U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5047

Prepared in Cooperation with the Utah Department of Environmental Quality

Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids 

from Selected Wells in Utah
Cover images: Left, Utah Valley, 1986, Google Earth

Right, Utah Valley, 2016, Google Earth.
Quantifying Trends in Arsenic, Nitrate, 

and Dissolved Solids from Selected 

Wells in Utah

By Olivia L. Miller

Prepared in Cooperation with the Utah Department of Environmental Quality

Scientific Investigations Report 2020–5047

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Miller, O.L., 2020, Quantifying trends in arsenic, nitrate, and dissolved solids from selected wells in Utah: 

U.S. Geological Survey Scientific Investigations Report 2020–5047, 80 p., https://doi.org/10.3133/sir20205047.

ISSN 2328-0328 (online)
iii

Acknowledgments

Thanks to Paul Inkenbrandt at the Utah Geological Survey for assistance with obtaining Safe 

Drinking Water Information System data and to Emily Frary Nettles at the Utah Department 

of Environmental Quality for assistance with Safe Drinking Water Information System data 

inspection and verification.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Factors that Affect Water Quality ......................................................................................................2

Effects and Regulation of Groundwater Contamination .................................................................3

Purpose and Scope ..............................................................................................................................6

Methods...........................................................................................................................................................6

Study Area..............................................................................................................................................6

Data Sets Used ......................................................................................................................................6

Data Preparation ...................................................................................................................................7

Comparison Between Data Collection and Analysis Methods for Data from National 

Water Information System and Safe Drinking Water Information System 

Databases.................................................................................................................................7

Data Analysis.........................................................................................................................................8

Results: Identification and Quantification of Groundwater-Quality Trends .........................................9

Data Summary and Database Comparison ......................................................................................9

Arsenic...........................................................................................................................................9

Nitrate..........................................................................................................................................19

Dissolved Solids .........................................................................................................................28

Trends in Arsenic, Nitrate, and Dissolved Solids from Combined Datasets .............................37

Arsenic.........................................................................................................................................38

Nitrate..........................................................................................................................................38

Dissolved Solids .........................................................................................................................38

Trends in Arsenic, Nitrate, and Dissolved Solids from Safe Drinking Water 

Information System Data .....................................................................................................43

Linking Trends to Land-Use Change ................................................................................................48

Trends Across Analytes and Land-Use Change ...................................................................73

Summary........................................................................................................................................................75

References Cited..........................................................................................................................................77

Figures

1. Map showing study basins in Utah ............................................................................................5

2. Graphs showing number of arsenic samples over time in select Utah basins 

and sub-basins in the National Water Information System and Safe Drinking 

Water Information System datasets ........................................................................................10

3. Boxplots of arsenic concentrations in select Utah basins and sub-basins for 

data from the National Water Information System and Safe Drinking Water 

Information System, and combined datasets ........................................................................15

4. Graphs showing arsenic concentrations over time by dataset in select Utah 

basins and sub-basins ...............................................................................................................16

5. Map showing location of wells with sample concentrations that exceed the 

maximum contaminant level for arsenic in select basins in Utah ......................................17
vi

6. Graphs showing decadal and sub-decadal median arsenic concentration in 

select basins and sub-basins in Utah .....................................................................................18

7. Graphs showing number of nitrate samples over time in select Utah basins and 

sub-basins in the National Water Information System and Safe Drinking Water 

Information System datasets ....................................................................................................23

8. Boxplots of nitrate concentrations in select Utah basins and sub-basins for 

data from the National Water Information System and Safe Drinking Water 

Information System, and combined datasets ........................................................................24

9. Graphs showing nitrate concentrations over time by dataset in select Utah 

basins and sub-basins ...............................................................................................................25

10. Map showing location of wells with samples that exceed the Maximum 

Contaminant Level for nitrate in select basins in Utah .........................................................26

11. Graphs showing decadal and sub-decadal median nitrate concentration in 

select basins and sub-basins in Utah .....................................................................................27

12. Graphs showing number of dissolved solids samples over time in select Utah 

basins and sub-basins in the National Water Information System and Safe 

Drinking Water Information System datasets ........................................................................32

13. Boxplots of dissolved-solids concentrations in select Utah basins and 

sub-basins for data from the National Water Information System and Safe 

Drinking Water Information System, and combined datasets ............................................33

14. Graphs showing dissolved-solids concentrations over time by dataset in select 

Utah basins and sub-basins ......................................................................................................34

15. Map showing location of wells with samples that exceed the secondary 

maximum contaminant level and maximum contaminant level for dissolved 

solids in select basins in Utah ..................................................................................................35

16. Graphs showing decadal and sub-decadal median dissolved-solids 

concentration in select basins and sub-basins in Utah .......................................................36

17. Maps showing spatial patterns of trends in arsenic, nitrate, and dissolved 

solids data from the National Water Information System and the Safe Drinking 

Water Information System in select basins and sub-basins of Utah .................................40

18. Maps showing spatial patterns of trends in arsenic, nitrate, and dissolved 

solids data from the Safe Drinking Water Information System in select basins 

and sub-basins of Utah ..............................................................................................................45

19. Graphs showing decadal and sub-decadal median arsenic concentration in 

select basins and sub-basins by land-use change category in Utah ................................51

20. Graphs showing decadal and sub-decadal median nitrate concentration in 

select basins and sub-basins by land-use change category in Utah ................................59

21. Graphs showing decadal and sub-decadal median dissolved-solids 

concentration in select basins and sub-basins by land-use change 

category in Utah ..........................................................................................................................67

22. Bar charts showing number and direction of trends for each analyte in 

each basin for National Water Information System and Safe Drinking Water 

Information System data combined and the Safe Drinking Water Information 

System data .................................................................................................................................74
vii

Tables

1. Primary and secondary drinking water standards .................................................................4

2. Number of wells and arsenic samples; period of record; and minimum, 

maximum, and median concentration in select Utah basins and sub-basins for 

data from the National Water Information System (NWIS), Safe Drinking Water 

Information System (SDWIS), and combined NWIS and SDWIS data ..............................11

3. Number of wells and nitrate samples; period of record; and minimum, 

maximum, and median concentration in select Utah basins and sub-basins for 

data from the National Water Information System (NWIS) and Safe Drinking 

Water Information System (SDWIS), and combined NWIS and SDWIS data ..................20

4. Number of wells and dissolved solids samples; period of record; and minimum, 

maximum, and median concentration in select Utah basins and sub-basins for 

data from the National Water Information System (NWIS) and Safe Drinking 

Water Information System (SDWIS), and combined NWIS and SDWIS data ..................29

5. Arsenic trend test results for National Water Information System and Safe 

Drinking Water Information System data combined in select basins and 

sub-basins in Utah between 1975 and 2015 ...........................................................................39

6. Nitrate trend test results for National Water Information System and Safe 

Drinking Water Information System data combined in select basins and 

sub-basins in Utah between 1975 and 2015 ...........................................................................41

7. Dissolved solids trend test results for National Water Information System and 

Safe Drinking Water Information System data combined in select basins and 

sub-basins in Utah between 1975 and 2015 ...........................................................................42

8. Arsenic trend test results for Safe Drinking Water Information System data in 

select basins and sub-basins in Utah between 1975 and 2015 ...........................................44

9. Nitrate trend test results for Safe Drinking Water Information System data in 

select basins and sub-basins in Utah between 1975 and 2015 ...........................................46

10. Dissolved solids trend test results for Safe Drinking Water Information System 

data in select basins and sub-basins in Utah between 1975 and 2015 ..............................47

11. Number of wells; period of record; number of arsenic measurements; and 

minimum, maximum, and median arsenic concentration in each basin for each 

land-use change category ........................................................................................................49

12. Trend test results for arsenic in basins for each land-use change category ..................52

13. Number of wells; period of record; number of nitrate measurements; and 

minimum, maximum, and median nitrate concentration in each basin for each 

land-use change category ........................................................................................................56

14. Trend test results for nitrate in basins for each land-use change category ....................60

15. Number of wells; period of record; number of dissolved solids measurements; 

and minimum, maximum, and median dissolved-solids concentration in each 

basin for each land-use change category .............................................................................64

16. Trend test results for dissolved solids in basins for each land-use change 

category .......................................................................................................................................69
viii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

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

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

°F = (1.8 × °C) + 32.

Datum

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

Supplemental Information

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

IQR interquartile range

MCL maximum contaminant levels

NTU Nephelometric Turbidity Units

NWIS National Water Information System

SDWA Safe Drinking Water Act

SDWIS Safe Drinking Water Information System

SMCL secondary maximum contaminant level

USGS U.S. Geological Survey
Quantifying Trends in Arsenic, Nitrate, and Dissolved 

Solids from Selected Wells in Utah

By Olivia L. Miller

Abstract

Groundwater makes up a primary portion of the water 

supply in many parts of Utah, with annual withdrawals 

estimated at more than 1,000,000 acre-feet per year. Increases 

to groundwater withdrawal and land use may negatively 

impact water availability. Ensuring availability of clean water 

requires understanding how water quality has changed over 

time and how natural and human activities and processes 

influence water quality. Changes in arsenic, nitrate, and 

dissolved-solids concentrations in the groundwater in 

basins with high groundwater withdrawals were evaluated 

between 1975 and 2015 as indicators of basinwide water 

quality and the suitability of water for drinking. Data were 

used from the U.S. Geological Survey’s National Water 

Information System (NWIS) database and the Safe Drinking 

Water Information System (SDWIS) maintained by the 

Utah Department of Environmental Quality, Division of 

Drinking Water. Mann-Kendall trend tests were used to 

assess temporal trends in decadal and 5-year (sub-decadal) 

median analyte concentrations in basins. Trends also 

were assessed in smaller parts of larger basins to focus on 

changes occurring at a smaller spatial scale. To evaluate 

the relationship between land-use change and water-quality 

changes, trends also were evaluated for wells where land 

use has changed. Trends in decadal and sub-decadal median 

arsenic, nitrate, and dissolved-solids concentrations over time 

were identified throughout the basins and sub-basins in this 

study. For combined NWIS and SDWIS data, rates of median 

arsenic concentration change in basins and sub-basins ranged 

between decreases of –0.24 microgram per liter (ug/L) per 

year and increases of 0.48 ug/L per year. Rates of median 

nitrate-concentration change ranged between decreases 

of –0.08 milligram per liter (mg/L) per year and increases 

of 0.02 mg/L per year. Rates of median dissolved solids 

concentration change ranged between decreases of –5 mg/L 

per year and increases of 7 mg/L per year. The rates of change 

for nitrate and dissolved solids were similar to or less than 

rates of change observed in other parts of the country. Trends 

were not directly related to land-use change approximal to 

a well, although more data from wells where land use has 

changed would improve this evaluation. These findings 

highlight that water quality at a well is related to a range of 

factors including land, demographics, and water use over a 

larger area surrounding and up-gradient from the well; rates 

and direction of groundwater movement; and geologic and 

hydrologic conditions.

Introduction

Groundwater withdrawals in Utah have increased over 

time, mostly due to increased irrigation and industrial use 

(Burden, 2015). Groundwater also is used for public supply 

and serves as buffer for water suppliers when surface-water 

supplies decrease (for example, during summer months 

or drier years). Groundwater use is expected to play an 

even bigger role in meeting growing water demand as the 

population of Utah grows. The Utah Governor’s Office of 

Planning and Budget estimates indicate that the population 

in Salt Lake County will nearly double from approximately 

1 million people in 2010 to 1.8 million people by 2050 

(Utah Governor’s Office of Planning and Budget, 2012). 

Groundwater quality becomes increasingly important for 

supplying clean water to a growing population. Degradation of 

groundwater quality can have long-term negative implications 

for the viability of groundwater as a source of drinking water.

Groundwater has several advantages as a source for 

public water supply. Although surface-water supplies may 

be sensitive to precipitation and temperature variability 

on weekly to monthly timescales, groundwater integrates 

climatic conditions over multi-year timescales, making it 

a more constant supply. Groundwater also can be harder to 

contaminate than surface-water bodies because contaminants 

introduced at the land surface must travel through the 

subsurface to reach aquifers. Finally, groundwater withdrawal 

often occurs proximal to areas of demand, whereas surface 

water often requires conveyance over long distances (Price, 

1985). These advantages, in addition to the relatively large 

volumes of groundwater relative to surface water, make 

groundwater an important source of water for future water 

use and management plans. However, for groundwater to 

continue to be a viable supply into the future, groundwater 

resources must be carefully managed by using knowledge of 

the groundwater conditions. Excessive withdrawals can result 

in declines in water levels leading to increased costs to drill 

wells, land-surface subsidence, water-quality deterioration, 

and conflicts over water rights.
2 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

This report investigates spatial and temporal trends in 

arsenic, nitrate, and dissolved-solids concentrations in basins 

that have experienced significant groundwater development in 

Utah. These analytes were selected for several reasons. Each 

analyte is regulated by the U.S. Environmental Protection 

Agency (EPA) and reflects different natural and anthropogenic 

processes. Increased concentrations of nitrate and dissolved 

solids, resulting from human activities, also are a common 

water-quality issues in the southwestern United States (U.S.; 

Thiros and others, 2010). Characterizing temporal and spatial 

patterns and trends in these analytes is important for regulatory 

compliance and for understanding impacts of different natural 

and anthropogenic influences.

Arsenic, a toxic element of concern for human and 

animal health, has been predicted to exceed drinking water 

standards in 43 percent of the area of basin-fill aquifers in 

the southwestern U.S. (Anning and others, 2012; Beisner and 

others, 2012). Arsenic often occurs naturally in aquifers from 

interactions between water and arsenic-bearing minerals in 

rocks. Generally, at local scales, human alteration of aquifer 

geochemical conditions such as pH or oxidation-reduction 

conditions can mediate arsenic concentrations in groundwater; 

this could occur through groundwater pumping or artificial 

recharge, or the addition or removal of an acid or base to the 

groundwater system. Increased loading of arsenic, through 

leaching of mining tailings, for example, also can impact 

groundwater arsenic concentrations.

Nitrate can occur naturally in groundwater through 

dissolution of geologic deposits or from desert legume soil 

processes; or it can be introduced to water by human activity 

through fertilizer use, manure production, and agricultural 

and urban land development (Anning and others, 2012). 

Nitrate can cause a range of negative human and animal 

health impacts.

Dissolved solids occur naturally in water through 

dissolution of geologic deposits, or through anthropogenic 

processes including land development, wastewater treatment 

plant discharge, and irrigation and other agricultural practices. 

Dissolved-solids concentrations can increase through water 

consumption (for example, diversion of clean water out of 

a basin or evapotranspiration), which reduces the amount of 

water available for dilution. High concentrations of dissolved 

solids can impact aquatic ecosystems, and agricultural, 

municipal, industrial, and domestic water users who require or 

prefer water with low dissolved solids.

