Assessments of Aquifer Sensitivity on Navajo

Nation and Adjacent Lands and Ground-Water

Vulnerability to Pesticide Contamination on the

Navajo Indian Irrigation Project, Arizona,

New Mexico, and Utah

Prepared in cooperation with the

NAVAJO NATION ENVIRONMENTAL PROTECTION

AGENCY PESTICIDES PROGRAM

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4051
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4051

Assessments of Aquifer Sensitivity on Navajo

Nation and Adjacent Lands and Ground-Water

Vulnerability to Pesticide Contamination on the

Navajo Indian Irrigation Project, Arizona,

New Mexico, and Utah

Prepared in cooperation with the

NAVAJO NATION ENVIRONMENTAL PROTECTION

AGENCY PESTICIDES PROGRAM

By Paul J. Blanchard

Albuquerque, New Mexico

2002
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225-0286

District Chief

U.S. Geological Survey

Water Resources Division

5338 Montgomery NE, Suite 400

Albuquerque, NM 87109-1311

from:

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on the Internet via the World Wide Web. You may connect to the Home Page for the New Mexico District

Office using the URL http://nm.water.usgs.gov.
iii

CONTENTS

Page

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

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

Purpose and scope ....................................................................................................................................................... 2

Geohydrologic setting.................................................................................................................................................. 2

Review of selected, previously used assessment methods........................................................................................... 4

Aquifer sensitivity on Navajo Nation and adjacent lands...................................................................................................... 6

Method developed for assessment of aquifer sensitivity ............................................................................................. 7

Geology ............................................................................................................................................................. 7

Precipitation....................................................................................................................................................... 8

Soil properties.................................................................................................................................................... 8

Slope of the land surface ................................................................................................................................... 10

Stream courses................................................................................................................................................... 10

Conducting the assessment: combining characteristics..................................................................................... 10

Results of assessment of aquifer sensitivity ................................................................................................................ 13

Ground-water vulnerability to pesticide contamination on the Navajo Indian Irrigation Project ......................................... 13

Method developed for assessment of ground-water vulnerability............................................................................... 13

Results of assessment of ground-water vulnerability .................................................................................................. 16

Summary and conclusions ..................................................................................................................................................... 22

References cited..................................................................................................................................................................... 23

Appendix: Selected characteristics of pesticides used on block I of the Navajo Indian Irrigation Project, 1997-99............ 25

FIGURES

1-9. Maps showing:

1. Navajo Nation study area.................................................................................................................................. 3

2. Categories of geology in the Navajo Nation study area.................................................................................... 5

3. Categories of annual precipitation in the Navajo Nation study area................................................................. 9

4. Categories of soil hydrologic group in the Navajo Nation study area .............................................................. 11

5. Categories of land-surface slope in the Navajo Nation study area ................................................................... 12

6. Results of assessment of aquifer sensitivity in the Navajo Nation study area.................................................. 14

7. Results of assessment of aquifer sensitivity on the Navajo Indian Irrigation Project (NIIP) ........................... 15

8. Navajo Indian Irrigation Project fields for which pesticide application data are available, 1997-99 ............... 17

9. Results of assessment of ground-water vulnerability to pesticide contamination on the Navajo Indian

Irrigation Project, 2000................................................................................................................................ 18

TABLES

1. Ratings and weights of selected factors that contribute to potential for contamination .......................................... 6

2. Relative pesticide leaching potential of pesticides and number of fields in block I of the Navajo Indian

Irrigation Project on which each pesticide was applied, 1997-99...................................................................... 19

3. Range of percentage of fields on which “typically applied” and “not typically applied” pesticides were

used for each crop grown in block I of the Navajo Indian Irrigation Project, 1997-99...................................... 21

4. Ground-water vulnerability and crop ranking based on number of typically applied pesticides having

very high, high, and moderate relative pesticide leaching potential .................................................................. 21
iv

CONVERSION FACTORS AND VERTICAL DATUM

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929—a

geodetic datum derived from a general adjustment of the first-order level nets of the United States

and Canada, formerly called Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above sea level.

Multiply By To obtain

Length

inch 2.540 centimeter

inch 25.4 millimeter

foot 0.3048 meter

mile 1.609 kilometer

Area

acre 4,047 square meter

square mile 259.0 hectare

square mile 2.590 square kilometer
1

ABSTRACT

The U.S. Environmental Protection Agency

requested that the Navajo Nation conduct an

assessment of aquifer sensitivity on Navajo Nation

lands and an assessment of ground-water

vulnerability to pesticide contamination on the

Navajo Indian Irrigation Project. Navajo Nation

lands include about 17,000 square miles in

northeastern Arizona, northwestern New Mexico,

and southeastern Utah. The Navajo Indian

Irrigation Project in northwestern New Mexico is

the largest area of agriculture on the Navajo

Nation. The Navajo Indian Irrigation Project

began operation in 1976; presently (2001) about

62,000 acres are available for irrigated agriculture.

Numerous pesticides have been used on the

Navajo Indian Irrigation Project during its

operation.

Aquifer sensitivity is defined by the U.S.

Environmental Protection Agency as "The relative

ease with which a contaminant [pesticide] applied

on or near a land surface can migrate to the aquifer

of interest. Aquifer sensitivity is a function of the

intrinsic characteristics of the geologic material in

question, any underlying saturated materials, and

the overlying unsaturated zone. Sensitivity is not

dependent on agronomic practices or pesticide

characteristics." Ground-water vulnerability is

defined by the U.S. Environmental Protection

Agency as "The relative ease with which a

contaminant [pesticide] applied on or near a land

surface can migrate to the aquifer of interest under

a given set of agronomic management practices,

pesticide characteristics, and aquifer sensitivity

conditions.”

The results of the aquifer sensitivity

assessment on Navajo Nation and adjacent lands

indicated relative sensitivity within the boundaries

of the study area. About 22 percent of the study

area was not an area of recharge to bedrock

aquifers or an area of unconsolidated deposits and

was thus assessed to have an insignificant potential

for contamination. About 72 percent of the Navajo

Nation study area was assessed to be in the

categories of most potential or intermediate

potential for contamination. About 6 percent of the

study area was assessed to have the least potential

for contamination, mostly in areas where the slope

of the land surface is more than 12 percent. Nearly

all fields on the Navajo Indian Irrigation Project

were assessed to have the most potential for

contamination.

The assessment of ground-water

vulnerability to pesticide contamination on the

Navajo Indian Irrigation Project was based on

pesticide application to various crops on part of the

Navajo Indian Irrigation Project during 1997-99.

The assessment indicated that ground water

underlying fields of beans, wheat, barley, and

alfalfa was most vulnerable to pesticide

contamination; ground water underlying fields of

corn and potatoes was intermediately vulnerable to

pesticide contamination; and ground water

underlying fields of hay was least vulnerable to

pesticide contamination.

INTRODUCTION

The U.S. Environmental Protection Agency

(USEPA) requested that the Navajo Nation conduct

assessments of aquifer sensitivity on Navajo Nation

lands and ground-water vulnerability to pesticide

contamination on the Navajo Indian Irrigation Project

(NIIP). The results of these assessments are necessary

in order for the Navajo Nation to develop a Pesticide

Management Plan as required by the USEPA. Aquifer

sensitivity is defined by the USEPA as "The relative

ease with which a contaminant [pesticide] applied on

or near a land surface can migrate to the aquifer of

interest. Aquifer sensitivity is a function of the intrinsic

characteristics of the geologic material in question, any

ASSESSMENTS OF AQUIFER SENSITIVITY ON NAVAJO NATION AND

ADJACENT LANDS AND GROUND-WATER VULNERABILITY TO

PESTICIDE CONTAMINATION ON THE NAVAJO INDIAN IRRIGATION

PROJECT, ARIZONA, NEW MEXICO, AND UTAH

By Paul J. Blanchard
2

underlying saturated materials, and the overlying

unsaturated zone. Sensitivity is not dependent on

agronomic practices or pesticide characteristics" (U.S.

