U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2019–1032

Prepared in cooperation with the Idaho Department of Environmental Quality

Groundwater Quality and Nutrient Trends near Marsing, 

Southwestern Idaho, 2018
Cover: Main image: Groundwater quality sampling, level measurement, and elevation 

survey at well 433449116504401, Marsing, Idaho, March 27, 2018. Photograph by 

Kenneth Skinner, U.S. Geological Survey. 

Upper inset picture: Groundwater quality sampling, level measurement, and elevation 

survey at well 433322116515601, Marsing, Idaho, March 28, 2018. Photograph by 

Kenneth Skinner, U.S. Geological Survey.

Middle inset picture: Groundwater quality sampling, level measurement, and elevation 

survey at well 433441116511201, Marsing, Idaho, April 3, 2018. Photograph by Kenneth 

Skinner, U.S. Geological Survey.

Lower inset picture: Three wells including well 433515116524601 used for groundwater 

quality sampling and elevation survey, Marsing, Idaho, March 28, 2018. Photograph by 

Kenneth Skinner, U.S. Geological Survey.
Groundwater Quality and Nutrient Trends near 

Marsing, Southwestern Idaho, 2018

By Kenneth D. Skinner

Prepared in cooperation with the Idaho Department of Environmental Quality

Open-File Report 2019-1032

U.S. Department of the Interior

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

DAVID BERNHARDT, Acting Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

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Although this information product, for the most part, is in the public domain, it also may

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must be secured from the copyright owner.

Suggested citation:

Skinner, K.D., 2019, Groundwater quality and nutrient trends near Marsing, southwestern Idaho, 2018: U.S. 

Geological Survey Open-File Report 2019-1032, 23 p., https://doi.org/10.3133/ofr20191032. 

ISSN 2331-1258 (online) 
iii

Contents

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

Introduction.................................................................................................................................................... 2

Study Area ................................................................................................................................................. 2

Purpose and Scope.................................................................................................................................... 4

Methods......................................................................................................................................................... 4

Groundwater-Quality Sampling Methods.................................................................................................... 4

Ammonia and Nitrate Concentration Trends Analysis Methods ................................................................. 6

Quality Assurance and Quality Control Methods ........................................................................................ 6

Results........................................................................................................................................................... 6

Groundwater Levels ................................................................................................................................... 6

Oxidation/Reduction State.......................................................................................................................... 8

Trace Elements ........................................................................................................................................ 11

Arsenic................................................................................................................................................. 11

Iron and Manganese............................................................................................................................ 13

Sulfate and Total Dissolved Solids ........................................................................................................... 13

Piper Plots ........................................................................................................................................... 13

Dissolved Gases ...................................................................................................................................... 15

Nutrients................................................................................................................................................... 16

Trends Analysis ........................................................................................................................................... 18

Selection of Nutrient Trends Concentration Data ..................................................................................... 18

Nutrient Trends Results ....................................................................................................................... 18

Quality Assurance and Quality Control Results........................................................................................ 20

Summary ..................................................................................................................................................... 21

Acknowledgments........................................................................................................................................ 22

References Cited......................................................................................................................................... 22

Figures

Figure 1. Map showing the study area and site identification numbers for 15 wells sampled near Marsing, 

southwestern Idaho, 2018. ............................................................................................................................. 3

Figure 2. Map showing groundwater potentiometric surface and flow direction derived from nine 

groundwater-level measurements near Marsing, southwestern Idaho, 2018.................................................. 7

Figure 3. Map showing groundwater-quality sampling locations with site identification numbers and 

oxidation/reduction process and type for 15 wells near Marsing, southwestern Idaho, 2018........................10

Figure 4. Arsenic concentrations from 15 water-quality samples collected from wells near Marsing, 

southwestern Idaho, 2018. ...........................................................................................................................12

Figure 5. Map (A) and piper plot (B) of 15 groundwater-quality samples collected near Marsing, 

southwestern Idaho, 2018. ...........................................................................................................................14

Figure 6. Map showing dissolved oxygen presence and nutrient type in groundwater-quality samples 

from 15 wells near Marsing, southwestern Idaho, 2018................................................................................17

Figure 7. Graphs showing ammonia and nitrate concentrations from water-quality samples collected 

at well 433422116511601 near Marsing, southwestern Idaho, 2018............................................................20
iv

Tables

Table 1. Groundwater-quality parameters sampled from 15 wells near Marsing, southwestern Idaho, 

2018............................................................................................................................................................... 5

Table 2. Oxidation/reduction (redox) state and predominant process, with selected groundwater-quality 

results from 15 wells near Marsing, southwestern Idaho, 2018. .................................................................... 9

Table 3. Ammonia and nitrate trend evaluations from water-quality samples collected at 15 wells near 

Marsing, southwestern, Idaho, 2018............................................................................................................ 19

Conversion Factors

International System of Units to U.S. customary units 

Multiply By To obtain

Volume

liter (L) 33.81402 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

cubic decimeter (dm3) 61.02 cubic inch (in3) 

liter (L) 61.02 cubic inch (in3) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

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

°F = (1.8 × °C) + 32.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

Supplemental Information

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

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

Abbreviations

EPA U.S. Environmental Protection Agency

IDEQ Idaho Department of Environmental Quality

ISDA Idaho State Department of Agriculture

MCL maximum contaminant level (EPA)

NWQL National Water Quality Laboratory (USGS)

SMCL secondary maximum contaminant level (EPA)

USGS U.S. Geological Survey
1 

Groundwater Quality and Nutrient Trends near Marsing, 

Southwestern Idaho, 2018

By Kenneth D. Skinner

Abstract

The U.S. Geological Survey, in cooperation with the Idaho Department of Environmental 

Quality, sampled groundwater from 15 wells during spring 2018 near the city of Marsing in rural 

northwestern Owyhee County, southwestern Idaho. Samples were analyzed for field parameters, 

nutrients, trace elements, major inorganics, and dissolved gas, including methane. To examine trends in 

individual wells and in the region, ammonia and nitrate results from the spring 2018 sampling were 

compared with data collected from 1996 to 2015 by the Idaho Department of Environmental Quality 

and the Idaho State Department of Agriculture. 