Factors that Affect Water Quality

Natural and human factors can influence groundwater 

quality. In the southwestern U.S., several natural and 

human factors have been identified as important controls on 

groundwater quality including the quality of recharge water, 

the composition of geologic material in contact with water, 

land and water use, and chemical spills or leaks (Thiros 

and others, 2010). Bexfield and others (2011) described in 

detail common natural and human contamination sources to 

basin-fill aquifers in the southwestern U.S. Because these 

processes vary in time and space, and can work constructively 

or destructively, disentangling the effects of specific processes 

on water quality poses a unique challenge. In addition, 

assessment and process attribution of changing groundwater 

quality over time is further complicated by the multiyear 

to millennial timeframes of groundwater movement. The 

following paragraphs broadly describe natural and human 

factors, including changes to the hydrologic flow system and 

changes to constituent sources, that can influence arsenic, 

nitrate, and dissolved-solids concentrations in groundwater.

Natural factors can influence groundwater quality. The 

chemical composition and amount of recharge water can 

influence groundwater quality. The geologic composition 

of porous media through which water passes, the contact 

time, and geochemical conditions can greatly influence 

concentrations of dissolved solids and metals (for example, 

arsenic and uranium; Anning and others, 2007; Bexfield 

and others, 2011). In the Southwest, volcanic bedrock 

surrounding basin-fill aquifers, low rates of natural recharge 

from precipitation, high potential evapotranspiration, minimal 

basin outflow, and geochemical conditions all contribute 

to increased vulnerability of an aquifer to high arsenic 

concentrations (Anning and others, 2012). Recharge from 

mountain streams to basin-fill aquifers typically originates 

as snowmelt runoff and is generally of high quality (low 

dissolved solids). Dissolved-solids concentrations typically 

increase along flow paths through interactions with basin-fill 

sediments and evapotranspiration (Anning and others, 2007). 

Evapotranspiration and nitrate fixation by vegetation also can 

concentrate nitrate in soils, which can subsequently dissolve 

in recharge passing through soil and moving downward 

to aquifers.
Introduction 3

Humans have influenced groundwater quality through 

alteration of the hydrologic flow system in Utah (Thiros 

and others, 2010). As groundwater pumping and use have 

increased, human-mediated recharge (for example, through 

infiltration of excess irrigation water and seepage from leaky 

canals, pipes, or ponds) has become an important component 

of the hydrologic system with potentially important impacts 

on groundwater quality. Increased pumping can alter flow 

patterns within an aquifer, leading to higher flows and thus 

higher connectivity, particularly from the land surface to 

shallow aquifers, thereby increasing the risk of contamination 

from the surface (Thiros and others, 2010). Recharge of water 

exposed to surface contamination can transport contaminants 

to aquifers. Historically, recharge has occurred through 

mountain block recharge or as infiltration through streams and 

alluvial fans at the base of mountains. However, as irrigation 

and development has increased, excess water from fields and 

yards and leaking canals and pipes has become a source of 

recharge to aquifers (Lambert, 1995). Excess irrigation and 

artificial recharge (for example, seepage from unlined canals, 

leaky pipes, or septic systems; or engineered recharge facilities 

including percolation ponds) can contribute substantially to 

increased concentrations of nitrate and dissolved solids in 

groundwater (Bexfield and others, 2011). Recharge of this 

kind poses a risk for degrading water quality in underlying 

aquifers because the water quality can be poor at the surface 

and this water is more susceptible to surface contamination. 

Recharge and flow rates, which also depend on sediment 

type and the presence of large fractures, control how quickly 

contaminated surface water moves into and through an 

aquifer. Coarser sediments with well-connected pore space 

allow for higher flows, whereas finer sediments with poorly 

connected pore space impede or even prevent flow. Flow rates 

determine the duration of contact between groundwater and 

aquifer material, and longer contact times can result in greater 

interaction between water and porous media, which controls 

constituent concentration.

Humans have also influenced groundwater quality 

through activities related to constituent source. Mining 

and mineral processing waste and leachate from landfills 

can contribute to increases in concentration of metals and 

dissolved solids in groundwater (Waddell and others, 1987). 

Commercial fertilizer application is the dominant source of 

nitrogen in agricultural areas of the western U.S. (Puckett, 

1994) and in some urban areas (Hamlin and others, 2002). 

In agricultural areas, nitrate can be added to groundwater 

through infiltration of irrigation drainage containing nitrate 

(Edmonds and Gellenbeck, 2002), whereas in urban areas 

this can occur through recharge from leaky septic systems, 

water lines, septic systems, or lawn irrigation (Thiros, 2003). 

Regions with cropland and well drained soils are at greater 

risk for high nitrate levels in groundwater, particularly where 

irrigation is necessary (Spalding and Exner, 1993). Older or 

poorly constructed wells can exhibit increased nitrate in well 

water (Spalding and Exner, 1993). These processes also would 

contribute dissolved solids to groundwater.

Recharge of urban runoff and leaky infrastructure to 

aquifers can affect groundwater quality (Carlson and others, 

2011). Road salt application has been proposed as a source 

of chloride in groundwater (Waddell and others, 1987). 

Broadly, numerous factors associated with urbanization could 

contribute to water-quality degradation, including changes in 

amount and type of water use, which could impact infiltrations 

patterns, irrigation with reclaimed wastewater, fertilizer 

and pesticide application, mining activities, septic system 

use, and water system infrastructure. Aging of urban water 

infrastructure such as sewage system pipes also makes it more 

susceptible to leaks, which can affect groundwater.

Effects and Regulation of Groundwater 

Contamination

Degradation of groundwater quality can result in human 

and animal health problems. Arsenic exposure can result in 

skin lesions, circulatory system problems, neuropathy, and 

increased risks of cancer and diabetes (Yu and others, 2003; 

Ahamed and others, 2006). Ingestion of nitrate in drinking 

water can cause methemoglobinemia (blue baby syndrome), 

which can be fatal for infants and livestock (Campbell and 

others, 1954; Ward and others, 2005). Nationally, nitrate 

is one of the most frequent anthropogenic contaminants to 

exceed human health standards in water from public-supply 

wells (Toccalino and others, 2010). High levels of dissolved 

solids in water can affect the taste and color of water, lead to 

mineral deposits on pipes and other infrastructures, and impact 

plants and animals that cannot tolerate saline water. Although 

dissolved solids can have limited impact on health, their 

presence can result in an aversion to the public water supply 

and be costly to treat.

To reduce the risks to human health arising from poor 

public-supply water quality, the EPA has defined Maximum 

Contaminant Levels (MCL) for a range of constituents in the 

National Primary Drinking Water Regulations, pursuant to 

the Safe Drinking Water Act (SDWA; U.S. Environmental 

Protection Agency, 2009). Primary drinking water regulations 

apply to a range of microorganisms, disinfectants and 

their byproducts, inorganic and organic chemicals, and 

radionuclides. Non-enforceable secondary maximum 

contaminant levels (SMCL), established in the National 

Secondary Drinking Water Regulations, have been developed 

to assist public water suppliers in managing water for color, 

taste, and odor qualities (U.S. Environmental Protection 

Agency, 2009). Secondary standards apply to dissolved solids, 

some metals and foaming agents, and pH. The State of Utah 

has primary and secondary standards consistent with federal 

regulations and has additional standards for dissolved solids 

(Utah Administrative Code, 2019; table 1).
4 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Prior work has been completed to analyze conditions and 

trends of arsenic, nitrate, and dissolved solids in groundwater 

at regional, well, or basin-specific scales in Utah. Since 1964, 

the U.S. Geological Survey (USGS) has published yearly 

reports describing annual groundwater conditions, including 

annual water levels and water-quality measurements, although 

temporal changes in water quality are not generally addressed 

(Burden, 2017). A few examples of prior work that focused on 

arsenic, nitrate, or dissolved solids conditions, conducted at 

either the regional or local scale, are described below. These 

studies tended to examine shorter periods (years to decades) 

than the analysis presented in this report.

In a regional study of arsenic, Anning and others (2012) 

used statistical models to predict arsenic concentrations 

throughout basin-fill aquifers in the southwestern U.S. Of 

the total area of basin-fill aquifers in Utah, approximately 

53 percent were predicted to have low arsenic concentrations 

(less than 10 micrograms per liter, μg/L), 24 percent were 

predicted to have concentrations between 10 and 24 μg/L, and 

23 percent were predicted to have concentrations greater than 

or equal to 25 μg/L (Anning and others, 2012). Many of the 

high concentration areas were in western Utah.

A few studies have been conducted on arsenic conditions 

at the basin or well scale in Utah. For example, sources of 

arsenic have been investigated in Goshen Valley (in the Utah 

Valley basin in this study); geothermal springs had the highest 

arsenic concentrations, and groundwater interactions between 

alluvial or carbonate rocks also were associated with moderate 

arsenic concentrations (Selck and others, 2018). Arsenic 

in areas of residential development in the Salt Lake Valley 

was characterized, and no correlation between percentage 

of residential land use surrounding a well and arsenic 

concentration in well water was determined (Thiros, 2003). 

This study also reported higher arsenic concentration on the 

western and northwestern sides of the Salt Lake Valley than 

the eastern side, which were possibly related to sedimentology, 

redox conditions and reactions, proximity to faults and 

geothermal water, high concentrations of arsenic in canals, and 

the presence of volcanic rocks. Arsenic trends in the Great Salt 

Lake have been assessed and although mean concentrations 

were greater than 100 ug/L, consistent evidence for temporal 

trends was not identified (Adams and others, 2015).

In a regional study of nitrate concentrations in the 

Southwest, Anning and others (2012) used statistical models 

to predict nitrate concentrations throughout basin-fill aquifers. 

Nitrate concentrations were generally predicted to be less 

than 5 milligrams per liter (mg/L; Anning and others, 2012). 

Approximately 65 percent of the total area of basin-fill 

aquifers in Utah were predicted to have nitrate concentrations 

less than 0.5 mg/L, 10 percent were predicted to have 

concentrations between 0.5 and 0.99 mg/L, and 20 percent 

were predicted to have concentrations between 1.0 and 

1.9 mg/L (Anning and others, 2012). Higher concentrations 

were predicted for shallower wells (Anning and others, 2012). 

A mapper also was developed to display spatio-temporal 

trends in nitrate in public-supply systems across the state 

(Wallace and Inkenbrandt, 2013).

Many studies have been conducted on nitrate conditions 

at the basin or well scale in Utah (fig. 1 shows a map of 

Utah). For example, geologic sources, septic-tank systems, 

and agricultural activities have been identified as potential 

sources of nitrate in groundwater in Cedar City Valley (Lowe 

and Wallace, 2001). Sources also were evaluated in Goshen 

Valley (part of Utah Valley in this study) where the highest 

nitrate concentrations occurred in agricultural areas, with 

manure being the major source (Selck and others, 2018). 

Nitrate conditions and sources in the Salt Lake Valley 

public-supply wells were characterized, and human influence 

(for example, from fertilizer application, or leaky septic 

systems or sewer pipes) was implicated in areas where nitrate 

concentrations were greater than 2–3 mg/L (39 percent of 

sampled public-supply wells; Thiros and Manning, 2004). 

Table 1. Primary and secondary drinking water standards.

[*Maximum total dissolveds solids levels are given in the Utah Primary Drinking Water Standards. Adapted from R309-200 (Monitoring and water quality: 

Drinking water standards, Utah Administrative Code) and U.S. Environmental Protection Agency (2009). Abbreviations: mg/L, milligrams per liter; >, greater 

than]

Contaminant Maximum contaminant level

(mg/L)

Secondary maximum 

contaminant level

(mg/L)

Arsenic 0.010 (0.05 mg/L prior to 1/23/2006) None

Nitrate 10 (as nitrogen) None

Nitrite 1 (as nitrogen) None

Total nitrate and nitrite 10 (as nitrogen) None

Total dissolved solids* 2,000 (if concentration >1,000 mg/L, supplier must meet additional requirements) 500
Introduction 5

38°

39°

40°

41°

42°

114° 113° 112° 111° 110° 109°

37°

Study basin

Basin and sub-basin boundary

Highway

Base from U.S. Geological Survey 30-meter National Elevation 

Dataset Universal Transeverse Mercator projection, Zone 12, 

North American Datum of 1983 (NAD 83)

EXPLANATION

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Cache Valley

Cache Valley South

Lower Bear River East Basin

East Shore Area East

Salt Lake Valley Northeast

Cache Valley North

Salt Lake Valley

Salt Lake Valley Southeast Salt Lake Valley Southwest

Tooele Valley

Utah Valley

Utah Valley Northwest Utah Valley Northeast

Utah Valley Southeast

Utah Valley Southwest

Goshen

Sevier Valley

Desert Northern

Juab Valley

Pahvant

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar City

Valley

Cedar City

Valley North

Cedar City 

Valley South

Lower Bear

River Basin

Lower Bear

River West Basin

East Shore Area

East Shore Area West

Salt Lake Valley Northwest

Milford

Valley

Figure 1. Study basins in Utah.
6 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Nitrate occurrence and distribution in the Great Salt Lake 

Basins and Tooele Valley was described in Thiros (2000) and 

Susong (2005). Throughout the Great Salt Lake Basins, water 

from wells in agricultural or urban areas had higher nitrate 

concentrations than water from wells in rangeland areas, and 

in urban areas shallower wells had higher median nitrate 

concentrations than deeper wells. Groundwater quality in 

Cache Valley has been classified by nitrate concentration and 

shallower wells, and wells in discharge zones tended to have 

higher nitrate concentrations (Lowe and others, 2003). Nitrate 

concentrations and trends at several wells in Milford Valley 

were evaluated, and trends varied by well (Susong, 1996).

Anning and others (2007) conducted a regional study of 

dissolved-solids concentrations and trends in basin-fill aquifers 

and streams across the Southwest. Nearly 40 percent of the 

area of basin-fill aquifers in the southwestern U.S., including 

Utah, exceeded the SMCL for dissolved solids of 500 mg/L 

from the 1960s through the 1980s (Anning and others, 2007). 

Anning and others (2007) assessed dissolved solids trends 

in select wells in basin-fill aquifers across the Southwest 

for 1974–88, 1989–2003, and 1974–2003 and reported that 

concentrations of dissolved solids did not increase over time 

in most groundwater-quality monitoring wells. Of wells with 

trends, more showed increasing trends than decreasing trends 

(Anning and others, 2007).

Several studies have been conducted on dissolved solids 

conditions at the basin or well scale in Utah. For example, 

changes in dissolved-solids concentrations in wells in the 

Salt Lake Valley were determined (Waddell and others, 1987; 

Thiros and Manning, 2004; Thiros and Spangler, 2010). 

Among public-supply wells, dissolved-solids concentrations 

were generally lower on the eastern side of the valley than 

the western side, and the southeastern side of the valley had 

the lowest concentrations, although concentrations were 

increasing in some areas (Thiros and Manning, 2004; Thiros 

and Spangler, 2010). Increasing trends were identified in 

wells completed in the principal aquifer in the Salt Lake 

Valley between 1962 and 1984; seepage from reservoirs, 

evaporation ponds, and tailings piles contributed to increased 

dissolved-solids concentrations (Waddell and others, 1987). 

Dissolved solids have been used to classify groundwater in 

Cedar Valley, where 80 percent of the basin, primarily in 

the central and western parts, had concentrations less the 

500 mg/L (Lowe and others, 2010). Although year-to-year 

fluctuations have occurred, few substantial changes in 

dissolved-solids concentrations over time were observed in 

the East Shore Area wells between 1960 and 1969 (Bolke and 

Waddell, 1972).

