South Airport Production well; USGS, U.S. Geological Survey; ft, foot; μg/L, microgram per liter; mg/L, milligram per liter; <, less than]

Local well name

Station 

identification 

No.

Date

Screened 

interval below

 land surface 

(ft)

Trace-element concentrations (μg/L)

pH

Dissolved 

oxygen 

(mg/L) Arsenic Arsenate Arsenite Iron 

SAP#1 385926119451201 11-07-05 150 to 405 31.1 0.9 28.8 <6 8.8 0.19

EXP-1 shallow 385926119451103 11-14-05 73 to 93 27.7 24.3 <.6 <6 8.2 .12

EXP-1 middle 385926119451102 11-10-05 201 to 221 37.8 8.9 24.9 <6 8.3 .19

EXP-1 deep 385926119451101 11-09-05 339 to 359 32.3 .6 29.1 <6 8.9 .22

USGS AG 385927119451301 11-15-05 15 to 25 84.8 78.0 <.6 12.2 7.6 .35

EXP-2 shallow 385927119445603 11-15-05 100 to 125 30.2 26.5 <.6 <6 8.4 .23

EXP-2 middle 385927119445602 11-08-05 185 to 205 30.7 17.3 9.5 <6 8.5 .24

EXP-2 deep 385927119445601 11-08-05 305 to 325 35.6 2.6 29.0 <6 8.7 .25
6 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

tac09-4161_fig02

Depth of changing

redox condition

Screen

305 to 325 feet

0

FEET BELOW 

LAND SURFACE

42

52

80

102

132

167

172

187

250

287

345

Sand, gravel, brown clay

Sand and clay layers

Sand, clay, gravel

Brown clay

Brown clay

Brown clay

Rounded gravel, coarse sand

Gravel, coarse sand

Coarse sand

Green-grey clay

Green-grey clay

Figure 2. Lithology of core obtained November 2007 when drilling Arsenic 2-inch (As-2) and 

Arsenic 6-inch (As-6) wells for Phase II, in-situ arsenic remediation experiments, Douglas 

County, Nevada, June and October 2008.
Phase I and II Approach and Experiments 7

Phase I and II Approach and 

Experiments

The approach included (1) oxidation by aeration and 

chlorination of water pumped from the aquifer, (2) addition of 

acid to lower the pH, (3) addition of iron to coprecipitate with 

arsenic and enhance aquifer adsorption properties, (4) injection 

of the treated water back into the aquifer, and (5) withdrawal 

of water from either the same well or adjacent well(s) for 

evaluation of the effectiveness of in-situ remediation. Aeration 

of the pumped water is needed as a source of oxygen for the 

formation of iron oxides. Unfortunately, the conversion of 

As(III) to As(V) by oxygen is slow (Eary, 1987; Lowry and 

Lowry, 2002) and can take months to years (Stollenwerk, 

2003). Alternatively, chlorine is an effective oxidizer of 

arsenite (Frank and Clifford, 1986), but its use as an oxidant 

requires the removal of free chlorine prior to injection to 

avoid formation of carcinogenic disinfection by-products. In 

these in-situ experiments, the reduced form of iron, ferrous 

iron [Fe(II)], was used as the source of iron. Conceptually, 

the injected Fe(II) travels through the aquifer reacting with 

oxygen and becomes ferric iron, [Fe(III)], at which point it 

precipitates with arsenic from the groundwater onto aquifer 

materials (Rott and Friedle, 1999). Removal efficiency was 

calculated by dividing the volume of remediated water (with 

concentrations of arsenic less than 10 micrograms per liter) 

recovered by the volume of groundwater treated.

Phase I Field Experiment Design

The phase I experiment was designed to evaluate the 

effectiveness of in-situ arsenic remediation in Carson Valley at 

the south end of the Minden-Tahoe airport where production 

well SAP #1 produces water with arsenic concentrations 

exceeding the drinking-water standard (fig. 1A). Phase I work 

included sampling of existing wells and field experiments 

in 2006 (Welch and others, 2008). Samples were collected 

from eight wells with varying depths near the remediation 

site including SAP#1 production well; USGS-AG well, a 

25-ft deep monitoring well about 200 ft northwest of SAP#1; 

and three nested piezometers (one each representing shallow, 

middle, and deep) at EXP-1 and EXP-2 well sites (fig. 1B, 

table 1). EXP-1 and EXP-2 are located about 60 ft north and 

1,600 ft east of SAP#1, respectively. 

Water was piped from the SAP#1 well to the EXP-2 

shallow and deep wells. The water from SAP#1 was 

chlorinated at the well head by Douglas County with free 

chlorine concentrations in the treatment stream ranging from 

0.4 to 0.7 mg/L, as determined using a HACH DR890 field 

meter (HACH, 2000). Using a diffuser about 100 ft from the 

SAP#1 well head, air was injected into the pipeline attaining 

dissolved-oxygen concentrations of 9 to 11 mg/L before 

reaching the EXP-2 wells. A series of in-line valves at the 

EXP-2 wells were used to remove excess air from the injection 

water and a 2-ft3 activated-carbon filter was used to remove 

chlorine prior to additional treatment and injection. After 

free chlorine was removed, hydrochloric acid and a ferrous 

chloride were added to reduce groundwater pH to between 4.5 

and 6.5 and increase iron concentration. 

The experiments consisted of injecting water with 

different iron concentrations and pH into the EXP-2 shallow 

and deep wells for periods of about 1 hour. Following injection 

and a reaction period of 1–2 hours, water was withdrawn 

from the well. This cycle was repeated several times because 

other studies have shown that efficiency increases with time 

(Rott and Friedle, 1999; Appelo and deVet, 2003). Water 

was withdrawn for periods of 1.5–4 hours until pH and 

specific conductance values reached pre-treatment conditions. 

Injection and withdrawal rates were about 5–6 gal/min. 

During withdrawal, water was sampled to determine changes 

in concentrations of arsenic and other constituents. All 

samples were analyzed by the USGS National Water Quality 

Laboratory and Geologic Division, Denver, Colorado.

Phase I Field Experiment Results

Phase I experiments used iron concentrations ranging 

from 0.37 to 6.4 mg/L, and pH values from 5.3 to 6.0. 

Experiments using the low iron concentration (0.37 mg/L) 

removed little arsenic, whereas the high iron concentration 

(6.4 mg/L) reduced arsenic concentrations to as low as 1 

μg/L in the shallow well and as low as 5.7 μg/L in the deep 

well (figs. 3 and 4). In the shallow and deep wells, removal 

efficiency of groundwater with arsenic concentrations at or 

less than 10 μg/L ranged from 0 to 1.9 and from 0 to 0.5, 

respectively. The experiments showed that arsenic removal 

was greater in the shallower well where the primary form 

of arsenic, prior to chlorination, was As(V) indicating a 

more oxidizing environment than in the deep well where 

As(III) predominated. This difference in redox condition was 

further indicated by the slower decline in dissolved-oxygen 

concentration during withdrawal of water from the shallow 

well compared to the deep well (Welch and others, 2008). 

After treatment using the shallow well, arsenic 

concentration was less than drinking-water standard during 

the first hour of pumping using moderate (3.3 mg/L) and high 

(6.4 mg/L) iron treatment (fig. 3). The volumes of treated 

water withdrawn during these treatments were 406–554 gal, 

indicating removal efficiencies of 1.0 and 1.9, respectively. 

Although arsenic concentrations were less than10 μg/L during 

the first hour of pumping, the moderate to high iron treatments 

resulted in iron concentrations greater than the secondary 

drinking-water standard of 300 μg/L. Iron concentrations 

generally decreased to less than detection (50 μg/L) after 

about one hour of withdrawal. Iron concentrations were below 
8 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

detection in the low-iron treatments. Manganese concentration 

in groundwater prior to treatment was approximately 

17 μg/L. However, manganese remained above the Nevada 

secondary drinking-water standard of 100 μg/L during most 

of the withdrawal period. The high iron and manganese 

concentrations likely resulted from the incomplete oxidation 

and precipitation of these elements with oxygen. 

