Groundwater Level and 

Nitrate Concentration 

Trends on Mountain 

Home Air Force Base, 

Southwestern Idaho

U.S. Department of the Interior

U.S. Geological Survey

Open File-Report 2014-1022

Prepared in cooperation with the U.S. Air Force
Overview

• The Problem

• Study Area in Context

• Background and Past Studies

• Analysis Method and Criteria

• Monitoring Period

• Key Findings

• Summary
The Problem

• The regional aquifer is primary drinking-water 

resource for Mountain Home Air Force Base 

(MHAFB)

– MHAFB is located in Mountain Home 

Groundwater Management Area

– MHAFB is adjacent to the Cinder Cone Butte 

Critical Groundwater Area

– Federal and State studies show that regional 

groundwater levels are declining
The Problem

• Nitrate (measured as nitrogen) in excess of 10 milligrams per 

liter (mg/L) is a health risk for infants <6 months of age and 

for the elderly

• MHAFB is #14 on Idaho Department of Environmental 

Quality’s nitrate priority area list

– Two production wells are blended for base drinking water 

because of high nitrate concentration

– Another production well recently exceeded the U.S. 

Environmental Protection Agency’s 

Maximum Contaminant Level (MCL )

of 10 mg/L 

Kevin Dennehy
Study Area in Context

Study 

Area
Monitoring Well Locations

Source: ESRI, USGS
Purpose of the Study

• The 366 CES Environmental Flight requested 

the USGS to perform water level and nitrate 

trend analysis to help USAF leadership make 

drinking-water resource decisions
Background

• USGS and MHAFB have a 

long-standing relationship 

that began in late 1980s

• USGS monitors the 

following groundwater 

wells:

– 25 wells for water levels

– 17 wells for water quality, 

primarily nutrients
Past Analysis and Studies

Report found high levels of 

nitrate, fecal bacteria, and 

some VOCs in some MWs
WWTP Effluent

MW7-2 

MW30

MW36

MW23

-10

-5 

0 

5 

10

15

20

5 10 15 20

Delta O-18 (permil) 

Delta N-15 (permil) 

Stable Isotope Analysis, October 2004, Mountain Home AFB, Idaho

Definitive Manure and 

Septic Waste Signature

Overlap of Signatures

Kendall, C. (1998) USGS

(D.J. Parliman, U.S. Geological Survey, unpub. data, 2004) 

Past Analysis and Studies
Past Analysis and Studies

“...clean, unambiguous 

indication that sewer 

wastewater is a source of 

some, if not all of the 

nitrate.” 
Trend Analysis

• USGS developed a program for Mann-Kendall 

Family of trend tests

– Long-term datasets are analyzed to determine 

trends

– Objectives of the analysis:

• Whether or not there is a trend

• Direction of the trend (increasing or decreasing)

• Level of confidence in the findings (probability value)

• And a trend slope indicating the magnitude of change
Criteria

• Mann-Kendall Test

– Run on nitrogen data unless plot indicated a seasonal 

variance

• Seasonal Kendall Test

– Run on water levels and nitrogen data with seasonal 

variance

• Regional Kendall Test

– Combined results from all wells to determine overall 

regional trend in water level

• Linear relationship between X (time) and Y (measured 

value) were considered statistically significant if the 

Kendall’s Tau was >0.6, and had a p-value of <0.05
Years of Water-Level Monitoring

Measurement

month

Years of record Number of wells

March 12 25

June 12 25

September 13 25

December 13 25
Years of Water-Quality Monitoring

Local well No.

Years 

monitored* Local well No.

Years 

monitored*

MW 3-2 12 MW 22 11

MW 6-2 11 MW 23 11

MW 7-2 13 MW 29 9

BPW 8 10 MW 30 9

MW 11-2 9 MW 34 9

MW 16-2 11 MW 35 9

MW 17-2 12 MW 36 9

MW 18-2 11 MW 40 3

MW 21 11

 * data through October 2012
Key Findings

• MHAFB groundwater levels are declining at an average rate of 

about 1.08 feet per year based on Kendall Regional Test

• Nitrate concentration trend results:

– 3 wells (18 percent) show an increasing nitrate 

concentration trend 

– 3 wells (18 percent) show a decreasing nitrate 

concentration trend

– And 11 wells (64 percent) show no nitrate 

concentration trend

• 6 wells (35 percent) exceed the MCL of 10 mg/L for nitrate 

(measured as nitrogen)
2600

2602

2604

2606

2608

2610

2612

2614

2616

2618

2620

2622

2624

2626

2628

2630

2632

02/01/0002/01/0102/01/0202/01/0302/01/0402/01/0502/01/0602/01/0702/01/0802/01/0902/01/1002/01/1102/01/1202/01/13