Fourteen of the 15 samples collected in 2018 contained arsenic (0.13–33.8 micrograms per liter 

[μg/L]), with 7 arsenic concentrations greater than the U.S. Environmental Protection Agency (EPA) 

maximum contaminant level (MCL) of 10 μg/L. Iron (465–4,180 μg/L), manganese (54–693 μg/L), 

sulfate (300–624 milligrams per liter [mg/L]), and total dissolved solids (511–1,350 mg/L) were 

detected at concentrations greater than EPA secondary maximum contaminant levels (SMCL) in waterquality samples from 6, 10, 4, and 14 of the 15 wells, respectively. Fourteen of the 15 samples contained 

ammonia concentrations from 0.12 to 7.34 milligrams per liter (mg/L). Six samples contained nitrate

concentrations from 0.08 to 24.6 mg/L, with one sample greater than the EPA MCL of 10 mg/L for 

drinking water. The presence of both ammonia and nitrate in four samples indicated multiple nutrient 

and groundwater sources and varying redox states. Ammonia concentrations tended to increase 

downgradient throughout the study area.

Nutrient trend analysis identified water-quality samples from 2 of the 15 wells with increasing 

nitrate concentrations from 1999–2018 and 2005–2018. The well with increasing nitrate concentrations 

from 2005–2018 showed a decreasing trend in ammonia concentrations during the same time period. 

Groundwater-quality samples from the 13 remaining wells showed no temporal trends. A Regional 

Kendall test, which evaluates trends at numerous wells across the study area to determine if a consistent 

trend exists for the area, was done to analyze 539 ammonia concentrations from 91 wells over 20 years 

(1999–2018) and 591 nitrate concentrations from 107 wells over 23 years (1996–2018). The Regional 

Kendall Test for ammonia had a tau correlation coefficient of -0.073 with a p-value of 0.072, and nitrate 

had a tau correlation coefficient of -0.041 with a p-value of 0.198, both indicating no statistically

significant trends.
2 

Introduction

In March 1999, the Idaho Department of Environmental Quality (IDEQ) and the Idaho State 

Department of Agriculture (ISDA) initiated a cooperative, 5-year study of groundwater quality in

northwestern Owyhee County near the town of Marsing, southwestern Idaho. The study was prompted 

by citizens’ concerns about increased dairy operations in the area and by previous detections of elevated 

concentrations of nitrate in groundwater. The study identified several water-quality parameters of 

concern, including nitrate, chloride, sulfate, arsenic, and specific conductance. As the study progressed, 

IDEQ and ISDA continued sampling groundwater, well owners became more aware of groundwater 

quality issues, and the ISDA updated waste containment rules and created nutrient management plans

(Boyle and others, 2002). 

A subsequent 10-year ISDA study from 1999 to 2008 (Bahr, 2009) indicated that the shallow 

alluvial aquifer was being impacted by nitrate and pesticides and detected elevated ammonia 

concentrations in the deeper aquifer. During the 10 years of sampling, 4–12 percent of the wells 

sampled had nitrate concentrations greater than 10 milligrams per liter (mg/L), the U.S. Environmental 

Protection Agency (EPA) maximum contaminant level (MCL). The ISDA study also reported stable

nitrate and ammonia concentrations during the 10-year study period. Nitrogen isotope results suggested 

that fertilizer and animal waste were sources of nitrate. The ISDA study recommended additional 

groundwater-quality sampling and implementing measures to reduce nutrient impacts on groundwater 

such as evaluating nutrient management plans for all agricultural types in the area, growers, 

agrichemical professionals, and dairy and feedlot facilities. 

Study Area

The study area is located near Marsing in rural northwestern Owyhee County (fig. 1). The study 

area is bordered to the north and east by the Snake River, to the south by State Highway 55, and to the 

west by Jump Creek (fig. 1). The topography in the study area is predominantly flat, sloping gently 

towards the Snake River. The area is used primarily for agriculture, with irrigated alfalfa fields and large 

concentrated dairy farms (Idaho Department of Environmental Quality, 2010). The predominant crops 

in the area are corn, alfalfa, and wheat (U.S. Department of Agriculture, 2018). 
3 

Figure 1. Map showing the study area and site identification numbers for 15 wells sampled near Marsing, 

southwestern Idaho, 2018.
4 

The geologic map of Owyhee County indicates the study area is made up of Quaternary gravels 

and terraces on western Snake River Plain with Quaternary alluvial deposits along the Snake River 

(Link, 2002). Groundwater primarily occurs within fractured basalt, with sedimentary sequences of 

gravel, sand, silt, and clay (Idaho Department of Environmental Quality, 2010).

Most residents in the study area rely on domestic wells for water and are on septic systems. 

Domestic wells in the area have typical water uses, such as bathing, washing, and watering yards and 

gardens. Many of the residents do not use water from their wells for drinking due to aesthetic issues 

such as a strong sulfur smell. 

Purpose and Scope

This report documents groundwater-quality sampling at 15 wells near Marsing, Idaho, in 

southwestern Idaho, during spring 2018. The sampling was done to assess groundwater quality in the 

area. 

This report also evaluates nutrient trends and provides the data necessary for future trend 

analyses of nutrients and other contaminants of interest. This report focuses on the occurrence and 

trends of ammonia and nitrate in groundwater. Any groundwater quality parameters greater than EPA 

MCL or secondary maximum contaminant levels (SMCLs) are also discussed. 

