U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with

Idaho Department of Environmental Quality

Analysis of Nitrate (NO3–N) Concentration Trends 

25 Ground-Water-Quality Management Areas, 

Idaho, 1961–2001

Water-Resources Investigations Report 02–4056
Analysis of Nitrate (NO3–N) Concentration Trends 

25 Ground-Water-Quality Management Areas, Ida

1961–2001 

By D.J. Parliman 

Water-Resources Investigations Report 02–4056 

Prepared in cooperation with


Idaho Department of Environmental Quality


Boise, Idaho


April 2002

U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endors

by the U.S. Government. 

Additional information can be obtained from: Copies of this report are available in PDF forma

can be viewed using Adobe Acrobat Reader, at 

District Chief 

U.S. Geological Survey http://idaho.usgs.gov/public/reports.html 

230 Collins Road 

Boise, ID 83702-4520 

http://idaho.usgs.gov 
CONTENTS 

Abstract .............................................................................................................................................................................

Introduction .......................................................................................................................................................................

Data compilation ...............................................................................................................................................................

Trend analyses...................................................................................................................................................................

Time-period trends by decades or selected years.......................................................................................................

Time-series trends for data from individual wells .....................................................................................................

Additional information needed..........................................................................................................................................

Selected references............................................................................................................................................................

Appendix A. Boxplots, summary statistics, and Mann-Whitney test p-values for 25 ground-water-quality managemen

areas in Idaho, 2001 ...................................................................................................................................................

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 

1961–2001 .................................................................................................................................................................

FIGURES 

1–2. Maps showing: 

1. Locations of 33 priority ground-water-quality management areas in Idaho, July 2000 ................................

2. Locations and identification of 25 ground-water-quality management areas in Idaho selected for nitrate 

trend analyses, February 2001........................................................................................................................

3–6. Diagrams showing: 

3.	 Number of nitrate analyses in relation to number of wells in 25 ground-water-quality management areas 

in Idaho, 1961–2001.......................................................................................................................................

4. Ranges of nitrate concentrations for 25 ground-water-quality management areas in Idaho, 1961–2001......

5.	 Mean (average) nitrate concentrations in relation to median (50th percentile) concentrations for 

25 ground-water-quality management areas in Idaho, 1961–2001................................................................

6.	 Nitrate concentrations in relation to total well depth, selected wells in ground-water-quality management 

areas in Idaho, 1961–2001 .............................................................................................................................

TABLES 

1. Number of nitrate analyses per year for 25 ground-water-quality management areas in Idaho, 1961–2001 .....

2.	 Long-term (decades) and short-term (selected years) nitrate trend summaries for 25 ground-water-quality 

management areas in Idaho, 1970–2000 .............................................................................................................

CONVERSION FACTORS 

Multiply By To obtain 

foot 0.3048 meter 

mile 1.609 kilometer 

Temperature in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) as follows: 

°C=(°F–32)/1.8 
Analysis of Nitrate (NO3–N) Concentration Trends 

25 Ground-Water-Quality Management Areas, Ida

1961–2001 

By D.J. Parliman 

Abstract 

In Idaho, drinking-water supplies are pumped 

from relatively shallow ground-water zones where 

water quality has great potential for degradation 

by land- and water-use activities. One indicator of 

water quality, and one of the most widespread contaminants in Idaho ground water related to land 

and water uses, is dissolved nitrate. 

In December 2000, the U.S. Geological Survey, in cooperation with the Idaho Department of 

Environmental Quality, began a study to compile 

and assess nitrate data for ground water in 25 

ground-water-quality management areas in Idaho. 

The primary objective of the study was to determine whether statistically significant trends in 

ground-water concentrations were discernible. Data 

included ground-water analyses but not analyses 

of water from springs, drains, or thermal water 

sources. A total of 8,465 nitrate analyses were 

compiled from 2,931 wells in the 25 priority areas; 

analyses dates ranged from June 1961 to February 

2001. 

A time-period comparison was used to assess 

general trends in nitrate concentrations within 

individual priority areas. A time-series comparison 

was used to assess trends in nitrate concentrations 

from selected wells in each priority area. 

In time-period assessments, general trends in 

an area were evaluated by compiling nitrate data 

for selected time intervals to determine whether 

the population distributions were significantly different between one period and another. Data within each priority area were sorted into decades— 

1970s, 1980s, and 1990s—for long-term trend 

assessment. The 1990s data also were divided into 

sets of selected years corresponding to Statewide 

Ambient Ground-Water Quality Program sam

cycles—1991 through 1994, 1995 through 

and a partial cycle, 1999 through 2000—for 

term trend assessment. Data were analyzed 

summary statistics, boxplots, and the Mann

Whitney statistical test. 

Long-term increasing trends in nitrate c

trations were evident for 6 of 25 priority are

long-term decreasing trends were evident fo

25 areas. Short-term increasing trends were e

for 7 of 25 areas, and a short-term decreasin

was evident for only 1 area. No long-term n

trends were evident for 7 of 25 areas, and no 

term trends were evident for 15 of 25 areas

were insufficient for long-term trend assessm

8 areas and for short-term assessment in 2 a

Time-series trend analyses were conduc

data from wells with 7 or more nitrate analy

and longest periods of record, at least 10 ye

between oldest and most recent analyses. B

long-term records were available for so few 

and well construction data were not availab

several of these wells, time-series analyses 

not helpful to the nitrate trend assessment s

Trend results may be strongly affected b

construction, hydrogeologic environments, 

changes in density and areal distribution of 

and analyses. The utility of nitrate trend ass

ments in priority areas would be improved b

consistent and specific well location descrip

between agencies; well construction and ma

water-yielding zone information to accompa

water-quality data for each well; and additio

historical nitrate data to data bases, particul

analyses prior to about 1990. Investigations 

possible effects of changing priority area bo

aries and time periods on both long-term and 

A
term trend assessments are needed. Addition of 

these kinds of information would allow assessment of trends associated with hydrology and 

geology of each area and would provide a much 

stronger basis for trend assessment than currently 

possible. 

INTRODUCTION 

In Idaho, ground water currently is the principal 

source of about 95 percent of public water supplies and 

most rural-domestic and livestock supplies (E. Hagan, 

Idaho Department of Water Resources, IDWR, oral 

commun., 2001). Many drinking-water supplies are 

pumped from relatively shallow zones where groundwater quality has great potential for degradation by landand water-use activities. One indicator of water quality, 

and one of the most widespread contaminants 1 in Idaho 

ground water related to land and water uses, is dissolved 

nitrate-nitrogen (NO3

–N) concentrations. 2 

The maximum U.S. Environmental Protection 

Agency (EPA, 2000, p. 9) limit for nitrate concentrations in public water supplies is 10 mg/L. Concentrations of nitrate in Idaho ground water prior to land and 

water development probably were less than 1 mg/L 

(Parliman, 2000, p. 1), but elevated nitrate concentrations in ground water have been reported for more than 

40 years (see Selected References). The median concentration (50th percentile) of nitrate in Idaho ground 

water from 1990 through 2000, for example, was about 

1.5 mg/L, but the range of concentrations was less than 

0.05 to 110 mg/L. 

Widespread elevated nitrate concentrations historically have been detected in some areas of Idaho more 

frequently than in other areas. In 1998, personnel with 

the Idaho Department of Environmental Quality (IDEQ) 

conducted a preliminary assessment of statewide nitrate 

concentrations and delineated 33 ground-water-quality 

management areas (nitrate priority areas) where more 

than 25 percent of nitrate concentrations exceeded 5 

mg/L (S. Short, IDEQ, written commun., 2000; fig. 1). 

In July 2000, IDEQ convened the Ground-Water 

Monitoring Technical Committee to discuss groundwater-quality management issues in Idaho. One goal of 

1Contaminants are components of ground-water quality that can limit the 

water’s suitability for use or can represent degradation of water quality. 2In this report, nitrate is reported as nitrogen, NO3

–N, in milligrams per 

liter (mg/L), equivalent to parts per million. 

the committee was to prioritize the 33 ground-w

quality management areas where nitrate has sig

cantly degraded ground-water quality; one com

of the prioritization process was identification of 

concentration trends within each area. In Decem

2000, the U.S. Geological Survey (USGS), in c

tion with IDEQ, began a study to compile and a

nitrate data for ground water in the 33 priority a

The primary objective of the study was to deter

whether statistically significant trends in nitrate 

centrations were discernible. On the basis of sta

trend analysis results, nitrate trends would be d

as increasing, decreasing, or neither increasing 

decreasing (no trend). Areas would be identified 

data were insufficient for statistical assessment. 

purpose of this report is to present findings of th

trend analyses. 