Purpose and Scope

This report presents the result of an analysis of trends 

in groundwater arsenic, nitrate, and dissolved-solids 

concentrations between 1975 and 2015 in selected basins 

characterized by high groundwater development. This analysis 

was conducted with support from the Utah Department 

of Environmental Quality, Division of Water Quality. The 

objectives of the analysis were to (1) compare data from two 

different databases and their combination to determine if 

samples from each database are comparable, and (2) identify 

and interpret trends in groundwater quality in select basins 

across Utah. Water-quality data come from the National 

Water Information System (NWIS) and Safe Drinking Water 

Information System (SDWIS) databases. This analysis 

provided a more temporally and spatially comprehensive 

assessment of the state of water quality in the selected basins 

throughout Utah.

Methods

A description of the study area, datasets used, data 

preparation, and statistical methods applied to the data are 

included in the following sections.

Study Area

Selected basins analyzed for this study included Cache 

Valley, Cedar City Valley, East Shore Area, Lower Bear River 

Basin, Milford Valley, Northern Juab Valley, Pahvant Valley, 

Parowan Valley, Salt Lake Valley, Sevier Desert, Tooele 

Valley, and Utah Valley (fig. 1). Milford Valley is similar to the 

area called the Milford area of Escalante Valley in the annual 

“Groundwater Conditions in Utah” reports (for example, 

Burden and others, 2017).

Data Sets Used

Water-quality data from two sources were used 

in this study: the USGS NWIS database and the EPA 

SDWIS database maintained by the Utah Department 

of Environmental Quality, Division of Drinking Water 

(Utah Division of Drinking Water, 2017). NWIS data were 

obtained from the USGS NWIS database (U.S. Geological 

Survey, 2017, https://waterdata.usgs.gov), and the 

SDWIS data were obtained from the SDWIS database 

(http://www.drinkingwater.utah.gov/).

The NWIS database contains water-quality data, 

beginning in 1911 through the time of this study from more 

than 6,000 wells in Utah, collected for local and regional 

studies or as part of an annual groundwater monitoring 

program in cooperation with the Utah Department of Natural 

Resources, Division of Water Rights, and Utah Department 

of Environmental Quality, Division of Water Quality. Wells 

were not necessarily sampled at regular time intervals. Data 

in NWIS represent samples taken at individual wells. Wells 

included had a wide range of depths and uses, from irrigation 

to monitoring to public supply; therefore, the source of water 

may have varied substantially.
Methods 7

The SDWIS database contains water-quality data 

from regular sampling of nearly 700 public-supply wells in 

accordance with the SDWA. Public-supply wells must be 

sampled every 3 years for inorganic and metal contaminants 

and sampled annually for nitrate unless a waiver is obtained. 

Samples for arsenic, total dissolved solids, nitrate, and 

nitrite are taken at the source. This study used SDWIS data 

from samples that were taken at single source wells before 

treatment or distribution. Data within the SDWIS database 

come from public-supply wells, which may bias the results 

toward cleaner water from potentially deeper wells, although 

exceptions may occur.

Data Preparation

Water-quality data from the NWIS and SDWIS databases 

were compiled (hereinafter referred to as NWIS samples 

and SDWIS samples). Data from the SDWIS database 

were limited to single source wells before treatment or 

distribution. Delineations of basin-fill aquifers (McKinney 

and Anning, 2009) were modified to focus on areas of 

substantial groundwater and agricultural development. 

Basins were further subdivided into sub-basins to evaluate 

trends on a smaller spatial scale. Subdivision was based 

on hydrologic unit code eight boundaries, and river and 

municipality locations.

For the trend analysis, datasets were limited to the years 

1975–2015 for two reasons: (1) much of the SDWIS data 

were collected after the Safe Drinking Water Act of 1974 

was enacted, and (2) to divide data into sub-decades of equal 

length. Arsenic data from 1,337 wells (598 NWIS wells and 

739 SDWIS wells) were used. Nitrate data from 1,857 wells 

(1,051 NWIS wells and 806 SDWIS wells) were used. 

Dissolved solids data from 1,955 wells (1,173 NWIS wells 

and 782 SDWIS wells) were used.

Duplicate sample entries within datasets were excluded 

from the analysis. Additionally, some samples had multiple 

results reported for the same analyte (for example, dissolved 

solids reported as the sum of constituents and the residual 

on evaporation at 180 degrees Celsius; °C). For nitrate data, 

the order of preference was filtered nitrate, unfiltered nitrate, 

filtered nitrate plus nitrite, and finally unfiltered nitrate plus 

nitrite following Oelsner and others (2017). For dissolved 

solids data, values obtained from both methods were used, 

although the sum of a constituent’s value was preferentially 

selected over the residual on evaporation value (Liebermann 

and others, 1989).

Data were manually and visually inspected for unlikely 

measurement values such as concentrations in multiple orders 

of magnitude above other values from the same well, samples 

collected during drilling operations, or probable typographical 

errors. Suspect data were compared to original lab reports and 

other concentration data for a given site and were eliminated 

if obvious errors were identified. In the SDWIS database, data 

from one site were sometimes assigned to multiple wells in 

a basin. Such group assignments are coded into the SDWIS 

dataset during data reporting. However, for older data (1980s 

and older), group assignment of measurements was not coded. 

To eliminate replication, and thus artificial weighting of data 

that were sampled at one site but assigned to multiple sites, 

identical concentrations taken on the same date in the same 

basin and stored in the same database were filtered out and 

only one value was retained.

Comparison Between Data Collection and 

Analysis Methods for Data from National Water 

Information System and Safe Drinking Water 

Information System Databases

The water data in the NWIS and SDWIS databases differ 

in several ways. In addition to the challenges of combining 

water-quality data described by Sprague and others (2017), 

including missing or ambiguous sample fraction, chemical 

form, parameter name, units of measurement, precise 

numerical value, or remark codes, several differences between 

NWIS and SDWIS data were identified. Sample collection 

and analysis methods differ for sample filtration and well 

purging and pumping practices for NWIS and SDWIS data. 

NWIS samples are collected in accordance with the sampling 

procedures described in the USGS National Field Manual 

(U.S. Geological Survey, 2006) and analyzed according to 

a range of standardized methods. The SDWIS samples are 

collected according to 40 CFR 141.23 (U.S. Environmental 

Protection Agency, 1996). Some sampling and laboratory 

methods have changed over time because of technology 

advances and improved method development.

NWIS and SDWIS samples have different practices for 

sample filtration, well purging and pumping, and potentially 

different depths of sample collection. NWIS samples are 

generally collected after well purging and then are filtered 

in the field. Some NWIS groundwater samples for smaller 

studies are collected with low-flow pumps following well 

purging. Purging is meant to ensure samples are representative 

of ambient formation water and filtering is done for analysis 

of dissolved ions in water. Explicit purging of wells may not 

occur before collection of SDWIS samples, although wells 

used for public supply are generally pumped more frequently 

and for longer duration, and samples are not field filtered, 

although some lab filtering may occur. NWIS samples come 

from wells with a wide range of uses, from irrigation to 

monitoring to public supply, and can therefore come from 

shallow or deep wells. The SDWIS samples come from 

public-supply wells, which can bias the results toward cleaner 

water, and in many cases, deeper wells. The main sampling 

differences (filtration, pumping rate, purging, and well depth) 

influence particulate matter or turbidity in water, which can 

alter constituent measurements. Specifically, constituents 

can interact with particulate matter, thereby altering 

measured concentrations.
8 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Although variation in sample collection and laboratory 

analysis procedures between NWIS and SDWIS samples 

exists, comparing data from the NWIS and SDWIS databases 

is justified because the sampling differences generally result in 

lower-turbidity samples that are more comparable. According 

to R309-200 (Monitoring and Water Quality: Drinking Water 

Standards) of the Utah Administrative Code, turbidity in 

samples of groundwater for public supply must be below 

5 Nephelometric Turbidity Units (NTU). This is lower than 

the 100 NTU that Puls and Powell (1992) observed contribute 

to significant metal concentration differences between filtered 

and unfiltered water samples. Therefore, samples in SDWIS 

are biased toward low turbidity (unless they are in violation 

of that standard) so they should be comparable to filtered 

NWIS samples even if the lab does not filter samples (Puls and 

Powell, 1992). For samples with low turbidity, difference in 

filtration should not bias contaminant measurements between 

the two databases. In a nationwide study of trends in rivers 

and streams, concentrations of nitrate and nitrate plus nitrite 

from filtered samples were indistinguishable from unfiltered 

samples (Oelsner and others, 2017).

Because the data from the two databases were determined 

to be comparable (in other words, an analyte from one 

database is comparable to the same analyte in the other 

database), combining the datasets was therefore justified. Data 

stored in the NWIS and SDWIS databases were combined 

to increase the number of samples available for analysis 

and expand the temporal and spatial data coverage. Using 

information about groundwater conditions from multiple 

sources improves the robustness of the analysis against biases 

arising from different sampling strategies and protocols and 

provides a more comprehensive analysis of water quality. 

Accounting for all the variation and temporal changes in 

sampling and analysis is beyond the scope of this report, but 

it should be acknowledged to potentially induce variability 

and bias into datasets, which can make trend determination 

more difficult.

Data Analysis

Before trend analysis, the data from each database, and 

the combination of datasets, were compared to understand 

differences between datasets and how that may influence trend 

results. The Mann-Kendall trend test was used to identify 

and quantify monotonic trends in decadal and sub-decadal 

median concentrations of arsenic, nitrate, and dissolved solids 

in groundwater over time. Monotonic trends were of interest 

because they identify overarching, consistent changes in 

water quality over time. Trends were identified in decadal 

and sub-decadal median concentrations within each basin 

and smaller portions of some basins (sub-basins). To evaluate 

the effect of land-use change on water quality, trends were 

identified among wells in each basin that had experienced 

different kinds of land-use change.

Water-quality data are often censored (reported as 

less than a certain value). Data can be reported at multiple 

censoring limits because labs and analysis techniques 

change. The purpose of sample collection can even determine 

censoring limit; some concentrations in the SDWIS database 

are reported as less than the MCL instead of reported as 

the measured value. Although censored values contain 

information about water quality, they complicate common 

statistical calculations. A range of statistical techniques have 

been employed by various researchers to deal with censored 

data including substitution, maximum likelihood, regression 

on order statistics, and nonparametric treatments.

Trends were evaluated using the Mann-Kendall trend 

test, which uses Kendall’s tau, a nonparametric correlation 

coefficient statistic that indicates the monotonic association 

between two variables (in this study, time and analyte 

concentration). Water-quality data rarely follow a normal 

distribution, which is required for parametric trend tests (for 

example, linear regression). The nonparametric Mann-Kendall 

trend test can determine a trend regardless of whether or not 

the data follow a normal distribution. Kendall’s Tau, which 

ranges from 1 to –1, depends on the number of increases 

and decreases in concentration over time. If all median 

concentrations increased over time, tau would equal +1 and 

if all median concentrations decreased over time, tau would 

be –1. Consequently, noise in the concentration data reduces 

tau toward zero (similar number of increase and decreases 

over time). The Theil-Sen slope estimate of the trend line, a 

nonparametric analog to linear regression commonly used in 

environmental analysis, also was used and can be interpreted 

in this study as the rate of median-concentration changes over 

time. Trends were considered significant at the 90-percent 

confidence level if the two-sided p-value was less than 0.1.

To identify basin-wide trends in groundwater quality, 

decadal and sub-decadal nonparametric Kaplan-Meier 

estimates of summary statistics for each basin were calculated 

using a single concentration per well per year (Helsel, 

2012). At least three concentrations per basin per decade or 

sub-decade were required to calculate a median concentration. 

Calculation of summary statistics and trend tests all account 

for censoring at multiple levels through the application of 

survival analysis methods to water-quality data (Helsel, 2012; 

Lee, 2017). In this study, the recommended nonparametric 

Kaplan-Meier technique for datasets with up to 50 percent 

censored observations was used to calculate decadal and 

sub-decadal summary statistics (for example, medians; Helsel, 

2012). The relatively short period of record, low sampling 

frequency, or frequent occurrence of censored values for some 

analytes in some basins made identifying trends using decadal 

medians difficult. To address this issue, sub-decadal medians 

were calculated and used for trend analysis. This increased the 

number of observations at the expense of increased variability. 
Results: Identification and Quantification of Groundwater-Quality Trends 9

Mann-Kendall trend tests were then applied to the decadal 

and sub-decadal median concentrations in each basin for 

each constituent to identify trends in groundwater quality. 

Some basins also were sub-divided and trends were assessed 

in sub-basins of larger basins to focus on changes in water 

quality at a smaller spatial scale (fig. 1). Results from trend 

tests on the combined NWIS and SDWIS data and the SDWIS 

data are presented. The SDWIS trend results are included 

because they represent drinking water sources (before 

any treatment) and may therefore be of interest to public 

water suppliers.

Identified trends were compared to land-use change 

in each basin. To identify the connection between surface 

practices and groundwater quality, trends in wells where land 

use has changed in each basin were evaluated to determine 

the relationship between land-use change and trends in 

concentrations of arsenic, nitrate, and dissolved solids. 

Land-use changes in each basin were identified throughout the 

study area by comparing land use in 2012 to land use in 1974. 

The USGS National Water-Quality Assessment Wall-to-Wall 

Anthropogenic Land Use Trends dataset contains national 

60-meter, 19-class mapping of anthropogenic land uses for 

five periods between 1974 and 2012 (Falcone, 2015). The 

dataset contains six broad land-use classes including water, 

developed, semi-developed, production, low use, and very 

low use/conservation. Developed land includes the built 

environment such as residences, places of employment, and 

recreation. Production land includes areas where natural 

resources are produced such as agricultural or natural resource 

extraction. These classes were lumped so that urban included 

developed and semi-developed land, and low use included 

low use and very low use/conservation in order to increase 

the number of wells in each class. Wells were classified based 

on the kind of land-use change (including no change) that 

had occurred directly at the well location (within 60-m grid 

cell) from 1974 to 2012. Mann-Kendall trend tests were then 

applied to decadal and sub-decadal medians in each basin for 

each constituent for all land-use change classes.

Well characteristics can change as land-use changes. 

For example, as more development of an area occurs, water 

demand may increase, prompting an increase in the number of 

wells or in the depth to which wells are drilled. Water quality 

can change with depth in a well. To avoid the confounding 

effects resulting from a potential increase in deeper wells as an 

area develops over time, NWIS wells shallower than 200 feet 

depth also were tested. Depth data was not available for many 

SDWIS wells and so SDWIS data was therefore not used for 

this part of the analysis. These shallow wells were expected 

to be the first to experience possible impacts from land-use 

change as well.

Results: Identification and 

Quantification of Groundwater-Quality 

Trends

Results of a comparison of data from each database and 

the combination of datasets is presented below, followed by a 

description of the trends analysis, and a comparison of trends 

to land-use change patterns.

Data Summary and Database Comparison

Generally, there are more data from the SDWIS database 

than the NWIS database. These results show differences and 

similarities between datasets from each database and how 

these differences may influence trend results. Variability across 

datasets introduces variability into the trend tests, which 

makes trend identification more difficult.