Methods

Groundwater-Quality Sampling Methods

At each well, the same methods were used to document, survey, sample, and measure the 

groundwater level. Upon arrival at a well, photographs were taken to document the setting and any 

possible nearby sources of contamination. If possible, the location and altitude of the well were 

surveyed, and the groundwater level was measured following the procedures of Cunningham and Schalk 

(2011). 

Groundwater-quality sampling followed U.S. Geological Survey (USGS) protocols for 

collecting water-quality data (U.S. Geological Survey, variously dated), including specific methods for 

determining adequate well purge prior to sampling, such as flushing at least three well bore volumes of 

water prior to sampling and monitoring filed parameters until stabilized (Wilde, 2006). Most sites 

sampled in this study are heavily used domestic wells. The wells were purged for a minimum of 25 

minutes prior to sampling. Field parameters (pH, water temperature, specific conductance, dissolved 

oxygen, and turbidity) were measured every 5 minutes until they became stable, indicating adequate 

well purge. Once the well purge was completed, groundwater was transferred directly from the well 

faucet to a mobile laboratory through Teflon® tubing, and all samples were collected in an isolated 

processing chamber where sampling occurred. When sampling was completed, samples were preserved 

if required and shipped overnight to the USGS National Water Quality Laboratory (NWQL) in 

Lakewood, Colorado, except samples for dissolved gas analysis. Five dissolved gas samples were 

collected and stored in a cool dark place until sampling was complete, and then samples were shipped to 

the Isotech Laboratory in Champaign, Illinois. A complete list of sampled constituents is shown in table 

1. 
5 

Table 1. Groundwater-quality parameters sampled from 15 wells near Marsing, southwestern Idaho, 2018. 

[Reporting levels for field parameters determined using U.S. Geological Survey National Field Manual protocols. All other 

reporting levels determined from analyses of samples at the laboratory indicated in that constituent’s spanner. 

Abbreviations: mg/L, milligram per liter; μg/L, microgram per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 

degrees Celsius; cc/L, cubic centimeter per liter] 

Parameter Reporting level Parameter Reporting level

Field parameters Groundwater major inorganics

Alkalinity 1 mg/L (National Water Quality Laboratory Schedule 2750)

Bicarbonate 1 mg/L Bromide 0.01 mg/L

Carbonate 0.1 mg/L Calcium 0.022 mg/L

Dissolved oxygen 0.2 mg/L Chloride 0.02 mg/L

pH 0.1 Fluoride 0.01 mg/L

Specific conductance 1 μS/cm at 25 °C Iron 5 μg/L

Temperature 0.1 °C Magnesium 0.011 mg/L

Turbidity 0.1 Manganese 0.2 μg/L

Groundwater nutrients Potassium 0.06 mg/L

(National Water Quality Laboratory Schedule 2755) Residue, 180 degrees Celsius (TDS) 20 mg/L

Nitrogen, ammonia 0.01 mg/L Silica 0.018 mg/L

Nitrogen, nitrite 0.001 mg/L Sodium 0.1 mg/L

Nitrogen, nitrite + nitrate 0.04 mg/L Sulfate 0.02 mg/L

Phosphorus, phosphate, ortho 0.004 mg/L Parameter Reporting units

Total nitrogen 

(NH3+NO2+NO3+organic) 0.05 mg/L Dissolved gas analysis 

(Isotech Laboratories, Inc.) 

Groundwater trace elements Argon (Ar) Mole percent

(National Water Quality Laboratory Schedule 2710) Carbon dioxide (CO2) Mole percent

Aluminum 3 μg/L Carbon monoxide (CO) Mole percent

Antimony 0.03 μg/L Ethane (C2H6) Mole percent, cc/L, and mg/L

Arsenic 0.05 μg/L Ethylene (C2H4) Mole percent

Barium 0.1 μg/L Hexanes + (C6+) Mole percent

Beryllium 0.01 μg/L Hydrogen (H2) Mole percent

Boron 5 μg/L Iso-butane (n-C4H10) Mole percent

Cadmium 0.03 μg/L Iso-pentane (n-C5H12) Mole percent

Chromium 0.5 μg/L Methane (CH4) Mole percent, cc/L, and mg/L

Cobalt 0.03 μg/L n-Butane (i-C4H10) Mole percent

Copper 0.2 μg/L Nitrogen (N2) Mole percent

Lead 0.02 μg/L n-Pentane (i-C5H12) Mole percent

Lithium 0.15 μg/L Oxygen (O2) Mole percent

Manganese 0.4 μg/L Propane (C3H8) Mole percent, cc/L, and mg/L

Molybdenum 0.05 μg/L Propylene (C3H6) Mole percent

Nickel 0.2 μg/L

Selenium 0.05 μg/L

Silver 1 μg/L

Strontium 0.5 μg/L

Thallium 0.02 μg/L

Uranium, natural 0.01 μg/L

Vanadium 0.1 μg/L

Zinc 2 μg/L
6 

Ammonia and Nitrate Concentration Trends Analysis Methods

A Regional Kendall test (Helsel and others, 2006) was conducted to assess areal trends in 

ammonia and nitrate concentrations. The Regional Kendall is a nonparametric test for consistent 

regional trend across an area utilizing multiple sites. It evaluates a Mann-Kendall test for individual 

sampling sites and combines the results into one overall test for the regional trend (Helsel and Frans, 

2006). Statistical power comparisons of the Seasonal Kendall test (which also applies to the Regional 

Kendall Test) with other trend tests determined that the Seasonal Kendall test is more powerful or nearly 

as powerful (96 percent) as other trend tests (Helsel and Frans, 2006). However, the Regional Kendall 

test is better in that it can directly use “non-detection” values because the test requires only a change 

between values and not the magnitude of change. 