DATA COMPILATION 

The first task of the nitrate trends study wa

identify sources of electronically transferable n

data in Idaho and combine these data into one d

Data were compiled from data bases maintained 

following agencies: (1) Idaho Department of Ag

ture (IDAG)—ground-water-monitoring networ

a one-time analysis of water from wells at dairi

wide, (2) IDEQ—individual ground-water-qua

investigations statewide and the Drinking Water 

mation Management System (DWIMS) public w

supply data base, (3) USGS—individual groun

water-quality investigations statewide by Idaho 

Utah Districts, and (4) the USGS/IDWR Statew

Ambient Ground-Water Quality Monitoring Pro

(Statewide GW-QW Program).3 

Data included ground-water analyses but n

yses of water from springs, drains, or thermal w

(greater than 84.5°F) sources. Nitrite plus nitrat

nitrogen (NO2+NO3

–N) and nitrate-nitrogen (N

data4

 were moved into one Microsoft Excel 200

spreadsheet. Minimum required information for 

analysis included sample date and agency sourc

imum required location information for each w

3A description of this program is available at 

http://www.idwr.state.id.us/planpol/techserv/gwmon/statewide.ht

4USGS laboratory analyses are reported in units of nitrite p

as nitrogen (NO2+NO3

–N). Idaho State Laboratory analyses are r

units of nitrate as nitrogen (NO3

–N). Concentrations of nitrite in 

water generally are negligible. For purposes of this study, USGS 

analyses are comparable. 

2 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
117° EXPLANATION 

Nitrate priority area 

116° 

115° 

114° 

113° 

112° 

44° 

45° 

46° 

47° 

48° 

49° 

BOUNDARY 

BONNER 

KOOTENAI 

SHOSHONE 

BENEWAH 

LATAH 

CLEARWATER 

IDAHO 

NEZ 

PERCE 

LEWIS 

ADAMS 

VALLEY 

WASHINGTON 

PAYETTE

GEM BOISE 

LEMHI 

CUSTER 

CANYON 

OWYHEE 

ADA 

ELMORE 

CAMAS 

BLAINE 

GOODING 

LINCOLN 

JEROME MINIDOKA 

TWIN 

FALLS 

CASSIA 

POWER BANNOCK 

ONEIDA 

BUTTE 

BINGHAM 

CLARK 

JEFFERSON 

FREMONT 

TETON 

MADISON 

BONNEVILLE 

CARIBOU 

FRANKLIN 

BEAR 

LAKE 

Coeur 

d' Alene 

Lewiston 

BOISE 

Twin 

Falls 

Pocatello 

Idaho 

Falls 

43° 

42° 

Base from U.S. Geological Survey digital data, 0 25 50 75 100 MILESÄ

1:250,000, 1994; Albers Equal-Area projectionÄ

Standard parallels 43° 30', 47° 30', 0 25 50 75 100 KILOMETERSÄ and -114° 00', 41° 45'Ä

No false easting or false northingÄ

Figure 1. Locations of 33 priority ground-water-quality management areas in Idaho, July 2000. (Nitrate contamination areas from S. 

Short, Idaho Department of Environmental Quality, written commun., 2000) 

Data Compilation 3

117° EXPLANATION 

Area 

name 

14 Bliss 

15 Twin Falls 

16 Rupert 

17 Burley/Marsh Creek 

18 Pocatello 

19 Fort Hall 

20 Preston/Cache Valley 

21 Soda Springs/Bear Lake 

(upper Portneuf/Blackfoot River) 

22 Mud Lake 

23 Hibbard 

24 St. Anthony 

25 Ashton, Drummond, Teton 

River 

43° 

42° 

116° 

115° 

114° 

113° 

112° 

44°

45° 

46° 

47° 

48° 

49° 

BOUNDARY 

BONNER 

KOOTENAI 

SHOSHONE 

BENEWAH 

LATAH 

CLEARWATER 

IDAHO 

NEZ 

PERCE 

LEWIS 

ADAMS 

VALLEY 

WASHINGTON 

PA YETTE

GEM BOISE 

LEMHI 

CUSTER 

CANYON 

OWYHEE 

ADA 

ELMORE 

CAMAS 

BLAINE 

GOODING 

LINCOLN 

JEROME MINIDOKA 

TWIN 

FALLS 

CASSIA 

POWER BANNOCK 

ONEIDA 

BUTTE 

BINGHAM 

CLARK 

JEFFERSON 

FREMONT 

TETON 

MADISON 

BONNEVILLE 

CARIBOU 

FRANKLIN 

BEAR 

LAKE 

Coeur 

d' Alene 

Lewiston 

BOISE 

Twin 

Falls 

Pocatello 

Idaho 

Falls 

1 

2 

3 

7 

8 

9 

10 

11 

12 

4 

5 

6 

13 14 

15 16 

17 

18 

20 

19 

21 

22 

23 

25 24 

Nitrate priority area and area number 

22 

Genesee/Cow Creek 

Lapwai Creek

Camas Prairie 

Weiser 

Payette 

Lower Boise River Basin, 

Canyon County 

Eagle/Star 

Homedale/Marsing 

Meridian 

Grand View 

Bruneau 

Mountain Home 

Hammett 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Area 

number 

Area 

name 

Area 

number 

0 25 50 75 100 MILES 

0 25 50 75 100 KILOMETERS 

Figure 2. Locations and identification of 25 ground-water-quality management areas in Idaho selected for nitrate trend analyses, 

February 2001. 

4 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
included latitude-longitude, township-range-section, 

and county. Total well depth and identifying name were 

requested for each well, but this information was not 

available for most wells. 

Because shapes of the 33 priority areas were based 

on approximate boundaries of areas where nitrate concentrations were relatively large, data for wells in and 

near (within approximately 1 mile) each priority area 

were used for trend analyses. Adding approximately 

1 mile to some boundaries, however, caused several 

priority areas to overlap. When priority areas with overlapping boundaries were located in one major drainage 

basin and hydrogeologic characteristics of the areas 

were similar, the multiple areas were combined. The 

original 33 nitrate priority areas thereby were reduced 

to 25 areas for trend analyses. Locations and names for 

the final 25 priority areas are shown in figure 2. 

Reporting formats for remaining data were standardized—latitude and longitude to North American 

Datum of 1927 (NAD27) and decimal degrees; township-range-section to uniform columns, spaces, and 

characters; county names to numbers; and sample dates 

to yyyymmdd numeric format. Priority area numbers, 

shown in figure 2, were assigned to each datum, and 

the data were sorted by priority area. When identified, 

duplicate data (identical data from two or more agency 

sources) and replicate data (multiple analyses of water 

from a single well on one sample date) were removed 

from the data sets. Zeros and “less-than” concentrations (concentrations reported as less than a minimum 

laboratory reporting limit) were important components 

of the data sets, indicating that, although nitrate concentrations were extremely small, laboratory analyses 

had been completed for those ground-water samples. 

A number of approaches are available for use of 

less-than concentrations in data analyses (Helsel and 

Hirsch, 1995, p. 359–376). In this study, less-than 

qualifiers were removed, and 0.001 mg/L was subtracted from the reported nitrate concentration. A concentration of <0.05 mg/L, for example, was replaced in 

the data set by 0.049 mg/L. For purposes of this study, 

use of this approach and assignment of very small concentrations to less-than data should have little effect on 

the overall outcome of the trend analyses. 

A total of 8,465 nitrate analyses were compiled 

from 2,931 wells in the 25 priority areas: dates for 

nitrate data ranged from June 1961 (USGS data) to 

February 2001 (DWIMS data). General characteristics 

of the data for each of the 25 priority areas are presented in table 1 and figures 3–6. 

The number of analyses by year and priori

are shown in table 1. Prior to about 1990, numb

analyses and periods of time when data were co

were not evenly distributed for individual areas 

throughout all the areas. Data generally were th

of relatively small-scale ground-water studies b

vidual agencies. After about 1990 and the begin

the Statewide GW-QW Program, data were mo

formly collected from all parts of the State each 

The number of analyses in relation to the n

of wells in each priority area is shown in figure 

Areas 6, 9, 15, 16, and 17 have a relatively larg

ber of wells with analyses, and many of these w

have multiple water analyses. In most other are

analyses are available and few multiple analyse

available for individual wells. The ranges of nit

concentrations are shown in figure 4, and mean 

age) versus median (50th percentile) values for 

priority area are shown in figure 5. Nitrate conc

tions exceed 10 mg/L in nearly all areas. Large 

ences between mean and median concentration

area often indicate the presence of anomalously 

nitrate concentrations in the data set. Nitrate con

tions in relation to total well depth for selected w

all priority areas are shown in figure 6. For well

total depth information, all nitrate concentration

greater than 10 mg/L were from wells less than 

400 feet total depth. 