Arsenic

Widespread measurement of arsenic concentrations 

in wells began in the mid to late 1970s, roughly coincident 

with enactment of the Safe Drinking Water Act of 1974 

(fig. 2; table 2). The SDWIS database contained more 

arsenic concentration data than the NWIS database, and 

generally covered a longer period of record. The number of 

measurements varied greatly by basin. Some sub-basins had 

fewer than 10 wells and fewer than 20 samples, and the period 

of record may only have extended back to the late 1980s, 

which increases the uncertainty in interpreting results for 

those areas.

Generally, the percentage of censored data in each basin 

and sub-basin was low, although many basins had between 30 

and 50 percent censored data (table 2). The SDWIS data had 

a higher percentage of censored values than NWIS arsenic 

data and several basins had more than 50 percent censoring. 

This violates the recommendations for fewer than 50 percent 

censoring for the methods used in this study and therefore 

the results for these data are less reliable. When combining 

the NWIS and SDWIS data, there were fewer than 50 percent 

censored data in each basin except the East Shore Area. The 

NWIS data have fewer censored values than the SDWIS data.
10 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

National Water Information System data 

Safe Drinking Water Information System data

2

4

6

8

10

12

5

10

15

1

2

3

4

5

6

2

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5

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50

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Lower

Bear

River

Basin

Lower

Bear

River

W Basin

Lower

Bear

River

E Basin

Cache

Valley

N

Cache

Valley

S

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Cedar

City

Valley S

Cedar

City

Valley N

Utah

Valley

SW

Utah

Valley

SE

Utah

Valley

NE

Utah

Valley

NW

Salt

Lake

Valley SW

Salt

Lake

Valley SE

Salt

Lake

Valley NE

Salt

Lake

Valley NW

East

Shore

Area E

East

Shore

Area W

A. Basins B. Sub-basins

Number of arsenic samplesNumber of arsenic samples

0

5

10

15

20

25

2

4

6

8

10

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0

0

10

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1980 1990 2000 2010

Year

1980 1990 2000 2010

Year

EXPLANATION

N = north E = east S = south W = west

Figure 2. Number of arsenic samples over time in select Utah A, basins and B, sub-basins in the National Water 

Information System and Safe Drinking Water Information System datasets.
Results: Identification and Quantification of Groundwater-Quality Trends 11

Table 2. Number of wells and arsenic samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS), Safe Drinking Water Information System (SDWIS), 

and combined NWIS and SDWIS data.

[Number in parentheses indicates the total number of wells. Abbrevations: μg/L, micrograms per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(μg/L)

Maximum 

concentration 

(μg/L)

Median 

concentration 

(μg/L)

Basins

NWIS and SDWIS data combined (1,337)

Beryl-Enterprise Area 23 1978 2015 90 15 17 0.04 95.7 3.8

Cache Valley 74 1975 2015 293 116 40 0.02 42.4 0.9

Cedar City Valley 58 1978 2015 212 70 33 0.1 15.7 2

East Shore Area 150 1976 2015 544 306 56 0.1 50 0.7

Lower Bear River Basin 80 1978 2015 356 116 33 0.1 106 2

Milford Valley 43 1978 2015 176 6 3 1 39 6.6

Northern Juab Valley 21 1978 2015 63 27 43 0.19 10 0.7

Pahvant Valley 61 1978 2015 115 17 15 0.21 19 2

Parowan Valley 17 1978 2015 53 6 11 0.5 11.3 3.8

Salt Lake Valley 412 1975 2015 1,814 499 28 0.005 360 2.1

Sevier Desert 78 1978 2015 231 20 9 0.08 730 8

Tooele Valley 125 1977 2015 421 108 26 0.005 206 1.5

Utah Valley 195 1977 2015 704 286 41 0.1 72.9 1.1

NWIS data (598)

Beryl-Enterprise Area 18 2005 2015 52 1 2 0.04 95.7 3.9

Cache Valley 25 1989 2015 59 7 12 0.02 23.5 1

Cedar City Valley 11 2005 2015 38 0 0 0.3 6.4 0.88

East Shore Area 24 1989 2015 56 6 11 0.1 44 3.7

Lower Bear River Basin 29 1989 2015 51 2 4 0.1 95 1

Milford Valley 23 2005 2015 50 0 0 1.4 34.7 3.2

Northern Juab Valley 9 2005 2015 17 0 0 0.19 2.2 0.68

Pahvant Valley 54 1985 2015 94 7 7 0.21 19 2.3

Parowan Valley 12 2005 2015 36 0 0 1.5 11.3 4

Salt Lake Valley 182 1983 2015 423 48 11 0.005 360 5

Sevier Desert 51 1979 2015 79 2 3 0.08 730 8

Tooele Valley 87 1977 2015 251 30 12 0.005 206 1.8

Utah Valley 73 1989 2015 128 6 5 0.1 18 2.1

SDWIS data (739)

Beryl-Enterprise Area 5 1978 2014 38 14 37 0.1 10 2.9

Cache Valley 49 1975 2015 234 109 47 0.3 42.4 0.8

Cedar City Valley 47 1978 2015 174 70 40 0.1 15.7 2.4

East Shore Area 126 1976 2015 488 300 61 0.1 50 0.7

Lower Bear River Basin 51 1978 2015 305 114 37 0.2 106 2.3

Milford Valley 20 1978 2015 126 6 5 1 39 9

Northern Juab Valley 12 1978 2014 46 27 59 0.4 10 0.7

Pahvant Valley 7 1978 2015 21 10 48 0.5 10 1

Parowan Valley 5 1978 2013 17 6 35 0.5 8 2
12 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 2. Number of wells and arsenic samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS), Safe Drinking Water Information System (SDWIS), 

and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: μg/L, micrograms per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(μg/L)

Maximum 

concentration 

(μg/L)

Median 

concentration 

(μg/L)

Basins—Continued

SDWIS data (739)—Continued

Salt Lake Valley 230 1975 2015 1,391 451 32 0.1 50 1.9

Sevier Desert 27 1978 2015 152 18 12 0.5 62 8

Tooele Valley 38 1978 2015 170 78 46 0.1 23 1.1

Utah Valley 122 1977 2015 576 280 49 0.1 72.9 0.9

Sub-basins

NWIS and SDWIS data combined (969)

Cache Valley N 22 1978 2015 105 41 39 0.02 42.4 2.0

Cache Valley S 52 1975 2015 188 75 40 0.09 25.0 0.7

Cedar City Valley N 21 1978 2015 74 27 36 0.5 11.7 3.0

Cedar City Valley S 37 1981 2015 138 43 31 0.1 15.7 1.6

East Shore Area E 124 1976 2015 462 282 61 0.1 50.0 0.6

East Shore Area W 26 1978 2015 82 24 29 0.5 42.5 3.1

Lower Bear River Basin 

E

48 1978 2015 231 84 36 0.1 106.0 1.7

Lower Bear River Basin 

W

32 1978 2015 125 32 26 0.2 95.0 2.2

Salt Lake Valley NE 66 1978 2015 259 108 42 0.1 60.0 0.8

Salt Lake Valley NW 23 1983 2014 33 4 12 1 360.0 20.0

Salt Lake Valley SE 153 1976 2015 634 233 37 0.1 60.0 0.9

Salt Lake Valley SW 170 1975 2015 888 154 17 0.005 110.0 6.0

Utah Valley NE 97 1977 2015 325 145 45 0.1 72.9 1.0

Utah Valley NW 7 1997 2014 17 1 6 0.5 34.0 4.0

Utah Valley SE 72 1978 2015 272 137 50 0.1 53.0 0.7

Utah Valley SW 19 1980 2015 90 3 3 0.5 18.0 9.2

NWIS data (344)

Cache Valley N 7 1989 2015 20 3 15 0.02 17.3 5.9

Cache Valley S 18 1989 2015 39 4 10 0.09 23.5 0.9

Cedar City Valley N 2 2007 2013 4 0 0 2 3.0 2.3

Cedar City Valley S 9 2005 2015 34 0 0 0.3 6.4 0.9

East Shore Area E 12 1989 2015 22 6 27 0.1 44.0 0.7

East Shore Area W 12 1989 2015 34 0 0 0.84 42.5 14.0

Lower Bear River Basin 

E

10 1989 2015 18 2 11 0.1 7.3 1.7

Lower Bear River Basin 

W

19 1989 2015 33 0 0 0.66 95.0 1.0

Salt Lake Valley NE 26 1983 2015 52 6 12 0.34 60.0 1.1

Salt Lake Valley NW 23 1983 2014 33 4 12 1 360.0 20.0

Salt Lake Valley SE 59 1983 2015 115 13 11 0.12 60.0 1.0
Results: Identification and Quantification of Groundwater-Quality Trends 13

Table 2. Number of wells and arsenic samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS), Safe Drinking Water Information System (SDWIS), 

and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: μg/L, micrograms per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(μg/L)

Maximum 

concentration 

(μg/L)

Median 

concentration 

(μg/L)

Sub-basins—Continued

NWIS data (344)—Continued

Salt Lake Valley SW 74 1983 2015 223 25 11 0.005 110.0 8.0

Utah Valley NE 40 1989 2015 61 3 5 0.1 6.0 1.7

Utah Valley NW 3 1998 2004 3 0 0 0.8 4.0 2.7

Utah Valley SE 15 1989 2015 30 3 10 0.4 11.0 0.6

Utah Valley SW 15 1989 2015 34 0 0 0.93 18.0 6.4

SDWIS data (625)

Cache Valley N 15 1978 2014 85 38 45 0.5 42.4 1.5

Cache Valley S 34 1975 2015 149 71 48 0.3 25.0 0.7

Cedar City Valley N 19 1978 2015 70 27 39 0.5 11.7 3.4

Cedar City Valley S 28 1981 2014 104 43 41 0.1 15.7 2.0

East Shore Area E 112 1976 2015 440 276 63 0.1 50.0 0.6

East Shore Area W 14 1978 2013 48 24 50 0.5 34.0 1.0

Lower Bear River Basin 

E

38 1978 2015 213 82 38 0.3 106.0 1.6

Lower Bear River Basin 

W

13 1978 2015 92 32 35 0.2 62.0 3.2

Salt Lake Valley NE 40 1978 2015 207 102 49 0.1 11.0 0.7

Salt Lake Valley SE 94 1976 2015 519 220 42 0.1 23.0 0.9

Salt Lake Valley SW 96 1975 2015 665 129 19 0.1 50.0 5.0

Utah Valley NE 57 1977 2015 264 142 54 0.5 72.9 0.7

Utah Valley NW 4 1997 2014 14 1 7 0.5 34.0 10.8

Utah Valley SE 57 1978 2015 242 134 55 0.1 53.0 0.7

Utah Valley SW 4 1980 2015 56 3 5 0.5 14.8 10.6
14 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

The maximum concentration in most basins and all 

sub-basins was at or above the arsenic MCL of 10 ug/L for 

NWIS and SDWIS data combined. The NWIS data in several 

basins including Sevier Valley, Salt Lake Valley, and Tooele 

Valley had maximum concentrations of more than 100 ug/L. 

However, for NWIS data, SDWIS data, and combined data, 

the median concentration in all basins was below 10 ug/L and 

most were below 5 ug/L. A paired two-sided t-test indicated 

that the medians of the NWIS and SDWIS datasets, the 

SDWIS and combined datasets, and the NWIS and combined 

datasets were not significantly different (p-value greater than 

0.05). Among NWIS data, some sub-basins had maximum 

concentrations below the MCL such as Cedar City Valley 

North, Cedar City Valley South, Lower Bear River Basin East, 

Utah Valley Northeast, and Utah Valley Northwest. Among 

SDWIS data, all sub-basins had maximum concentrations 

above the MCL.

The distribution of concentrations in individual and 

combined datasets is shown for each basin and sub-basin 

in figure 3. Most concentrations in each basin fell below 

the MCL. However, concentrations in the Sevier Desert, 

Milford Valley, and Beryl-Enterprise Area were generally 

elevated relative to the other basins and had more regulatory 

exceedances. The distribution of concentrations in individual 

and combined datasets was generally similar within a given 

basin. However, in some basins the distributions vary; for 

example, in Milford Valley, the NWIS interquartile range 

(IQR) was completely below and outside the IRQ of the 

SDWIS and combined datasets, indicating that the NWIS data 

was distinctly lower than the SDWIS and combined datasets 

in this area. The NWIS distribution extended higher than the 

SDWIS distribution in some basins (for example, the East 

Shore Area) and lower in others (for example, Milford Valley 

and Cedar City Valley); the differences between datasets were 

not systematic across basins. The variability of concentrations 

also differed by basin. For example, Parowan Valley had 

a much narrower range of concentrations than the Salt 

Lake Valley.

The distribution of arsenic concentrations of NWIS, 

SDWIS, and combined NWIS and SDWIS data is shown for 

each sub-basin in figure 3. There are several sub-basins that 

had IQRs that exceed the MCL including the East Shore Area 

East, Salt Lake Valley Northwest, Salt Lake Valley Southwest, 

Utah Valley Northwest, and Utah Valley Southwest.

Arsenic concentration data in each basin for each 

dataset over time are shown in figure 4; concentrations varied 

substantially by basin. At the time of this study, widespread 

exceedance of the MCL of 10 ug/L occurred in the studied 

basins (figs. 3, 4). Some basins had many exceedances 

(for example, Lower Bear River Basin, East Shore Area, 

Utah Valley, and Milford Valley), whereas some basins had 

concentrations that exceeded the regulatory standard by a 

factor of ten (for example, Tooele Valley, Salt Lake Valley, 

and Sevier Desert). In some basins, regulatory exceedances 

were rare (for example, Northern Juab Valley, Pahvant Valley, 

and Parowan Valley). The MCL changed in 2002 from 50 to 

10 ug/L. Many basins had data that exceeded the old standard 

as well. The locations of wells with samples that exceeded the 

MCL are shown in figure 5. The Salt Lake Valley had many 

instances of regulatory exceedance. The greater number of 

samples taken may account for some of the high number of 

regulatory exceedances relative to the other basins.

Arsenic concentration data in each sub-basin for each 

dataset over time are shown in figure 4. In Cache Valley, 

Cache Valley North had higher concentration data than Cache 

Valley South, although the high concentration data is generally 

only from 2000 to 2015; whereas in Cache Valley South, more 

regulatory exceedances occurred in the period from 1975 to 

2000 than in Cache Valley North. In Cedar City Valley, the 

number, magnitude, and timing of regulatory exceedances 

was similar. In the East Shore Area, the western sub-basin 

had more exceedances among NWIS data. Lower Bear River 

Basin East had fairly regular regulatory exceedances, and 

the concentrations could be greater than 50 ug/L. In Lower 

Bear River Basin West, regulatory exceedances were rare, 

but they could be greater than 50 ug/L when they did occur. 

In the Salt Lake Valley, the Northwest sub-basin had the 

highest arsenic concentrations, followed by the southwestern 

sub-basin. Higher arsenic concentrations on the western and 

northwestern part of the Salt Lake Valley were consistent with 

the findings of Thiros (2003); the Northeast and Southeast 

had similar concentrations, with a few high concentration 

regulatory exceedances in the mid-1980s and a few infrequent 

exceedances between the 1980s and 2010s. In Utah Valley, the 

northeastern part had the highest concentrations, followed by 

the southeastern part; the northwestern sub-basin has a much 

shorter period of record compared to the rest of the basin.