Environmental Protection Agency, 1993, p. 9).

Ground-water vulnerability is defined by the USEPA as

"The relative ease with which a contaminant [pesticide]

applied on or near a land surface can migrate to the

aquifer of interest under a given set of agronomic

management practices, pesticide characteristics, and

aquifer sensitivity conditions" (U.S. Environmental

Protection Agency, 1993, p. 9).

Navajo Nation lands include about 17,000

square miles in northeastern Arizona, northwestern

New Mexico, and southeastern Utah (fig. 1). These

lands consist of the contiguous Navajo Reservation,

three non-contiguous reservations (the Alamo Navajo

Reservation, the Cañoncito Navajo Reservation, and

the Ramah Navajo Reservation), and, in northwestern

New Mexico, land purchased by the Navajo Nation and

land allotments to individual Navajo Tribal members.

Land ownership in northwestern New Mexico is

mixed: in addition to Navajo Tribal members, other

land owners include the U.S. Government, the State of

New Mexico, and non-Navajo people.

The largest area of agriculture on Navajo Nation

lands is the NIIP in northwestern New Mexico (fig. 1),

which is operated by the Navajo Agricultural Products

Industries (NAPI). All cropland on this project is

irrigated. The NIIP began operation in 1976; presently

(2001) about 62,000 acres are available for irrigated

agriculture. Numerous pesticides have been used on the

NIIP during its operation.

In addition to the NIIP, at least 11 community

irrigation systems have been operated or are under

consideration on the Navajo Nation. There is no known

documentation of pesticide sales or use regarding these

community irrigation systems, but pesticide use on

these systems is reported to be minimal. Each of these

systems typically includes less than 5,000 acres.

Purpose and Scope

This report has three purposes. The first is to

describe the methods developed and used to assess

aquifer sensitivity on Navajo Nation and adjacent lands

and ground-water vulnerability to pesticide

contamination on the NIIP. The second purpose is to

present the results of these assessments. The third

purpose is to provide assessment tools that the Navajo

Nation can use with minor modification to assess

aquifer sensitivity at specific locations and groundwater vulnerability under differing agricultural

practices.

Methods development included locating sources

of and obtaining, reviewing, and assessing available

data and electronic databases on geology, precipitation,

soils, topography, hydrologic characteristics in the

study area, and pesticide characteristics and use of

pesticides on the NIIP. Subsequently, each database

that was evaluated to be useful for conducting the

assessments was modified as necessary and converted

into a geographic information system (GIS) coverage.

These coverages included geology (areas of

unconsolidated deposits and areas where bedrock

aquifers are recharged), precipitation, soil properties,

slope of the land surface, stream courses, and pesticide

use on part of the NIIP. The assessments consisted of

assigning ratings or scores to values of the

characteristics represented on the GIS coverages,

overlaying the GIS overages, and combining the scores

from each contributing coverage.

Lands assessed included the contiguous Navajo

Indian Reservation; the non-contiguous Alamo,

Cañoncito, and Ramah Navajo Reservations; nonNavajo lands between and (or) adjacent to the

reservations; and lands of mixed ownership in

northwestern New Mexico. These lands are referred to

in this report as the “Navajo Nation study area.” All

lands in northwestern New Mexico were assessed,

regardless of ownership. The entire area under

consideration in this study included about 21,500

square miles.

Geohydrologic Setting

The Navajo Nation study area is part of the

Colorado Plateaus Physiographic Province (Fenneman,

1931), which is characterized by thick sequences of

sedimentary rocks with deep canyons downcut into the

sedimentary strata. The strata typically are flat lying or

dipping at a low angle. These flat-lying or nearly flatlying strata are interrupted by broad anticlinal,

synclinal, and monoclinal folds and, locally, by

intrusive and extrusive igneous rocks.

The United States has been divided into 15

ground-water regions (Heath, 1984), and the entire

study area is within the Colorado Plateau and Wyoming

Basin Region. The Colorado Plateau and Wyoming

Basin Region is described as being typically composed

of thin, patchy, and rocky soils overlying sedimentary

strata and having sparse vegetation.
40

Figure 1. Navajo Nation study area.

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CHUSKA MOUNTAINS

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160

Navajo Indian

Irrigation Project

CONTIGUOUS NAVAJO RESERVATION 

NAVAJO INDIAN IRRIGATION 

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ALAMO NAVAJO, CAÑONCITO NAVAJO, 

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3
4

Ground water is in shallow, unconfined,

unconsolidated aquifers and in underlying bedrock

aquifers, which may be either confined or unconfined.

Aquifers in unconsolidated deposits are in alluvium

and terrace deposits along drainages, in eolian deposits

in upland areas, and in landslide deposits along the

margins of upland areas. Areas within the approximate

boundaries of the contiguous Navajo Reservation that

are unconsolidated deposits and (or) are recharge areas

to bedrock aquifers are identified in figure 2 as areas

having significant potential to facilitate contamination

of ground water.

In the areas identified as having significant

potential to facilitate contamination of ground water

(fig. 2), the land surface and underlying aquifers are

hydrologically connected; therefore, in these areas the

quality of ground water can be affected by human

activities at land surface, including agriculture.

Agricultural chemicals applied at land surface can be

transported downward to the ground-water system by

infiltration and percolation of precipitation or irrigation

water.

Review of Selected, Previously Used

Assessment Methods

The USEPA has compiled information on 11

aquifer sensitivity assessment methods and 22 groundwater-vulnerability assessment methods (U.S.

Environmental Protection Agency, 1993). Aquifer

sensitivity methods are described by the USEPA as the

most appropriate screening tool of large areas, and

ground-water-vulnerability methods are described by

the USEPA as the most appropriate for use at the field

or plot level.

The most well-known and commonly used

method to assess aquifer sensitivity is a system that

assigns ratings and weights to hydrogeologic factors

titled “DRASTIC: a standardized system for evaluating

ground-water pollution potential using hydrogeologic

settings” (Aller and others, 1985). DRASTIC is an

acronym for factors that are considered by the method:

depth to ground water, recharge, aquifer media, soil

media, topography, impact of the vadose zone, and

hydraulic conductivity of the aquifer. Aller and others

(1985) also developed agricultural DRASTIC to be

used when considering the potential effects of

agriculture on underlying ground water. Slightly

different weights have been assigned to soil media,

topography, impact of the vadose zone, and hydraulic

conductivity of the aquifer in agricultural DRASTIC.

DRASTIC and agricultural DRASTIC specify

ratings weights for each of the seven factors. Each

factor is divided into ranges of values, and ranges are

assigned a rating from 1 to 10. For example, recharge,

in inches per year, is divided into five ranges: 0 to 2, 2

to 4, 4 to 7, 7 to 10, and more than 10, and these ranges

are assigned ratings of 1, 3, 6, 8, and 9, respectively.

The weight of each factor depends on its relative

significance. In agricultural DRASTIC, depth to water

and soil media were assigned a weight of 5; recharge

and impact of the vadose zone were assigned a weight

of 4, aquifer media and topography were assigned a

weight of 3, and conductivity was assigned a weight

of 2.