TREND ANALYSES 

Two principal statistical approaches were a

to the compiled data set. A time-period compar

was used to assess general trends in nitrate conc

tions within individual priority areas. A time-se

comparison was used to assess trends in nitrate 

trations from selected wells in each priority are

Time-Period Trends by Decades or 

Selected Years 

In time-period assessments, general trends 

area were evaluated by compiling nitrate data fo

selected time intervals to determine whether the 

lation distributions were significantly different b

one period and another. Data within each priori

were sorted into decades—1970s (1/1/70–12/3

1980s (1/1/80–12/31/89), and 1990s (1/1/90– 

Trend An
6 

Table 1. Number of nitrate analyses per year for 25 ground-water-quality management areas in Idaho, 1961–2001 

Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 Ground-water-quality management area (locations and names shown in figure 2) 

Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 

1961 . . . . . . . . . . . . . 1 1964 . . . . . . . . . . . . . 1968 . . . . . . . . . . . . . 1 1970 . . . . . . . . . . . . . 14 7 1972 . . . . . . . . . . . . . 1 1 9 1973 . . . . . . . . . . . . . 6 4 5 44 1974 . . . . . . . . . . . . . 3 2 1975 . . . . . . . . . . . . . 1 3 17 1 5 1 4 1976 . . . . . . . . . . . . . 7 6 12 2 1977 . . . . . . . . . . . . . 5 6 1 1 1978 . . . . . . . . . . . . . 1 18 8 1 5 1979 . . . . . . . . . . . . . 17 1 10 4 4 1 1980 . . . . . . . . . . . . . 19 11 7 9 13 12 1 3 1981 . . . . . . . . . . . . . 26 1 17 2 10 4 1982 . . . . . . . . . . . . . 8 15 8 1 1983 . . . . . . . . . . . . . 4 16 1984 . . . . . . . . . . . . . 1 1 3 18 1985 . . . . . . . . . . . . . 17 19 1986 . . . . . . . . . . . . . 1 12 17 15 1987 . . . . . . . . . . . . . 1 1 5 2 24 84 21 1988 . . . . . . . . . . . . . 1 1 14 20 17 22 1 1989 . . . . . . . . . . . . . 1 2 19 1 24 50 1990 . . . . . . . . . . . . . 4 1 11 9 1 13 1 4 4 2 5 21 1 1991 . . . . . . . . . . . . . 1 3 3 2 33 2 1 5 1 2 4 2 1 14 7 8 28 5 2 1992 . . . . . . . . . . . . . 1 1 4 2 21 40 1 19 2 1 1 20 8 9 9 1 4 1993 . . . . . . . . . . . . . 1 4 14 7 16 105 2 6 150 3 3 23 3 30 63 33 46 1 4 1994 . . . . . . . . . . . . . 2 13 11 6 121 2 8 295 3 4 27 4 57 22 31 40 7 1995 . . . . . . . . . . . . . 1 3 11 18 17 209 12 5 208 4 4 31 6 1 61 26 34 43 4 8 1996 . . . . . . . . . . . . . 1 4 10 47 19 291 10 106 3 5 33 4 1 55 40 32 47 4 9 1997 . . . . . . . . . . . . . 2 4 12 44 25 236 12 12 116 6 6 35 5 109 104 68 47 1 10 1998 . . . . . . . . . . . . . 1 4 68 42 26 146 2 11 102 14 8 43 4 144 104 83 49 2 15 1999 . . . . . . . . . . . . . 1 5 13 44 46 154 2 49 104 5 5 33 5 3 192 111 99 50 6 17 2000 . . . . . . . . . . . . . 2 4 13 50 28 163 2 62 81 6 5 48 3 175 120 90 50 4 53 2001 . . . . . . . . . . . . . 1 11 36 5 5 2 2 6 5 8 1 Number of 

analyses . . . . . . . . 11 31 165 281 230 1,670 38 173 1,270 57 63 331 52 6 901 709 666 564 136 134 

Number of wells. . . . 3 6 88 75 93 613 24 78 328 26 28 98 20 3 345 254 267 78 61 70 Total number of 

analyses . . . . . . . . 8,465 

Total number 

of wells . . . . . . . . . 2,931 
1,700 

1,600 

1,500 

1,400 

1,300 

1,200 

1,100 

1,000 

900 

800 

700 

600 

500 

400 

300 

200 

100 

0 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2

Number of analyses 

Number of wells 

AREA NUMBER 

Figure 3. Number of nitrate analyses in relation to number of wells in 25 ground-water-quality management areas in Idaho, 

1961–2001. NUMBER OF ANALYSES 

Trend An
8 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001

115

110

105

100

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

1 3 4 6 7 8 10 11 12 3 14 15 16 17 18 19 20 21 22 23 24 25

AREA NUMBER

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

Figure 4. Ranges of nitrate concentrations for 25 ground-water-quality management areas in Idaho, 1961–2001. Figure 4. Ranges of nitrate concentrations for 25 ground-water-quality management areas in Idaho, 1961–2001.

2 5 9 1
management areas in Idaho, 1961–2001. 

10 

9


8


7


6


5


4


3


2


1


0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25


Mean 

Median 

AREA NUMBER 

Figure 5. Mean (average) nitrate concentrations in relation to median (50th percentile) concentrations for 25 ground-water-qu

management areas in Idaho, 1961–2001. DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Trend An
10 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961

–2001 

2,200 2,000 1,800 1,600 1,400 1,200 1,000 

800 600 400 200 

0 

0 5 10 15 20 25 30 35 

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

TOTAL WELL DEPTH, IN FEET 

Figure 6. Nitrate concentrations in relation to total well depth, selected wells in ground-water-quality management areas in Idaho, 1961–2001. 
12/31/99)—for long-term trend assessment. Data from 

the 1960s were not used in trend analyses because 

there were too few data (a total of four analyses) for 

trend comparisons. 

The 1990s data also were divided into sets of 

selected years corresponding to Statewide GW-QW 

Program sampling cycles—1991 through 1994, 1995 

through 1998, and a partial cycle, 1999 through 2000, 

for short-term trend assessment. During 1991, the first 

year of the Statewide Program’s initial 4-year sampling 

cycle, water samples were collected for laboratory 

analyses from approximately 400 wells throughout 

Idaho. Each year of the 4-year sampling cycle, samples 

from 400 additional wells were analyzed and, by the 

final year of the sampling cycle,1994, water samples 

from more than 1,600 wells had been analyzed. Wells 

originally included in the 1991 sampling were sampled 

again in 1995, and by December 2000, 2 1/2 sampling 

cycles had been completed. Data from these sampling 

cycles were useful for short-term nitrate trend assessment because, although only 2 1/2 sampling cycles had 

been completed, analyses were from a consistent and 

large number of wells for each cycle. The sampling 

cycles also provided a periodic, statewide overview of 

water-quality conditions in most major water-yielding 

zones and from wells with various well construction 

conditions and water uses. 

In both decade and selected year (sampling cycle) 

data assessments, only one nitrate analysis per well per 

year was used. For wells with multiple analyses per 

year, selection of one analysis per year primarily was 

based first on time of year, then on uniformity of sample dates. Samples from June through September, the 

general period of time when Statewide GW-QW Program samples are collected, were chosen over samples 

from other times of the year. If June-September samples were not available, samples from the same season 

of the year or month each year were chosen for individual wells. 

Several methods were used to analyze and compare nitrate data by these decades and sampling cycles. 

Microsoft Excel 2000 was used to calculate summary 

statistics (number of analyses, median or 50th percentile, mean or average, and range of data) for wells with 

four or more analyses and to prepare comparative compilations or plots of data (table 1 and figures 3–6) for 

analyses in the nitrate trend data set. SPSS Inc. SYSTAT Version 9 was used to create boxplot diagrams and 

perform Mann-Whitney statistical tests for analyses in 

the nitrate trend data set. 

A boxplot diagram (see half-title page for 

dix A) summarizes the distribution of a data set 

useful for comparing several related data sets, s

nitrate data by decades or selected ranges of yea

boxplot “box” shows the upper quartile (75th p

tile), median (50th percentile), and lower quartil

percentile) of data. One and one-half (1.5) time

height of the box (75th percentile to 25th percen

defined as a “step.” Lines extending above and 

the box (“whiskers”) are 1 step away from the b

Blue circles on the diagram represent values bet

and 2 steps above or below the box, and orange 

represent values more than 2 steps above or bel

box. 