Quality Assurance and Quality Control Methods

All quality-assurance (QA) and quality-control (QC) samples were collected following protocols 

described in the USGS NFM (U.S. Geological Survey, variously dated). All water-quality samples

(except those for dissolved gas analysis) were analyzed by the NWQL. The NWQL uses a Quality 

Management System (D.L. Stevenson, U.S. Geological Survey, unpub. data, 2013) and Quality 

Assurance and Quality Control Manual (D.L. Stevenson and A.R. Barnard, U.S. Geological Survey, 

unpub. data, 2013) as guidelines for the analytical work done at the laboratory. Isotech Laboratories 

does QA/QC using internal check standards and duplicates of submitted samples. The bias and precision 

of groundwater-quality sample results were evaluated through the collection of a field blank (except for 

dissolved gasses) and a sequential replicate sample. 

Results

Groundwater Levels 

Groundwater levels were only measured at 9 of the 15 wells due to access constraints in the 

other 6 wells, and a groundwater potentiometric surface and corresponding groundwater flow path were

determined from the measurements (fig. 2). Previous studies by the IDEQ showed that groundwater 

flows in a northeasterly direction from the mountains in the south through the study area toward the 

Snake River (Idaho Department of Environmental Quality, 2018). The groundwater flow path direction 

in figure 2 matches that previously identified by the IDEQ (2018). 
7 

Figure 2. Map showing groundwater potentiometric surface and flow direction derived from nine groundwater-level 

measurements near Marsing, southwestern Idaho, 2018. Figure made by Christina Andry, Natural Resources 

Conservation Service. 
8 

Oxidation/Reduction State

The oxidation/reduction (redox) state of groundwater can affect the concentration, transport, and 

fate of groundwater-quality constituents and can generate undesirable byproducts such as manganese, 

iron, hydrogen sulfide, and methane (Jurgens and others, 2009; McMahon and others, 2009). The redox 

state of groundwater is denoted by the dominant type of redox process occurring in the groundwaterquality sample and can vary locally by the type of dominant redox process such as dissolved oxygen, 

nitrate, manganese, iron, or sulfate. The groundwater-quality redox state was evaluated using the 

Microsoft® Excel® workbook created by Jurgens and others (2009). 

Groundwater oxidation/reduction conditions were anoxic in 11 of the 15 groundwater samples; 2 

were oxic, 1 suboxic, and 1 mixed (anoxic) (table 2). The primary reduction process was Ferric 

iron/SO4

2– in the anoxic samples. The anoxic water-quality state occurred throughout the study area (fig. 

3), with no discernable spatial pattern for redox state or process. Oxic conditions occurred in two of the 

deeper wells; however, these conditions were inconsistent among other deep wells. Groundwater quality 

from two wells seemed to be experiencing mixing between the shallow and deep aquifers, as their redox 

states were either suboxic or mixed (anoxic) (table 2).
9 

Table 2. Oxidation/reduction (redox) state and predominant process, with selected groundwater-quality results from 15 wells near Marsing, southwestern

Idaho, 2018. 

[Shaded constituents are greater than U.S. Environmental Protection Agency secondary maximum contaminant levels. Abbreviations: mg/L, milligram per liter; μg/L, 

microgram per liter; <, less than] 

Well No.

Well depth 

(feet below 

land surface)

General 

redox category

Redox 

process

Dissolved 

oxygen 

(mg/L)

Iron 

(μg/L)

Manganese 

(μg/L)

Sulfate 

(mg/L)

Total 

dissolved 

solids 

(mg/L)

Dissolved gas

433335116485101 142 Anoxic CH4gen, 

methanogenesis

<0.01 193 28 0.16 564

433449116504401 145 Anoxic CH4gen, 

methanogenesis

<0.01 309 54 0.13 538

433249116500301 144 Anoxic Fe(III)/SO4 <0.01 4,180 687 624 1,350

433322116515601 164 Anoxic Fe(III)/SO4 <0.01 2,030 158 239 648 Sampled, methane detected

433335116533401 185 Anoxic Fe(III)/SO4 <0.01 593 693 553 1,030 Sampled, methane detected

433410116510201 80 Anoxic Fe(III)/SO4 <0.01 465 136 37 529

433441116511201 120 Anoxic Fe(III)/SO4 <0.01 290 265 228 659

433458116531401 42 Anoxic Fe(III)/SO4 <0.01 108 36 4.3 492

433515116524601 130 Anoxic Fe(III)/SO4 <0.01 198 197 132 516

433521116524001 117 Anoxic Fe(III)/SO4 <0.01 445 101 28 509

433416116535601 200 Anoxic Mn(IV) <0.01 45.0 202 156 511 Sampled, methane detected

433241116514601 130 Mixed (anoxic) NO3-Fe(III)/SO4 <0.01 142 275 300 798

433402116503201 256 Suboxic Suboxic <0.01 61.7 38.2 12 689 Sampled, ethane detected and 

elevated concentrations of 

methane

433349116510301 184 Oxic O2 3.61 <10.0 <0.40 64 554

433422116511601 530 Oxic O2 4.09 15.6 7.04 403 1,260 Sampled, methane detected
10 

Figure 3. Map showing groundwater-quality sampling locations with site identification numbers and 

oxidation/reduction process and type for 15 wells near Marsing, southwestern Idaho, 2018. 
11 

Trace Elements

Arsenic 

Arsenic is a nonmetallic trace element with natural geologic sources and is used in some 

pesticides and industrial activities. Long-term exposure to arsenic in drinking water can result in skin 

damage or circulatory system problems, as well as increased risk of cancer of the skin, bladder, liver, 

kidney, nasal passages, prostate, and lungs (U.S. Environmental Protection Agency, 2018a). The EPA 

MCL for arsenic is 10 micrograms per liter (μg/L) or parts per billion. Elevated arsenic concentration in 

groundwater are related to complex geochemical conditions in the aquifer. The most common cause of 

elevated arsenic concentrations in groundwater is the release of arsenic from iron oxides in the aquifer 

materials (Welch and others, 2000). Other causes of elevated arsenic concentrations are low dissolved 

oxygen (anoxic redox conditions), high pH (alkaline conditions), and arsenic being concentrated by 

evaporation (Ayotte and others, 2011). 