The decadal and sub-decadal medians for each dataset 

and combined datasets are shown for each basin and 

sub-basin in figure 6. In general, the medians for individual 

and combined datasets were similar over time; there was no 

obvious systematic bias. The medians for NWIS, SDWIS, 

and combined data largely agreed, and the sub-decadal 

medians were more variable over time, whereas the decadal 

medians were smoother over time. Several basins had median 

concentrations that exceeded the MCL. The NWIS medians 

were consistently higher than the other medians in several 

basins (for example, the East Shore Area and Salt Lake 

Valley). The variation among medians was greatest in the 

Sevier Desert area. In Milford Valley, the NWIS period of 

record was much shorter than the SDWIS records and so the 

medians were less comparable to other medians.

In Cache Valley North, the NWIS data had a much 

shorter period of record than the SDWIS data. In the East 

Shore Area West, the NWIS medians were consistently higher 

than the SDWIS or combined data medians. In Utah Valley 

Northwest there were fewer data and the period of record was 

shorter, resulting in fewer medians than in other sub-basins, 

over a shorter period of time. There were only NWIS data in 

the Salt Lake Valley Northwest.
Results: Identification and Quantification of Groundwater-Quality Trends 15

0

1

10

100

BerylEnterprise

Area

Cache 

Valley

Cedar 

City 

Valley

East

Shore 

Area

Lower 

Bear 

River 

Basin

Northern 

Juab 

Valley

Pahvant 

Valley

Parowan 

Valley

Salt

Lake 

Valley

Sevier 

Desert

Tooele 

Valley

Utah 

Valley

Milford 

Valley

Cache 

Valley

N

Cache 

Valley

S

Cedar 

City 

Valley

N

Cedar 

City 

Valley

S

East 

Shore 

Area

E

East 

Shore 

Area

W

Lower 

Bear 

River 

Basin

W

Lower 

Bear 

River 

Basin

E

Salt 

Lake 

Valley

NE

Salt 

Lake 

Valley

NW

Salt 

Lake 

Valley

SE

Salt 

Lake 

Valley

SW

Utah 

Valley

NE

Utah 

Valley

NW

Utah 

Valley

SE

Utah 

Valley

SW

Maximum contaminant level

Maximum contaminant level

A. Basins

B. Sub-basins

0

1

10

100

1,000

Arsenic concentration, in micrograms per liter Arsenic concentration, in micrograms per liter

N = north

E = east

S = south

W = west

EXPLANATION

Combined datasets

National Water 

 Information System

Safe Drinking Water 

 Information System

Median

95th percentile

Individual observation

 above 95th percentile

Individual observation

 below 5th percentile

75th percentile

25th percentile

5th percentile

Figure 3. Arsenic concentrations in select Utah A, basins and B, sub-basins for data from the National Water Information System 

and Safe Drinking Water Information System, and combined datasets.
16 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells Arsenic concentration, in micrograms per liter

1980 1990 2000 2010

Maximum contaminant level

Censored data National Water Information System data 

Safe Drinking Water Information System data

N = north E = east S = south W = west

EXPLANATION

A. Basins B. Sub-basins

Lower

Bear

River

Basin

Lower

Bear

River

Basin W

Lower

Bear

River

Basin E

Cache

Valley

N

Cache

Valley

S

East

Shore

Area W

East

Shore

Area E

Salt

Lake

Valley

NW

Salt

Lake

Valley

NE

Salt

Lake

Valley

SE

Salt

Lake

Valley

SW

Cedar

City

Valley

N

Cedar

City

Valley

S

Utah

Valley

NW

Utah

Valley

NE

Utah

Valley

SE

Utah

Valley

SW

0

20

40

60

80

100

0

10

20

30

40

0

10

20

30

40

50

0

50

100

150

200

0

100

200

300

60

Arsenic concentration, in micrograms per liter

10

0

20

40

0

2

4

6

8

0

200

400

600

0

5

10

15

0

10

20

30

40

0

20

40

60

80

100

0

2

4

6

8

10

0

5

10

15

0

20

40

60

80

0

20

40

60

80

100

0

10

20

30

40

0

5

10

15

20

25

0

10

20

30

40

0

10

20

30

40

50

0

100

200

300

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0

20

40

60

80

100

0

10

20

30

0

20

40

60

0

10

20

30

40

50

0

5

10

15

0

2

4

6

8

10

12

0

5

10

15

1980 1990 2000 2010

Decade Decade

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Figure 4. Arsenic concentrations over time by database in select Utah A, basins and B, sub-basins.
Results: Identification and Quantification of Groundwater-Quality Trends 17

Cache

Valley

Salt Lake

Valley

Tooele Valley

Utah

Valley

Sevier

Desert Northern

Juab Valley

Pahvant

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar City

Valley

Lower Bear

River Basin

East Shore

Area

Milford

Valley

38°

39°

40°

41°

42°

114° 113° 112° 111° 110° 109°

37°

EXPLANATION

Basin boundary

Highway Concentration exceeds arsenic

 maximum contaminant level

Study basin

Base from U.S. Geological Survey 30-meter National Elevation 

Dataset Universal Transeverse Mercator projection, Zone 12, 

North American Datum of 1983 (NAD 83)

0 20 40 60 MILES

0 30 60 KILOMETERS

Figure 5. Location of wells with sample concentrations that exceed the maximum contaminant level for 

arsenic in select basins in Utah.
18 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

NWIS = National Water Information System

SDWIS = Safe Drinking Water Information System

N = north E = east S = south W = west

EXPLANATION

A. Basins B. Sub-basins

Arsenic median concentration, in micrograms per literArsenic median concentration, in micrograms per liter

2

4

6

1.0

1.5

2.0

2.5

3.0

0

5

10

15

2.5

5.0

7.5

0

5

10

2

4

6

0.6

0.7

0.8

0.9

5

10

15

1.5

2.0

2.5

3.0

5

10

15

20

0

5

10

15

2

4

6

0

1

2

3

2

4

6

8

3

6

9

12

2

4

6

2

4

6

0

5

10

15

20

0.25

0.50

0.75

1.00

0

50

100

1

2

3

4

0

5

10

5

10

15

5

10

2

4

6

8

0.5

1.0

1.5

2.0

2.5

6

8

10

12

2.0

2.5

3.0

3.5

0

1

2

1970 1980 1990 2000 2010

Decade

1970 1980 1990 2000 2010

Decade

 Combined decadal median

Combined sub-decadal median

NWIS decadal median

NWIS sub-decadal median

SDWIS decadal median

SDWIS sub-decadal median

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Lower

Bear

River

Basin W

Lower

Bear

River

Basin E

Cache

Valley

N

Cache

Valley

S

East

Shore

Area W

East

Shore

Area E

Salt

Lake

Valley

NW

Salt

Lake

Valley

NE

Salt

Lake

Valley

SE

Salt

Lake

Valley

SW

Cedar

City

Valley

N

Cedar

City

Valley

S

Utah

Valley

NW

Utah

Valley

NE

Utah

Valley

SE

Utah

Valley

SW

Figure 6. Decadal and sub-decadal median arsenic concentration in select A, basins and B, sub-basins in Utah.
Results: Identification and Quantification of Groundwater-Quality Trends 19

Nitrate

Widespread measurement of nitrate concentrations in 

wells began in the mid to late 1970s, roughly coincident with 

enactment of the Safe Drinking Water Act of 1974, although 

the purpose of many USGS studies (NWIS data) at that 

time was to document the suitability of water resources for 

use (table 3; fig. 7). The SDWIS database contained more 

nitrate concentration data than the NWIS database, although 

there were more NWIS data in some basins including the 

Beryl-Enterprise Area, Pahvant Valley, and Parowan Valley. 

The number of wells and measurements varied greatly by 

basin and sub-basin. Several sub-basins had fewer than 

10 wells including Utah Valley Northwest (combined NWIS 

and SDWIS data, NWIS data, and SDWIS data), and Utah 

Valley Southwest (SDWIS data only); this reduces the ability 

to detect trends in these areas. Generally, unfiltered nitrate 

samples from the SDWIS database, filtered nitrate plus nitrite 

samples from the NWIS database, and unfiltered nitrate plus 

nitrite samples from the SDWIS database were the most 

numerous sample types in each basin (fig. 7).

Generally, the percentage of censored measurements in 

each basin and sub-basin was low and was below 50 percent 

in all basins (table 3). For the combined datasets, the Sevier 

Desert had the highest percentage of censored values with 

19 percent censored values.

Maximum concentrations exceeded the nitrate MCL of 

10 mg/L in all basins except Parowan Valley for NWIS and 

SDWIS combined data. The NWIS maximum concentration 

was above the nitrate MCL in most basins, whereas the 

SDWIS maximum concentration was above the MCL in 6 out 

of 13 basins. The median concentrations in all basins for 

NIWS data, SDWIS data, and combined NWIS and SDWIS 

data were below 5 mg/L. Many sub-basins had maximum 

concentrations that exceeded the MCL, although the median 

in all basins for all datasets was well below the MCL. A paired 

two-sided t-test indicated that the medians of the NWIS and 

SDWIS datasets and the SDWIS and combined datasets are 

statistically different (p-value less than 0.05). The medians 

of the NWIS and combined datasets were not significantly 

different (p-value greater than 0.05).

The distribution of concentrations in individual and 

combined datasets is shown for each basin in figure 8. The 

IQR of concentrations in each basin and for individual and 

combined datasets fell below the MCL. The distribution of 

concentrations for individual and combined datasets was 

generally similar within a given basin. In some basins, the 

distributions of datasets varied. In Cache Valley and the East 

Shore Area, the NWIS IQR extended much lower than the 

IQR of SDWIS or combined datasets. Northern Juab Valley 

and Pahvant Valley had the highest IQRs relative to the other 

basins, whereas the Salt Lake Valley had the highest outlier 

values. The variability of concentrations also differed by basin. 

For example, the Beryl-Enterprise Area had a much narrower 

range of concentrations than the Sevier Desert.

The distribution of nitrate concentrations of NWIS, 

SDWIS, and combined NWIS and SDWIS data also is shown 

for each sub-basin in figure 8. The IQR of all data types fell 

below the MCL except NWIS data in Utah Valley Southwest. 

Most basins had some data above the MCL. The IQR for 

each data type within a sub-basin were generally similar with 

a few notable exceptions. The IQR of NWIS data in Cache 

Valley North and East Shore Area West was much lower than 

the SDWIS or combined datasets. The IQR of NWIS and 

combined datasets in the Salt Lake Valley Northwest were the 

same because there is no SDWIS data from this area. The Salt 

Lake Valley Northeast had the highest concentration data.

Nitrate concentration data in each basin and sub-basin for 

each database over time are shown in figure 9. Concentrations 

varied substantially by basin. Some basins had many or severe 

MCL exceedances (for example, Cache Valley, Tooele Valley, 

Salt Lake Valley, Utah Valley, and Pahvant Valley). In some 

basins, regulatory exceedances were rare or non-existent (for 

example, Northern Juab Valley, Beryl-Enterprise Area, and 

Parowan Valley). Exceedances occur in SDWIS and, more 

commonly, NWIS data. The locations of wells with nitrate 

samples that exceeded the MCL are shown in figure 10.

Concentrations exceeded the MCL in nearly every 

sub-basin except Cache Valley South, Cedar City Valley 

South, Salt Lake Valley Northwest, Utah Valley Northeast, and 

Utah Valley Northwest. In Cache Valley, Cache Valley North 

had more high concentration data than Cache Valley South. In 

Cedar City Valley, the concentration data were similar except 

for some higher concentration NWIS data from the early 

2000s in the northern sub-basin. In the East Shore Area, the 

West sub-basin had overall lower concentration data, and both 

areas had very few samples that exceeded the MCL. In the 

Lower Bear River Basin, the West sub-basin had more data 

that exceeds the MCL, although these samples were NWIS 

data that may not represent water used for drinking water. In 

the Salt Lake Valley, the Northeast and Southwest sub-basins 

had the highest nitrate concentration data, although these data 

were from only a few samples. The data in the Northwest 

sub-basin were all below the MCL. The Southeast sub-basin 

had some data that exceeded the MCL, although it was all 

NWIS data that may not represent water used for drinking 

water. In Utah Valley, the Southwest and Southeast sub-basins 

had the highest concentrations. All of the data that exceeded 

the MCL in the Southwest sub-basin came from the NWIS 

database and may not represent drinking water.

The decadal and sub-decadal median nitrate 

concentrations for individual and combined datasets in each 

basin and sub-basin are shown in figure 11. Medians were 

below the MCL of 10 mg/L in all basins, although they were 

above the MCL in several sub-basins (Cache Valley North, 

Utah Valley Southwest, and Cedar City Valley North). In 

general, the medians for individual and combined datasets 

were similar within a basin or sub-basin. The NWIS medians 

were higher than the other medians in several basins (for 

example, Milford Valley and Cedar City Valley). The 

variation among medians for different datasets was greatest in 

Pahvant Valley.
20 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 3. Number of wells and nitrate samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number 

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

NWIS and SDWIS data combined (1,857)

Beryl-Enterprise Area 35 1975 2015 306 2 1 0.04 10.0 1.93

Cache Valley 97 1975 2015 818 41 5 0.01 18.9 1.26

Cedar City Valley 107 1975 2015 720 15 2 0.02 19.5 1

East Shore Area 212 1975 2015 1,731 212 12 0.01 18.0 1

Lower Bear River Basin 96 1975 2015 809 45 6 0.001 27.9 1.08

Milford Valley 55 1975 2015 377 23 6 0.01 40.3 0.77

Northern Juab Valley 40 1975 2015 208 1 0 0.01 42.0 3.4

Pahvant Valley 78 1975 2015 363 4 1 0.02 43.3 3.2

Parowan Valley 44 1975 2015 160 20 13 0.01 6.4 1.35

Salt Lake Valley 486 1975 2015 3,934 262 7 0.01 86.0 1.4

Sevier Desert 87 1975 2015 412 79 19 1.00E–06 22.0 0.37

Tooele Valley 223 1975 2015 1,032 12 1 0.02 36.9 1.7

Utah Valley 297 1975 2015 2,344 153 7 9.00E–04 46.0 0.83

NWIS data (1,051)

Beryl-Enterprise Area 29 1975 2015 206 0 0 0.04 10.0 1.96

Cache Valley 37 1979 2015 86 14 16 0.02 8.9 0.6

Cedar City Valley 58 1975 2015 149 0 0 0.035 19.5 2.02

East Shore Area 77 1975 2015 171 54 32 0.01 18.0 0.3

Lower Bear River Basin 42 1975 2015 120 17 14 0.01 27.9 1.68

Milford Valley 34 1975 2015 177 1 1 0.08 40.3 2.49

Northern Juab Valley 28 1975 2015 88 0 0 0.46 42.0 4.9

Pahvant Valley 71 1975 2015 297 3 1 0.05 43.3 3.2

Parowan Valley 39 1975 2015 109 3 3 0.04 6.4 1.71

Salt Lake Valley 239 1976 2015 626 86 14 0.01 86.0 1.43

Sevier Desert 58 1975 2015 127 5 4 0.01 22.0 0.58

Tooele Valley 175 1975 2015 437 8 2 0.02 36.9 2.53

Utah Valley 164 1975 2015 321 36 11 0.02 46.0 1.3

SDWIS data (806)