In the deep well, arsenic was not remediated to below 

10 μg/L, where iron and manganese concentrations were 

less than their respective secondary drinking-water standards 

(fig. 4). The high iron treatment (5.0 mg/L) did result in the 

removal of arsenic to less than 10 μg/L; however, the volume 

of remediated water recovered (183 gal) was only about 

one-half the volume of treated water injected. In the deep 

well, iron and manganese concentrations were above the 

secondary drinking-water standards during the initial period 

of the withdrawal period for the moderate (3.1 mg/L) and 

high (4.9 mg/L) iron treatments. Iron concentrations were 

below detection in the low-iron treatments in the deep aquifer 

experiments. Reducing arsenic to concentrations less than 

tac09-4161_fig03 ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

0

5

10

15

20

25

Iron 0.37 milligrams per liter (mg/L); pH 5.4

Iron 3.3 mg/L; pH 5.6

Iron 6.4 mg/L; pH 5.3

A.

B.

Removal efficiency of 1.92

Arsenic drinking-water

standard

Removal efficiency

of 1.04

Iron secondary drinking-water standard

Manganese secondary drinking-water standard

Withdrawal initiated

Withdrawal initiated

EXPERIMENT TIME, IN HOURS

IRON AND MANGANESE CONCENTRATIONS,

IN MICROGRAMS PER LITER

10

100

1,000

10,000

Iron 0.37 mg/L; pH 5.4 

Iron 3.3 mg/L; pH 5.6 

Iron 3.3 mg/L; pH 5.6 

Iron 6.4 mg/L; pH 5.3

Iron 6.4 mg/L; pH 5.3 

Iron

Manganese

2.0 2.5 3.0 3.5 4.0 4.5

Figure 3. Concentrations of (A) arsenic and (B) iron 

and manganese withdrawn from the EXP-2 shallow 

well after treatment with low (0.37 mg/L), moderate (3.3 

mg/L), and high (6.4 mg/L) iron concentrations at pH 

values ranging from 5.3 to 5.6 during Phase 1, in-situ 

arsenic remediation experiments, Douglas County, 

Nevada, September 2006.

tac09-4161_fig04

EXPERIMENTAL TIME, IN HOURS

1 2 3 4 5 6

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

0

5

10

15

20

25

30

35

40

Iron 0.76 milligrams per liter (mg/L); pH 5.4 

Iron 3.1 mg/L; pH 6.0 

Iron 5.0 mg/L; pH 5.3 

Arsenic Drinking-Water Standard

A.

Removal efficiency

of 0.5

IRON AND MANGANESE CONCENTRATIONS,

IN MICROGRAMS PER LITER

10

100

1,000

10,000

Manganese Secondary

Drinking-Water Standard

Iron Secondary

Drinking-Water Standard

Iron 3.1 mg/L; pH 6.0 

Iron 3.1 mg/L; pH 6.0

Iron 5.0 mg/L; pH 5.3 

Iron 5 mg/L; pH 5.3

Iron 0.76 mg/L; pH 5.4 

Iron

Manganese

B.

Withdrawal initiated

Figure 4. Concentrations of (A) arsenic and (B) iron 

and manganese withdrawn from the EXP-2 deep well 

after treatment with low (0.76 mg/L), moderate (3.1 

mg/L), and high (5.0 mg/L) iron concentrations at pH 

values ranging from 5.3 to 6.0, during Phase I, in-situ 

arsenic remediation experiments, Douglas County, 

Nevada, September 2006.
Phase I and II Approach and Experiments 9

10 μg/L was less effective in the deeper well because of the 

more reducing environment with depth. Dissolved oxygen and 

pH changed rapidly during early withdrawal, with dissolved 

oxygen being consumed to below detection. The dissolved 

oxygen likely was reacting with iron in the aquifer materials, 

which is consistent with the greenish gray color of the deeper 

sediments (Welch and others, 2008).

The experiments were limited by the relatively small 

volumes of treated water that were injected, generally about 

300–400 gal. Assuming a porosity of about 30 percent for 

aquifer sediments and uniform hydraulic properties, these 

volumes penetrated the aquifer to a radial distance of less than 

0.33 ft around the 20-ft screened interval of the wells (Welch 

and others, 2008). It is hypothesized that repeated cycles of 

injection and withdrawal could lower the high concentrations 

of iron in withdrawn water if ferrous iron adsorbed onto newly 

formed iron oxide.

Phase II Laboratory Column Experiments

A core of aquifer material from the As-2 well was 

obtained from 310–315 ft below land surface. The core 

material was processed under a nitrogen-gas atmosphere to 

prevent changes in the redox condition of the aquifer material 

during column preparation. Under the inert nitrogen-gas 

atmosphere, aquifer material was extracted from the 310‐ft 

end of the core and packed into an oxygen impermeable 

Plexiglas® column, 0.98 ft long with a 0.083 ft inside 

diameter (volume of 0.005 ft3). Each end of the column 

contained filter disks to prevent the escape of fines and capped 

to prevent the entry of air. The column containing wet aquifer 

material was removed from the nitrogen-gas atmosphere and 

weighed prior to starting the experiment. After the experiment 

was completed the aquifer material was extracted and dried 

and column porosity calculated (41 percent). A pore volume 

is the amount of liquid that resides in the void (pore) space 

within a given volume of porous material. In these laboratory 

experiments, the pore volume (63 mL) was the amount of fluid 

contained in the column of aquifer material at one time.

During the laboratory experiments, water was pumped 

from the bottom of the column to the top. In this report, water 

pumped into the column is called source water, and water 

flowing out of the column is called leachate. The column 

used in the experiment was a closed system. Because the 

source water used in the column experiment was saturated 

with oxygen the column experiment was under ambient 

atmospheric conditions. Leachate was collected from the 

column in polypropylene bottles modified to allow sample 

collection under a nitrogen-gas atmosphere. The column was 

wrapped with aluminum foil to prevent any photocatalyzed 

reactions. At the end of each day, the column was sealed to 

prevent oxygen diffusion into the sediment.

Two different source waters were used during the 

experiment. Source-water 1 was aerated but otherwise 

untreated, simulating pre-aerated water used during Phase 

II field experiments where groundwater was pumped to the 

surface, oxygenated, and then injected back into the aquifer. 

The composition of source-water 1 (table 2) was similar to that 

reported by Welch and others (2008). Source-water 1 was not 

treated with iron or acid; dissolved-oxygen was about 6 mg/L, 

and pH was 8.62. About five pore volumes of this source water 

were pumped through the column at a rate of about 1.5 pore 

volumes per day.

Following the pumping of the 5 pore-volumes of sourcewater 1 through the column, the source water was changed 

to source-water 2. The composition of source-water 2 was 

similar to that expected following aeration, pH adjustment, 

and iron addition (table 2). Ferrous iron was about 7 mg/L, 

arsenic was added as arsenate, As(V), pH was about 5.5, and 

dissolved oxygen was about 6 mg/L. Ferrous iron in sourcewater 2 was stable only for about 10 hours before measurable 

oxidation began; therefore, source-water 2 was prepared 

daily. Initial flow rates were adjusted to elute about 1–1.5 

pore volumes during a 5–7 hour period. Eventually, flow rates 

gradually were increased until about 3–6 pore volumes per day 

of leachate were collected. The experiment ran for 147 days 

and at the end of each day the column was sealed to prevent 

oxygen diffusion into the sediment. 

Table 2. Source-water composition for laboratory column 

experiments using core material from wells drilled as part of 

the Phase II, in-situ arsenic remediation study, Douglas County, 

Nevada.

[Abbreviations: O2, diatomic oxygen; mg/L, milligram per liter; μg/L, 

microgram per liter; SiO2, silica dioxide; ~, approximately]

Constituent and 

unit of measure Source-water 1 Source-water 2

Dissolved oxygen, 

mg/L as O2

~6 ~6

pH, standard units 8.62 ~5.5

Calcium, mg/L 16.0 16.0

Sodium, mg/L 47.0 47.0

Potassium, mg/L 0.9 .9

Silica, mg/L as SiO2 ~42 ~42

Sulfate, mg/L 38.4 38.4

Chloride, mg/L ~50 1~90

Bicarbonate, mg/L 9.2 22.3

Arsenite, μg/L 30 0

Arsenate, μg/L 0 30

Ferrous iron, mg/L 0 ~7

1

 Chloride concentrations were calculated from the amount of hydrochloric 

acid needed to adjust pH to 8.62 and 5.5.