All 15 water-quality samples had mid-range pH values (between 7 and 8), and 13 of the 15 

samples had no dissolved oxygen. The redox state of the 15 water-quality samples varied from anoxic to 

oxic conditions. Concentrations of total dissolved solids, silica, and alkalinity varied with the arsenic 

concentrations in the 15 water-quality samples, indicating that arsenic occurrence is not related to 

geochemical changes resulting from evaporative concentration (Welch and others, 2000). 

Seven of the 15 wells sampled for arsenic had concentrations greater than the EPA MCL of 10 

μg/L (11.2–33.8 μg/L; fig. 4). Arsenic concentrations do not appear to be related spatially, to well 

depth, or to other water-quality constituents. 
12 

Figure 4. Arsenic concentrations from 15 water-quality samples collected from wells near Marsing, southwestern 

Idaho, 2018.
13 

Iron and Manganese

For several water-quality constituents, the EPA has secondary drinking water standards to 

account for aesthetic qualities of drinking water such as taste and odor. These SMCLs are not 

considered a risk to human health. Effects of both iron and manganese at concentrations greater than the 

SMCL are a metallic taste to the drinking water and staining of items in contact with the water (U.S. 

Environmental Protection Agency, 2018b). 

Concentrations of both iron and manganese exceeded SMCLs in water-quality samples collected 

for this study. Iron concentrations were greater than the SMCL of 300 μg/L in 6 of the 15 samples, with 

concentrations from 16 to 4,180 μg/L, more than 6 times the SMCL (table 2). Manganese 

concentrations were greater than the SMCL of 50 μg/L in 10 of the 15 samples, with concentrations 

from 7 to 693 μg/L, more than 13 times the SMCL (table 2). 

Sulfate and Total Dissolved Solids

Along with iron and manganese, sulfate and total dissolved solids (TDS) have SMCLs 

established by the EPA. Effects of both sulfate and TDS greater than the SMCL are a salty taste to 

drinking water; TDS concentrations greater than the SMCL also result in high hardness of water, 

deposits on plumbing fixtures, and staining of items in contact with the water. Sulfate concentrations 

were greater than the SMCL of 250 mg/L in 4 of the 15 samples, with concentrations from 0.13 to 624 

mg/L, more than double the SMCL. TDS concentrations were greater than the SMCL of 500 mg/L in 14 

of the 15 samples, with concentrations from 492 to 1,350 mg/L, more than double the SMCL (table 2). 

Piper Plots

The groundwater-quality samples were characterized by a piper plot (trilinear diagram), which 

visualizes the relative abundance of ions in groundwater. Groundwater quality samples with similar 

chemistry tend to plot in groups. The group location within the piper plot provides the description of the 

chemical composition of the water; for example, a sodium-bicarbonate type water plots close to the 

bottom corner of the central diamond shape (Winston, 2000; Appelo and Postma, 2004). The piper plot 

identified two wells with calcium bicarbonate water, five wells with sodium bicarbonate water, and 

eight wells with calcium sulfate water (fig. 5). However, the piper plot groupings do not correlate with 

well depth or any of the groundwater quality parameters of interest discussed in other sections. 
14 

Figure 5. Map (A) and piper plot (B) of 15 groundwater-quality samples collected near Marsing, southwestern 

Idaho, 2018.
15 

Figure 5.—Continued. 

Dissolved Gases

Five groundwater-quality samples were collected for dissolved gases, including percent 

composition of argon, oxygen, carbon dioxide, helium, dinitrogen, carbon monoxide, methane, and 

higher-chain hydrocarbons (table 1). All five dissolved-gas samples had mole percentages of argon, 

oxygen, carbon dioxide, dinitrogen, and methane gas. One sample also contained ethane. Four of the 

five dissolved gas samples had methane concentrations below 0.015 mg/L, but one sample from well 

433402116503201 had a methane concentration of 41 mg/L. The dissolved gas sample with the elevated 

methane concentration also contained the one detection of ethane (0.0032 mg/L). The EPA has not 

established a MCL for methane, but the U.S. Office of Surface Mining action level for dissolved 

methane is 28 mg/L (Eltschlager and others, 2001). U.S. Office of Surface Mining indicates that 

methane concentrations greater than 28 mg/L result in potentially explosive or flammable quantities of 

gas being degassed from the water. 
16 

Nutrients

Nutrients in groundwater were of specific interest in this study because of the predominance of 

agriculture in the study area and evaluations of nutrients in previous studies (1999–2009) (Bahr, 2009; 

Idaho Department of Environmental Quality, 2018). While the production of most crops grown in 

Owyhee County have remained stable from 2002 to 2012, corn production has increased four-fold, and 

milk output from dairies has more than tripled (U.S. Department of Agriculture, 2018). These increases 

result in increased application of nutrients for crops and increased nutrients from dairy cattle. The form 

of nutrients observed in the groundwater samples depends on the oxidation/reduction conditions within 

the aquifer. Ammonia occurs in a reducing environment since oxygen is not present to facilitate 

nitrification (the conversion of ammonium to nitrate by bacteria). 