The Mann-Whitney test is a nonparametric 

sum test (Helsel and Hirsch, 1995, p. 118) for w

one population set tends to produce larger or sm

observations than another set. This nonparamet

is an alternative to the parametric two-sample t 

(Ott, 1988, p. 183) and has less stringent assum

of normality and equal variances of the two sets 

compared.5 No assumptions are needed in this t

cerning the distribution of data in either set. Re

the Mann-Whitney test are expressed as p-value

significance level attained by the data. For grou

nitrate data, p-values less than or equal to 0.050 

cate there is a 95-percent-or-greater confidence 

data from the two decades or selected years com

are different. When statistical comparison meth

indicated that two sets of data were different, su

statistics and boxplot comparisons were used to 

mine increasing or decreasing trends in the nitra

Boxplots, summary statistics, and Mann-W

test values for each priority area are shown in ap

A. A summary of Mann-Whitney p-values for c

sons of data by decade and selected years is pre

in table 2 and indicates increasing, decreasing, 

trend (p-values greater than 0.050) assessments 

data. Data for areas 1 and 14 were insufficient (

than four analyses per decade and selected year

either long- or short-term trend assessments. No 

existed from the 1970s and 1980s or were insuf

5Helsel and Hirsch (1995, p. 118) state that if the only inter

data is to determine whether one group tends to produce higher 

observations—the objective of this analysis—the two groups do n

have the same distribution. Application of the Mann-Whitney test 

specific purposes, such as determining whether two groups come f

same population, assumes that the shapes of the distributions are t

In fact, the shapes of the distributions of data in groups used in th

are generally similar (figs. 4 and 5 and boxplots in appendix A), d

ing nonnormality (distribution skewed to the right) and extreme o

Trend An
1

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12 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 Table 2. Long-term (decades) and short-term (selected years) nitrate trend summaries for 25 ground-water-quality management areas in Idaho, 1970–2

[Mann-Whitney test p-value for decade (1970, 1980, 1990) or selected years (1991–94, 1995–98, 1999–2000); 2 groups compared, 4 samples or more each group; p-valevel: “+” increasing trend; “-” = decreasing trend; x = insufficient data] 

1970 1970 1970 1970 1970 1980 1980 1980 1980 Long-term 

through through through through through through through through through trend 

Area Name 1980 1990 1991 – 94 1995–98 1999 – 2000 1990 1991 – 94 1995 – 98 1999 – 2000 (decades) 

Genesee/Cow Creek. . . . . . . . x Lapwai Creek. . . . . . . . . . . . . x 

Camas Prairie. . . . . . . . . . . . . x 

eiser . . . . . . . . . . . . . . . . . . . . 0.002+ 0.619+ 0.003+ 0.003+ increasing 

ayette . . . . . . . . . . . . . . . . . . 0.352 .555 .609 .480 .717 0.266 0.493 0.206 0.305 no trend 

ower Boise-Canyon . . . . . . . .162 .757 .267 .839 .702 .161 .512 .071 .219 no trend 

Eagle/Star. . . . . . . . . . . . . . . . x Homedale/Marsing . . . . . . . . x 

Meridian. . . . . . . . . . . . . . . . . .352 .359 .925 .310 .007- .119 .415 .086 .001- decreasing 

Grand 

View . . . . . . . . . . . . . . .239 .019+ .803 .010+ .002+ .084 .847 .048+ .016+ increasing 

Bruneau . . . . . . . . . . . . . . . . . .561 .492 .955 .439 .164 .614 .643 .462 .129 no trend Mountain Home . . . . . . . . . . . .211 .686 .762 .627 .769 .005- .030- .007- .012- decreasing 

Hammett . . . . . . . . . . . . . . . . .000+ .029+ .001+ .004+ increasing 

Bliss . . . . . . . . . . . . . . . . . . . . x 

Twin Falls . . . . . . . . . . . . . . . .032+ .000+ .001+ .000+ .000+ .010+ .050+ .032+ .001+ increasing 

Rupert. . . . . . . . . . . . . . . . . . .004+ .923 .808 .936 .678 .000- .006- .000- .002- decreasing 

Burley/Marsh Creek . . . . . . . .000+ .002+ .008+ .003+ .001+ .000- .000- .003- .017- decreasing 

Pocatello . . . . . . . . . . . . . . . . .433 .058 .138 .060 .029+ .031+ .174 .067 .029+ increasing 

Fort Hall. . . . . . . . . . . . . . . . . .398 .480 .214 .762 no trend Preston/Cache 

Valley . . . . . . . x 

Soda Springs/Bear Lake . . . . .849 .315 .234 .455 .580 .397 .369 .450 .511 no trend 

Mud Lake. . . . . . . . . . . . . . . . .522 .403 .877 .446 .456 .514 1.000 .575 .456 no trend 

Hibbard . . . . . . . . . . . . . . . . . x 

St. Anthony . . . . . . . . . . . . . . .477 .131 .268 .447 no trend 

Ashton-Teton River . . . . . . . . .214 .000+ .045+ .000+ .001+ .270 .707 .195 .468+ increasing WPL
for long-term trend assessment for areas 2, 3, 7, 8, 20, 

and 23. In priority areas with a mix of increasing and 

no trend, decreasing and no trend, or increasing/decreasing and no trend indications, the overall trend was 

based on assessments from longest periods of time— 

1970s or 1980s to 1999–2000, for example. Short-term 

assessments were made for data between only 1991– 

94 and 1995–98 or 1999–2000. No assessments were 

made for data between 1995–98 and 1999–2000 because there were relatively few years between the two 

sample groups and few sites with multiple analyses 

during this time. 

Long-term increasing trends in nitrate concentrations were evident for areas 4, 10, 13, 15, 18, and 25. 

Short-term increasing trends during either 1991–94 to 

1995–98 or during 1991–94 to 1999–2000 were evident for areas 3, 4, 6, 10, 13, 15, and 17, but short-term 

increasing trends in both sets of selected years were 

evident only for areas 4 and 10. 

Long-term decreasing trends in nitrate concentrations were evident for areas 9, 12, 16, and 17. A shortterm decreasing trend was evident only for area 9 and 

for both sets of selected years. 

No long-term nitrate trend was evident for areas 5, 

6, 11, 19, 21, 22, and 24. No short-term trends were 

evident for areas 2, 5, 7, 8, 11, 12, 16, and 18 through 

25 for either set of selected years. 

Time-Series Trends for Data From 

Individual Wells 

Time-series trend analyses were conducted on 

data from 24 wells with 7 or more nitrate analyses and 

longest periods of record, at least 10 years between 

oldest and most recent analyses. The Mann-Kendall 

test statistic was used to assess whether nitrate concentrations tended to increase or decrease with time (Helsel and Hirsch, 1995, p. 326) at individual wells. The 

statistic was calculated for each data set by using an 

Excel spreadsheet. Results of the Mann-Kendall test 

are expressed as p-values, the significance level 

attained by the data. A p-value less than or equal to 

0.050 indicated a 95-percent-or-greater confidence that 

there was a change in nitrate concentration over time at 

an individual well. 

Plots of nitrate concentration in relation to time 

were used to determine increasing, decreasing, or no 

trend for data from a particular well. Plots of nitrate 

concentration, p-values, and trend determination; and 

well location, priority area, county, source of ni

data, and total well depth for the 24 wells are sh

appendix B. 

Long-term increasing trends were evident 

selected wells in priority areas 5, 9, 17, 18, and 

Decreasing trends were evident for selected we

area 9, and no trend was evident for selected we

areas 6, 9, 12, 16, 17, 18, and 21. Time-series tre

water analyses from individual wells reflect hyd

logic conditions specific to the location and we

struction. Data from a well may show increasin

decreasing, or no trend in nitrate concentrations 

time, but the trend for that well may or may not 

resentative of general area water-quality conditi

Because long-term records were available for so 

wells (fewer than 1 percent of 2,931 wells) and 

construction data were available for fewer than 

these wells, time-series analyses were of interes

not helpful to the current nitrate trend study. As 

periods of record and well construction informa

become available for increasing numbers of we

the priority areas, time-series analyses may bec

more valuable tool for assessment of nitrate tren

ground-water-quality management areas. 

ADDITIONAL INFORMATION NEEDE

To improve the utility of nitrate trend asses

in priority areas, additional well location and co

tion information is needed. Some duplicate data 

have been included in the data set for this study, 

inventory information for wells often was not co

ble from agency to agency. For example, wells 

located to 1/4-

1/4-1/4 section in some data bases but 

section in others, and total depth information wa

able for only a few wells. Without more specific 

descriptions or well depth information, decision

not be made on whether the wells were the sam

methods for determining latitude-longitude loca

for wells have changed over time, and reported 

tions for wells in different data bases were incon

from agency to agency. 

Well construction information (including t

depth, completion date, and screen/perforations

borehole data) and major water-yielding zone in

tion to accompany the water-quality data are ne

Addition of these kinds of information would a

assessment of trends associated with hydrology 

geology of each area and would provide a much 

Additional Information N
ger basis for trend assessment than analyzing values 

distributed over a map area. 