Several States and at least one private entity have

conducted aquifer sensitivity assessments using the

DRASTIC method (Soller, 1992). Soller compared the

results of DRASTIC and several other assessment

methods at the county level in the Midwest. Soller

commented that data required by the DRASTIC

assessment method typically are not available or are

inadequate, especially for depth to ground water,

recharge, and hydraulic conductivity. Soller also

reported that in areas where data were inadequate and

standard values were used instead of real data, the

assignment of standard values varied considerably with

different investigators. This variability adversely

affected comparability between areas investigated by

different individuals. Soller recommended that the

DRASTIC method be modified to include only

characteristics for which adequate data are available.

An example of a modified DRASTIC method is

that used in the Denver Basin by Hearne and others

(1995) to determine sensitivity of the uppermost

ground water in the greater Denver, Colorado, area.

This study covered about 2,500 square miles.

Characteristics assessed included depth to ground

water, soil media, topography of the land surface, and

geohydrologic unit (impact of the vadose zone) but did

not include recharge, aquifer media, or hydraulic

conductivity. Aquifer media and hydraulic

conductivity of the aquifer, however, address the fate of

water and its solutes within the saturated zone rather

than in the overlying unsaturated zone. Recharge was

generally considered to be in the lowest category of

recharge defined by Aller and others (1985); variation

of recharge in the study area was not quantified.

Ratings for selected characteristics developed by Aller

and others (1985) and Hearne and others (1995) are

listed in table 1.

6

The resulting map contained 158 “vulnerability

response units,” with each response unit appearing in 1

to 92 areas on the map. Hearne and others (1995)

labeled these areas as “vulnerability response units”

even though no agronomic management practices or

pesticide characteristics were included in the

assessment. According to USEPA definitions (U.S.

Environmental Protection Agency, 1993, p. 9), these

units would be called “sensitivity response units.”

Ground-water-vulnerability assessment methods

were subdivided by the USEPA (1993) into computer

simulation methods (15 studies), pesticide leaching

methods (5 studies), and pesticide loading methods (2

studies). Computer simulation methods typically

require programming expertise and computers

containing large amounts of memory, each of which

may limit their use by the general population.

Therefore, these methods were not considered for this

study. Pesticide leaching methods typically evaluate

pesticide properties. Three of the methods compiled by

the USEPA (1993) also incorporate soil,

hydrogeologic, and (or) climate characteristics. None

consider pesticide application amounts.

One of the two pesticide loading methods simply

adds a pesticide application amount to the aquifer

sensitivity assessment. The other pesticide loading

method, relative aquifer vulnerability evaluation

(RAVE) (DeLuca and Johnson, 1990), includes

irrigation practices, frequency of pesticide application,

and a relative pesticide leaching potential based on the

sorption coefficient and half-life of the applied

pesticide. RAVE was developed and is used by the

Montana Department of Agriculture and is intended for

onsite determinations.

AQUIFER SENSITIVITY ON NAVAJO

NATION AND ADJACENT LANDS

This section describes development of the

assessment method and the results of the assessment of

aquifer sensitivity on Navajo Nation and adjacent

lands. Development included selecting characteristics

to include in the assessment method and sources of data

used to describe these characteristics, developing a

method to combine data describing each characteristic,

and interpreting the results of this combination.

Table 1. Ratings and weights of selected factors that contribute to potential for contamination

[Rating, potential for contamination, ranging from 1 to 10; weight, comparative potential

for contamination among factors; >, greater than; <, less than]

Depth to

water (feet) Rating

Annual

recharge

(inches) Rating

Slope of

land surface

(percent) Rating

Aller and others, 1985

0 to 5 10 >10 9 0 to 2 10

5 to 10 9 7 to 10 8 2 to 6 9

15 to 30 7 4 to 7 6 6 to 12 5

30 to 50 5 2 to 4 3 12 to 18 3

50 to 75 3 0 to 2 1 >18 1

75 to 100 2

>100 1

Weight = 5 Weight = 4 Weight = 3

Hearne and others, 1995

<5 10 <6 10

5 to 20 9 6 to 12 5

>20 7 >12 3
7

Method Developed for Assessment of

Aquifer Sensitivity

The Navajo Nation study area, because of its size

and relative isolation, typically is lacking in detailed

information regarding geology, soils, and hydrology

that is required to use a rigorous scoring method such

as DRASTIC. For example, soil surveys have been

conducted on only small parts of the Navajo Nation.

Although 965 data points are available regarding depth

to ground water, the data were still too sparse (on

average, one data point for about every 20 square

miles) to determine areas of relative potential to

facilitate contamination. The method developed for

this study, therefore, required optimal use of limited

data. Characteristics for which data were considered

adequate included geology within the approximate

boundaries of the contiguous Navajo Reservation only

(impact of the vadose zone), precipitation (a surrogate

for recharge), soil properties, slope of the land surface

(topography), and location of stream courses.

Aquifer media and hydraulic conductivity of the

aquifer (Aller and others, 1985) address the fate of

water and its solutes within the saturated zone rather

than in the overlying unsaturated zone. Although the

definition of aquifer sensitivity includes characteristics

of saturated materials (U.S. Environmental Protection

Agency, 1993), these factors, along with hydraulic

gradient, describe the direction and rate of the

movement of water and its solutes once they have

entered the aquifer. Therefore, they are important for

assessing where the solutes can be expected to travel.

The emphasis of this assessment of aquifer sensitivity,

however, is to assess the relative protection the

overlying material affords the aquifer. Aquifer media

and hydraulic conductivity of the aquifer, therefore,

were not considered in this assessment.

The largest limitation of the method developed

for assessment of aquifer sensitivity on Navajo Nation

and adjacent lands is the lack of adequate information

to describe depth to the uppermost ground water

throughout the study area. The rating for depth to water

is included in table 1 to demonstrate its importance

regarding potential for contamination. Aller and others

(1985) and Hearne and others (1995) indicated a rating

of 10 where ground water is within 5 feet of land

surface; Aller and others (1985) indicated a rating of 1

where ground water is more than 100 feet below land

surface. Depth to the uppermost ground water in the

study area ranges from less than 10 feet along some

stream courses to more than 1,300 feet in the

northwestern part of the study area.

In addition to the lack of adequate information

needed to describe depths to ground water throughout

the study area geographically, the available depth-towater data do not reliably indicate depth to the

uppermost ground water. An example is on the north

part of Black Mesa (fig. 1) where wells that supply a

coal slurry operation penetrate several aquifers prior to

accessing a deep, confined aquifer because the deep,

confined aquifer provides the large quantity of water

required by the slurry. The overlying aquifers are

capable of supplying only much smaller quantities of

water. Depth to water in the deep, confined aquifer is

more than 700 feet. The uppermost ground water,

however, is as shallow as 10 feet.

Geology

Cooley and others (1969) identified areas within

the approximate boundaries of the contiguous Navajo

Reservation where consolidated rocks are recharged

and where unconsolidated deposits are at the surface.

Unconsolidated deposits included alluvium, eolian

deposits, terrace deposits, and landslide deposits.

In this assessment, geology served as a surrogate

for impact of the vadose zone (Aller and others, 1985)

and was described in terms of potential to facilitate

contamination of ground water. Two categories were

used, significant and insignificant, and were assigned

numerical values of 1 and 0, respectively. Within the

approximate boundaries of the contiguous Navajo

Reservation, areas identified by Cooley and others

(1969) where consolidated rocks are recharged and

where unconsolidated deposits are at land surface were

assigned to the category significant potential to

facilitate contamination of ground water and assigned

a numerical value of 1. The remaining areas within the

approximate boundaries of the contiguous Navajo

Reservation were assigned to the category insignificant

potential to facilitate contamination of ground water

and assigned a numerical value of 0. These areas are

shown in figure 2.