Fourteen of the 15 nutrient samples contain ammonia, and 6 of the 15 nutrient samples contain 

nitrate (fig. 6). The presence of both ammonia and nitrate in four samples with no dissolved oxygen 

indicates multiple nutrient sources, because nitrification cannot occur without dissolved oxygen. One 

nitrate concentration (24 mg/L) was greater than the EPA MCL of 10 mg/L for drinking water and was 

also collected from the deepest well sampled. The EPA has not established a MCL for ammonia in 

drinking water. Ammonia concentrations increase downgradient toward the Snake River through the 

study area. 
17 

Figure 6. Map showing dissolved oxygen presence and nutrient type in groundwater-quality samples from 15 wells 

near Marsing, southwestern Idaho, 2018.
18 

Trends Analysis

Selection of Nutrient Trends Concentration Data 

Ammonia and nitrate concentration trends were evaluated from groundwater-quality samples 

from individual wells and for the entire study area. Water-quality data for ammonia and nitrate were 

compiled from ISDA, IDEQ, and the USGS National Water Information System databases. Waterquality data were filtered to include only samples collected nearest to the sampling month of this study 

(March and April). Eleven of the 15 sampled wells had sufficient prior water-quality data to evaluate 

trends at each well. One of the wells with water-quality data did not have enough ammonia 

concentrations above the laboratory detection limit to evaluate a trend. The same was true for nitrate 

concentrations from 7 of the 11 wells. Regional nutrient water-quality data are available for 1996–2018, 

with a varying amount of data available for each well. The Regional Kendall test input concentration 

data applied a similar data restriction used for the individual well trend tests (samples collected in the 

spring); however, all available well locations in the Marsing area were used, including the 2018 

sampling done for this study. 

Nutrient Trends Results 

Eight of the wells with ammonia concentrations showed no temporal trend. Two of the wells 

with nitrate concentrations also showed no trend. Well 433349116510301 indicated an increasing trend 

in nitrate concentration from 1999 to 2018 (table 3), Mann-Kendall test p-value 0.009. Well 

433422116511601 also had an increasing trend in nitrate concentrations (Mann-Kendall test p-value 

0.056), but not statistically significant trends for ammonia concentrations for the period of record 

(2000–18), even though ammonia concentrations are decreasing since 2003 (fig. 7). The increase of 

nitrate and decrease of ammonia concentrations could occur from an increase of dissolved oxygen 

concentrations or a change of nutrient input from the surface. Ammonia concentrations from well 

433335116533401 do not indicate a statistically significant trend for all samples, 1999–2018, but have 

increasing concentrations since 2002. Seasonal effects are not present in the trend analysis because all 

samples were collected during spring. Also, trend analysis did not account for possible climatic affects, 

such as droughts or floods. 
19 

Table 3. Ammonia and nitrate trend evaluations from water-quality samples collected at 15 wells near Marsing, 

southwestern, Idaho, 2018.

Well No. Site name Ammonia trend evaluation Nitrate evaluation

433335116485101 03N 04W 33AAA1 No trend Concentrations below laboratory 

detection limit

433449116504401 03N 04W 20CAB1 No trend Concentrations below laboratory 

detection limit

433249116500301 03N 04W 32DDD1 Insufficient data for analysis Insufficient data for analysis

433322116515601 03N 04W 31BDD1 No trend Concentrations below laboratory 

detection limit

433335116533401 03N 05W 35AAA1 Increasing concentrations Concentrations below laboratory 

detection limit

433410116510201 03N 04W 29ADA2 Insufficient data for analysis Insufficient data for analysis

433441116511201 03N 04W 19CAB1 No trend Concentrations below laboratory 

detection limit

433458116531401 03N 05W 24BBA1 No trend Concentrations below laboratory 

detection limit

433515116524601 03N 05W 24ADA1 No trend Concentrations below laboratory 

detection limit

433521116524001 03N 05W 13DCD1 Insufficient data for analysis Insufficient data for analysis

433416116535601 03N 05W 26ABC1 No trend No trend

433241116514601 02N 04W 06BAA1 No trend No trend

433402116503201 03N 04W 29DBB1 Insufficient data for analysis Insufficient data for analysis

433349116510301 3N 04W 29ADA1 Concentrations below 

laboratory detection limit

Statistically significant 

increasing trend

433422116511601 03N 04W 30CCD2 Decreasing concentrations Statistically significant 

increasing trend
20 

Figure 7. Graphs showing ammonia and nitrate concentrations from water-quality samples collected at well 

433422116511601 near Marsing, southwestern Idaho, 2018.

The Regional Kendall Test for ammonia trends analyzed 539 ammonia concentrations from 91 

wells over 20 years (1999–2018). The Regional Kendall Test for ammonia resulted in no significant 

regional trend with a tau correlation coefficient of -0.073 and a p-value of 0.072. The Regional Kendall 

Test for nitrate trends analyzed 591 nitrate concentrations from 107 wells over 23 years (1996–2018). 

The Regional Kendall test for nitrate resulted in no significant regional trend with a tau correlation 

coefficient of -0.041 and a p-value of 0.198. 

Quality Assurance and Quality Control Results

The field blank resulted in non-detections for all constituents except silica and ammonia. Field 

blank detections for silica and ammonia were slightly greater than the detection limits. The silica 

concentration was 0.025 mg/L, which is 0.007 mg/L greater than the detection level of 0.018 mg/L, 

indicating an insignificant bias considering silica concentrations in the 15 water-quality samples were 

42–79 mg/L. The ammonia concentration was 0.02, which is 0.01 mg/L greater than the detection limit 

of 0.01 mg/L. The low concentration ammonia detection is likely due to laboratory method processes 

and not field procedures (Fulford, 2018). The ammonia concentrations in the water-quality samples 

range from a non-detection to 9.5 mg/L, indicating a minimal bias for small ammonia concentrations. 