2

 Bicarbonate is low because of acidification to pH 5.5.
10 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

Laboratory Column Results

Arsenic concentrations in leachate for the first five pore 

volumes were about 30 μg/L (fig. 5). Source-water 1 contained 

30 μg/L arsenite; however, the leachate contained about 7–8 

μg/L arsenite and 22–24 μg/L arsenate. Arsenite is relatively 

stable in the presence of oxygen (Eary, 1987; Lowry and 

Lowry, 2002); therefore, something in the sediment, possibly 

manganese oxide minerals, likely catalyzed the oxidation of 

arsenite (Moore and others, 1990). The high concentrations of 

arsenic in the first 5 pore volumes of leachate indicates a lack 

of coprecipitation and sorption, probably because of the high 

pH, low iron, and lack of adsorption sites.

After changing to source-water 2 (table 2), arsenic 

concentrations in leachate decreased rapidly at first and then 

more slowly (fig. 5). Arsenic was reduced to below drinkingwater standards after about 30 pore volumes, when the 

concentration apparently reached equilibrium ranging from 

about 6–8 μg/L for the next 300 pore volumes (fig. 5). Most 

arsenic was in the oxidized form, As(V), indicating no change 

in arsenic speciation during the experiment. The decrease in 

arsenic concentrations likely is a result of coprecipitation with 

the fresh hydrous ferric oxide (HFO) near the influent end 

of the column. After 150 pore volumes, Fe(II) was excluded 

from the source water, arsenic concentrations slowly began 

to increase because of the discontinued coprecipitation with 

ferric oxide and the lack of available sorption sites. At pore 

volume 200, arsenic concentration in the leachate reached 

20 mg/L. At this point Fe(II) was then added back to the source 

water and arsenic concentrations decreased. The laboratory 

column results indicate that arsenic concentrations can be 

decreased to values below the drinking-water standard by 

aeration and the addition of ferrous iron and acid.

TOTAL ARSENIC IN, IN MICROGRAMS PER LITER

PORE VOLUME

Arsenic in leachate

Arsenic in source water

10 micrograms per liter 

 maximum contaminant level 

0

0

10

20

30

40

100 200 300 400

Figure 5. Arsenic concentrations in leachate and source water from column experiments, Douglas 

County, Nevada.
Phase I and II Approach and Experiments 11

The pH of source-water 1 used in the first five pore 

volumes was 8.62, similar to the groundwater at the south 

end of the Minden-Tahoe Airport, Douglas County, Nevada. 

The pH of the source water was then adjusted to about 5.5 

with hydrochloric acid. After adjustment, the pH of the first 

pore volume of leachate was 8.31 then gradually increased 

to values of about 9.5 (fig. 6). The secondary drinking-water 

standard for pH ranges from 6.5 to 8.5 (U.S. Environmental 

Protection Agency, 2008); therefore, pH changes during 

in-situ treatment should be monitored. However, this 

secondary standard was exceeded before the treatment 

process (pre-treatment pH ranging from 8.1 to 8.9). One 

possible explanation for the increase in pH is dissolution of 

carbonate minerals. The narrow inlet and outlet openings of 

the column and the close packing of sediment grains in the 

column prevents rapid degassing of carbon dioxide (CO2) and 

could have resulted in hydrolysis to carbonic acid (H2CO3), 

causing an increase in pH. This set of reactions was simulated 

using the geochemical modeling program PHREEQC 

(Parkurst and Appelo, 1999). Sufficient calcium carbonate 

was allowed to dissolve to obtain an aqueous bicarbonate 

concentration similar to that measured in leachate, about 

23 mg/L as bicarbonate. This reaction resulted in a pH of 9.5, 

and a partial pressure of carbon dioxide (pCO2) of 10-5 atm, 

compared to atmospheric pCO2 of 10-3.5 atm. To increase 

the rate of dissolution of carbonate phases, source-water pH 

was decreased to about 5.2 at pore volume 100. Measured 

pH gradually decreased to 7.8 after about 340 pore volumes, 

likely a result of a decrease in carbonate minerals (fig. 6).

pH, IN STANDARD UNITS

PORE VOLUME

pH of leachate

pH of source water

0

4

5

6

7

8

9

10

100 200 300 400

Figure 6. pH of leachate from column experiments, Douglas County, Nevada.
12 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

PHREEQC simulations indicated that the solubility 

of hydrous ferric oxide at the highest pH measured in 

leachate, 9.5, would be 5 μg/L as iron. Although, ferrous-iron 

concentration in source-water 2 was about 7 mg/L, no iron was 

detected in the leachate for the 340 pore volume laboratory 

experiment. Visual inspection of the column showed that 

a front of reddish brown ferric oxide moved through the 

column. A decrease in flow rate was not evident as a result 

of iron oxide precipitation, which is in agreement with field 

observations during in-situ remediation efforts elsewhere (Rott 

and Friedle, 1999; Mettler and others, 2001). Manganese was 

not added to the source water; however, an initial manganese 

concentration of 134 μg/L occurred in the first pore volume 

(fig. 7). Manganese concentration rapidly decreased to about 

1 μg/L after 18 pore volumes and remained between 1 and 2 

μg/L for the remainder of the experiment. Manganese phases 

were not determined in core material analyzed for mineralogy 

(John McCormick, Nevada Bureau of Mines, University 

of Nevada, Reno, written commun., May 2008) suggesting 

that, if present, manganese oxides may be heterogeneously 

dispersed within the aquifer sediments. These results suggest 

limited dissolution of a manganese solid phase or desorption 

of manganese from aquifer materials under these experimental 

conditions.

Phase II Field Experiment Design

Phase II experiments were designed to refine methods 

of arsenic remediation in the deeper part of the aquifer where 

arsenite is the predominant arsenic species and reducing 

conditions prevail. The EXP-2 wells are sufficiently close 

(1,600 ft) to the SAP#1 well to provide similar hydrologic 

and geochemical conditions, yet sufficiently distant to avoid 

affecting water quality in the production well. 

Figure 7. Manganese concentrations in leachate from column experiments, Douglas County, Nevada.

MANGANESE, IN MICROGRAMS PER LITER

PORE VOLUME

Manganesse in leachate

Manganesse in source water 

0

0

50

100

150

100 200 300 400
Phase I and II Approach and Experiments 13

Work for Phase II began in November 2007 and included 

the drilling of two wells in the deep aquifer near the EXP-2 

wells, laboratory experiments using aquifer material obtained 

during drilling, and field experiments using the EXP-2 deep 

well and the two new wells. These wells were drilled to a 

depth of about 345 ft and screened from 305 to 325 ft, similar 

to the construction of the existing EXP-2 deep well. A 6-in. 

diameter well (As-6) was drilled 15 ft from the EXP-2 deep 

well and a 2-in. diameter well (As-2) was drilled 10 ft from 

the 6-in. well and 15 ft from the EXP-2 deep well (figs. 

1B and 8). Assuming a radius of influence around the As-6 

well of 13 ft, an area of approximately 530 ft2 theoretically 

would be affected during the in-situ experiments. Assuming a 

porosity of 10–30 percent and a 20-ft aquifer thickness (equal 

to the screened interval of the wells), 1 pore volume could 

range from approximately 7,900 to 23,800 gal (more than 

approximately 10,600 ft3 of aquifer volume). During the initial 

Phase II experiment, water was withdrawn, by air-lift, from 

the As-6 well and injected into the As-2 and EXP-2 deep wells 

to create cross-flow among them (fig. 8A). In a second Phase 

II experiment, water was withdrawn from the As-2 and EXP-2 

deep wells and injected into the As-6 well (fig. 8B). Airlifting was used to oxygenate water during withdrawal. Such 

experiments allowed larger volumes of groundwater to be 

treated than the Phase I injection and withdrawal experiments, 

more closely simulating actual conditions at a functioning 

in-situ arsenic remediation site.