Beryl-Enterprise Area 6 1978 2015 100 2 2 0.1 7.2 1.64

Cache Valley 60 1975 2015 732 27 4 0.01 18.9 1.35

Cedar City Valley 49 1977 2015 571 15 3 0.02 10.6 0.9

East Shore Area 135 1976 2015 1,560 158 10 0.01 11.6 1.02

Lower Bear River Basin 54 1977 2015 689 28 4 0.001 15.6 1

Milford Valley 21 1978 2015 200 22 11 0.01 6.4 0.4

Northern Juab Valley 12 1978 2015 120 1 1 0.01 9.7 2.9

Pahvant Valley 7 1978 2015 66 1 2 0.02 9.1 3.2

Parowan Valley 5 1978 2015 51 17 33 0.01 1.2 0.2

Salt Lake Valley 247 1975 2015 3,308 176 5 0.01 70.0 1.4
Results: Identification and Quantification of Groundwater-Quality Trends 21

Table 3. Number of wells and nitrate samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

SDWIS data (806)—Continued

Sevier Desert 29 1978 2015 285 74 26 1.00E–06 6.2 0.3

Tooele Valley 48 1977 2015 595 4 1 0.02 5.3 1.15

Utah Valley 133 1976 2015 2,023 117 6 9.00E–04 23.1 0.8

Sub-basins

NWIS and SDWIS data combined (1,295)

Cache Valley N 28 1977 2015 318 23 7 0.01 18.85 1.7

Cache Valley S 69 1975 2015 500 18 4 0.01 8.84 0.88

Cedar City Valley N 37 1977 2015 291 3 1 0.035 19.5 1.1

Cedar City Valley S 70 1975 2015 429 12 3 0.02 8.98 0.9

East Shore Area E 176 1975 2015 1,549 156 10 0.01 11.6 1.1

East Shore Area W 36 1975 2015 182 56 31 0.01 18 0.2

Lower Bear River Basin E 60 1977 2015 563 42 7 0.001 15.6 0.8

Lower Bear River Basin W 36 1975 2015 246 3 1 0.05 27.9 2.25

Salt Lake Valley NE 82 1976 2015 615 42 7 0.01 86.0 1.6

Salt Lake Valley NW 25 1976 2014 41 29 71 0.01 5.0 0.0

Salt Lake Valley SE 179 1976 2015 1,715 64 4 0.01 21.0 1.2

Salt Lake Valley SW 200 1975 2015 1,563 127 8 0.01 70.0 1.7

Utah Valley NE 165 1976 2015 1,020 47 5 9.00E–04 5.9 0.8

Utah Valley NW 9 1980 2015 32 1 3 0.06 4.6 1.1

Utah Valley SE 92 1975 2015 1,151 89 8 0.01 23.1 0.8

Utah Valley SW 31 1975 2015 141 16 11 0.01 46.0 1.5

NWIS data (617)

Cache Valley N 10 1979 2015 31 7 23 0.02 8.9 0.1

Cache Valley S 27 1979 2015 55 7 13 0.037 6.7 1.2

Cedar City Valley N 18 1977 2013 39 0 0 0.035 19.5 1.5

Cedar City Valley S 40 1975 2015 110 0 0 0.25 9.0 2.1

East Shore Area E 57 1975 2015 109 17 16 0.01 11.2 1.0

East Shore Area W 20 1975 2014 62 37 60 0.01 18.0 0.0

Lower Bear River Basin E 19 1977 2015 57 16 28 0.01 11.0 0.4

Lower Bear River Basin W 23 1975 2015 63 1 2 0.05 27.9 2.6

Salt Lake Valley NE 39 1976 2015 94 8 9 0.019 86.0 4.3

Salt Lake Valley NW 25 1976 2014 41 29 71 0.01 5.0 0.0

Salt Lake Valley SE 77 1976 2015 208 12 6 0.01 21.0 1.2

Salt Lake Valley SW 98 1976 2015 283 37 13 0.03 25.0 1.9

Utah Valley NE 102 1976 2015 166 24 14 0.02 4.4 0.9

Utah Valley NW 5 1980 2004 5 1 20 0.06 3.1 2.0

Utah Valley SE 30 1975 2015 74 8 11 0.02 15.4 1.5

Utah Valley SW 27 1975 2015 76 3 4 0.05 46.0 4.3
22 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 3. Number of wells and nitrate samples; period of record; and minimum, maximum, and median concentration in select Utah 

basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

Sub-basins—Continued

SDWIS data (678)

Cache Valley N 18 1977 2015 287 16 6 0.01 18.9 2.0

Cache Valley S 42 1975 2015 445 11 2 0.01 8.8 0.8

Cedar City Valley N 19 1977 2015 252 3 1 0.07 10.6 1.1

Cedar City Valley S 30 1977 2015 319 12 4 0.02 6.8 0.7

East Shore Area E 119 1976 2015 1,440 139 10 0.01 11.6 1.1

East Shore Area W 16 1977 2015 120 19 16 0.01 5.4 0.7

Lower Bear River Basin E 41 1977 2015 506 26 5 0.001 15.6 0.8

Lower Bear River Basin W 13 1978 2015 183 2 1 0.2 4.0 2.2

Salt Lake Valley NE 43 1977 2015 521 34 7 0.01 31.7 1.4

Salt Lake Valley SE 102 1976 2015 1,507 52 3 0.01 6.0 1.2

Salt Lake Valley SW 102 1975 2015 1,280 90 7 0.01 70.0 1.7

Utah Valley NE 63 1977 2015 854 23 3 9.00E–04 5.9 0.8

Utah Valley NW 4 1997 2015 27 0 0 0.1 4.6 1.1

Utah Valley SE 62 1976 2015 1,077 81 8 0.01 23.1 0.8

Utah Valley SW 4 1977 2014 65 13 20 0.01 2.2 1.1
Results: Identification and Quantification of Groundwater-Quality Trends 23

A. Basins B. Sub-basins

0

10

20

30

0

10

20

30

0

20

40

60

20

40

0

0

50

100

150

0

50

10

15

5

0

10

20

0

10

20

20

0

10

5

10

0

10

20

30

30

0

10

20

Number of nitrate samplesNumber of nitrate samples

5

10

0

10

20

5

10

15

0

10

20

2

4

6

8

0

20

40

60

5

0

10

20

0

20

40

60

0

20

40

60

3

1980 1990 2000 2010

1

2

0

20

40

60

0

20

40

60

5

0

10

20

0

10

20

N = north E = east S = south W = west

EXPLANATION Decade

1980 1990 2000 2010

Decade

1970

 Nitrate, filtered (NWIS) Nitrate + nitrite, filtered (NWIS)

 Nitrate + nitrite, filtered (SDWIS)

 Nitrate + nitrite, unfiltered (NWIS)

 Nitrate + nitrite, unfiltered (SDWIS)

 Nitrate, unfiltered (NWIS)

 Nitrate, unfiltered (SDWIS)

NWIS = National Water Information System SDWIS = Safe Drinking Water Information System

Lower

Bear

River

Basin W

Lower

Bear

River

Basin E

Cache

Valley

N

Cache

Valley

S

East

Shore

Area W

East

Shore

Area E

Salt

Lake

Valley

NW

Salt

Lake

Valley

NE

Salt

Lake

Valley

SE

Salt

Lake

Valley

SW

Cedar

City

Valley

N

Cedar

City

Valley

S

Utah

Valley

NW

Utah

Valley

NE

Utah

Valley

SE

Utah

Valley

SW

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Figure 7. Number of nitrate samples over time in select Utah A, basins and B, sub-basins in the National Water Information System 

and Safe Drinking Water Information System datasets.
24 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

0

0

0

10

N = north

E = east

S = south

W = west

EXPLANATION

Combined datasets

National Water 

 Information System

Safe Drinking Water 

 Information System

Median

95th percentile

Individual observation

 above 95th percentile

Individual observation

 below 5th percentile

75th percentile

25th percentile

5th percentile

0

0

0

10

100

100

Maximum contaminant level

Maximum contaminant level

A. Basins

B. Sub-basins

BerylEnterprise

Area

Cache 

Valley

Cedar 

City 

Valley

East

Shore 

Area

Lower 

Bear 

River 

Basin

Northern 

Juab 

Valley

Pahvant 

Valley

Parowan 

Valley

Salt

Lake 

Valley

Sevier 

Desert

Tooele 

Valley

Utah 

Valley

Milford 

Valley

Cache 

Valley

N

Cache 

Valley

S

Cedar 

City 

Valley

N

Cedar 

City 

Valley

S

East 

Shore 

Area

E

East 

Shore 

Area

W

Lower 

Bear 

River 

Basin

W

Lower 

Bear 

River 

Basin

E

Salt 

Lake 

Valley

NE

Salt 

Lake 

Valley

NW

Salt 

Lake 

Valley

SE

Salt 

Lake 

Valley

SW

Utah 

Valley

NE

Utah 

Valley

NW

Utah 

Valley

SE

Utah 

Valley

SW

Nitrate concentration, in micrograms per liter Nitrate concentration, in micrograms per liter

Figure 8. Nitrate concentrations in select Utah A, basins and B, sub-basins for data from the National Water Information 

System and Safe Drinking Water Information System, and combined datasets.
Results: Identification and Quantification of Groundwater-Quality Trends 25

0

10

20

0

5

10

15

0

5

10

15

0

10

20

30

0

25

50

75

0

10

20

30

40

0

10

20

30

40

0

5

10

15

20

0

10

20

30

40

0

10

20

30

40

0.0

2.5

5.0

7.5

10.0

0.0

2.5

5.0

7.5

10.0

0

5

10

15

20

0

10

20

0

5

10

15

0

5

10

15

0.0

2.5

5.0

7.5

10.0

0

5

10

15

0

3

6

9

12

0.0

2.5

5.0

7.5

10.0

0

25

50

75

0

5

10

15

20

0

20

40

60

0.0

2.5

5.0

7.5

10.0

0.0

2.5

5.0

7.5

10.0

0

5

10

15

20

0

10

20

30

40

0

5

10

15

20

0.0

2.5

5.0

7.5

10.0

A. Basins B. Sub-basins

1980 1990 2000 2010

Decade

1980 1990 2000 2010

Decade

Lower

Bear

River

Basin W

Lower

Bear

River

Basin E

Cache

Valley

N

Cache

Valley

S

East

Shore

Area W

East

Shore

Area E

Salt

Lake

Valley

NW

Salt

Lake

Valley

NE

Salt

Lake

Valley

SE

Salt

Lake

Valley

SW

Cedar

City

Valley

N

Cedar

City

Valley

S

Utah

Valley

NW

Utah

Valley

NE

Utah

Valley

SE

Utah

Valley

SW

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Nitrate concentration, in milligrams per literNitrate concentration, in milligrams per liter

Maximum contaminant level

Censored data National Water Information System data 

Safe Drinking Water Information System data

N = north E = east S = south W = west

EXPLANATION

Figure 9. Nitrate concentrations over time by dataset in select Utah A, basins and B, sub-basins.
26 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Cache

Valley

Salt Lake

Valley

Tooele Valley

Utah

Valley

Sevier

Desert Northern

Juab Valley

Pahvant

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar City

Valley

Lower Bear

River Basin

East Shore

Area

Milford

Valley

38°

39°

40°

41°

42°

114° 113° 112° 111° 110° 109°

37°

EXPLANATION

Basin boundary

Highway Concentration exceeds nitrate

 maximum contaminant level

Study basin

Base from U.S. Geological Survey 30-meter National Elevation 

Dataset Universal Transeverse Mercator projection, Zone 12, 

North American Datum of 1983 (NAD 83)

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Figure 10. Location of wells with samples that exceed the Maximum Contaminant Level for nitrate in select 

basins in Utah.
Results: Identification and Quantification of Groundwater-Quality Trends 27

1

2

0

1

2

3

0.0

0.5

1.0

1.5

1

2

3

4

1

2

3

1.0

1.5

2.0

2

3

4

5

6

0.4

0.8

1.2

1.6

2

4

6

0

1

2

3

4

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

1

2

3

4

A. Basins B. Sub-basins

Nitrate median concentration, in milligrams per literNitrate median concentration, in milligrams per liter

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

1970 1980 1990 2000 2010 1970 1980 1990 2000 2010

Decade Decade

2

0.00

0.25

0.50

0.75

1.00

1.25

0

5

10

15

0.8

1.2

1.6

0.0

0.2

0.4

0.6

0.8

0

2

4

6

8

0.02

0.03

0.04

0.05

0.06

0.07

2

4

6

0.6

0.8

1.0

1.2

1.4

1.6

1

2

3

4

0.4

0.8

1.2

0.5

1.0

0.5

1.0

1.5

2.0

2.5

3.0

5

10

15

2.5

5.0

7.5

10.0

1

2

4

6 Lower

Bear

River

W Basin

Lower

Bear

River

E Basin

Cache

Valley

N

Cache

Valley

S

Cedar

City

Valley S

Cedar

City

Valley N

Utah

Valley

SW

Utah

Valley

SE

Utah

Valley

NE

Utah

Valley

NW

Salt

Lake

Valley SW

Salt

Lake

Valley SE

Salt

Lake

Valley NE

Salt

Lake

Valley NW

East

Shore

Area E

East

Shore

Area W

EXPLANATION

NWIS = National Water Information System

SDWIS = Safe Drinking Water Information System

N = north E = east S = south W = west

 Combined decadal median

 Combined sub-decadal median NWIS sub-decadal median SDWIS sub-decadal median

 NWIS decadal median SDWIS decadal median

Figure 11. Decadal and sub-decadal median nitrate concentration in select A, basins and, B sub-basins in Utah.
28 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Dissolved Solids

The period of record and number of measurements 

of dissolved-solids concentrations in wells is shown in 

table 4 and figure 12. The SDWIS database contained more 

dissolved solids concentration data than the NWIS database, 

although there were more NWIS measurements in some 

basins including the Beryl-Enterprise Area, Milford Valley, 

Northern Juab Valley, Pahvant Valley, Parowan Valley, Sevier 

Desert, and Tooele Valley. Utah Valley Northwest was the 

only sub-basin with fewer than 10 wells for combined NWIS 

and SDWIS data, and there were only 18 samples in this area. 

This makes trend identification more difficult. The number of 

measurements varied greatly by basin. Generally, dissolved 

solids as the residual of evaporation from the SDWIS 

database and as the sum of constituents from the NWIS 

database were the most numerous sample types in each basin 

(fig. 12). The sum of constituents depends on the number of 

constituents measured.

None of the dissolved solids data were censored (table 4). 

The maximum concentration in all basins was above the 

dissolved solids SMCL of 500 mg/L. However, the median 

concentration in many basins was below 500 mg/L. Among 

NWIS data, eight basins had medians greater than the SMCL 

of 500 mg/L, whereas among SDWIS data, no basins had 

medians greater than 500 mg/L. A paired two-sided t-test 

indicates that the medians of the NWIS and SDWIS datasets, 

the NWIS and combined datasets, and the SDWIS and 

combined datasets were statistically different (p-value less 

than 0.05). The increased variability this introduces makes 

trend identification more difficult when combining NWIS and 

SDWIS data.