The sequential replicate sample was collected on the same date as the field blank. The 

constituents with the highest relative percent differences (Mueller and others, 2015) between the 

original water-quality sample and the sequential replicate sample were (1) lead, 84 percent, sample and 

replicate concentrations of 0.14 and 0.06 μg/L; (2) copper, 48 percent, sample and replicate 

concentrations of 2.5 and 4.0 μg/L; (3) zinc 39 percent, sample and replicate concentrations of 2.0 and 

3.0 μg/L; (4) carbonate 18 percent, sample and replicate concentrations of 0.6 and 0.5 mg/L; (5) nickel, 

14 percent, sample and replicate concentrations of 0.52 and 0.45μg/L; and (6) cobalt, 9 percent, sample 

and replicate concentrations of 0.049 and 0.045μg/L. The elevated relative percent differences of these 

six constituents have minimal impact on the trends analysis because these constituents are of minimal 

importance given the scope of the study. 
21

For all other constituents, the replicate analysis yielded values that were less than 5 percent 

difference between the original field sample and sequential replicate sample, or the constituents were at 

concentrations less than the laboratory reporting levels hence not comparable.

Summary

The U.S. Geological Survey, in cooperation with the Idaho Department of Environmental 

Quality, sampled groundwater for field parameters, nutrients, trace elements, major inorganics, and 

dissolved gas, including methane from 15 wells during spring 2018 near the city of Marsing in rural 

northwestern Owyhee County, southwestern Idaho. Groundwater levels were measured at 9 of the 15 

wells from which a groundwater potentiometric surface map and corresponding flow paths were 

determined. Groundwater-quality samples were characterized by piper plots and evaluated for 

oxidation/reduction status and type. Ammonia and nitrate trends were also performed at individual wells 

and for the region.

Groundwater-flow paths confirmed previous direction findings. Piper plots identified three 

groundwater chemistry types. Groundwater oxidation/reduction (redox) conditions were anoxic in 11 of 

15 the groundwater samples; 2 were oxic, 1 suboxic, and 1 mixed (anoxic). The redox condition of 

groundwater can affect the concentration, transport, and fate of groundwater-quality constituents and 

generate undesirable byproducts such as manganese, iron, hydrogen sulfide, and methane. The anoxic 

redox condition occurs throughout the study area with no discernable spatial pattern or depth 

relationship for redox condition or process.

Groundwater-quality contaminants of interest include arsenic where 7 of the 15 wells sampled 

had concentrations greater than the EPA MCL of 10 μg/L. Arsenic concentrations ranged from 11.2 to

33.8 μg/L in samples from these 7 wells. Many groundwater-quality samples from wells had 

contaminants greater than the EPA SMCLs, which are not considered to be a risk to human health but 

affect the aesthetic qualities of drinking water such as taste and odor and can cause staining of items in 

contact with the water. Water-quality samples from the 15 wells were greater than the SMCL for iron (6 

samples with concentrations from 16 to 4,180 μg/L), manganese (10 samples with concentrations from 

7 to 693 μg/L), sulfate (4 samples with concentrations from 0.13 to 624 mg/L), and total dissolved 

solids (14 samples with concentrations from 492 to 1,350 mg/L).

Groundwater-quality nutrient analyses shows that 14 of the 15 samples contain ammonia, and 6

of the 15 nutrient samples have nitrate with 1 nitrate sample (24 mg/L) exceeding the EPA maximum 

contaminant level of 10 mg/L for drinking water. The presence of both ammonia and nitrate in four 

samples with no dissolved oxygen indicates multiple nutrient sources as nitrification cannot occur 

without dissolved oxygen. Ammonia concentrations increase downgradient toward the Snake River 

through the study area. Nitrate concentrations do not have a spatial trend in the study area.

Nutrient trend analyses evaluated water-quality data for the months of March and April for the 

period 1996 to 2018 and identified an increasing trend in nitrate concentration from water-quality 

samples from one well (Mann-Kendall test p-value 0.009). Water-quality samples from another well do 

not have statistically significant trends but have increasing nitrate concentrations since 2005 and have 

decreasing ammonia concentrations since 2003. For all other wells, water-quality samples do not 

indicate the presence of a temporal trend. The Regional Kendall test indicated no significant regional 

trends for either ammonia or nitrate, p-values of 0.072 and 0.198, respectively.
22

Acknowledgments

This study would not have been possible without the cooperation of well owners in the area. 

Access to their wells is essential for collecting groundwater-quality samples. The author would also like 

to thank Christina Andry of the U.S. Department of Agriculture Natural Resources Conservation 

Service who assisted with collection of the groundwater-quality samples and analyzed groundwater 

levels to create the groundwater-flow map shown in figure 2.

References Cited

Appelo, C.A.J., and Postma, D., 2004, Geochemistry, groundwater and pollution: CRC Press, 668 p.

Ayotte, J.D., Gronberg, J.M., and Apodaca, L.E., 2011, Trace elements and radon in groundwater across 

the United States, 1992–2003: U.S. Geological Survey Scientific Investigations Report 2011–5059, 

115 p., accessed May 15, 2018, at https://pubs.usgs.gov/sir/2011/5059/.

Bahr, G., 2009, Ground water quality of northern Owyhee County aquifers: Boise, Idaho State 

Department of Agriculture Technical Results Summary # 42, 20 p., accessed May 15, 2018, at 

http://www.idahoag.us/agri/Categories/Environment/water/waterPDF/gwreports/NorthOwyhee_Repor

t_April_2009_Final_Draft.pdf.