During Phase I experiments with the EXP-2 deep well, 

dissolved oxygen was consumed rapidly, resulting in the 

incomplete precipitation of iron oxide. To improve arsenic 

remediation and prevent this rapid consumption of dissolved 

oxygen, the aquifer was first oxidized by introducing dissolved 

oxygen into the system prior to treatment with iron and 

acid. Pre-aeration of the aquifer was achieved by air-lifting 

water from one well and injecting the oxygenated water into 

adjacent wells.

First Phase II Field Experiment and Results

Water was air-lifted from a depth of about 140 ft in the 

As-6 well into a 250-gal settling tank elevated about 5 ft above 

the ground which drained into a 75-gal stock tank. Water 

from the stock tank was pumped through three 2-ft3 activatedcarbon filters prior to injection by gravity feed into the As-2 

and EXP-2 deep wells. A sampling pump was placed in the 

As-6 well about 100 ft below the level of air-lifting (As-6 

Sampling Port, fig. 8A) to allow sampling of water unaffected 

by air-lifting and to detect breakthrough of injected water. 

Beginning June 4, 2008, water was injected without 

treatment (pre-aeration) on four separate days prior to the 

initial Phase II experiment. Initial injection rates were 

12–15 gal/min at the As-2 well and EXP-2 deep well, 

respectively. After about 1 hour of injection, however, water 

began to overflow from the wells, which indicated clogging, 

possibly caused by air entrained in the injected water. Air 

lifting of the injection wells after a day of injection was 

required to regain the initial injection rates of 12–15 gal/min. 

Additionally, the carbon filters became clogged after 2–3 

hours of operation requiring back flushing to maintain a 

total flow rate of about 20 gal/min. Over the 4 days of air-lift 

and injection without treatment, the combined injection rate 

at both wells ranged from 5 to 25 gal/min, and 13,400 gal 

(approximately 0.6–1.7 pore volumes) was injected. 

Oxygenated water was not detected at the As-6 well. 

On June 23, 2008, injection of treated water began. 

Treatment proceeded in a step-wise fashion starting with (1) 

aeration by air lifting, (2) conversion of As(III) to As(V) using 

chlorination, (3) activated -charcoal filtration to remove free 

chlorine, (4) addition of acid to achieve a pH of approximately 

5.5, and, finally, (5) addition of iron to achieve a concentration 

of 7 mg/L, in the injected water (fig. 8A, B). Rates of reagent 

addition were adjusted until the target treatment concentrations 

were achieved (table 3). Combined injection rates ranged from 

5 to 19 gal/min and a total volume of 5,740 gal were injected 

(table 4). 

Prior to injection of treated groundwater, As(V) 

composed 93 percent (35.9 μg/L) of the arsenic in treated 

groundwater (fig. 8A, between steps 3 and 4; table 5). 

These results indicate that the chlorination was effective in 

oxidizing As(III) to As(V). Arsenic concentrations in samples 

collected from the deep sampling port in the As-6 well were 

predominately As(III) indicating that oxidized conditions did 

not reach the well. Dissolved-oxygen concentrations ranged 

from 6.3 to 9.0 mg/L in groundwater airlifted from As-6, 

approximately 65–94 percent of saturation (fig. 8A, at step 1). 

Dissolved oxygen did not break through to the As-6 well over 

the 9.5-hour experiment, and was always less than 1 mg/L.

Although the target treatment pH (5.5) was achieved, 

acidic pH was not observed at the As-6 well (fig. 9A). 

Groundwater pH did not exceed the upper limit of the 

secondary drinking-water criterion of 8.5. Although alkalinity 

did decrease somewhat at the As-6 well, the differences 

among alkalinity measurements were not outside the analytical 

variability of 2.5 percent (fig. 9B).
14 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

tac09-4161_fig08

Sodium Hypochlorite

(Chlorination) 

As-2 Sampling

Port 

15 ft 

10 ft 

 100 ft 

As-6 Well 

EXP-2

Deep Well 

As-2 Well 

EXP-2 Sampling

Port 

Treated Injectate 

1 

1 

3 2 

5 4 

Activated carbon filters

Acid and iron addition

 (full treatment)

Underground flow

Step used for treatment 

Sodium Hypochlorite

(Chlorination) 

As-6 Sampling

Port 

15 ft 

15 ft 

10 ft 

 100 ft 

As-6 Well 

EXP-2

Deep Well 

As-2 Well 

Treated Injectate 

Treated Injectate 

1 

2 3 

4 5 

5 

A. Experiment No. 1, June 23, 2008

B. Experiment No. 2, October 16, 2008

EXPLANATION

15 ft 

Figure 8. Schematic diagrams of 

treatment and sampling ports for Phase 

II in-situ arsenic remediation, Douglas 

County, Nevada. Numbers indicate 

steps in treatment.
Phase I and II Approach and Experiments 15

Table 3. Summary of treatment and concentrations of substances used during first Phase II field experiment No. 1, in-situ remediation 

of arsenic remediation, Douglas County, Nevada, June 23, 2008.

[Treatment: Sodium hypochlorite, used to oxidize arsenite to arsenate; Muriatic acid, HCl acid used to lower pH of groundwater to a target pH of 5.5; Ferrous 

chloride, source of iron to precipitate iron-oxide. Feed solution concentration: Concentrations determined based on information from manufacturer (ferrous 

chloride solution is 11 percent ferrous iron and specific gravity 1.28 gm/mL; sodium hypochlorite solution is 12.5 percent NaOCl, density 1.209 g/mL; muriatic 

acid is 31.4 percent HCl). Density of muriatic acid determined during this study was 1.16 g/mL. Total amount of muriatic acid used during treatment expressed 

as muriatic acid. Total amount of treatment: Total amount of “treatment” used was adjusted for mass losses due to overflow of water from the injection well, 

EXP-2, during the experiment. Abbreviations: mg/mL, milligram per milliliter; mL/min, milliliter per minute; mg/L, milligram per liter; g, gram; g/mL, gram 

per milliliter; gal/min, gallon per minute. Symbols: HCl, hydrochloric acid; NaOCl, sodium hypochlorite, –, not available]

Treatment Feed solution 

concentration (mg/mL) Feed Rate (mL/min) In-stream concentration (mg/L)

Total amount of 

treatment used (g)

Sodium hypochlorite, as chloride 1.71 68 12.19 337

Muriatic acid – – – 1,474

Ferrous chloride, as iron 2.10 – 27.5 57

1

 Based on a median flow rate of 14 gal/min (range of flow rates during treatment 12–15 gal/min).

2

 Two samples collected after addition of ferrous chloride had unfiltered-iron concentrations of 6.8 and 8.2 mg/L, respectively; the average concentration of 

7.5 mg/L is reported in the table.

3

 Free chlorine from the sodium hypochlorite was removed, before the introduction of acid and iron, by passing the treated groundwater through activatedcarbon filters. These activated-carbon filters effectively removed free chlorine. Samples collected after flowing through the activated-carbon filters contained free 

chlorine concentrations less than 0.1 mg/L as determined in the field using a portable DR890 colorimeter (HACH, 2000).

Samples collected during treatment with ferrous iron 

contained an average total-iron concentration of 7.5 mg/L, 

indicating the target concentration for in-stream iron treatment 

was achieved. Full treatment (chlorination, acid, and iron 

addition) was maintained for approximately 2.5 hours, similar 

to Phase I experiments. Upon the termination of the injection 

experiment, and as long as 3 days after, iron concentrations 

measured in the deep EXP-2 injection well, remained below 

detection (less than 50 μg/L). Manganese concentrations 

(0.5–36.6 μg/L) measured in this same well remained 

below the secondary drinking-water criterion of 100 μg/L. 

These results indicate that enough oxygen was available to 

precipitate iron and manganese from solution. Pre-aeration 

of the aquifer prior to treatment was effective in preventing 

concentrations of iron and manganese from exceeding their 

respective secondary drinking-water criteria during this 

experiment.