The part of the study area that extends beyond

approximately the eastern and southeastern boundaries

of the contiguous Navajo Reservation was not included

in the analysis of Cooley and others (1969), and areas

of recharge to consolidated rocks and areas of

unconsolidated deposits therefore were not identified

beyond these boundaries. Regarding geology, the

potential to facilitate contamination of ground water in

this part of the study area has been identified in figure 2

as undetermined. In order to not underestimate the

potential of this part of the study area to facilitate
8

contamination, however, it was included in the

category significant potential to facilitate

contamination and assigned a numerical value of 1

(fig. 2).

Precipitation

Because precipitation provides the solvent

(water) in which contaminants are transported from

land surface to an underlying aquifer, larger

precipitation amounts typically increase the potential

for contaminants to infiltrate the land surface and move

downward toward underlying aquifers. In this

assessment, precipitation was used as a surrogate for

recharge. Actual recharge, however, typically is a small

percentage of precipitation.

The percentage of precipitation that becomes

recharge has been estimated by several investigators.

Eychaner (1983) estimated recharge to be 1 to 3

percent of precipitation on and north of Black Mesa,

respectively. In eastern Colorado, two estimates of the

percentage of annual precipitation that becomes annual

recharge were 4 percent (Erker and Romero, 1967) and

less than 10 percent (Hurr and others, 1975).

The Parameter-Elevation Regressions on

Independent Slopes Model (PRISM) database,

developed by Oregon State University, was used to

provide precipitation data for this assessment (Daly

and Taylor, 1998). Using this model, Daly and Taylor

estimated annual precipitation in the study area to

range from 5 to 35 inches. By using 10 percent as the

maximum percentage of precipitation that becomes

recharge (Hurr and others, 1975), estimated recharge in

the study area would range from 0.5 to 3.5 inches. This

range is within the two smallest (0 to 2 inches and 2 to

4 inches) of five categories described by Aller and

others (1985). The five categories of recharge described

in DRASTIC (Aller and others, 1985) exceed the range

of recharge in the study area because DRASTIC was

designed for use throughout the United States.

In this assessment, precipitation was divided into

three categories to determine relative potential to

facilitate contamination of ground water: least,

intermediate, and most, having corresponding

numerical values of 1, 2, and 3, respectively.

Precipitation was categorized as follows: less than

8 inches, “least potential to facilitate contamination”;

8 to 16 inches, “intermediate potential to facilitate

contamination”; and more than 16 inches, “most

potential to facilitate contamination.” The potential for

precipitation to facilitate contamination of ground

water throughout the study area is shown in figure 3.

Soil Properties

Texture, infiltration rate, drainage, and organic

content are principal soil properties that control the

potential for contaminant movement from land surface

through soils to underlying geologic materials. Soil

texture controls hydraulic conductivity, the rate at

which water and the solutes it contains move through

the soil. Fine-grained soils (silts and clays) also have

sorption sites to which contaminants can attach.

Infiltration rate controls the amount of water entering

the soil from land surface. Drainage rate controls the

time that water and its solutes are in contact with the

soil prior to leaving the soil. Organic material can

decrease the amount of pesticides in solution by

microbial activity and sorption. A soil that has a coarsegrained texture, a high infiltration rate, a low organic

content, and is well drained has a larger potential for

contaminant delivery from land surface to the

underlying geologic material than a soil that has a finegrained texture, a low infiltration rate, a high organic

content, and is poorly drained.

The STATe Soil GeOgraphic (STATSGO)

database was created by the U.S. Department of

Agriculture, Natural Resources Conservation Service

(1994) and subsequently was modified for ease of use

on U.S. Geological Survey (USGS) computer systems

(Schwarz and Alexander, 1995). The STATSGO

database was digitized at a scale of 1:250,000 and was

designed to be used at the multicounty, State, and

regional level.

The STATSGO database contains a calculated

classification of soils called hydrologic group, which

was used in this assessment. Characteristics combined

to create hydrologic group (class) include texture,

infiltration rate, and drainage classification (Schwarz

and Alexander, 1995). Class 1 soils are sands and

gravels having high infiltration rates and are well

drained to excessively drained. Class 2 soils are

moderately coarse grained, have moderate infiltration

rates, and are moderately well drained to well drained.

Class 3 soils are moderately fine to fine grained or have

layers that impede downward movement of water, have

low infiltration rates, and are poorly drained. Class 4

soils are high in clay content and have low infiltration

rates, may be underlain by a confining layer, and are

very poorly drained.

In the STATSGO database, a soil hydrologic

group (1, 2, 3, or 4) was assigned to each component of

a soil mapping unit. A weighted average of these values

was then calculated to determine a soil hydrologic

group score for each mapping unit. These values

ranged from 1 to 4.
40

37° 

UTAH

COLORADO

ARIZONA

A

N

O

ZI R

A

O

CI X

E

M

W

E

N

0

0

10

10

20 Miles

20 Kilometers

Lake

Powell

RAINBOW

PLATEAU

San Juan

 River

SAN

JUAN

MCKINLEY

APACHE

COCONINO

NAVAJO

Colorado

Little

Teec

Nos Pos

Round

 Rock

Chinle

Ganado

Gallup

Pueblo

Wash

Co

ol

ar

do

Polacca

Wash

CIBOLA

D ni

nebi ot

Wash

Navajo

Tuba

City

Cameron

l o C ar o

od

evi R r

Leupp

Shiprock

r evi R ocah C

Puerco River

264

87

77

53

108°

109°

110°

111° 

36° 

35° 

89

160

32

32

Figure 3. Categories of annual precipitation in the Navajo Nation study area.

MORE THAN 16 INCHES: MOST POTENTIAL TO FACILITATE

 CONTAMINATION OF GROUND WATER

8 TO 16 INCHES: INTERMEDIATE POTENTIAL TO FACILITATE

 CONTAMINATION OF GROUND WATER

LESS THAN 8 INCHES: LEAST POTENTIAL TO FACILITATE 

 CONTAMINATION OF GROUND WATER

 

 

EXPLANATION

CHUSKA

MOUNTAINS

ZUNI MOUNTAINS

160

 Wash

 Moenkopi

hsa W el ni h C

Creek

Kaibito 

 Creek

River

666

163

9

BLACK 

MESA
10

Organic content in soils was not included in the

determination of hydrologic group (Schwarz and

Alexander, 1995). Data describing organic material,

however, were included in the STATSGO database and

were evaluated as part of this assessment. Soil organic

content was less than 2 percent in all soils throughout

the study area. Therefore, microbial activity and

sorption were considered to be minimal, and all soils

were assessed to be in the category most potential to

facilitate contamination regarding soil organic content.

Because the entire area was assessed to have the same

potential to facilitate contamination and, therefore, to

have little relative difference throughout the area, no

value for organic content was included in the

assessment.

In this assessment, soil properties also were

described in terms of potential to facilitate

contamination of ground water. Three categories were

used: least, intermediate, and most, having

corresponding numerical values of 1, 2, and 3,

respectively. Categories of hydrologic group were:

more than 3, least potential to facilitate contamination;

2 to 3, intermediate potential to facilitate

contamination; and less than 2, most potential to

facilitate contamination. The potential for soils to

facilitate contamination of ground water throughout

the study area is shown in figure 4.

Slope of the Land Surface

The slope of the land surface affects the ability of

precipitation to infiltrate soils and geology.