Addition of historical nitrate data, particularly 

analyses prior to about 1990, to data bases is needed. 

Public water-supply analyses from the 1950s, 1960s, 

1970s, and 1980s, for example, may be available but 

have not been entered into data bases. Addition of these 

types of data would be invaluable to both long-term 

nitrate trend assessment and time-trend assessments 

within priority areas. 

Investigation of possible effects of increasing or 

decreasing the size of priority area boundaries on 

nitrate trend assessment results is needed. Trend results 

may be strongly affected by changes in numbers of 

wells, numbers of analyses, density and areal distribution of wells and analyses, well construction, and 

hydrogeologic environments represented by wells and 

water samples. 

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14 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
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water near Mountain Home, Elmore County, 

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Fork and Mud Lake areas (hydrologic dat

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office of the U.S. Geological Survey, 1 pl

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2000, City of Ashton—water supply syste

nitrate mitigation study: Jackson, Wyo., u

lished, 42 p. 

Kilburn, Chabot, 1964, Ground water in the upp

of the Teton Valley, Teton Counties, Idaho 

Wyoming: U.S. Geological Survey Water-

Paper 1789, 60 p., 4 pls. 

Littleton, R.T., and Crosthwaite, E.G., 1957 [19

Ground water geology of the Bruneau-Gran

area, Owyhee County, Idaho: U.S. Geolog

Survey Water-Supply Paper 1460–D, p. 1

198. 

Mahler, R.L., Porter, Ernestine, and Taylor, Roy

Quality water for Idaho—nitrate and grou

ter: Moscow, University of Idaho, College 

Agriculture, Current Information Series n

2 p. 

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farms in Gooding, Jerome, and Twin Falls 

ties, Idaho, 1990–93: U.S. Geological Su

Fact Sheet FS–111–96, 2 p. 

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study of levels in southeast Idaho: Idaho F

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48 p., 2 apps. 

Meisler, Harold, 1958, Preliminary report on gr

water in the Bonanza Lake area, Power an

Blaine Counties, Idaho: U.S. Geological S

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Selected Refer
Moffatt, R.L., and Jones, M.L., 1984, Availability and 

chemistry of ground water on the Bruneau Plateau and adjacent eastern plain in Twin Falls 

County, south-central Idaho: U.S. Geological 

Survey Water-Resources Investigations Report 

84–4065, 43 p. 

Mundorff, M.J., 1967, Ground water in the vicinity of 

American Falls Reservoir, Idaho: U.S. Geological Survey Water-Supply Paper 1846, 58 p. 

Nace, R.L., West, S.W., and Mower, R.W., 1957, Feasibility of ground-water features of the alternate 

plan for the Mountain Home project, Idaho: U.S. 

Geological Survey Water-Supply Paper 1376, 

121 p. 

Neely, K., 2001a, Current status of the ground water 

quality in the Treasure Valley, July 2001: Idaho 

Department of Water Resources, accessed December 26, 2001, at URL http://www.idwr.state. 

id.us/planpol/techserv/gwmon/statewide.htm 

——— 2001b, Ground water quality in the Twin Falls 

hydrogeologic subarea, 1991–2000: Idaho 

Department of Water Resources, Water Information Bulletin no. 50, Part 4, 69 p. 

——— 2001c, Nitrate results from the statewide program, 1991–2000: Idaho Department of Water 

Resources Technical Summary, 28 p. 

Neely, K.W., and Crockett, J.K., 1998, Ground water 

quality characterization and initial trend analyses 

for the Treasure Valley shallow and deep hydrogeologic subareas: Idaho Department of Water 

Resources, Water Information Bulletin no. 50, 

Part 3, 76 p., 5 apps. 

——— 1999, Nitrate in Idaho’s ground water: Idaho 

Department of Water Resources, Technical 

Results Summary no. 1, 12 p. 

Ott, Lyman, 1988, An introduction to statistical methods and data analysis, 3d ed.: Boston, PWS-Kent 

Publishing Company, 835 p., 1 app. 

Parliman, D.J., 1982, Compilation of ground-water 

quality data for selected wells in Elmore, Owyhee, Ada, and Canyon Counties, Idaho, 1945 

through 1982: U.S. Geological Survey Open-File 

Report 83–39, 152 p. 

——— 1983a, Ground-water quality in the western 

Snake River Basin, Swan Falls to Glenns Ferry, 

Idaho: U.S. Geological Survey Water-Resources 

Investigations Report 83–4062, 85 p. 

——— 1983b, Reconnaissance of ground-water quality, eastern Snake River Basin, Idaho: U.S. Geological Survey Water-Resources Investiga

Report 82–4004, 100 p. 

——— 1986a, Compilation of well and ground

quality data, Groveland-Collins and surro

areas near Blackfoot, Bingham County, Id

U.S. Geological Survey Open-File Report 

127, 113 p. 

——— 1986b, Quality of ground water in the P

River Basin, Idaho: U.S. Geological Surv

Water-Resources Investigations Report 86

85 p. 

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Water Summary 1986—Ground-water qu

U.S. Geological Survey Water-Supply Pap

2325, p. 229–236. 

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the Henrys Fork Basin, eastern Idaho: U.S

logical Survey Fact Sheet FS–029–00, 6 

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Selected well and ground-water chemistry 

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water quality in northern Ada County, low

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quality data for the Murtaugh Lake area, s

central Idaho, June 1987: U.S. Geological 

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——— 1989a, Selected ground-water quality d

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tion, southeastern Idaho, June 1989: U.S. 

logical Survey Open-File Report 89–594, 

1 sheet, scale 1:100,000. 

——— 1989b, Selected water-quality data for t

Hall Indian Reservation, southeastern Idah

1988: U.S. Geological Survey Open-File 

88–496, 1 sheet. 

——— 1990, Hydrologic and chemical data fro

selected wells and springs in southern Elm

County, including Mountain Home Air Fo

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logical Survey Open-File Report 90–112, 

Ralston, D.R., and Chapman, S.L., 1968, Groun

water resource of the Mountain Home are

Elmore County, Idaho: Idaho Department 

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16 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
——— 1969, Ground-water resource of northern 

Owyhee County, Idaho: Idaho Department of 

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organic compounds in ground water of the upper 

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–N) in ground water 

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673, 34 p. 

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quality in Bannock, Bear Lake, Caribou, and part 

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Spinazola, J.M., Tungate, A.M., and Rogers, T.L., 

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Preliminary report on ground water in the 

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water standards and health advisories: U.S

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disposal wells, eastern Snake River Plain, 

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water in Idaho: U.S. Geological Survey W

Supply Paper 2272, 53 p. 

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77–108, 40 p. 

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ground-water quality data for the southern 

of the Fort Hall Indian Reservation, south

Idaho, July 1989: U.S. Geological Survey 

File Report 89–593, 1 sheet, scale 1:1,00

Young, H.W., Parliman, D.J., and Jones, M.L., 

Selected water-quality data for the Burley 

tion District, south-central Idaho, March-A

1987: U.S. Geological Survey Open-File 

87–240, 1 sheet. 

——— 1992, Seasonal changes in ground-wate

ity and ground-water levels and directions 

Selected Refer
ground-water movement in southern Elmore Young, H.W., Parliman, D.J., and O’Dell, I., 19

County, southwestern Idaho, including Mountain Selected water-quality data for the Minido

Home Air Force Base, 1990–91: U.S. Geological Irrigation District, south-central Idaho, Ju

Survey Water-Resources Investigations Report 1987: U.S. Geological Survey Open-File 

92–4027, 2 sheets, scale 1:1,000,000. 87–465, 1 sheet. 

18 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Appendix A. Boxplots, summary statistics, and Mann-Whitney test p-values for 

ground-water-quality management areas in Idaho, 2001 

EXPLANATION 

Interquartile range (IQR) 

(step) 

More than 2 steps above or 

below box 

1 to 2 steps above or below 

box 

75th percentile plus 1.5 

times the IQR 

Upper quartile (75th percentile) 

Median (50th percentile) 

Lower quartile (25th percentile) 

25th percentile minus 1.5 

times the IQR 

Area 1 Genesee/Cow Creek 

8 

7 

6 

3.5 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

No 

data 

No 

data 

Insufficient 

data 

Insufficient 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 11 0.21 1.1 7.1 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 3 .20 1.1 3.1 

1995–98 (1/1/95 to 12/31/98) . . . . . 5 .23 1.6 7.1 

1999–2000 (1/1/99 to 12/31/00) . . . . . 3 .21 .16 .21 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995–98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 

1991–94 to 1999–2000 . . . . . . . 