Table 4. Volumes of groundwater through treatment system 

during first Phase II field experiment No. 1, in-situ arsenic 

remediation, Douglas County, Nevada, June 23, 2008.

[All values are in gallons. Treated groundwater injected: Volume of fully 

treated water (chlorination, acid, and iron) injected into the As-2 injection well 

was 690 gallons and into the EXP-2 well was 1,970 gallons (totaling 2,660 

gallons); 1,100 gallons were treated only by chlorination and acid. Symbol: –, 

not applicable]

Well Groundwater 

pumped

Untreated 

aerated 

groundwater 

injected

Treated 

groundwater 

injected

Total 

groundwater 

injected

As-6 6,200 – – –

As-2 – 380 1,140 1,520

EXP-2 – 1,600 2,620 4,220

Total 6,200 1,980 3,760 5,740
16 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

During pre-aeration and the June 23 Phase II in-situ 

remediation experiment, the injection capacity of the 2-in. 

wells (EXP-2 and As-2 well) decreased over 4–5 hours and the 

wells overflowed. This decrease in injection capacity likely 

was caused by entrained air from treated water cascading into 

the injection wells. The total volume of water pumped from 

the As-6 well during the 9.5-hour experiment was 6,200 gal. 

About 1,980 gal of aerated water was injected into injection 

wells As-2 and EXP-2 deep, followed by about 3,760 gal of 

treated injectate, for a combined total of 5,740 gal (table 4). 

An estimated 460 gal overflowed from the injection wells. 

Given that one pore volume ranges from about 7,900 to 

23,800 gal, the total volume of groundwater pumped from the 

As-6 well during the 9.5-hour experiment ranged from 0.3 

to 0.8 pore volume. The estimated amount of treated water 

injected was equivalent to 0.2 to 0.5 pore volume.

During the initial Phase II, arsenic remediation was 

not apparent in water sampled from the As-6 well (fig. 10). 

Although the injected water had a pH of 5.5, the pH of 

water in the As-6 well did not change over the course of the 

experiment (fig. 9A). Dissolved oxygen did not break through 

to the As-6 well during the 9.5-hour experiment, and was 

always less than 1 mg/L. This lack of change in pH likely 

was a result of the small amount of groundwater treated 

(0.2–0.5 pore volume), the relatively high pH within the 

aquifer, and short treatment time. 

At the conclusion of the first Phase II in-situ arsenic 

remediation experiment, samples were collected from the 

EXP-2 deep injection well. The EXP-2 deep well was selected 

for sampling because it received most of the injected water 

(4,200 gal) during the 9.5-hour experiment. Arsenic speciation 

data indicated that arsenic was primarily As(V) in the injectate 

(table 5). Arsenate was 15 μg/L in the EXP-2 deep injection 

well 1 day after the experiment, about one-half the initial 

concentration (30 μg/L). The iron probably precipitated near 

the site of injection, coprecipitating with arsenic. Arsenic 

concentrations within the EXP-2 deep well increased to about 

27 μg/L within 3 days after treatment (table 5; fig. 10).

The lack of arsenic remediation in the As-6 well 

likely was a result of the small amount of iron used in 

this experiment relative to the amount of water requiring 

treatment. The reduction in arsenic concentration in the sample 

collected from the EXP-2 deep injection well, 1 day after the 

experiment suggests that longer reaction times could result in 

greater arsenic remediation. 

Second Phase II Field Experiment and Results

The problems with clogged injection wells and activatedcarbon filters influenced the design of the second Phase II 

field experiment. In an attempt to alleviate the clogging of 

the well, the experimental plan was altered such that water 

Table 5. Concentration of various arsenic species determined for samples collected during first Phase II field experiment No. 1, 

in-situ arsenic remediation, Douglas County, Nevada, June 23–24, 2008.

[Abbreviations: As, arsenic; As(V), arsenate (the oxidized form of arsenic); As(III), arsenite (the reduced form of arsenic); DMA, dimethylarsenic acid 

(dimethylated organic arsenic species); MMA, monomethylarsonic acid (monomethylated organic arsenic species); USGS, U.S. Geological Survey; 

NWQL, National Water Quality Laboratory; GD, Geologic Discipline Laboratory; μg/L, microgram per liter; nd, not determined; E, estimated]

Sample 

collection 

Site 

identification 

No.

Date Time

USGS NWQL (μg/L) USGS GD

As(V) As(III) DMA MMA Sum of As 

species As (μg/L)

As-6 (well) 385927119445605 06-23-08 0940 4.0 31.2 0.71 E 0.26 36.2 34.9

06-23-08 1400 5.7 30.1 .65 E .23 36.7 35.8

06-23-08 1600 7.1 30.2 .67 E .22 38.2 36.7

Post-chlorination

(treatment stream)

nd 06-23-08 1130 17.5 9.9 E .33 E .12 27.8 27.1

06-23-08 1645 35.9 E .23 E .16 E .02 36.3 38.7

EXP-2 (well) 385927119445601 06-23-08 1755 22.7 6.6 E .27 E .09 29.7 29.5

06-24-08 1000 8.7 6.6 E .05 E .25 15.6 15.0

06-26-08 0945 nd nd nd nd nd 26.9
Phase I and II Approach and Experiments 17

tac09-4161_fig10

TIME, IN HOURS

05 10 15

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

10

15

20

25

30

35

40

30 45 60 75 90

Full Treatment

As-6 Well

EXP-2

DURING EXPERIMENT POST-EXPERIMENT

Figure 10. Remediation of arsenic during and after 

Phase II experiment No. 1, in-situ arsenic remediation, 

Douglas County, Nevada, June 23–26, 2008.

tac09-4161_fig09

EXPERIMENT TIME, IN HOURS

2 3 4 5 6 7 8 9 10

pH, IN STANDARD UNITS

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

Full treatment

As-6 well

A.

PRETREATMENT TREATMENT

POST

TREATMENT

ALKALINITY, IN MILLIGRAMS PER LITER AS CALCIUM CARBONATE

65

66

67

68

69

70

71

72

Chlorination treatment

As-6 well 

B.

Figure 9. Measured (A) pH and (B) alkalinity during 

Phase II experiment No. 1, in-situ arsenic remediation, 

Douglas County, Nevada, June 23, 2008. Full treatment 

is chlorination and addition of ferrous iron and acid.

was air-lifted from a depth of 120 ft in the As-2 and EXP-2 

deep wells and injected into the larger diameter As-6 well 

(fig. 8B). With the exception of the reversal of flow direction, 

experimental set up and treatment were the same as for the 

first Phase II experiment, and sampling pumps were placed 

about 80 ft below the point of air-lift (fig. 8B) in the As-2 

well and EXP-2 deep well to detect breakthrough of injected 

water. Additionally, plans were made to continue air-lifting, 

treating, and injecting continuously for several days to achieve 

the treatment of approximately 30 pore volumes (237,000–

714,000 gal), estimated by the laboratory column experiments 

as necessary to remove arsenic to concentrations less than the 

drinking-water standard.

Preparation for the second experiment began in October 

2008 with air-lift testing of the As-2 well and EXP-2 deep 

well, and injection without treatment into the As-6 well. Initial 

air-lift rates were about 9 and 11 gal/min from the EXP-2 deep 

well and As-2 well, respectively. The maximum pumping rate 

obtained through the activated-carbon filters was limited to 

12–13 gal/min, thereby limiting the injection rate into the As-6 

well, with 7–8 gal/min being lost to waste. 

When determining the rates of sodium hypochlorite, acid, 

and iron deliveries, the rate of flow through the activatedcarbon filters began to decrease to less than 5 gal/min. The 

decrease in flow through the filters likely was caused by 

clogging with entrained air. After backflushing the filters, 

the original injection rate of about 13 gal/min was restored; 
18 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

however, backflushing eventually became necessary every 

35-minutes (fig. 11A). After about 5 hours of air-lift and 

injection, air-lift rates from the EXP-2 deep and As-2 wells 

decreased to 7 and 9 gal/min, respectively (fig. 11B). The 

reason for the decrease in air-lift rates is not certain, but it may 

have been caused by the greater rate of air-lift than injection. 