Boyle, L., Carlson, R., and Bahr, G., 2002, 2001 annual ground water quality monitoring summary 

results for northwest Owyhee County, Idaho: Boise, Idaho State Department of Water Quality, 38 p., 

accessed November 6, 2017, at http://www.deq.idaho.gov/media/474027-

_water_data_reports_ground_water_owyhee_county_monitoring.pdf.

Cunningham, W.L., and Schalk, C.W., comps., 2011, Groundwater technical procedures of the U.S. 

Geological Survey: U.S. Geological Survey Techniques and Methods 1–A1, 151 p., accessed March 

3, 2018, at https://pubs.usgs.gov/tm/1a1/.

Eltschlager, K.K., Hawkins, J.W., Ehler, W.C., and Baldassare, Fred, 2001, Technical measures for the 

investigation and mitigation of fugitive methane hazards in areas of coal mining: Pittsburgh, Office of 

Surface Mining Reclamation and Enforcement, 124 p., accessed July 18, 2018, at 

https://www.osmre.gov/resources/library/ghm/methane.pdf.

Fulford, J., 2018, Low-level detections of ammonia in surface-water field-blank samples: Water Mission 

Area Technical Note Number 18, accessed July 18, 2018, at https://sites.google.com/a/usgs.gov/wmawiki/news/technote/18.

Helsel, D.R., and Frans, L.M., 2006, Regional Kendall Test for trend: Environmental Science & 

Technology, v. 40, no. 13, 8 p., https://doi.org/10.1021/es051650b.

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 2005-5272, 4 p., accessed June 16, 

2018, at https://pubs.usgs.gov/sir/2005/5275/.

Idaho Department of Environmental Quality, 2010, Owyhee County ground water quality improvement 

and drinking water source protection plan: Boise, Idaho Department of Environmental Quality, 229 p., 

accessed November 6, 2017, at http://www.deq.idaho.gov/media/473984-

_water_data_reports_ground_water_owyhee_co_gwq_dwsp_plan.pdf.

Idaho Department of Environmental Quality, 2018, Owyhee County ground water quality improvement 

and drinking water source protection plan: Idaho Department of Environmental Quality, accessed 

August 3, 2018, at http://www.deq.idaho.gov/media/60181879/owyhee-county-ground-water-qualityimprovement-drinking-water-source-protection-plan.pdf.

Jurgens, B.C., McMahon, P.B., Chapelle, F.H., and Eberts, S.M., 2009, An Excel® workbook for 

identifying redox processes in ground water: U.S. Geological Survey Open-File Report 2009–1004, 

8 p., accessed August 3, 2018, at http://pubs.usgs.gov/of/2009/1004/.
23

Link, P.K., 2002, Digital atlas of Idaho: Pocatello, Idaho State University, Geosciences Department, 

accessed June 28, 2018, at https://imnh.iri.isu.edu/digitalatlas/counties/geomaps/geomap.htm.

McMahon, P.B., Cowdery, T.K., Chapelle, F.H., and Jurgens, B.C., 2009, Redox conditions in selected 

Principal Aquifers of the United States: U.S. Geological Survey Fact Sheet 2009–3041, 6 p., accessed 

June 28, 2018 at https://pubs.usgs.gov/fs/2009/3041/.

Mueller, D.K., Schertz, T.L., Martin, J.D., and Sandstrom, M.W., 2015, Design, analysis, and 

interpretation of field quality-control data for water-sampling projects: U.S. Geological Survey 

Techniques and Methods, book 4, chap. C4, 54 p., http://dx.doi.org/10.3133/tm4C4.

U.S. Department of Agriculture National Agricultural Statistics Service, 2018, CropScape—Cropland 

data layer: U.S. Department of Agriculture, accessed June 3, 2018, at 

https://nassgeodata.gmu.edu/CropScape/.

U.S. Department of Agriculture National Agricultural Statistics Service, 2018, Quick stats: U.S. 

Department of Agriculture National Agriculture Statistics Service, accessed August 28, 2018, at 

https://quickstats.nass.usda.gov/

U.S. Environmental Protection Agency, 2018a, Drinking water requirements for states and public water 

systems—Chemical contaminant rules: U.S. Environmental Protection Agency, accessed June 3, 

2018, at https://www.epa.gov/dwreginfo/chemical-contaminant-rules.

U.S. Environmental Protection Agency, 2018b, Secondary drinking water standards: Guidance for 

nuisance chemicals, accessed August 14, 2018, at 

https://www.epa.gov/dwstandardsregulations/secondary-drinking-water-standards-guidance-nuisancechemicals.

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: 

U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1–A10, 

accessed January 18, 2018, at http://pubs.water.usgs.gov/twri9A.

Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 2000, Arsenic in ground water of the 

United States—Occurrence and geochemistry: Ground Water, v. 38, no. 4, p. 589–604

https://doi.org/10.1111/j.1745-6584.2000.tb00251.x.

Wilde, F.D., ed., 2006, Collection of water samples (version 2.0): U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 9, chap. A4, section 4.2.3., 5 p., accessed December 18, 

2017, at http://pubs.water.usgs.gov/twri9A4/.

Winston, R.B., 2000, Graphical user interface for MODFLOW-GUI, version 4: U.S. Geological Survey 

Open-File Report 00-315, 27 p., accessed December 18, 2017, at 

https://water.usgs.gov/nrp/gwsoftware/mfgui4/modflow-gui.html.
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For more information concerning the research in this report, contact the

Director, Idaho Water Science Center

U.S. Geological Survey

230 Collins Rd

Boise, Idaho 83702-4520

https://www.usgs.gov/centers/id-water
Skinner—Groundwater Quality and Nutrient Trends near Marsing, Southwestern Idaho, 2018—Open-File Report 2019–1032

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20191032