Precipitation falling on a nearly flat land surface

remains nearly stationary, maximizes the time available

for infiltration because of storage time on the land

surface, and therefore maximizes the potential for

infiltration. Conversely, precipitation falling on a steep

land surface has little storage time on the land surface

and will begin moving downslope as soon as the

precipitation rate exceeds the infiltration capacity of

the upper soil layer. The potential for infiltration,

therefore, is minimized.

In this assessment, USGS digital elevation

models (DEM’s) were used to determine slope of the

land surface, and slope of the land surface was

described in terms of potential to facilitate

contamination of ground water. Three categories were

used: least, intermediate, and most, having

corresponding numerical values of 1, 2, and 3,

respectively. Slopes of more than 12 degrees were

considered to have the least potential to facilitate

contamination; slopes of 6 to 12 degrees were

considered to have intermediate potential to facilitate

contamination; and slopes of less than 6 degrees were

considered to have the most potential to facilitate

contamination. These categories of land-surface slope

are similar to those of Aller and others (1985) and

Hearne and others (1995) (table 1). The potential for

slope of the land surface to facilitate contamination of

ground water throughout the study area is shown in

figure 5.

Stream Courses

Flood plains and terraces bordering streams

typically are areas of the most potential to facilitate

contamination of ground water compared to

surrounding areas because shallow ground water is

stored in and moves through these deposits. These

deposits most often have a sandy texture and a

corresponding high infiltration rate and low organic

content. Underlying bedrock may perch water in these

deposits because of its lower infiltration rate, or the

water may recharge an underlying aquifer.

In several locations, flood-plain deposits were

not identified in the STATSGO database because of

their small geographical area. Stream courses

throughout the study area, therefore, were identified

using DEM’s. Strahler fourth-order and higher stream

courses (Strahler, 1963, p. 504) were generated using

the ARC/INFO grid module. Areas identified by this

procedure were categorized as having the most

potential to facilitate contamination and were assigned

a numerical score of 9.

Although actual precipitation may be less than

16 inches on the stream course, runoff following

precipitation typically concentrates in stream courses.

Soils on flood plains and terraces bordering streams

typically are composed of materials that place them in

hydrologic group 1, and the topography of stream

courses typically is nearly flat, both parallel and

perpendicular to the stream. Stream courses are shown

in figures 1 through 6.

Conducting the Assessment: Combining

Characteristics

The aquifer sensitivity assessment was

conducted by overlaying GIS coverages of geology,

precipitation, soil properties, and slope. For each

geographic area (GIS polygon) in the combined

coverage, the scores for precipitation, soil properties,

and slope were summed and then multiplied by the

score for geology (either 0 or 1) to determine the final
37° 

40

 Wash

 Moenkopi

River

COLORADO

ARIZONA

UTAH

A

N

O

ZI R

A

O

CI X

E

M

W

E

N

hsa W el ni h C

0

0

10

10

20 Miles

20 Kilometers

Lake

Powell

RAINBOW

PLATEAU

San Juan

 River

SAN

JUAN

MCKINLEY

APACHE

COCONINO

NAVAJO

Colorado

Little

Teec

Nos Pos

Round

 Rock

Chinle

Ganado

Gallup

Pueblo

Wash

Co

ol

ar

do

Polacca

Wash

CIBOLA

D ni

nebi ot

Wash

Navajo Creek

Kaibito Creek

Tuba

City

Cameron

l o C ar o

od

evi R r

Leupp

Shiprock

r evi R ocah C

Puerco River

264

87

77

53

108°

109°

110°

111° 

36° 

35° 

163

89

160

666

32

32

Figure 4. Categories of soil hydrologic group in the Navajo Nation study area.

LESS THAN 2: MOST POTENTIAL TO FACILITATE 

 CONTAMINATION OF GROUND WATER

2 TO 3: INTERMEDIATE POTENTIAL TO FACILITATE

 CONTAMINATION OF GROUND WATER

MORE THAN 3: LEAST POTENTIAL TO FACILITATE

 CONTAMINATION OF GROUND WATER

 

 

EXPLANATION

CHUSKA MOUNTAINS

ZUNI MOUNTAINS

160

Puerco River

11

BLACK 

MESA
Figure 5. Categories of land-surface slope in the Navajo Nation study area.

LESS THAN 6 DEGREES: MOST POTENTIAL TO FACILITATE 

 CONTAMINATION OF GROUND WATER

6 TO 12 DEGREES: INTERMEDIATE POTENTIAL TO FACILITATE

 CONTAMINATION OF GROUND WATER

MORE THAN 12 DEGREES: LEAST POTENTIAL TO FACILITATE 

 CONTAMINATION OF GROUND WATER

 

 

EXPLANATION

Lake

Powell

RAINBOW

PLATEAU

San Juan

 River

SAN

JUAN

MCKINLEY

APACHE

COCONINO

NAVAJO

Colorado

Little

Teec

Nos Pos

Round

 Rock

Chinle

Ganado

Gallup

Pueblo

Wash

Co

ol

ar

do

Polacca

Wash

CIBOLA

D ni

nebi ot

Wash

Navajo

Tuba

City

Cameron

l o C ar o

od

evi R r

Leupp

Shiprock

r evi R ocah C

Puerco River

264

87

77

53

108°

109°

110°

111° 

36° 

35° 

160

89

160

666

32

32

0

0

10

10

20 Miles

20 Kilometers

CHUSKA MOUNTAINS

ZUNI MOUNTAINS

ZUNI MOUNTAINS

40

sa W el ni h C h

Creek

Kaibito Creek

 Wash

 Moenkopi

River

37° 

163

UTAH

COLORADO

ARIZONA

A

N

O

ZI R

A

O

CI X

E

M

W

E

N

12

BLACK 

MESA
13

score. Scores for polygons within the boundaries of the

contiguous Navajo Reservation that were neither areas

of recharge to bedrock aquifers nor areas of

unconsolidated deposits were zero (geology score = 0).

Scores for polygons that were areas of recharge to

bedrock aquifers and (or) areas of unconsolidated

deposits ranged from 3 where the least potential to

facilitate contamination occurred for precipitation, soil

properties, and slope to 9 where the most potential to

facilitate contamination occurred for these

characteristics. No scores were greater than 0 or less

than 3. Stream courses were the exception to this

assessment method; they were assigned a score of 9.

Finally, the aquifer sensitivity assessment was

completed by converting the numerical scores to

categories of “potential for contamination.” These

categories are defined as: 0, insignificant potential; at

least 3 but less than 5, least potential; 5 to 7,

intermediate potential; and more than 7, most potential

for contamination.

The resulting score for any area within the

boundaries of the study area is comparable to that of

any other area and indicates relative potential to

facilitate contamination. The resulting scores, however,

are not comparable to any other assessment conducted

either within or beyond the boundaries of this study,

unless the same data sources and the same assessment

method are used.

Results of Assessment of Aquifer

Sensitivity

Results of the assessment of aquifer sensitivity

on Navajo Nation and adjacent lands are shown in

figure 6. The results indicated that about 22 percent of

the study area has an insignificant potential for

contamination, being neither bedrock recharge areas

nor areas of unconsolidated deposits. The results

indicate potential for contamination based on actual

precipitation conditions (fig. 3). If this assessment

method were used to determine the potential for

contamination of an existing or proposed irrigatedagriculture project, it would need to be modified to

adjust precipitation to the category most potential to

facilitate contamination. This adjustment would

account for the increased potential recharge resulting

from irrigation.

About 71 percent of the Navajo Nation study

area was assessed to be in the categories most potential

(more than 40 percent) or intermediate potential (more

than 31 percent) for contamination (fig. 6). About 6

percent of the study area was assessed to be in the

category least potential for contamination, generally in

areas where the slope of the land surface is greater than

12 percent (fig. 5). In the northwestern part of the study

area near Navajo Creek and the Rainbow Plateau,

precipitation is less than 8 inches; this also contributes

to parts of this area being in the category least potential

for contamination (figs. 3 and 6).