For combined NWIS and SDWIS data, the maximum 

dissolved solids concentration in each sub-basin was greater 

than 500 mg/L. The median in each sub-basin was below 

the SMCL in all basins except 6 out of 16 sub basins, and 

the highest median was below the MCL of 2,000 mg/L 

at 1,270 mg/L in the Salt Lake Valley Northwest. The 

distribution of concentrations in individual and combined 

datasets is shown for each basin in figure 13. The IQR of 

concentrations in each basin and for individual and combined 

datasets fell below the MCL except in Pahvant Valley. The 

IQR exceeded the supplier requirements level of 1,000 mg/L 

in Lower Bear River Basin, Tooele Valley, Sevier Desert, 

and Pahvant Valley although this was often only for NWIS 

samples, which are taken from wells with a range of purposes, 

not just drinking-water supply. Lower water quality may be 

acceptable when the water is not used for public supply. The 

IQRs of all basins exceeded the SMCL except in Cache Valley 

and Parowan Valley, where no IQR exceeded the SMCL. 

For SDWIS data, the IQR exceeded the SMCL in Cedar City 

Valley, Northern Juab Valley, Salt Lake Valley, and Tooele 

Valley. The distribution of concentrations in individual and 

combined datasets was generally similar within a given basin.

However, in some basins, the distributions of particular 

datasets vary. The NWIS IQR often extended higher than 

the SDWIS IQR. This is generally expected because SDWIS 

samples come from wells used for public supply, and may 

therefore be biased toward higher quality, whereas NWIS 

samples come from wells used for a range of purposes 

including agriculture irrigation or industrial applications where 

quality considerations are different.

Among sub-basins, the distributions of dissolved-solids 

concentrations varied substantially, although within each basin 

the distributions for NWIS, SDWIS, and combined datasets 

generally aligned (fig. 13). The IQRs for all sub-basins were 

below the MCL of 2,000 mg/L except in the Salt Lake Valley 

Northeast. The Salt Lake Valley Northwest had the highest 

IQR and highest concentration (20,900 mg/L from a shallow 

well near the Great Salt Lake). The SDWIS IQRs for many 

sub-basins were below the MCL of 500 mg/L, except in 

Lower Bear River Basin West; Salt Lake Valley Northeast and 

Southwest; Utah Valley Northwest and Southwest; and Cedar 

City Valley South. The NWIS IQR often extended higher than 

the SDWIS IRQ, although exceptions occurred and there was 

often substantial overlap.

Dissolved solids concentration data in each basin for each 

database over time are shown in figure 14. Concentrations 

varied substantially by basin and sub-basin. Some basins 

had many or severe MCL exceedances (for example, Tooele 

Valley, Salt Lake Valley, Sevier Desert, and Pahvant Valley). 

In some basins, concentrations exceeding the MCL were 

rare or non-existent (for example, Northern Juab Valley, 

Beryl-Enterprise Area, and Parowan Valley). Exceedances 

occurred in SDWIS and, more commonly, NWIS data. The 

locations of wells with dissolved solids samples that exceeded 

the MCL are shown in figure 15.

Within and among sub-basins, dissolved-solids 

concentrations varied (fig. 14). In Cache Valley, the northern 

sub-basin had more high-concentration data than the 

southern part. In Cedar City Valley and East Shore Area, both 

sub-regions had relatively few concentrations greater than 

2,000 mg/L. There were a few concentrations greater than 

2,000 mg/L in Lower Bear River West and none in Lower 

Bear River East. In the Salt Lake Valley, the Northwest 

and Northeast had several samples with concentrations of 

more than 10,000 mg/L; these were all NWIS data. In the 

Salt Lake Valley Southeast, all SDWIS data were below 

2,000 mg/L and in the Salt Lake Valley Southwest there were 

only a few SDWIS concentrations greater than 2,000 mg/L. 

Concentrations were generally lower than 1,000 mg/L in Utah 

Valley sub-basins, although there were limited data over a 

shorter period of record in the northeastern area.
Results: Identification and Quantification of Groundwater-Quality Trends 29

Table 4. Number of wells and dissolved solids samples; period of record; and minimum, maximum, and median concentration in select 

Utah basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number 

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

Basins

NWIS and SDWIS data combined (1,955)

Beryl-Enterprise Area 36 1975 2015 262 0 0 125 1,950 410

Cache Valley 88 1975 2015 352 0 0 150 1,986 288

Cedar City Valley 104 1975 2015 386 0 0 110 3,070 426

East Shore Area 248 1975 2015 958 0 0 28 4,000 298

Lower Bear River Basin 95 1975 2015 414 0 0 88 2,360 324

Milford Valley 57 1975 2015 370 0 0 156 10,200 456

Northern Juab Valley 40 1975 2015 165 0 0 18 2,940 794

Pahvant Valley 79 1975 2015 340 0 0 10 6,520 961

Parowan Valley 46 1975 2015 134 0 0 135 672 310

Salt Lake Valley 511 1975 2015 2,719 0 0 10 20,900 512

Sevier Desert 96 1975 2015 286 0 0 162 24,300 352

Tooele Valley 246 1975 2015 678 0 0 143 17,000 652

Utah Valley 309 1975 2015 1,083 0 0 55 2,560 314

NWIS data (1,173)

Beryl-Enterprise Area 31 1975 2015 224 0 0 125 1,950 432

Cache Valley 38 1979 2015 91 0 0 174 1,730 295

Cedar City Valley 60 1975 2015 160 0 0 183 3,070 541

East Shore Area 112 1975 2015 245 0 0 122 4,000 371

Lower Bear River Basin 42 1975 2015 130 0 0 118 1,920 521

Milford Valley 36 1975 2015 194 0 0 189 10,200 547

Northern Juab Valley 28 1975 2015 106 0 0 262 2,940 827

Pahvant Valley 72 1975 2015 311 0 0 305 6,520 1,050

Parowan Valley 41 1975 2015 115 0 0 148 672 304

Salt Lake Valley 265 1976 2015 692 0 0 71 20,900 701

Sevier Desert 69 1975 2015 159 0 0 193 24,300 553

Tooele Valley 201 1975 2015 454 0 0 143 17,000 771

Utah Valley 178 1975 2015 369 0 0 91 2,560 355

SDWIS data (782)

Beryl-Enterprise Area 5 1978 2014 38 0 0 160 723 304

Cache Valley 50 1975 2015 261 0 0 150 1,986 286

Cedar City Valley 44 1977 2015 226 0 0 110 2,720 368

East Shore Area 136 1976 2015 713 0 0 28 1,656 288

Lower Bear River Basin 53 1977 2015 284 0 0 88 2,360 288

Milford Valley 21 1978 2015 176 0 0 156 948 385

Northern Juab Valley 12 1978 2015 59 0 0 18 1,040 436

Pahvant Valley 7 1978 2015 29 0 0 10 882 380

Parowan Valley 5 1978 2013 19 0 0 135 504 402
30 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 4. Number of wells and dissolved solids samples; period of record; and minimum, maximum, and median concentration in select 

Utah basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

Basins—Continued

SDWIS data (782)—Continued

Salt Lake Valley 246 1975 2015 2,027 0 0 10 2,222 445

Sevier Desert 27 1978 2015 127 0 0 162 1,520 280

Tooele Valley 45 1977 2015 224 0 0 196 2,970 410

Utah Valley 131 1976 2015 714 0 0 55 1,290 298

Sub-basins

NWIS and SDWIS data combined (1,355)

Cache Valley N 27 1977 2015 124 0 0 156 1,986 260

Cache Valley S 61 1975 2015 228 0 0 150 576 295

Cedar City Valley N 35 1977 2015 131 0 0 112 2,510 385

Cedar City Valley S 69 1975 2015 255 0 0 110 3,070 485

East Shore Area E 200 1975 2015 785 0 0 28 2,960 296

East Shore Area W 48 1975 2015 173 0 0 158 4,000 308

Lower Bear River Basin E 59 1977 2015 269 0 0 88 1,142 249

Lower Bear River Basin W 36 1975 2015 145 0 0 307 2,360 896

Salt Lake Valley NE 81 1976 2015 463 0 0 84 16,800 582

Salt Lake Valley NW 40 1976 2014 68 0 0 336 20,900 1,270

Salt Lake Valley SE 182 1976 2015 1,046 0 0 10 2,430 268

Salt Lake Valley SW 208 1975 2015 1,142 0 0 10 8,550 696

Utah Valley NE 172 1976 2015 581 0 0 55 1,110 282

Utah Valley NW 9 1980 2014 18 0 0 387 1,510 949

Utah Valley SE 94 1975 2015 375 0 0 96 1,970 325

Utah Valley SW 34 1975 2015 109 0 0 348 2,560 719

NWIS data (695)

Cache Valley N 11 1979 2015 33 0 0 218 1,730 258

Cache Valley S 27 1979 2015 58 0 0 174 539 307

Cedar City Valley N 18 1977 2013 42 0 0 276 2,510 503

Cedar City Valley S 42 1975 2015 118 0 0 183 3,070 570

East Shore Area E 81 1975 2015 139 0 0 122 2,960 339

East Shore Area W 31 1975 2015 106 0 0 158 4,000 374

Lower Bear River Basin E 19 1977 2015 60 0 0 118 835 236

Lower Bear River Basin W 23 1975 2015 70 0 0 342 1,920 1,020

Salt Lake Valley NE 39 1976 2015 97 0 0 204 16,800 706

Salt Lake Valley NW 40 1976 2014 68 0 0 336 20,900 1,270

Salt Lake Valley SE 82 1976 2015 216 0 0 71 2,430 434

Salt Lake Valley SW 104 1976 2015 311 0 0 206 8,550 778

Utah Valley NE 109 1976 2015 192 0 0 91 1,110 312

Utah Valley NW 5 1980 2004 5 0 0 387 1,510 960
Results: Identification and Quantification of Groundwater-Quality Trends 31

The decadal and sub-decadal dissolved solids medians 

for each database grouping are shown in figure 16. Median 

concentrations did not exceed the MCL of 2,000 mg/L in any 

basin or sub-basin except the Salt Lake Valley Northwest, 

although medians in many basins exceeded the SMCL 

of 500 mg/L. In general, the medians for individual and 

combined datasets were similar within a basin or sub-basin. 

The NWIS medians were higher than the other medians 

in several basins (for example, Lower Bear River Basin, 

Salt Lake Valley, and Sevier Desert). The variation among 

medians for different databases was greatest in Pahvant Valley. 

Variations among medians for different datasets were low in 

Cache Valley, Utah Valley, and Parowan Valley. Agreement 

among medians increased with time in the Pahvant Valley, 

Milford Valley, and Beryl-Enterprise Area.

Table 4. Number of wells and dissolved solids samples; period of record; and minimum, maximum, and median concentration in select 

Utah basins and sub-basins for data from the National Water Information System (NWIS) and Safe Drinking Water Information System 

(SDWIS), and combined NWIS and SDWIS data.—Continued

[Number in parentheses indicates the total number of wells. Abbrevations: mg/L, milligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; 

NW, northwest; SE, southeast; SW, southwest]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

Sub-basins—Continued

NWIS data (695)—Continued

Utah Valley SE 34 1975 2015 87 0 0 196 1,970 340

Utah Valley SW 30 1975 2015 85 0 0 354 2,560 807

SDWIS data (660)

Cache Valley N 16 1977 2014 91 0 0 156 1,986 262

Cache Valley S 34 1975 2015 170 0 0 150 576 290

Cedar City Valley N 17 1977 2015 89 0 0 112 1,630 320

Cedar City Valley S 27 1977 2015 137 0 0 110 2,720 423

East Shore Area E 119 1976 2015 646 0 0 28 1,350 290

East Shore Area W 17 1977 2013 67 0 0 178 1,656 274

Lower Bear River Basin E 40 1977 2015 209 0 0 88 1,142 256

Lower Bear River Basin W 13 1978 2014 75 0 0 307 2,360 772

Salt Lake Valley NE 42 1977 2015 366 0 0 84 1,056 540

Salt Lake Valley SE 100 1976 2015 830 0 0 10 1,710 248

Salt Lake Valley SW 104 1975 2015 831 0 0 10 2,222 636

Utah Valley NE 63 1977 2015 389 0 0 55 998 275

Utah Valley NW 4 1997 2014 13 0 0 392 1,290 949

Utah Valley SE 60 1976 2015 288 0 0 96 1,200 320

Utah Valley SW 4 1977 2013 24 0 0 348 954 444
32 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

1980 1990 2000 2010 1980 1990 2000 2010

ROE = residual on evaporation SUM = sum of constituents 

ROE (NWIS) ROE (SDWIS) SUM (NWIS)

10

15

5

4

8

12

0

20

40

60

0

20

40

60

80

0

25

50

75

100

0

20

40

60

4

8

12

0

10

20

30

0

10

20

5

10

15

2.5

5.0

7.5

10.0

0

10

20

30

0

10

20

30

Number of dissolved solids samplesNumber of dissolved solids samples

2

4

6

5

10

2.5

5.0

7.5

2.5

5.0

7.5

10.0

0

5

10

15

20

0

10

20

30

40

5

10

15

0

5

10

15

20

0

10

20

30

40

0

10

20

30

40

50

1.0

1.5

2.0

2.5

3.0

0

20

40

60

5

10

15

2.5

5.0

7.5

10.0

2.5

5.0

7.5

0

10

20

30

N = north E = east S = south W = west

NWIS = National Water Information System

SDWIS = Safe Drinking Water Information System

A. Basins B. Sub-basins

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Decade Decade

Lower

Bear

River

W Basin

Lower

Bear

River

E Basin

Cache

Valley

N

Cache

Valley

S

Cedar

City

Valley S

Cedar

City

Valley N

Utah

Valley

SW

Utah

Valley

SE

Utah

Valley

NE

Utah

Valley

NW

Salt

Lake

Valley SW

Salt

Lake

Valley SE

Salt

Lake

Valley NE

Salt

Lake

Valley NW

East

Shore

Area E

East

Shore

Area W

EXPLANATION

Figure 12. Number of dissolved solids samples over time in select Utah A, basins and B, sub-basins in the National 

Water Information System and Safe Drinking Water Information System datasets.
Results: Identification and Quantification of Groundwater-Quality Trends 33

N = north

E = east

S = south

W = west

EXPLANATION

Combined datasets

National Water 

 Information System

Safe Drinking Water 

 Information System

Maximum contaminant level

Median

95th percentile

Individual observation

 above 95th percentile

Individual observation

 below 5th percentile

75th percentile

25th percentile

5th percentile

Supplier requirements

Secondary maximum 

 contaminant level 10

100

1,000

10,000

10

100

1,000

10,000

BerylEnterprise

Area

Cache 

Valley

Cedar City 

Valley

East

Shore 

Area

Lower 

Bear River 

Basin

Northern 

Juab 

Valley

Pahvant 

Valley

Parowan 

Valley

Salt

Lake 

Valley

Sevier 

Desert

Tooele 

Valley

Utah 

Valley

Milford 

Valley

A. Basins

B. Sub-basins

Dissolved solids concentration, in micrograms per liter Dissolved solids concentration, in micrograms per liter