Additionally, the rate of groundwater flow between the wells 

may have been less than the air-lift and injection rates, causing 

a decline in water level in the pumped wells. Lowering the 

depth of air-lift and sampling pumps in the EXP-2 deep well 

and As-2 well temporarily alleviated the decreasing air-lift 

rates, but available sampling pumps were limited to a depth of 

0 

2 

4 

6 

8 

10 

12 

14 

0 50 100 150 200 250 300 

FLOW THROUGH FILTER, IN GALLONS PER MINUTE 

TIME ELAPSED, IN MINUTES 

Backflush activated-carbon filters 

6 

7 

8 

9 

10 

11 

12 

13 

0 50 100 150 200 250 300 350 

GROUNDWATER AIR-LIFT RATE, IN GALLONS PER MINUTE 

TIME ELAPSED, IN MINUTES 

As 2-inch 

EXP-2 deep 

A.

B.

Figure 11. Groundwater flow through (A) activated carbon filters and (B) air-lift rates during the 

second Phase II field experiment, in-situ arsenic remediation, Douglas County, Nevada, October 

16, 2008.
Summary and Conclusions 19

200 ft. The decreasing air-lift rates, and the constantly varying 

rates of pumping through the activated-carbon filters, did not 

allow maintenance of the target concentrations for the addition 

of chlorine, acid, and iron. Continuous air-lift, treatment, and 

injection for several days would not be possible using this 

small-scale design and so the experiment was stopped. 

Discussion of Phase I and II Experiments 

Problems observed during the Phase II experiments 

generally were due to system engineering and may not 

be insurmountable obstacles to successful in-situ arsenic 

remediation. Results of the Phase I and laboratory column 

experiments suggest that the remediation of arsenic to less 

than the drinking-water standard can be attained. The problem 

of well clogging during injection might be solved by the use of 

larger vented settling tanks that would allow holding water for 

longer periods than is possible with the 250-gal tank used in 

the Phase II experiments. The large, vented tanks would allow 

for the escape of entrained gas and provide injection water 

less likely to clog the well and filter. Installation of multiple 

larger diameter injection wells with longer screened intervals 

would allow greater volumes of water to be pumped, treated, 

and re-injected, and would allow for alternating injection 

among wells, a method already applied for systems using 

in-situ remediation (Mettler and others, 2001). An injection 

method other than gravity feed may improve the ability of the 

aquifer surrounding the injection well to receive treated water. 

The decrease in air-lift rates may be alleviated by the use of 

longer and more easily lowered air-lift lines and sampling 

pumps with longer intakes, or by air-lifting from alternate 

wells. Similarly, clogging of carbon filters could be overcome 

by switching to large-capacity filters or by alternating between 

filters while clogged filters are backflushed, or both. 

Switching injection or air-lifting among wells and filters 

may alleviate many problems; however, the maintenance of 

such a system may be substantial. Problems may occur when 

using a production well and separate injection wells because 

wells may have different producing zones (Rott and Friedle, 

1999). Rott and Meyerhoff (1994) point out the advantage 

of using the same well for pumping, treatment, and injection 

because treated water enters the same zones producing water 

from the well. They note that two working remediation wells 

in an area can be used to provide a constant supply of potable 

drinking water by switching remediation and production 

between the two wells. A similar method is used at the La 

Neuveville Site, a municipal supply well, in Switzerland 

(Mettler and others, 2001). If the problems of clogging can be 

overcome and relatively constant rates of injection obtained, 

treatment chemicals can be added at constant rates to achieve 

target concentrations, reducing the maintenance required for 

an operational remediation system.

Summary and Conclusions

Arsenic is one of the most common contaminants in 

groundwater in the western United States. Because of the 

lowering of the arsenic drinking-water standard from 50 to 

10 μg/L, water purveyors in many areas within the United 

States have been searching for new cost-effective methods 

to remove arsenic in order to comply with the new standard. 

Conventional arsenic remediation strategies primarily involve 

above-ground treatment that includes costs of building large 

treatment plants and costs associated with the disposal of 

sludge material. Two primary advantages of using in-situ 

remediation are the relatively small-scale facilities needed and 

the elimination of sludge-related disposal issues and costs.

Arsenic concentrations in groundwater in Douglas 

County, Nevada, range from 1 to 85 μg/L. A two-phase study 

was implemented in 2005 and 2008 to address the possibility 

of using in-situ arsenic remediation. The first phase (Phase I) 

examined the feasibility of in-situ remediation, using one well 

for treatment and withdrawal at the south end of the MindenTahoe Airport. The second phase (Phase II) incorporated the 

results of Phase I and evaluated the effectiveness of in-situ 

remediation using a three-well system for injection and 

withdrawal to create cross flow between the wells, a method 

more applicable to full-scale implementation.

Phase I results indicate that arsenic concentrations in 

groundwater in Douglas County, Nevada, can be lowered to 

less than 10 μg/L using chlorination, aeration, iron, and pH 

adjustment. During Phase I, treated groundwater from the 

deep aquifer was injected into the more oxidizing shallow 

aquifer and reducing deep aquifer environments. Arsenic 

concentrations decreased to less than 10 μg/L from the shallow 

and deep aquifer when moderate to high iron (3–6 mg/L) 

treatments and pH adjustments to less than 6 were used. The 

primary arsenic species in the shallow aquifer was arsenate, 

As(V), for which the remediation of arsenic was more 

effective. Recovery of remediated groundwater originating 

from the shallow aquifer showed removal efficiencies ranging 

from 0, using low iron treatments, to 1.9, when treating at 6 

mg/L iron at a pH of 5.3. Although arsenic from groundwater 

originating from the deep aquifer was remediated, the 

removal efficiency was only about 0.5. Deep groundwater was 

remediated only when treated at 5 mg/L iron at a pH of 5.3. 

The primary arsenic species within the deep aquifer is arsenite, 

the reduced form of arsenic. Additionally, dissolved oxygen 

introduced into the system during the deep aquifer remediation 

experiments was consumed at a more rapid rate than during 

the shallow aquifer remediation experiments. In shallow 

and deep experiments, iron and manganese concentrations 

exceeded respective secondary drinking-water criteria, 

likely as a result of the incomplete precipitation of iron and 

manganese oxides.
20 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

Phase II experiments involved the drilling of two 

additional wells, laboratory column experiments using aquifer 

material derived from the drilling of these new wells, and 

field tests. The new wells were installed sufficiently close 

(radial distances of 10–15 ft ) to the well used during Phase I 

experiments to be hydraulically connected to each other yet 

far enough away from the production well so that productionwell operations would not be influenced. Laboratory column 

experiments simulated proposed treatment as elucidated 

from Phase I experiments. The proposed remediation 

strategy involved pre-aeration of groundwater, treatment 

with high iron concentration (7 mg/L), and adjustment of 

pH to approximately 5.5. The laboratory results indicated 

that approximately 30 pore volumes of water would have 

to be treated in order to sustain the removal of arsenic to 

concentrations less than 10 μg/L.

Extrapolation of the laboratory results to the field 

indicated that approximately 237,000–714,000 gallons of 

groundwater originating from the deep aquifer would require 

treatment to achieve this remediation goal. The first Phase II 

field experiment indicated that the chemistry-related aspects 

associated with the proposed treatment strategy would work 

provided that an adequate number of pore volumes were 

treated and that sufficient reaction time was allowed. During 

the experiment, 3,760 gallons of groundwater, approximately 

0.2–0.5 of 1 pore-volume (7,900–23,800 gallons), were 

treated. Although remediated groundwater was not observed 

at the well from which water was being withdrawn during the 

9.5-hour experiment, 1 day after the conclusion of this first 

Phase II experiment, arsenic concentration decreased from 30 

to 15 μg/L in water collected from one of the wells used for 

injection. Additionally, the primary arsenic species at the time 

of sampling was arsenate, indicating that during this 24-hour 

post-experiment period somewhat oxidizing conditions 

prevailed near this well. Arsenic concentrations increased to 

27 μg/L, 3 days after the conclusion of the experiment.