Nearly all the NIIP (about 96 percent) was

assessed to be in the category most potential for

contamination; the remaining 4 percent of the NIIP was

assessed to be in the category intermediate potential for

contamination (fig. 7). The areas of intermediate

potential for contamination are on the periphery of the

NIIP and along small areas of Gallegos Canyon, Ojo

Amarillo Canyon, and an unnamed drainage west of

Ojo Amarillo Canyon. Following an adjustment for

precipitation to account for the effects of irrigation, all

irrigated fields except a small part of one field along

Ojo Amarillo Canyon were assessed to be in the

category most potential for contamination.

GROUND-WATER VULNERABILITY TO

PESTICIDE CONTAMINATION ON THE

NAVAJO INDIAN IRRIGATION PROJECT

This section describes the method developed for

assessment of ground-water vulnerability to pesticide

contamination on the NIIP. Subsequently, it describes

the results of the assessment and additional

applications of the method.

Method Developed for Assessment of

Ground-Water Vulnerability

To quantitatively determine ground-water

vulnerability on the NIIP or how the application of

pesticides to a particular field affects underlying

ground water, the concentration of each pesticide in

ground water underlying that field would need to be

determined. In the absence of available laboratorydetermined concentrations of pesticides in ground

water underlying the NIIP, the vulnerability of ground

water was determined by using leachability data for

pesticides typically applied to each crop. Pesticide
0

0

10

10

20 Miles

20 Kilometer

160

37° 

UTAH

COLORADO

ARIZONA

A

N

O

ZI R

A

O

CI X

E

M

W

E

N

Figure 6. Results of assessment of aquifer sensitivity in the Navajo Nation study area.

MOST POTENTIAL FOR CONTAMINATION OF GROUND WATER

INTERMEDIATE POTENTIAL FOR CONTAMINATION OF GROUND WATER

LEAST POTENTIAL FOR CONTAMINATION OF GROUND WATER

INSIGNIFICANT POTENTIAL FOR CONTAMINATION OF GROUND WATER

STREAM COURSES: MOST POTENTIAL FOR CONTAMINATION OF GROUND WATER

EXPLANATION

Lake

Powell

RAINBOW

PLATEAU

San Juan

 River

SAN

JUAN

MCKINLEY

APACHE

COCONINO

NAVAJO

Colorado

Little

Teec

Nos Pos

Round

 Rock

Chinle

Ganado

Gallup

Pueblo

Wash

Co

ol

ar

do

Polacca

Wash

CIBOLA

Wa

hs

Dinnebito 

Navajo

Tuba

City

Cameron

l o C ar o

od

evi R r

Leupp

Shiprock

r evi R ocah C

Puerco River

264

87

77

53

108°

109°

110°

111° 

36° 

35° 

89

160

666

32

32

CHUSKA MOUNTAINS

ZUNI MOUNTAINS

163

hsa W el ni h C

Creek

Kaibito Creek

 Wash

 Moenkopi

River

40

14

BLACK 

MESA
Figure 7. Results of assessment of aquifer sensitivity on the Navajo Indian Irrigation Project 

AQUIFER SENSITIVITY

MOST POTENTIAL FOR CONTAMINATION

 OF GROUND WATER

INTERMEDIATE POTENTIAL FOR CONTAMINATION

 OF GROUND WATER

FIELDS, CROPS

IRRIGATION CANAL

ROAD

UTAH

ARIZON

NIIP

0

0

1

1

2

2

3

3

4

4

5

5

6 MILES

6 KILOMETERS

Ojo Amarillo

Canyon

t se W r oF

Gk ell a so

g na C no

y

Gallegos Canyon

36°

40 ́

36°

30 ́

108°30 ́

108°20 ́

108°10 ́

EXPLANATION

15
16

application data for the 1997-99 growing seasons on

block I of seven blocks on the NIIP were obtained from

the NAPI (James Dangler, written commun., 2000),

and pesticide leachability data were obtained from the

Montana Department of Agriculture (DeLuca and

Johnson, 1990; Donna Rise, written commun., 2001).

The location of block I fields is shown in figure 8.

The following steps were conducted to prepare

available pesticide application data for use in the

ground-water vulnerability assessment. First, for the

1997-99 growing seasons, fields in block I of the NIIP

were categorized by crop. The number of fields that

were planted in each crop during the 3-year period

were: beans, 57; corn, 45; potatoes, 37; wheat, 37; hay

(fields used for grazing), 16; barley, 8; and alfalfa, 6.

Second, the number of fields on which each pesticide

was used on each crop during 1997-99 was summed

(table 2).

Pesticides used during 1997-99 were then

divided into two groups: those “typically applied” and

those “not typically applied” to each crop (table 2).

Pesticides that were applied to 40 percent or more of

fields planted in a particular crop were considered to be

“typically applied” to that crop. The difference was

characteristically large between the percentage of

fields on which pesticides in the “typically applied”

group were applied (43 to 100 percent) and the

percentage of fields on which pesticides in the “not

typically applied” group were applied (2.2 to 35

percent). These percentages are listed in table 3.

Oregon State University has determined groundwater ubiquity scores for nearly all the pesticides

applied to block I of the NIIP during 1997-99 (Donna

Rise, written commun., 2001). Ground-water ubiquity

scores are determined by the equation:

Ground-water ubiquity score = (log t) (4 – log Koc)

where t = pesticide half-life, and

Koc = pesticide sorption

 coefficient.

The Montana Department of Agriculture has

assigned descriptive terms, called relative pesticide

leaching potentials, to ranges of ground-water ubiquity

scores to provide pesticide users a tool with which to

compare potential effects of pesticides on shallow

ground water (DeLuca and Johnson, 1990; Donna Rise,

written commun., 2001). These terms and their

respective ground-water ubiquity scores are: extremely

low, less than 0.1; very low, 0.1 to 1.0; low, 1.0 to 2.0;

moderate, 2.0 to 3.0; high, 3.0 to 4.0; and very high,

more than 4.0. In this assessment, crops were ranked by

the number of typically applied pesticides that had (1)

a very high relative pesticide leaching potential, then

by the number of typically applied pesticides that had

(2) a high relative pesticide leaching potential, and then

by the number of typically applied pesticides that had

(3) a moderate relative pesticide leaching potential.

Typically, a ground-water vulnerability

assessment is a combination of the results of an aquifer

sensitivity assessment and agricultural practices.

Because the results of the aquifer sensitivity

assessment were the same on all fields of the NIIP and,

therefore, had no relative difference throughout the

NIIP, the results of the ground-water vulnerability

assessment were used independently.

Results of Assessment of Ground-Water

Vulnerability

The results for crops ranked by the number of

typically applied pesticides are listed in table 4. The

relative vulnerability of ground water to contamination

by pesticides underlying fields on the NIIP, based on

crops grown on each field in 2000, is shown in figure 9.

This relative vulnerability of ground water is based on

expected pesticide applications to each crop.

One pesticide typically applied to beans, wheat,

barley, and alfalfa had a very high relative pesticide

leaching potential. Fields planted in these crops in 2000

were identified, and ground water underlying these

fields was considered most vulnerable to pesticide

contamination.

No pesticide typically applied to corn and

potatoes had a very high relative pesticide leaching

potential, but two pesticides typically applied to corn

and one pesticide typically applied to potatoes had high

relative pesticide leaching potentials. Fields planted in

these crops in 2000 were identified, and ground water

underlying these fields was considered intermediately

vulnerable to pesticide contamination.