Appe
Area 2 Lapwai Creek 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

No 

data 

No 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 27 3.3 4.4 12 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 7 2.6 3.1 8.4 

1995–98 (1/1/95 to 12/31/98) . . . . . 15 3.3 4.6 11 

1999–2000 (1/1/99 to 12/31/00) . . . . . 9 3.9 5.3 15 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.245 

1991–94 to 1999–2000 . . . . . . . .491 

22 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 3 Camas Prairie DISSOLVED NITRATE AS NITROGEN, 

90 

IN MILLIGRAMS PER LITER 

80 

70 

60 

50 

40 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

No 

data 

No 

data 

DECADES RANGE OF YEARS 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 152 0.93 5.1 80 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 34 .12 1.6 8.5 

1995–98 (1/1/95 to 12/31/98) . . . . . 101 2.8 6.7 80 

1999–2000 (1/1/99 to 12/31/00) . . . . . 26 .17 2.9 22 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991–94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.005 

1991–94 to 1999–2000 . . . . . . . .702 

Appe
Area 4 Weiser 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

No 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 11 2.6 3.0 6.3 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 219 7.6 7.9 30 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 23 3.6 4.8 18 

1995–98 (1/1/95 to 12/31/98) . . . . . 151 7.9 8.1 25 

1999–2000 (1/1/99 to 12/31/00) . . . . . 94 9.1 8.9 30 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 0.002 1980 to 1991–94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . .619 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . .003 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . .003 1991–94 to 1995–98 . . . . . . . . . 0.006 

1991–94 to 1999–2000 . . . . . . . .005 

24 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 5 Payette 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 4 2.6 2.6 4.8 

1980 (1/1/80 to 12/31/89) . . . . . 9 .24 3.5 16 

1990 (1/1/90 to 12/31/99) . . . . . 178 3.7 4.7 23 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 45 3.9 4.6 23 

1995–98 (1/1/95 to 12/31/98). . . . . 87 3.4 4.4 12 

1999–2000 (1/1/99 to 12/31/00). . . . . 74 3.3 5.2 19 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.352 1980 to 1990 . . . . . . . . . . . . . 0.266 

1970 to 1990 . . . . . . . . . . . . .555 1980 to 1991–94. . . . . . . . . . .493 

1970 to 1991–94 . . . . . . . . . .609 1980 to 1995–98. . . . . . . . . . .206 

1970 to 1995–98 . . . . . . . . . .480 1980 to 1999–2000. . . . . . . . .305 

1970 to 1999–2000 . . . . . . . .717	 1991–94 to 1995–98 . . . . . . . . . .548 

1991–94 to 1999–2000 . . . . . . . .661 

Appe
Area 6 Lower Boise River Basin, Canyon County 

40 

35 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 78 2.7 3.6 19 

1980 (1/1/80 to 12/31/89) . . . . . 47 2.1 2.5 8.2 

1990 (1/1/90 to 12/31/99) . . . . . 1,346 2.5 3.5 33 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 299 2.2 3.0 22 

1995–98 (1/1/95 to 12/31/98). . . . . 882 2.7 3.8 33 

1999–2000 (1/1/99 to 12/31/00). . . . . 317 2.4 3.4 21 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.162 1980 to 1990 . . . . . . . . . . . . . 0.161 

1970 to 1990 . . . . . . . . . . . . .757 1980 to 1991–94. . . . . . . . . . .512 

1970 to 1991–94 . . . . . . . . . .267 1980 to 1995–98. . . . . . . . . . .071 

1970 to 1995–98 . . . . . . . . . .839 1980 to 1999–2000. . . . . . . . .219 

1970 to 1999–2000 . . . . . . . .702	 1991–94 to 1995–98 . . . . . . . . . .011 

1991–94 to 1999–2000 . . . . . . . .272 

26 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 7 Eagle/Star 

50 

40 

30 

20 

10 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Insufficient 

data 

Insufficient 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 1 

1980 (1/1/80 to 12/31/89) . . . . . 1 

1990 (1/1/90 to 12/31/99) . . . . . 34 4.0 8.0 45 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 6 5.3 5.4 11 

1995–98 (1/1/95 to 12/31/98) . . . . . 26 2.5 8.7 45 

1999–2000 (1/1/99 to 12/31/00) . . . . . 4 6.5 7.2 16 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.717 

1991–94 to 1999–2000 . . . . . . . .670 

Appe
Area 8 Homedale/Marsing 

40 

30 

20 

10 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Insufficient 

data 

Insufficient 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 1 

1980 (1/1/80 to 12/31/89) . . . . . 2 

1990 (1/1/90 to 12/31/99) . . . . . 103 0.25 4.4 39 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 16 2.3 6.8 39 

1995–98 (1/1/95 to 12/31/98) . . . . . 38 mrl* 3.2 38 

1999–2000 (1/1/99 to 12/31/00) . . . . . 111 .20 4.6 34 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.289 

1991–94 to 1999–2000 . . . . . . . .131 

* mrl, minimum reporting limit 

28 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 9 Meridian 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 36 2.0 2.8 13 

1980 (1/1/80 to 12/31/89) . . . . . 33 2.6 2.8 8.3 

1990 (1/1/90 to 12/31/99) . . . . . 1,114 2.0 2.3 18 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 469 2.3 2.5 11 

1995–98 (1/1/95 to 12/31/98). . . . . 532 1.9 2.3 18 

1999–2000 (1/1/99 to 12/31/00). . . . . 185 1.4 1.7 9.3 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.352 1980 to 1990 . . . . . . . . . . . . . 0.119 

1970 to 1990 . . . . . . . . . . . . .359 1980 to 1991–94. . . . . . . . . . .415 

1970 to 1991–94 . . . . . . . . . .925 1980 to 1995–98. . . . . . . . . . .086 

1970 to 1995–98 . . . . . . . . . .310 1980 to 1999–2000. . . . . . . . .001 

1970 to 1999–2000 . . . . . . . .007	 1991–94 to 1995–98 . . . . . . . . . .032 

1991–94 to 1999–2000 . . . . . . . .000 

Appe
Area 10 Grand View 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 5 mrl* 0.8 3.6 

1980 (1/1/80 to 12/31/89) . . . . . 7 0.16 4.2 24 

1990 (1/1/90 to 12/31/99) . . . . . 39 6.8 6.8 24 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 7 3.1 3.1 6.8 

1995–98 (1/1/95 to 12/31/98). . . . . 27 7.2 7.1 24 

1999–2000 (1/1/99 to 12/31/00). . . . . 11 6.8 10 26 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.239 1980 to 1990 . . . . . . . . . . . . . 0.084 

1970 to 1990 . . . . . . . . . . . . .019 1980 to 1991–94. . . . . . . . . . .847 

1970 to 1991–94 . . . . . . . . . .803 1980 to 1995–98. . . . . . . . . . .048 

1970 to 1995–98 . . . . . . . . . .010 1980 to 1999–2000. . . . . . . . .016 

1970 to 1999–2000 . . . . . . . .002	 1991–94 to 1995–98 . . . . . . . . . .031 

1991–94 to 1999–2000 . . . . . . . .021 

*mrl, minimum reporting limit 

30 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 11 Bruneau DISSOLVED NITRATE AS NITROGEN, 

120 

IN MILLIGRAMS PER LITER 

100 

80 

60 

40 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Nitrate concentrations, milligrams per liter Number of 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 5 0.17 1.2 5.3 

1980 (1/1/80 to 12/31/89) . . . . . 11 mrl* 1.2 9 

1990 (1/1/90 to 12/31/99) . . . . . 40 .10 10 110 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 11 .20 18 110 

1995–98 (1/1/95 to 12/31/98). . . . . 23 mrl

* 7.8 67 

1999–2000 (1/1/99 to 12/31/00). . . . . 10 0 4.4 27 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.561 1980 to 1990 . . . . . . . . . . . . . 0.614 

1970 to 1990 . . . . . . . . . . . . .492 1980 to 1991–94. . . . . . . . . . .643 

1970 to 1991–94 . . . . . . . . . .955 1980 to 1995–98. . . . . . . . . . .462 

1970 to 1995–98 . . . . . . . . . .439 1980 to 1999–2000. . . . . . . . .129 

1970 to 1999–2000 . . . . . . . .164	 1991–94 to 1995–98 . . . . . . . . . .277 

1991–94 to 1999–2000 . . . . . . . .164 

*mrl, minimum reporting limit 

Appe
Area 12 Mountain Home 

40 

35 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 6 1.5 2.4 5.8 

1980 (1/1/80 to 12/31/89) . . . . . 34 4.2 5.2 27 

1990 (1/1/90 to 12/31/99) . . . . . 243 2.5 3.2 35 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 55 2.1 3.0 16 