The first Phase II remediation experiment was terminated 

at 9.5 hours because of treated-groundwater overflow from 

the injection wells, which began at about 4.5 hours into the 

experiment. To avoid this overflow a second experiment was 

attempted. In this second Phase II field experiment, water was 

air lifted from each of the 2-in. diameter wells (previously 

used for injection) and injected into the 6-in. diameter well 

(previously used for withdrawal). Although the 6-in. diameter 

well did not overflow, over time the air-lift rate from each of 

the 2-in. diameter wells decreased by 2 gallons per minute. 

Additionally, groundwater flow rates through the activatedcarbon filters decreased from 12–13 gallons per minute to less 

than 5 gallons per minute. Backflushing the activated-carbon 

filters restored groundwater flow rates through the filters 

temporarily; however, backflushing was becoming necessary 

more frequently. Because of these hydraulic-related obstacles 

the experiment was stopped.

The difficulties encountered during both Phase II 

experiments may be overcome if engineered properly and 

then in-situ arsenic remediation in reduced aquifers may be 

successful. Successful remediation of arsenic concentrations 

to less than drinking-water standards during Phase I and 

laboratory column experiments support this possibility. 

Solutions to some of the hydraulic complications encountered 

during the Phase II experiments may include (1) installation 

and use of large, vented settling tanks to permit a continuous 

exchange of air between the groundwater and atmosphere 

to allow the release of entrained gas bubbles resulting in 

injection water that is less likely to clog well screens and 

filters (2) installation of multiple large-diameter injection 

wells with screened intervals longer than 20 ft to provide the 

option of alternating injection among wells and increasing 

well capacity, (3) using a method other than gravity-feed 

injection to reintroduce treated water back into the aquifer, and 

(4) alternating the use of large-capacity or multiple activatedcarbon filters, or both. If the issues associated with well screen 

and filter clogging can be overcome, the necessary chemical 

treatment (chlorination, and addition of iron and acid) can be 

automated thereby reducing the maintenance requirements 

necessary for a conventional large-scale remediation system. 

In aquifers where ambient arsenic is As(V), chlorination would 

not be necessary thereby eliminating the need for subsequent 

activated-carbon filtration. In such aquifers, treatment would 

require only the addition of iron, aeration, and lowering of pH, 

further simplifying the in-situ treatment process.

Acknowledgments

The authors gratefully acknowledge the Carson Water 

Subconservancy District for funding this project; the Minden 

Tahoe Airport for allowing the experiments to be conducted 

on airport property; Douglas County for use of the production 

well and assistance; and James Wood, Carl Thodal, Timothy 

Rowe and Keith Halford, U.S. Geological Survey, for their 

participation in the field experiments and guidance.

References Cited

Anderson, M.A., Ferguson, J.F., and Gavis, Jerome, 1976, 

Arsenate adsorption on amorphous aluminum hydroxide: 

Journal of Colloid and Interface Science, v. 54, no. 3, p. 

391-399.

Appelo, C.A.J., and deVet, W.W.J.M., 2003, Modeling in-situ 

iron removal from groundwater with trace elements such 

as As, in Welch, A.H., and Stollenwerk, K.G., eds., Arsenic 

in ground water—Geochemistry and occurrence: Boston, 

Mass., Kluwer Academic Publishers, p. 381-401.
References Cited 21

Appelo, C.A.J., Drijver, B., Hekkenberg, R., and De Jonge, 

M., 1999, Modeling in-situ iron removal from ground 

water: Ground Water, v. 37, no. 6, p. 811-817.

Brown, R.A., Leahy, M.C., and Pyrih, R.Z., 1998, In situ 

remediation of metals comes of age–Remediation/Summer 

1998: John Wiley and Sons, Inc., p. 81-96.

Cadena, F., and Kirk, T.L., 1995, Arsenate precipitation 

using ferric iron in acidic conditions: New Mexico Water 

Resources Research Institute Technical Completion Report, 

17 p.

Chiu, V.Q., and Hering, J.G., 2000, Arsenic adsorption 

and oxidation at manganite surfaces—1. Method for 

simultaneous determination of adsorbed and dissolved 

arsenic species: Environmental Science and Technology, 

v. 34, no. 10, p. 2029-2034.

De Vitre, Richard, Belzile, Nelson, and Tessier, Andre, 1991, 

Speciation and adsorption of arsenic on diagenetic iron 

oxyhydroxides: Limnology and Oceanography, v. 36, no. 7, 

p. 1480-1485.

Drever, J.I., 1988, The geochemistry of natural waters: 

Englewood Cliffs, N.J.: Prentice Hall, 437 p.

Druhan, J.L., Conrad, M.E., Williams, K.H., N’Guessan, 

L., Long, P.E., and Hubbard, S.S., 2008, Sulfur isotopes 

as indicators of amended bacterial sulfate reduction 

processes influencing field scale uranium bioremediation: 

Environmental Science and Technology, v. 42, p. 7842-

7849.

Eary, L.E., 1987, Rates of As(III) oxidation by dissolved 

oxygen at 25 and 90°C: Geological Society of America, 

Abstracts with Programs, v. 19, no. 7, p. 650.

ECO:LOGIC Engineering, 2003, South Airport exploratory 

well no. 2, construction and testing: Reno, Nev., Consulting 

Engineers memorandum report to Douglas County 

Community Development, Douglas County, Nevada, 

March 28, 2003, ECO:LOGIC Engineering, LLC, 18 p.

Edwards, Mark, 1994, Chemistry of arsenic removal during 

coagulation and Fe-Mn oxidation: Journal American Water 

Works Association, v. 9, p. 64-78.

Ferguson, J.F., and Gavis, Jerome, 1972, A review of the 

arsenic cycle in natural waters: Water Research, v. 6, 

p. 1259-1274.

Ford, R., 2002, Rates of hydrous ferric oxide crystallization 

and the influence on coprecipitated arsenate: Environmental 

Science and Technology, v. 36, p. 2459-2463.

Frank, P.L., and Clifford, D.A., 1986, Arsenic(III) oxidation 

and removal from drinking water: U.S. Environmental 

Protection Agency Summary Report, EPA/600/S2-86/021, 

11 p.

Fuller, C.C., Davis, J.A., and Waychunas, G.A., 1993, 

Surface chemistry of ferrihydrite–Part 2. Kinetics of 

arsenate adsorption and coprecipitation: Geochimica et 

Cosmochimica Acta, v. 57, p. 2271-2282.

Grosse, Douglas, Richards, Marta, Puls, Robert, Howard, 

Ralph, Davis, Andy, Murarka, Ishwar, and Rouse, 

Jim, 2000, In situ treatment of soil and groundwater 

contaminated with chromium, technical resource guide: 

U.S. Environmental Protection Agency, Office of Research 

and Development, EPA/625/R-00/005, 84 p.

HACH, 2000, Method 10069, Free chlorine (high range 0 

to 5 mg/L)—Procedures Manual, DR890 Colorimeter: 

Loveland, Colo., Hach Company, p. 113-117. 

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 p.

Jones, C.A., Langner, H.W., Anderson, K., McDermott, T.R., 

and Inskeep, W.P., 2000, Rates of microbially mediated 

arsenate reduction and solubilization: Soil Science Society 

of America Journal, v. 64, p. 600-608.

Kartinen, E.O., and Martin, C.J., 1995, An overview of arsenic 

removal processes: Desalination, v. 103, p. 79-88.

Langmuir, Donald., 1997, Aqueous environmental 

geochemistry: Upper Saddle River, N.J., Prentice Hall, 

600 p.

Lowry, J.D., and Lowry, S.B., 2002, Oxidation of arsenic(III) 

by aeration and storage: U.S. Environmental Protection 

Agency, EPA/600/R-01/102, 44 p.

Maurer, D.K., and Halford, K.J., 2004, Updated estimates of 

the distribution of average annual precipitation in Carson 

Valley, 1971–2000, Douglas County, Nevada, and Alpine 

County, California: Journal of Nevada Water Resources 

Association, v. 1, p. 20-39.