Altitude of water level, in feet (ft), above NGVD 1929

Date

Altitude of water levels — MW 16-2, 18-2, 17-2, 3-2, 20, and 34,

March 2000–September 2013

MW16-2

MW18-2

MW17-2

MW 3-2

MW 34

MW 20

Regional Kendall Trend Test

Groundwater levels declining at a rate of 

1.08 ft per year, τ = 0.867, p = 0.0000
17.8

0.91

1.05

3.65

20.4

0.42

4.02

9.17

6.18

6.77

5.17

4.31

27.5

13.9

16.6

19.3

0.89

1.12

3.64

21.5

0.43

1.93

8.51

6.98

6.83

6.34

23.5

5.26

4.27

23.9

11.6

15.7

0

5

10

15

20

25

30

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Local Monitoring Well Identifier

Nitrate Concentration by Groundwater Monitoring Well — Spring and Autumn 2012

April 2012 October 2012

Red line indicates EPA's maximum nitrate (measured as nitrogen) contaminant 

level (MCL) for drinking water (10 mg/L) *

* Sampled in November 2012

6 wells currently exceed 

the EPA’s MCL
17.8

0.91

1.05

3.65

20.4

0.42

4.02

9.17

6.18

6.77

5.17

4.31

27.5

13.9

16.6

19.3

0.89

1.12

3.64

21.5

0.43

1.93

8.51

6.98

6.83

6.34

23.5

5.26

4.27

23.9

11.6

15.7

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5 

10

15

20

25

30

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Local Monitoring Well Identifier

Nitrate Concentration by Groundwater Monitoring Well — Spring and Autumn 2012

April 2012 October 2012

Red line indicates EPA's maximum nitrate contaminant level (MCL) for drinking 

water (10 mg/L) * 

* Sampled in November 2012

Wells with increasing nitrate 

0.40 mg/L/yr

0.53 mg/L/yr

0.006 mg/L/yr
17.8

0.91

1.05

3.65

20.4

0.42

4.02

9.17

6.18

6.77

5.17

4.31

27.5

13.9

16.6

19.3

0.89

1.12

3.64

21.5

0.43

1.93

8.51

6.98

6.83

6.34

23.5

5.26

4.27

23.9

11.6

15.7

0 

5 

10

15

20

25

30

Nitrate (measured as nitrogen) concentration, in milligrams per liter (mg/L)

Local Monitoring Well Identifier

Nitrate Concentration by Groundwater Monitoring Well — Spring and Autumn 2012

April 2012 October 2012

Red line indicates EPA's maximum nitrate (measured as nitrogen) contaminant 

level (MCL) for drinking water (10 mg/L) * 

* Sampled in November 2012

Wells with decreasing nitrate 

-0.01 mg/L/yr

-0.27 mg/L/yr

-0.09 mg/L/yr
Nitrate Concentration by MW Location

• Spatial distribution of 

nitrate concentration

• The general movement of 

groundwater is to the 

southwest with a 3 inches 

per mile gradient*

*Schwarz and Parliman (2010)
Nitrate Concentration by MW Location

• Three clustered areas of 

elevated nitrate 

concentration*

*Schwarz and Parliman (2010)
Nitrate Concentration by MW Location

• 3 out of 17 MW wells 

show nitrate increases

0.53 

mg/L/yr

0.40 

mg/L/yr

0.006 

mg/L/yr
Nitrate Concentration by MW Location

• 3 out of 17 wells 

decrease in nitrate 

concentration

-0.01 mg/L/yr

-0.27mg/L/yr -0.09 mg/L/yr
Nitrate Concentration by MW Location

-0.01 mg/L/yr

-0.12 mg/L/yr

-0.27mg/L/yr 0.53 

mg/L/yr

0.40 

mg/L/yr

0.006 

mg/L/yr
Summary Table

Local well 

No.

Groundwater 

level trend

Magnitude 

(ft/yr)

Nitrate

concentration trend

Magnitude 

(mg/L/yr)

Exceeds 

MCL of 10 

mg/L

MW 3-2 Declining -1.18 Decreasing -0.01 No

MW 6-2 Declining -1.17 No significant trend -- No

MW 7-2 Declining -1.25 No significant trend -- No

BPW 8 -- -- Increasing 0.40 Yes

MW 11-2 Declining -1.21 Increasing 0.53 Yes

MW 16-2 Declining -1.20 Increasing 0.01 No

MW 17-2 Declining -1.29 No significant trend -- No

MW 18-2 Declining -1.16 No significant trend -- No
Summary Table—Continued

Local Well 

No.