1995–98 (1/1/95 to 12/31/98). . . . . 142 2.9 3.2 35 

1999–2000 (1/1/99 to 12/31/00). . . . . 81 2.4 3.7 30 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.211 1980 to 1990 . . . . . . . . . . . . . 0.005 

1970 to 1990 . . . . . . . . . . . . .686 1980 to 1991–94. . . . . . . . . . .030 

1970 to 1991–94 . . . . . . . . . .762 1980 to 1995–98. . . . . . . . . . .007 

1970 to 1995–98 . . . . . . . . . .627 1980 to 1999–2000. . . . . . . . .012 

1970 to 1999–2000 . . . . . . . .769	 1991–94 to 1995–98 . . . . . . . . . .629 

1991–94 to 1999–2000 . . . . . . . .828 

32 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 13 Hammett 

10 

8 

6 

4 

2 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

No 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 12 0.67 1.3 4.6 

1990 (1/1/90 to 12/31/99) . . . . . 35 4.8 5.0 9.2 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 10 4.8 4.9 9.1 

1995–98 (1/1/95 to 12/31/98) . . . . . 19 4.4 4.9 9.2 

1999–2000 (1/1/99 to 12/31/00) . . . . . 8 6.2 5.6 8.9 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 0.000 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . .029 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . .001 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . .004 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . .801 

1991–94 to 1999–2000 . . . . . . . .004 

Appe
34 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001

Area 14 Bliss DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

6

5

4

3

2

1

0

1970s 1980s 1990s 1991–94 1995–98 1999–2000

No

data

No

data

DECADES RANGE OF YEARS

Insufficient

data

Insufficient

data

Insufficient

data

Number of

Selected summary statistics analyses Median Mean Maximum

Decade

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 6 3.9 3.5 5.8

Range of years

1991–94 (1/1/91 to 12/31/94) . . . . . 1

1995–98 (1/1/95 to 12/31/98) . . . . . 2

1999–2000 (1/1/99 to 12/31/00) . . . . . 3

Mann-Whitney test

Decade or range of years p-value Decade or range of years p-value

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . .

1970 to 1990 . . . . . . . . . . . . 1980 to 1991–94. . . . . . . . . .

1970 to 1991–94 . . . . . . . . . 1980 to 1995–98. . . . . . . . . .

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000. . . . . . . .

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . .

1991–94 to 1999–2000 . . . . . . .

Nitrate concentrations, milligrams per liter
Area 15 Twin Falls DISSOLVED NITRATE AS NITROGEN, 

50 

IN MILLIGRAMS PER LITER 

45 

40 

35 

30 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 6 0.52 0.88 2.4 

1980 (1/1/80 to 12/31/89) . . . . . 28 3.8 3.2 10 

1990 (1/1/90 to 12/31/99) . . . . . 666 4.5 4.6 41 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 121 4.1 4.9 41 

1995–98 (1/1/95 to 12/31/98). . . . . 369 4.2 4.3 17 

1999–2000 (1/1/99 to 12/31/00). . . . . 367 4.6 5.0 31 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.032 1980 to 1990 . . . . . . . . . . . . . 0.010 

1970 to 1990 . . . . . . . . . . . . .000 1980 to 1991–94. . . . . . . . . . .050 

1970 to 1991–94 . . . . . . . . . .001 1980 to 1995–98. . . . . . . . . . .032 

1970 to 1995–98 . . . . . . . . . .000 1980 to 1999–2000. . . . . . . . .001 

1970 to 1999–2000 . . . . . . . .000	 1991–94 to 1995–98 . . . . . . . . . .769 

1991–94 to 1999–2000 . . . . . . . .015 

Appe
Area 16 Rupert DISSOLVED NITRATE AS NITROGEN, 

110 

IN MILLIGRAMS PER LITER 

95 

80 

65 

50 

35 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 55 3.3 3.5 14 

1980 (1/1/80 to 12/31/89) . . . . . 40 5.0 9.7 76 

1990 (1/1/90 to 12/31/99) . . . . . 489 3.4 4.5 100 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 100 3.7 5.1 58 

1995–98 (1/1/95 to 12/31/98) . . . . . 274 3.3 4.2 83 

1999–2000 (1/1/99 to 12/31/00) . . . . . 231 3.6 4.4 100 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.004 1980 to 1990 . . . . . . . . . . . . . 0.000 

1970 to 1990 . . . . . . . . . . . . .923 1980 to 1991 –94. . . . . . . . . . .006 

1970 to 1991– 94 . . . . . . . . . .808 1980 to 1995 –98 . . . . . . . . . . .000 

1970 to 1995–98 . . . . . . . . . .936 1980 to 1999 –2000 . . . . . . . . .002 

1970 to 1999–2000 . . . . . . . .678 1991–94 to 1995–98 . . . . . . . . . .850 

1991–94 to 1999–2000 . . . . . . . .682 

36 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 17 Burley/Marsh Creek 

75 

70 

65 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 15 1.5 2.7 16 

1980 (1/1/80 to 12/31/89) . . . . . 154 6.3 6.5 29 

1990 (1/1/90 to 12/31/99) . . . . . 399 4.3 5.0 68 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 81 4.3 4.3 12 

1995–98 (1/1/95 to 12/31/98) . . . . . 217 4.2 4.9 68 

1999–2000 (1/1/99 to 12/31/00) . . . . . 189 4.8 5.5 20 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.000 1980 to 1990 . . . . . . . . . . . . . 0.000 

1970 to 1990 . . . . . . . . . . . . .002 1980 to 1991 –94. . . . . . . . . . .000 

1970 to 1991– 94 . . . . . . . . . .008 1980 to 1995 –98 . . . . . . . . . . .003 

1970 to 1995–98 . . . . . . . . . .003 1980 to 1999 –2000 . . . . . . . . .017 

1970 to 1999–2000 . . . . . . . .001 1991–94 to 1995–98 . . . . . . . . . .850 

1991–94 to 1999–2000 . . . . . . . .050 

Appe
Area 18 Pocatello 

70 

60 

50 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 13 2.0 2.2 4.2 

1980 (1/1/80 to 12/31/89) . . . . . 136 2.4 3.5 60 

1990 (1/1/90 to 12/31/99) . . . . . 364 2.9 3.5 25 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 123 2.8 3.5 18 

1995–98 (1/1/95 to 12/31/98). . . . . 186 2.9 3.4 25 

1999–2000 (1/1/99 to 12/31/00). . . . . 101 2.8 3.5 15 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.433 1980 to 1990 . . . . . . . . . . . . . 0.031 

1970 to 1990 . . . . . . . . . . . . .058 1980 to 1991–94. . . . . . . . . . .174 

1970 to 1991–94 . . . . . . . . . .138 1980 to 1995–98. . . . . . . . . . .067 

1970 to 1995–98 . . . . . . . . . .060 1980 to 1999–2000. . . . . . . . .029 

1970 to 1999–2000 . . . . . . . .029	 1991–94 to 1995–98 . . . . . . . . . .726 

1991–94 to 1999–2000 . . . . . . . .343 

38 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 19 Fort Hall 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

No 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 

1980 (1/1/80 to 12/31/89) . . . . . 87 7.8 8.5 24 

1990 (1/1/90 to 12/31/99) . . . . . 45 9.9 10 24 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 7 12 11 23 

1995–98 (1/1/95 to 12/31/98) . . . . . 11 11 12 24 

1999–2000 (1/1/99 to 12/31/00) . . . . . 10 9.3 9.6 22 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 0.398 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . .480 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . .214 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . .762 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . .786 

1991–94 to 1999–2000 . . . . . . . .696 

Appe
Area 20 Preston/Cache Valley 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Insufficient 

data 

Insufficient 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 2 

1980 (1/1/80 to 12/31/89) . . . . . 1 

1990 (1/1/90 to 12/31/99) . . . . . 77 1.8 2.3 14 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 17 1.9 1.7 4.5 

1995–98 (1/1/95 to 12/31/98) . . . . . 42 1.4 2.2 8.0 

1999–2000 (1/1/99 to 12/31/00) . . . . . 70 2.1 3.1 19 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.927 

1991–94 to 1999–2000 . . . . . . . .279 

40 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 21 Soda Springs/Bear Lake (upper Portneuf/Blackfoot River) 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 41 1.6 4.0 21 

1980 (1/1/80 to 12/31/89) . . . . . 7 1.2 2.2 5.9 

1990 (1/1/90 to 12/31/99) . . . . . 201 2.8 3.4 24 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 69 3.0 3.9 24 

1995–98 (1/1/95 to 12/31/98). . . . . 101 2.6 3.3 14 

1999–2000 (1/1/99 to 12/31/00). . . . . 51 2.7 2.8 8.3 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.849 1980 to 1990 . . . . . . . . . . . . . 0.397 