Maurer, D.K., Paul, A.P., Berger, D.L., and Mayers, C.J., 2008, 

Analysis of streamflow trends, ground-water and surfacewater interactions, and water quality in the Upper Carson 

River Basin, Nevada and California: U.S. Geological 

Survey Scientific Investigations Report 2008-5238, 192 p.

Masscheleyn, P.H., Delaune, R.D., and Patrick, W.H., Jr., 

1991, Arsenic and selenium chemistry as affected by 

sediment redox potential and pH: Journal of Environmental 

Quality, v. 20, p. 522-527.
22 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

Mettler, S., 2002, In-situ iron removal from ground water—

Fe(II) oxygenation, and precipitation products in a 

calcareous aquifer: Zurich, Swiss Federal Institute of 

Technology, Ph.D. dissertation, 146 p.

Mettler, S., Abdelmoula, M., Hoehn, E., Schönenberger, R., 

Weidler, P., and Von Gunten, U., 2001, Characterization 

of iron and manganese precipitates from an in-situ 

ground water treatment plant: Ground Water, v. 39, no. 6, 

p. 921‐930.

Moore, J.N., Walker, J.R., and Hayes, T.H., 1990, Reaction 

scheme for the oxidation of As(III) to As(V) by birnessite: 

Clays and Clay Minerals, v. 38, no. 5, p. 549-555.

Nordstrom, D.K., and Archer, D.G., 2003, Arsenic 

thermodynamic data and environmental geochemistry, in

Welch, A.H., and Stollenwerk, K.G., eds., Arsenic in ground 

water—Geochemistry and occurrence: Boston, Mass., 

Kluwer Academic Publishers, p. 1-25.

Oscarson, D.W., Huang, P.M., and Liaw, W.K., 1981, Role 

of manganese in the oxidation of arsenite by freshwater 

lake sediments: Clays and Clay Minerals, v. 29, no. 3, 

p. 219‐225. 

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide 

to PHREEQC (Version 2)—A computer program for 

speciation, batch reaction, one-dimensional transport, and 

inverse geochemical modeling: U.S. Geological Survey 

Water-Resources Investigations Report 99-4259, 312 p.

Raven, K.P., Jain, A., and Hoeppert, R.H., 1998, Arsenite and 

arsenate adsorption on ferryhydrite–Kinetics, equilibrium 

and adsorption envelopes: Environmental Science and 

Technology, v. 32, p. 344-349.

Rott, U., 1990, Protection of groundwater quality by 

biochemical treatment in the aquifer, in Symposium on 

Groundwater Monitoring and Management, Dresden, 

Germany, March 1987, Proceedings: Oxfordshire, UK, 

International Association of Hydrological Sciences 

Publication no. 173, p. 383-389.

Rott, U., and Friedle, M., 1999, Subterranean removal of 

arsenic from groundwater, in Chappell, W.R., Abernathy, 

C.O., and Calderon, R.L., eds., Arsenic exposure and health 

effects: Oxford, UK, Elsevier Science Ltd., p. 389-396.

Rott, U., and Meyerhoff, R., 1994, Physical, chemical, 

and biological processes in consequence of in-situ 

treatment of groundwater, in Conference on Groundwater 

Quality Management, Tallinn, Estonia, September 1993, 

Proceedings: Oxfordshire, UK, International Association of 

Hydrological Sciences Publication no. 220, p. 439-447.

Rouse, J., Benker, E., Daud, M., Lam, D., 1999, In-situ 

remediation of chromium contaminated soil and ground 

water: Proceedings, Challenges Posed by Urban and 

Industrial Contaminants, Conference in Freemantle, 

Western Australia, sponsored by Centre for Groundwater 

Studies, CSIRO Land and Water, March 21-25, 1999, 

p. 623-631.

Shacklette, H.T., and Boerngen, J.C., 1984, Element 

concentrations in soils and other surficial materials of 

the conterminous United States: U.S. Geological Survey 

Professional Paper 1270, p. 105.

Stollenwerk, K.G., 2003, Geochemical processes controlling 

transport of arsenic in groundwater—A review of 

adsorption, in Welch, A.H., and Stollenwerk, K.G., eds., 

Arsenic in ground water—Geochemistry and occurrence: 

Boston, Mass., Kluwer Academic Publishers, p. 67-100.

U.S. Environmental Protection Agency, 2001a, Technical 

fact sheet—Final rule for arsenic in drinking water: U.S. 

Environmental Protection Agency, EPA 815-F-00-016, 

January 2001, p. 1-5.

U.S. Environmental Protection Agency, 2001b, National 

primary drinking water regulations; arsenic and 

clarifications to compliance and new source contaminants 

monitoring—Final rule: Federal Register, January 22, 2001, 

v. 66, no. 14, p. 6975-7066

U.S. Environmental Protection Agency, 2008, 2006 drinking 

water standards and health advisories: U.S. Environmental 

Protection Agency, accessed September 8, 2008, at http://

www.epa.gov/waterscience/criteria/drinking.

Violante, Antonio, Del Gaudio, Stefania, Pigna, Massimo, 

Ricciardella, Mariarosaria, and Banerjee, Dipanjan, 

2007, Coprecipitation of arsenate with metal oxides–2. 

Nature, mineralogy, and reactivity of iron(III) precipitates: 

Environmental Science and Technology, v. 41, 

p. 8275‐8280. 

Walker, Mark, Seiler, R.L., and Meinert, Michael, 2008, 

Effectiveness of household reverse-osmosis systems in a 

Western U.S. region with high arsenic groundwater: Science 

of the Total Environment, v. 389, p. 245-252.

Water Research Commission South Africa, 2004, The 

feasibility of in-situ groundwater remediation as a robust 

low-cost water treatment option: Buckinghamshire, 

UK, Foundation for Water Research Report No. 

1355/1/04, accessed July 2, 2009, at http://www.fwr.org/

wrcsa/1325104.htm.
References Cited 23

Welch, A.H., 1994, Ground-water quality and geochemistry 

in Carson and Eagle Valleys, Western Nevada and Eastern 

California: U.S. Geological Survey Open-File Report 93-33, 

99 p.

Welch, A.H., Lico, M.S., and Hughes, J.L., 1988, Arsenic in 

ground water of the western United States: Ground Water, 

v. 26, no. 3, p. 333-347.

Welch, A.H., Stollenwerk, K.G., Maurer, D.K., and Feinson, 

L.S., 2003, In-situ arsenic remediation in a fractured, 

alkaline aquifer, in Welch, A.H., and Stollenwerk, K.G., 

eds., Arsenic in ground water—Geochemistry and 

occurrence: Boston, Mass., Kluwer Academic Publishers, 

p. 403-419.

Welch, A.H., Stollenwerk, K.G., Paul, A.P., Maurer, D.K., and 

Halford, K.J., 2008, In situ arsenic removal in an alkaline 

clastic aquifer: Applied Geochemistry, v. 23, p. 2477-2495.

White, E.L., 2009, Fallon, NV—Pooling resources to construct 

arsenic treatment facility: U.S. Environmental Protection 

Agency, accessed July 13, 2009, at http://www.epa.gov/

ogwdw000/arsenic/casestudies/pdfs/casestudy_fallon.pdf.

Zeng, H., Fisher, B., and Giammar, D.E., 2008, Individual 

and competitive adsorption of arsenate and phosphate to a 

high-surface-area iron oxide-based sorbent: Environmental 

Science and Technology, v. 42, p. 147-152.

Zobrist, J., Dowdle, P.P., Davis, J.A., and Oremland, R.S., 

2000, Mobilization of arsenite by dissimilatory reduction of 

adsorbed arsenate: Environmental Science and Technology, 

v. 34, p. 4747-4753. 
24 In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada

This page intentionally left blank.
Publishing support provided by the U.S. Geological Survey

Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Nevada Water Science Center

U.S. Geological Survey

2730 N. Deer Run Road

Carson City, Nevada 89701

http://nevada.usgs.gov/
2 Report Title

Paul and others— In-Situ Arsenic Remediation in Carson Valley, Douglas County, West-Central Nevada— Scientific Investigations Report 2010-5161