Groundwater 

level trend

Magnitude 

(ft/yr) 

Nitrate 

concentration trend

Magnitude 

(mg/L/yr)

Exceeds 

MCL of 10 

mg/L

MW 21 Declining -1.27 No significant trend -- No

MW 22 Declining -1.27 No significant trend -- No

MW 23 Declining -1.29 Decreasing -0.27 No

MW 29 Declining -1.15 No significant trend -- Yes

MW 30 Declining -1.11 No significant trend -- No

MW 34 Declining -1.12 Decreasing -0.09 No

MW 35 Declining -1.18 No significant trend -- Yes

MW 36 Declining -1.11 No significant trend -- Yes

MW 40 No trend -- No significant trend -- Yes
Summary of Findings

• MHAFB groundwater levels are declining at an average rate of 

about 1.08 feet per year based on Kendall Regional Test

• Nitrate concentration trend results:

– 3 wells (18 percent) show an increasing nitrate 

concentration trend 

– 3 wells (18 percent) show a decreasing nitrate 

concentration trend

– And 11 wells (64 percent) show no nitrate 

concentration trend

• 6 wells (35 percent) exceed the MCL of 10 mg/L for nitrate 

(measured as nitrogen)
Supplementary Information
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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 3-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2001–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Level

Nitrate Concentration Trend

τ= -0.736, p = 0.0000, concentration 

is decreasing at rate of 0.01 mg/L/yr

Groundwater Level Trend

τ= 0.941, p = 0.0000, water level

declining at a rate of 1.18 ft per year

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Groundwater level, in feet (ft) below land surface 

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 6-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Nitrate Concentration Trend

τ= -0.329, p = 0.0342, no trend in 

concentration

Groundwater Level Trend

τ= 0.917, p = 0.0000, levels declining

at a rate of 1.17 ft per year
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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 7-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, March 2000–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Nitrate Concentration Trend

τ= 0.040, p = 0.7969, no trend in 

concentration

Groundwater Level Trend

τ= 0.929, p = 0.0000, levels declining 

at a rate of 1.25 ft per year
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Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Base Production Well 8, Mountain Home AFB, Idaho

Nitrate Concentrations, April 1990–October 2012 

Data gap from October 1990 to October 2003 

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L
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Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Base Production Well 8, Mountain Home AFB, Idaho

Nitrate Concentrations, October 2003–October 2012

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L

Nitrate Concentration Trend

τ = 0.797, p = 0.0015, nitrogen concentration 

increasing at rate of 0.40 mg/L per year

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Groundwater level, in feet (ft) land surface

Nitrate + nitrite, as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 11-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2004–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Nitrate Concentration Trend

τ= 0.667, p = 0.0005, concentration is 

increasing at a rate of 0.53 mg/L per year

Groundwater Level Trend

τ= 0.924, p = 0.0000, levels declining 

at a rate of 1.21 ft per year
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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 16-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, August 2002–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.986, p = 0.0000, levels are declining at 

a rate of about 1.29 ft per year

Nitrogen Concentration Trend

τ = 0.603, p = 0.0006, nitrogen concentration 

increasing at about 0.006 mg/L per year

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 17-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2001–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Level

Nitrate Concentration Trend

τ= -0.167, p = 0.4000, no trend in nitrogen 

concentration, but there are seasonal swings that 

correlate to water-level fluctuation

Groundwater Level Trend

τ= 0.986, p = 0.0000, water level declining 

at a rate of about 1.29 ft per year
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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well MW 18-2, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, September 2002–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.873, p = 0.0001, levels are declining at 

a rate of about 1.16 ft per year

Nitrogen Concentration Trend

τ = -0.147, p = 0.4338, no trend in nitrate 

concentration

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well MW 21, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2002–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.939, p = 0.0000, levels are declining at 

a rate of about 1.27 ft/yr

Nitrate Concentration Trend

τ = -0.518, p = 0.0112, no trend in concentration 

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, milligrams per liter (mg/L)

Monitoring Well MW 22, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2001–March 2013 

Nitrate (measured as nitrogen), mg/L

Groundwater Level Trend

τ = 0.668, p = 0.0000, levels are declining at 

a rate of about 1.33 ft per year

Nitrate Concentration Trend

τ = -0.216, p = 0.2079, no trend in 

concentration

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 23, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, April 2002–March 2013

Nitrate, mg/L MCL of 10 mg/L GW Levels

Nitrate Concentration Trend

τ=−0.625, p=0.0000, decreasing at a rate of 0.27 

mg/L per year

Groundwater Level Trend

τ = -0.970, p = 0.0000, levels are declining at 

a rate of about 1.29 ft per year

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 29, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2004–March 2013