No pesticide typically applied to hay had either a

very high or a high relative pesticide leaching potential,

but two pesticides typically applied to hay had a

moderate relative pesticide leaching potential. Fields

planted in hay in 2000 were identified, and ground

water underlying these fields was considered least

vulnerable to pesticide contamination.
0

0

1

1

2

2

3

3

4

4

5

5

6 MILES

6 KILOMETERS

Figure 8. Navajo Indian Irrigation Project fields for which pesticide application data are availa

BLOCK I FIELDS: FIELDS FOR WHICH

 PESTICIDE APPLICATION DATA WERE 

 AVAILABLE, 1997-99

FIELDS, CROPS

IRRIGATION CANAL

ROAD

Ojo Amarillo

Canyon

t s

e W r oF

G

k

ell a

s

o

g

n

a C

n

oy

Gallegos Canyon

36°

40 ́

36°

30 ́

108°30 ́

108°20 ́

108°10 ́

EXPLANATION

17
Ojo Amarillo

Canyon

t se W r oF

Gk ell a so

g na C no

y

Gallegos Canyo

INTERMEDIATELY VULNERABLE TO PESTICIDE CONTAMINATION

 (corn and potatoes)

 

UNPLANTED FIELDS

CROPS FOR WHICH PESTICIDE DATA WERE NOT AVAILABLE

MOST VULNERABLE TO

IRRIGATION CANAL

ROAD

 PESTICIDE CONTAMINATION

 (alfalfa, barley, beans, wheat)

LEAST VULNERABLE TO PESTICIDE CONTAMINATION

 (hay)

Figure 9. Results of assessment of ground-water vulnerability to pesticide contamination on the Navajo India

EXPLANATION

0

0

1

1

2

2

3

3

4

4

5

5

6 MILES

6 KILOMETERS

36°

40 ́

36°

30 ́

108°30 ́

108°20 ́

108°10 ́

18
19

Pesticide

Relative pesticide

leaching

potential 1997 1998 1999 Total

Pesticide applications to alfalfa (1 pesticide typically applied to fields of alfalfa)

Velpar Very high 2 of 2 2 of 2 2 of 2 6 of 6

Zinc Phosphide Low 0 0 2 2

Zorial Rapid 80 Low 0 1 0 1

Pesticide applications to barley (3 pesticides typically applied to fields of barley)

Amine, 2,4-D Moderate 8 of 8 0 of 0 0 of 0 8 of 8

Banvel Very high 8 0 0 8

Penncap-M Very low 8 0 0 8

Pesticide applications to beans (6 pesticides typically applied to fields of beans)

Basagran High 0 of 14 8 of 43 0 of 0 8 of 57

Dual/Dual II1 High 10 21 0 31

Frontier 6.0 Undetermined 0 20 0 20

Frontier 7.5 Undetermined 0202

Gramoxone Extra Extremely low 0 25 0 25

Kocide 2000 Low 0404

Poast Low 4004

Pursuit Very high 0 42 0 42

Roundup Ultra Very low 0404

Rovral 4FL Low 0 42 0 42

Sonalan HFP Very low 10 0 0 10

Thiodan 3EC Extremely low 0 11 0 11

Topsin M 70 W Very low 0 42 0 42

Zinc Phosphide Low 0 27 0 27

Pesticide applications to corn (3 pesticides typically applied to fields of corn)

Accent SP Very high 1 of 17 0 of 5 4 of 23 5 of 45

Asana XL Very low 1102

Atrazine 4L High 1045

Basis Gold High 1001

Beacon High 8 0 16 24

Bladex 90DF Low 16 5 1 22

Buctril 4EC Very low 0 0 14 14

Clarity Undetermined 2 1 4 14

Dual II/Dual II

Magnum2 High 16 5 11 32

Glyfos Very low 6006

Lannate LV High 2013

Penncap-M Very low 1001

Roundup Ultra Very low 0044

Zinc Phosphide Low 6006

Table 2. Relative pesticide leaching potential of pesticides and number of fields in block I of the

Navajo Indian Irrigation Project on which each pesticide was applied, 1997-99

[Pesticide, trade or commonly accepted product name; typically used pesticides are in bold type. Relative

pesticide leaching potential: Montana Department of Agriculture pesticide leachability classification system

of very high, high, moderate, low, very low, and extremely low leachability (DeLuca and Johnson, 1990)]
20

1Dual used on 10 fields in 1997; Dual II used on 21 fields in 1998.

2Dual II applied to 16 fields in 1997 and 5 fields in 1998; Dual II Magnum applied to 11 fields in 1999.

Pesticide applications to hay (2 pesticides typically applied to fields of hay)

Amine 4 Moderate 7 of 14 0 of 0 0 of 2 7 of 16

Amine 2,4-D Moderate 7 0 0 7

LV-6 Ester Weed Kill Low 0 0 2 2

Pesticide applications to potatoes (8 pesticides typically applied to fields of potatoes)

Asana XL Very low 26 of 29 4 of 5 3 of 3 33 of 37

Bravo Weather Stik Low 0 0 3 3

Bravo Zinc Low 25 3 0 28

Bravo ZN Low 0033

Diquat Extremely low 26 3 3 32

Dithane-DF Low 27 3 0 30

Eptam 7E Low 27 4 3 34

Glyfos Very low 1001

Kocide-DF Low 3003

Manzate 75DF Low 0 0 3 3

Matrix Low to high 4 2 3 9

Penncozef 75DF Low 0 2 0 2

Poast Low 1001

Ridomil Bravo 81W Low 16 0 0 16

Ridomil Gold Bravo Low 0 4 0 4

Ridomil Gold MZ Low 0033

Trifluralin 4EC Very low 26 0 0 26

Trifluralin HF Very low 0 0 3 3

Trilin Very low 0 2 0 2

Trilin 5 Very low 0 2 0 2

Turbo 8E High 19 0 0 19

Turbo 8EC High 0 2 0 2

Pesticide applications to wheat (4 pesticides typically applied to fields of wheat)

Accent SP Very high 0 of 5 0 of 4 1 of 28 1 of 37

Amine 2,4-D Moderate 5 0 12 17

Banvel*L Very high 5 4 26 35

Clarity Undetermined 0011

Dual II Magnum High 0022

Lorsban 4ESG Very low 0 4 0 4

Nufos 4E-SG Very low 0 4 22 26

Roundup Ultra Very low 0011

Weedar 64 Moderate 0 4 15 19

Pesticide

Relative pesticide

leaching

potential 1997 1998 1999 Total

Table 2. Relative pesticide leaching potential of pesticides and number of fields in block I of the

Navajo Indian Irrigation Project on which each pesticide was applied, 1997-99--Concluded
21

Table 3. Range of percentage of fields on which “typically applied” and “not typically

applied” pesticides were used for each crop grown in block I of the

Navajo Indian Irrigation Project, 1997-99

[Crop: number in parentheses, number of fields planted in specified crop in block I of the

Navajo Indian Irrigation Project during 1997-99; --, not applicable]

Crop

Range of percentage

of fields on which

“typically applied”

pesticides were used

Range of percentage

of fields on which

“not typically

applied” pesticides

were used

Alfalfa (6) 100 16 - 33

Barley (8) 100 --

Beans (57) 44 - 74 3.5 - 35

Corn (45) 49 - 71 2.2 - 31

Hay (16) 44 12

Potatoes (37) 43 - 92 2.7 - 24

Wheat (37) 46 - 95 2.7 - 11

Table 4. Ground-water vulnerability and crop ranking based on number of typically applied

pesticides having very high, high, and moderate relative pesticide leaching potential

[Relative pesticide leaching potential: Montana Department of Agriculture pesticide classification

system of very high, high, moderate, low, very low, and extremely low relative pesticide

leaching potential (DeLuca and Johnson, 1990)]

Groundwater

vulnerability Crop Very high High Moderate

High Beans 1 1 0

Wheat 1 0 2

Barley 1 0 1

Alfalfa 1 0 0

Moderate Corn 0 2 0

Potatoes 0 1 0

Low Hay 0 0 2

Number of typically applied pesticides

having very high, high, or moderate

relative pesticide leaching potential
22

This assessment method can be combined with

the cropping plan for any year, whether it is a past year,

the present year, or a future year, to assess groundwater vulnerability to pesticide contamination in that

year. The results can be used to adjust the number of

fields planted in each crop to optimize the combination

of economic and environmental benefits. The results

also can be used to locate crops whose typical pesticide

applications make underlying ground water most

vulnerable to pesticide contamination in the least

environmentally sensitive areas, such as project drains

and natural drainages, to decrease the potential effects

of pesticides on fish, birds, and other fauna.