1970 to 1990 . . . . . . . . . . . . .315 1980 to 1991–94. . . . . . . . . . .369 

1970 to 1991–94 . . . . . . . . . .234 1980 to 1995–98. . . . . . . . . . .450 

1970 to 1995–98 . . . . . . . . . .455 1980 to 1999–2000. . . . . . . . .511 

1970 to 1999–2000 . . . . . . . .580	 1991–94 to 1995–98 . . . . . . . . . .516 

1991–94 to 1999–2000 . . . . . . . .311 

Appe
Area 22 Mud Lake 

15 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 9 0.24 3.8 14 

1980 (1/1/80 to 12/31/89) . . . . . 12 2.5 2.8 6.1 

1990 (1/1/90 to 12/31/99) . . . . . 85 2.3 2.7 13 

Range of years 

1991–94 (1/1/91 to 12/31/94). . . . . 18 2.4 3.2 13 

1995–98 (1/1/95 to 12/31/98). . . . . 43 2.4 2.6 6.5 

1999–2000 (1/1/99 to 12/31/00). . . . . 41 2.5 2.9 7.7 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.522 1980 to 1990 . . . . . . . . . . . . . 0.514 

1970 to 1990 . . . . . . . . . . . . .403 1980 to 1991–94. . . . . . . . . . 1.000 

1970 to 1991–94 . . . . . . . . . .877 1980 to 1995–98. . . . . . . . . . .575 

1970 to 1995–98 . . . . . . . . . .446 1980 to 1999–2000. . . . . . . . .456 

1970 to 1999–2000 . . . . . . . .456	 1991–94 to 1995–98 . . . . . . . . . .548 

1991–94 to 1999–2000 . . . . . . . .856 

42 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 23 Hibbard 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

Insufficient 

data 

Insufficient 

data 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 2 

1980 (1/1/80 to 12/31/89) . . . . . 1 

1990 (1/1/90 to 12/31/99) . . . . . 170 1.9 2.9 26 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 29 2.0 2.3 6.5 

1995–98 (1/1/95 to 12/31/98) . . . . . 87 1.9 2.8 21 

1999–2000 (1/1/99 to 12/31/00) . . . . . 76 1.9 3.0 26 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 1980 to 1991– 94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . 1980 to 1999–2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . 1991–94 to 1995–98 . . . . . . . . . 0.594 

1991–94 to 1999–2000 . . . . . . . .937 

Appe
Area 24 St. Anthony DISSOLVED NITRATE AS NITROGEN, 

40 

IN MILLIGRAMS PER LITER 

35 

30 

25 

20 

15 

12 

9 

6 

3 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

No 

data 

DECADES RANGE OF YEARS 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 4 

1980 (1/1/80 to 12/31/89) . . . . . 

1990 (1/1/90 to 12/31/99) . . . . . 50 1.5 5.4 35 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 6 1.6 8.1 35 

1995–98 (1/1/95 to 12/31/98) . . . . . 18 2.6 7.0 30 

1999–2000 (1/1/99 to 12/31/00) . . . . . 34 1.5 5.2 32 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 1980 to 1990 . . . . . . . . . . . . . 

1970 to 1990 . . . . . . . . . . . . 0.447 1980 to 1991 –94. . . . . . . . . . 

1970 to 1991– 94 . . . . . . . . . .131 1980 to 1995 –98 . . . . . . . . . . 

1970 to 1995–98 . . . . . . . . . .268 1980 to 1999 –2000 . . . . . . . . 

1970 to 1999–2000 . . . . . . . .447 1991–94 to 1995–98 . . . . . . . . . 1.000 

1991–94 to 1999–2000 . . . . . . . .677 

44 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Area 25 Ashton, Drummond, Teton River 

40 

35 

30 

25 

20 

15 

10 

5 

0 

1970s 1980s 1990s 1991–94 1995–98 1999–2000 

DECADES RANGE OF YEARS 

DISSOLVED NITRATE AS NITROGEN, 

IN MILLIGRAMS PER LITER 

Number of Nitrate concentrations, milligrams per liter 

Selected summary statistics analyses Median Mean Maximum 

Decade 

1970 (1/1/70 to 12/31/79) . . . . . 16 3.8 2.9 6.0 

1980 (1/1/80 to 12/31/89) . . . . . 5 5.1 4.2 6.8 

1990 (1/1/90 to 12/31/99) . . . . . 266 5.7 7.0 38 

Range of years 

1991–94 (1/1/91 to 12/31/94) . . . . . 26 5.0 5.8 21 

1995–98 (1/1/95 to 12/31/98) . . . . . 187 5.9 7.3 38 

1999–2000 (1/1/99 to 12/31/00) . . . . . 75 5.3 6.9 30 

Mann-Whitney test 

Decade or range of years p-value Decade or range of years p-value 

1970 to 1980 . . . . . . . . . . . . 0.214 1980 to 1990 . . . . . . . . . . . . . 0.270 

1970 to 1990 . . . . . . . . . . . . .000 1980 to 1991 –94. . . . . . . . . . .707 

1970 to 1991– 94 . . . . . . . . . .045 1980 to 1995 –98 . . . . . . . . . . .195 

1970 to 1995–98 . . . . . . . . . .000 1980 to 1999 –2000 . . . . . . . . .468 

1970 to 1999–2000 . . . . . . . .001 1991–94 to 1995–98 . . . . . . . . . .075 

1991–94 to 1999–2000 . . . . . . . .311 

Appe

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality 

management areas in Idaho, 1961 to 2001 

Appendix B.

Well 8N–5W–22 Area 5 Payette County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 3N–2W–7CBC1 Area 6 Canyon County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0


1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 196 feet 

p = 0.548 (no trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001. DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.03 (increasing trend) 

Appe
Well 3N–2W–26BAA1 Area 6 Canyon County U.S. Geological Survey 

18 

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 3N–2W–33CAD1 Area 6 Canyon County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 63 feet 

p = 0.062 (no trend) 

YEARS 

Well depth = 83 feet 

p = 0.688 (no trend) 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Conti

50 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Well 5N–5W–32CDC1 Area 6 Canyon County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 3N–1E–1BCD1 Area 9 Ada County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 42 feet 

p = 0.142 (no trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

Appe

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = 58 feet 

p = 0.476 (no trend) 
Well 3N–1E–22CCC1 Area 9 Ada County U.S. Geological Survey & Idaho Department of Agriculture 18 

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 3N–1E–23DAB1 Area 9 Ada County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 47 feet 

p = 0.001 (decreasing trend) 

YEARS 

Well depth = 86 feet 

p = 0.004 (increasing trend) 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

52 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Well 3N–1W–1 Area 9 Ada County Idaho Department of Environmental Quality 

18 

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 4S–6E–13AAA1 Area 12 Elmore County U.S. Geological Survey & Idaho Department of Agriculture 36 

32 

28 

24 

20 

16 

12 

8 

4 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 79 feet 

p = 0.03 (increasing trend) 

YEARS 

Well depth = unknown 

p = 0.005 (increasing trend) 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

Appe
Well 10S–23E–17 Area 16 Minidoka County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 10S–22E–23 Area 17 Cassia County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.156 (no trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

54 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.398 (no trend) 
Well 10S–22E–29 Area 17 Cassia County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 10S–23E–30 Area 17 Cassia County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.004 (increasing trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

Appe

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.028 (increasing trend) 
Well 11S–22E–14BAB1 Area 17 Cassia County U.S. Geological Survey 

18 

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 13S–22E–28 Area 17 Cassia County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.120 (no trend) 

YEARS 

Well depth = 206 feet 

p = 0.358 (no trend) 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

56 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 
Well 6S–34E–15 Area 18 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 6S–34E–15 Area 18 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.005 (increasing trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

Appe

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.007 (increasing trend) 
Well 6S–34E–15 Area 18 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 6S–34E–15 Area 18 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.280 (no trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

58 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.246 (no trend) 
Well 6S–34E–16 Area 18 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 10S–42E–5 Area 21 Bannock County Idaho Department of Environmental Quality 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = unknown 

p = 0.932 (no trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

Appe

DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = unknown 

p = 0.112 (no trend) 
Well 9N–42E–36ABA1 Area 25 Fremont County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Well 8N–43E–23DBC1 Area 25 Fremont County U.S. Geological Survey 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1960 1965 1970 1975 1980 1985 1990 1995 2000 

Well depth = 310 feet 

p = 0.005 (increasing trend) 

YEARS 

Appendix B. Nitrate trend analyses for selected wells in ground-water-quality management areas in Idaho, 1961–2001—Contin

60 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 DISSOLVED NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Well depth = 289 feet 

p = 0.002 (increasing trend) 
Printed on recycled paper