Nitrate, mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.827, p = 0.0000, levels are declining at 

a rate of about 1.15 ft per year

Nitrogen Concentration Trend

τ = -0.175 p = 0.4391, no trend, but 

concentration changes with water fluctuation

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 30, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2004–March 2013 

Nitrate, mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.852, p = 0.0004, levels are declining at 

a rate of about 1.11 ft per year

Nitrate Concentration Trend

τ = -0.308, p = 0.1047, no trend in 

concentration

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Groundwater levels, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 34, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2004–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.852, p = 0.0004, levels are declining at 

a rate of about -1.12 ft per year

Nitrate Concentration Trend

τ = -0.610, p = 0.0007, nitrogen concentration 

decreasing at a rate of about 0.09 mg/L per year

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Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 35, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2004–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.728, p = 0.0006, levels are declining at 

a rate of about 1.18 ft per year

Nitrogen Concentration Trend

τ = -0.321, p = 0.1370, no trend in 

concentration

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Oct-2011

Apr-2012

Oct-2012

Apr-2013

Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 36, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2004–2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels

Groundwater Level Trend

τ = 0.852, p = 0.0004, levels are declining at 

a rate of about -1.11 ft per year

Nitrate Concentration Trend

τ = -0.214, p = 0.3322, no trend in 

concentration, but there seems to be seasonal 

fluctuation in concentration

Back
406

408

410

412

414

416

418

0 420

2

4

6

8

10

12

14

16

18

Oct-2010

Apr-2011

Oct-2011

Apr-2012

Oct-2012

Apr-2013

Groundwater level, in feet (ft) below land surface

Nitrate + nitrite as nitrogen, in milligrams per liter (mg/L)

Monitoring Well 40, Mountain Home AFB, Idaho

Nitrate Concentration and Groundwater Levels, October 2010–March 2013

Nitrate (measured as nitrogen), mg/L MCL of 10 mg/L GW Levels Back
References

Helsel, D.R., Mueller, D.K., and Slack, J.R., 2006, Computer program for the Kendall family of trend tests: U.S. Geological Survey 

Scientific Investigations Report 2005-5275, 4 p., computer program, http://pubs.usgs.gov/sir/2005/5275/.

Hirsch, R.M., and Slack, J.R., 1984, A nonparametric trend test for seasonal data with serial dependence: Water Resources 

Research, v. 20, no. 6, p. 727–732, doi: 10.1029/WR020i006p00727.

Idaho Department of Environmental Quality, 2008, Idaho Nitrate Priority Areas: IDEQ , 108 p., accessed December 9, 2013, at 

http://www.deq.state.id.us/media/471611-ranking_2008.pdf.

Idaho Department of Water Resources, 2013, Critical Ground Water Areas, Ground Water Management Areas & Areas of Drilling 

Concern: IDWR Web site, accessed December 9, 2013, at 

http://www.idwr.idaho.gov/WaterInformation/GroundWaterManagement/.

Kendall, C., and McDonnell, J. J., eds., 1998, Isotope tracers in catchment hydrology: Elsevier Science BV, Amsterdam, The 

Netherlands, 839 p.

Norton, M.A., Ondrechen, W., and Baggs, J.L., 1982, Ground water investigation of the Mountain Home Plateau, Idaho: Idaho 

Department of Water Resources, accessed December 9, 2013, at 

http://www.idwr.idaho.gov/WaterInformation/Publications/ofr/ofr-gw_investigations_mthome_plateau_id.pdf.

Parliman, D.J., and Young, H.W., 1990, Hydrologic and chemical data from selected wells and springs in southern Elmore County, 

including Mountain Home Air Force Base, southwestern Idaho, fall 1989: U.S. Geological Survey Open-File Report 90-112, 

35 p., http://pubs.er.usgs.gov/publication/ofr90112.
References—Continued

Ralson, D.R., and Chapman, S.L., 1968, Ground water resources in the Mountain Home Area, Elmore County: Idaho Department of 

Reclamation Water Information Bulletin no. 4, 63 p.

Schwarz, D.L., and Parliman, D. J., 2010, Use of human pharmaceuticals to determine a sewer water nitrate source to the regional 

aquifer, Mountain Home Air Force Base, Idaho: URS Group, Inc., Idaho, 40 p.

U.S. Environmental Protection Agency, 2013, Drinking Water Contaminants: EPA Web site, accessed December 9, 2013, at 

http://water.epa.gov/drink/contaminants/#Inorganic.