SUMMARY AND CONCLUSIONS

The USEPA requested that the Navajo Nation

conduct an assessment of aquifer sensitivity on Navajo

Nation lands and an assessment of ground-water

vulnerability to pesticide contamination on the NIIP.

Navajo Nation lands include about 17,000 square miles

in northeastern Arizona, northwestern New Mexico,

and southeastern Utah. The NIIP, in northwestern New

Mexico, is the largest area of agriculture on the Navajo

Nation. The NIIP began operation in 1976; presently

(2001) about 62,000 acres are available for irrigated

agriculture. Numerous pesticides have been used on the

NIIP during its operation.

Aquifer sensitivity is defined by the USEPA as

"The relative ease with which a contaminant [pesticide]

applied on or near a land surface can migrate to the

aquifer of interest. Aquifer sensitivity is a function of

the intrinsic characteristics of the geologic material in

question, any underlying saturated materials, and the

overlying unsaturated zone. Sensitivity is not

dependent on agronomic practices or pesticide

characteristics." Ground-water vulnerability is defined

by the USEPA as "The relative ease with which a

contaminant [pesticide] applied on or near a land

surface can migrate to the aquifer of interest under a

given set of agronomic management practices,

pesticide characteristics, and aquifer sensitivity

conditions.”

The Navajo Nation study area includes about

21,500 square miles, and because of its size and relative

isolation, is lacking in detailed information regarding

geology, soils, and hydrology that typically is used to

conduct an aquifer sensitivity assessment.

Characteristics for which data were considered

adequate included geology (within the approximate

boundaries of the contiguous Navajo Reservation

only), precipitation, soil properties, slope of the land

surface, and location of stream courses.

Geology within the approximate boundaries of

the contiguous Navajo Reservation was divided into

areas of significant potential to facilitate contamination

of ground water and insignificant potential to facilitate

contamination of ground water. Areas within the

approximate boundaries of the contiguous Navajo

Reservation where consolidated rocks are recharged

and (or) where unconsolidated deposits are at land

surface were considered to have significant potential to

facilitate contamination of ground water. All other

areas within the approximate boundaries of the

contiguous Navajo Reservation were considered to

have an insignificant potential to facilitate

contamination of ground water. No data-defining areas

of recharge to bedrock aquifers or areas of

unconsolidated deposits were available for the part of

the study area south and southeast of the approximate

boundaries of the contiguous Navajo Reservation.

Therefore, regarding geology, the potential to facilitate

contamination in this part of the study area was

identified as undetermined. In order to not

underestimate the potential of this part of the study area

to facilitate contamination, however, its potential was

estimated to be significant, and it was assigned a

numerical value of 1.

Precipitation, soil properties, and slope of the

land surface were evaluated regarding potential to

facilitate contamination of underlying ground water.

Each of these characteristics was subdivided into three

categories: least, intermediate, and most potential to

facilitate contamination.

Stream courses typically receive runoff from

their surrounding drainage during and following

precipitation. Deposits in them typically are coarse

grained, have a high infiltration rate, low organic

content, and are well drained; slope of the land surface

typically is nearly flat. Therefore, stream courses were

considered to be in the category most potential to

facilitate contamination of ground water with regard to

precipitation, soil properties, and the slope of the land

surface.

The assessment of aquifer sensitivity on Navajo

Nation and adjacent lands was conducted by first

converting the areas of least, intermediate, and most

potential to facilitate contamination of ground water to

the respective numerical values of 1, 2, and 3 in the GIS

coverages of precipitation, soil properties, and slope of
23

the land surface. For the GIS coverage of geology, the

category insignificant potential to facilitate

contamination of ground water was assigned a score of

0, and the category significant potential to facilitate

contamination of ground water was assigned a score of

1. GIS coverages for precipitation, geology, soil

properties, and slope of the land surface were overlaid,

and the numerical values for each GIS polygon in the

resulting coverage were summed and then multiplied

by the score for geology (0 or 1). All stream courses

received a score of 9. Finally, the scores were converted

to potentials for contamination as follows: 0,

insignificant; at least 3 but less than 5, least; 5 to 7,

intermediate; and more than 7, most.

Results of the assessment indicated that about 22

percent of the study area was in the category

insignificant potential to facilitate contamination.

About 72 percent of the study area was assessed as

being in the categories most or intermediate potential

for contamination. About 6 percent of the study area

was assessed to be in the category least potential for

contamination, typically in areas where the slope of the

land surface is more than 12 percent.

About 96 percent of the NIIP was assessed to be

in the category most potential for contamination; the

remaining 4 percent of the NIIP was assessed to be in

the category intermediate potential for contamination.

All irrigated fields except a small part of one field along

Ojo Amarillo Canyon were assessed to be in the

category most potential for contamination.

The assessment of aquifer vulnerability to

pesticide contamination on the NIIP was based on the

relative pesticide leaching potential of pesticides

typically applied to each crop. First, pesticides

typically applied to each crop were determined.

Second, the relative pesticide leaching potential was

assigned to each typically applied pesticide. Third,

crops then were ranked by the number of typically

applied pesticides that had (1) a very high relative

pesticide leaching potential; then by the number of

typically applied pesticides that had (2) a high relative

pesticide leaching potential; and then by the number of

typically applied pesticides that had (3) a moderate

relative pesticide leaching potential.

The results of this assessment indicated relative

ground-water vulnerability to pesticide contamination

under all fields on the NIIP. Ground water underlying

fields planted in alfalfa, barley, beans, and wheat was

considered most vulnerable; ground water underlying

fields planted in corn and potatoes was considered

intermediately vulnerable; and ground water

underlying fields planted in hay was considered least

vulnerable to pesticide contamination. These

categories of vulnerability were substituted for the

respective crops in the NIIP 2000 cropping plan, and a

GIS coverage was created that shows relative groundwater vulnerability to pesticide contamination under all

fields on the NIIP for 2000.

The results of this assessment of ground-water

vulnerability to pesticide contamination on the NIIP

show relative vulnerability for 2000. This assessment

method can be combined with the cropping plan for

any year, whether it is a past year, the present year, or a

future year, to assess ground-water vulnerability to

pesticide contamination in that year. The results can be

used to adjust the number of fields planted in each crop

to optimize the combination of economic and

environmental benefits. The results also can be used to

locate crops whose typically applied pesticides make

underlying ground water most vulnerable away from

the most environmentally sensitive areas, such as

project drains and natural drainages, to decrease the

potential effects of pesticides on fish, birds, and other

fauna.

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