Cache 

Valley

N

Cache 

Valley

S

Cedar 

City 

Valley

N

Cedar 

City 

Valley

S

East 

Shore 

Area

E

East 

Shore 

Area

W

Lower 

Bear 

River 

Basin

W

Lower 

Bear 

River 

Basin

E

Salt 

Lake 

Valley

NE

Salt 

Lake 

Valley

NW

Salt 

Lake 

Valley

SE

Salt 

Lake 

Valley

SW

Utah 

Valley

NE

Utah 

Valley

NW

Utah 

Valley

SE

Utah 

Valley

SW

Figure 13. Dissolved-solids concentrations in select Utah A, basins and B, sub-basins for data from the National Water Information 

System and Safe Drinking Water Information System, and combined datasets.
34 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells Dissolved solids concentration, in milligrams per liter Dissolved solids concentration, in milligrams per liter

A. Basins B. Sub-basins

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

1980 1990 2000 2010

Decade

1980 1990 2000 2010

Decade

Lower

Bear

River

W Basin

Lower

Bear

River

E Basin

Cache

Valley

N

Cache

Valley

S

Cedar

City

Valley S

Cedar

City

Valley N

Utah

Valley

SW

Utah

Valley

SE

Utah

Valley

NE

Utah

Valley

NW

Salt

Lake

Valley SW

Salt

Lake

Valley SE

Salt

Lake

Valley NE

Salt

Lake

Valley NW

East

Shore

Area E

East

Shore

Area W

0

500

1,000

1,500

2,000

500

1,000

1,500

2,000

0

1,000

2,000

3,000

4,000

0

5,000

10,000

15,000

0

5,000

10,000

15,000

20,000

0

1,000

2,000

0

1,000

2,000

3,000

0

5,000

10,000

15,000

20,000

25,000

0

2,000

4,000

6,000

0

2,500

5,000

7,500

10,000

500

1,000

1,500

2,000

500

1,000

1,500

2,000

0

1,000

2,000

3,000

500

1,000

1,500

2,000

0

500

1,000

1,500

2,000

500

1,000

1,500

2,000

500

1,000

1,500

2,000

0

1,000

2,000

3,000

4,000

0

1,000

2,000

3,000

0

5,000

10,000

15,000

20,000

0

5,000

10,000

15,000

0

500

1,000

1,500

2,000

2,500

0

2,500

5,000

7,500

500

1,000

1,500

2,000

0

500

1,000

1,500

2,000

500

1,000

1,500

2,000

500

1,000

1,500

2,000

2,500

0

500

1,000

1,500

2,000

2,500

0

1,000

2,000

3,000

Maximum contaminant level

National Water Information System data 

Safe Drinking Water Information System data

N = north E = east S = south W = west

EXPLANATION

Figure 14. Dissolved-solids concentrations over time by dataset in select Utah A, basins and B, sub-basins.
Results: Identification and Quantification of Groundwater-Quality Trends 35

Cache

Valley

Salt Lake

Valley

Tooele Valley

Utah

Valley

Sevier

Desert Northern

Juab Valley

Pahvant

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar City

Valley

Lower Bear

River Basin

East Shore

Area

Milford

Valley

38°

39°

40°

41°

42°

114° 113° 112° 111° 110° 109°

37°

EXPLANATION

Concentration exceeds dissolved solids

 secondary maximum contaminant level

Concentration exceeds dissolved solids

 maximum contaminant level

Basin boundary

Highway

Study basin

Base from U.S. Geological Survey 30-meter National Elevation 

Dataset Universal Transeverse Mercator projection, Zone 12, 

North American Datum of 1983 (NAD 83)

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Figure 15. Location of wells with samples that exceed the secondary maximum contaminant level and 

maximum contaminant level for dissolved solids in select basins in Utah.
36 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

500

700

900

1,100

150

200

250

300

500

1,000

1,500

300

350

400

200

300

400

500

600

700

300

400

500

600

700

1,000

1,500

2,000

2,500

500

600

700

200

300

400

500

600

400

600

800

1,000

700

800

900

1,000

250

300

280

300

320

340

360

400

600

800

1,000

200

300

400

500

300

500

700

400

600

400

500

600

700

800

280

320

360

400

400

600

800

500

1,000

500

750

1,000

1,250

300

400

500

600

700

300

350

400

450

500

320

360

400

440

200

400

600

800

200

400

600

800

1,000

275

300

325

300

400

500

600

700

800

1,000

N = north E = east S = south W = west

 Combined decadal median

 Combined sub-decadal median NWIS sub-decadal median SDWIS sub-decadal median

 NWIS decadal median SDWIS decadal median NWIS = National Water Information System

SDWIS = Safe Drinking Water Information System

A. Basins B. Sub-basins

Dissolved solids median concentration, in milligrams per literDissolved solids median concentration, in milligrams per liter

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

1970 1980 1990 2000 2010

Decade

1970 1980 1990 2000 2010

Decade

Lower

Bear

River

W Basin

Lower

Bear

River

E Basin

Cache

Valley

N

Cache

Valley

S

Cedar

City

Valley S

Cedar

City

Valley N

Utah

Valley

SW

Utah

Valley

SE

Utah

Valley

NE

Utah

Valley

NW

Salt

Lake

Valley SW

Salt

Lake

Valley SE

Salt

Lake

Valley NE

Salt

Lake

Valley NW

East

Shore

Area E

East

Shore

Area W

EXPLANATION

Figure 16. Decadal and sub-decadal median dissolved-solids concentration in select A, basins and B, sub-basins in Utah.
Results: Identification and Quantification of Groundwater-Quality Trends 37

Trends in Arsenic, Nitrate, and Dissolved Solids 

from Combined Datasets

Overall, despite differences between the NWIS and 

SDWIS databases, the increased understanding of general 

basin-wide conditions justifies combining the datasets. 

In addition, combining the datasets generally reduces the 

percentage of censored values in each basin. Combining 

the NWIS and SDWIS datasets also increases the number 

of samples, which increases precision in estimates 

characterizing water-quality conditions. However, combining 

the datasets also can introduce variability because the range 

in concentrations can increase, particularly when there are 

larger differences between the datasets. Further, because 

there are generally more SDWIS data than NWIS data, the 

combined dataset results can be dominated by patterns in the 

SDWIS data.

Long-term trend analysis of groundwater arsenic, nitrate, 

and dissolved solids evaluated in basins that have experienced 

increased development and groundwater use show widespread 

changes in water quality. Trends evaluated in smaller regions 

of some basins highlight local water-quality conditions.

When there was an insufficient number of decadal 

medians, trend tests were often only possible using 

sub-decadal medians. Comparing results of trend tests 

on decadal and sub-decadal medians indicated that 

often sub-decadal medians have greater variability than 

decadal medians. This additional variability can make 

trend identification more difficult, although analysis of 

sub-decadal median data more frequently yielded significant 

groundwater-quality trends compared to analysis of decadal 

median data. This is in part because of the larger number 

of sub-decadal median data compared to decadal median 

data. In these cases, the magnitude and sign of Kendall’s 

tau (nonparametric correlation coefficient measuring the 

monotonic association between the dependent and independent 

variable) for both tests were generally similar, indicating 

that the additional data available in the sub-decadal median 

analysis provided additional statistical power without adding 

noise. This similarity between decadal and sub-decadal 

analyses supports the robustness of this analysis.

In a few cases, trends were identified in decadal 

medians, but not in sub-decadal medians. This may be due to 

the increased variability introduced in some cases by more 

frequent median calculations. The Mann-Kendall trend test 

identifies monotonic trends, which are obscured by increased 

variability. Increasing and decreasing trends were identified in 

all basins for some constituents except Tooele Valley. Results 

are presented below and comparisons to trends identified in 

other areas are described where applicable.

Sample replicate variability can influence the 

concentrations from which a decadal or sub-decadal median 

is calculated. Replicate variability of samples taken following 

USGS sampling and lab protocols has been assessed. For 

samples with dissolved-solids concentrations between 14 and 

1,000 mg/L, the standard deviation of replicates was 7 mg/L 

and for concentrations between 1,000 and 9,015 mg/L the 

relative standard deviation was 3 percent (Gross and others, 

2012). For samples with nitrate concentrations between 

0.05 and 1.0 mg/L, the standard deviation of replicates was 

0.043 mg/L and for concentrations between 1 and 58 mg/L 

the relative standard deviation was 2.9 percent (Mueller and 

Titus, 2005). The replicate variability is generally less than 

the variability among different samples from a single site or 

samples from different sites. The trend test looks at changes 

in median values over time and so replicate variability or even 

temporal changes in concentrations must be big enough to 

influence the median to contribute to a monotonic trend.

Evaluating trends in comparison to land-use change 

provides some insights into understanding trend drivers. 

However, when considering land-use change at a well, the 

number of wells in each land-use change category decreased 

relative to the number of wells in each basin, and the number 

of samples and period of record were also often smaller, 

making trend detection more difficult. Trends, specifically for 

nitrate and dissolved solids, can occur in areas of increased 

population and urbanization. However, land use directly 

surrounding wells is not always useful in identifying trends. 

Trends in arsenic, nitrate, and dissolved solids were commonly 

identified among wells in areas where land use did not change. 

This is in part because there were more wells in areas where 

land use did not change than there were in areas where land 

use changed.

Although land use is expected to have a substantial 

impact on water quality, these results highlight a more 

complex relationship between land use and water quality, 

with various spatial and temporal factors influencing surface 

to subsurface connectivity. Among wells where land use did 

not change over time, trends in arsenic, nitrate, and dissolved 

solids were still identified, indicating that factors other than 

land use directly at the well location impact water quality, 

including the combination of activities farther away from 

the well, groundwater travel time, and the timing of land-use 

transitions. For example, a lag in the time between conversion 

of land from low use to farming and an increase in nitrate 

concentration at a well several miles away is expected due to 

the time required for a sufficient nitrate load, from increased 

fertilizer application, to enter the groundwater system and 

move to the well. Changes in nitrate loads upgradient from a 

well may take decades or more to travel to a well and register 

as a change in concentration. Even land-use change occurring 

at a well can have a lag time as nitrate moves through the 

unsaturated zone.
38 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Arsenic

Evidence for statistically significant increases in decadal 

or sub-decadal median arsenic concentrations between 0.02 

and 0.17 μg/L per year was identified in the Beryl-Enterprise 

Area, East Shore Area, Utah Valley, Pahvant Valley, and 

Parowan Valley (table 5; fig. 17). Evidence for decreasing 

median concentrations of –0.24 μg/L per year was identified 

in the Sevier Desert. Within sub-basins, evidence for 

statistically significant increases in decadal or sub-decadal 

median arsenic concentrations between 0.01 and 0.48 μg/L 

per year was identified in the East Shore Area West, Salt 

Lake Valley Northwest, Salt Lake Valley Southeast, and Utah 

Valley Northeast (table 5; fig. 17). Evidence for decreasing 

median concentrations of –0.17 μg/L per year was identified 

in the Salt Lake Valley Southwest. Overall, the sub-basin 

trend results highlight areas that drive basinwide trends. 

The increasing trend in the East Shore Area West drove the 

basinwide increasing trend. The opposing trends in the Salt 

Lake Valley (increases in the Northwest and Southeast and 

a decrease in the Southwest) result in an overall result of no 

trend basinwide. The increase in Utah Valley Northeast drove 

the basinwide increasing trend.

Nitrate

Evidence for statistically significant increases between 

0.01 and 0.02 mg/L per year in decadal or sub-decadal median 

nitrate concentrations was identified in the East Shore Area 

and Salt Lake Valley (table 6; fig. 17). Evidence for decreasing 

median concentrations between –0.005 and –0.08 mg/L per 

year was identified in the Beryl-Enterprise Area, Milford 

Valley, Lower Bear River Basin, Northern Juab Valley, and 

Utah Valley. Within sub-basins, evidence for statistically 

significant increases between 0.01 and 0.02 mg/L per year 

in decadal or sub-decadal median nitrate concentrations was 

identified in the East Shore Area East and West, and Salt Lake 

Valley Southeast (table 6; fig. 17). Evidence for decreasing 

median concentrations of –0.01 mg/L per year was identified 

in Utah Valley Southeast. The increasing trend in the East 

Shore Area occurred in both the East and West sub-basins. 

The increasing trend in the Salt Lake Valley Southeast and the 

decreasing trend in Utah Valley Southeast drove the respective 

basinwide trends.

Nitrate trend results are similar to or smaller in magnitude 

than trends identified elsewhere using similar methods. 

Significant increases in median nitrate concentrations ranged 

from 0.01 to 0.02 mg/L per year and concentration decreases 

ranged from 0.005 to 0.08 mg/L per year. The rate of change 

for all trends was smaller in magnitude than nitrate trends 

identified in the Columbia Basin, Washington, where wells 

with high (greater than 10 mg/L) nitrate concentrations had 

median slopes of 0.35 and 0.46 mg/L per year (Helsel and 

Frans, 2006) and on a similar order of magnitude as trends 

in the Central Valley, California, where increases between 

0.005 and 0.06 mg/L per year were detected (Burow and 

others, 2013). The rates of change are within the ranges of 

increases and decreases calculated in a range of well networks 

representing a range of land-use and principal aquifers across 

the U.S., where between 1988 and 2010 nitrate concentrations 

increased between less than 0.01 and 0.28 mg/L per year 

and decreased between –0.42 and –0.01 mg/L per year 

(Lindsey and Rupert, 2012). For a comparison of trends in the 

southwestern region in this study, no statistically significant 

trend was identified in alluvial aquifers in the Upper Colorado 

River Basin, a significant increasing trend of 0.01 mg/L per 

year was identified in the Nevada Basin and Range basin-fill 

aquifers, and no significant trend and a decreasing trend of 

–0.01 mg/L per year were identified in different parts of the 

Rio Grande Aquifer System (Lindsey and Rupert, 2012). 

The period of record varies between these studies, but the 

comparison is meant to give some general context for the 

slope of the trend line.

Dissolved Solids

Evidence for statistically significant increases of 1 mg/L 

per year in decadal or sub-decadal median dissolved-solids 

concentrations was identified in Cache Valley (table 7; fig. 17). 

Evidence for decreasing median concentrations between –4 

and –5 mg/L per year was identified in Milford Valley. The 

larger differences between NWIS and SDWIS dissolved solids 

data may make trend identification more difficult, resulting in 

fewer trends detected.

Within sub-basins, evidence for statistically significant 

increases between 1 and 7 mg/L per year in decadal or 

sub-decadal median dissolved-solids concentrations was 

identified in Cache Valley South, Salt Lake Valley Northeast 

and Southeast, and Utah Valley Southwest (table 7; fig. 17). 

Evidence for decreasing median concentrations of between 

–1 and –3 mg/L per year was identified in the East Shore Area 

West and Utah Valley Southeast. Although no basinwide trend 

was identified in the East Shore Area, the western part of the 

basin had a decreasing trend. Similarly, no trend was identified 

in the Salt Lake Valley, although the eastern half of the basin 

had increasing trends. In Utah Valley Southeast and Southwest 

sub-basins, the opposing signs of trends may account for the 

lack of overall trend.
Results: Identification and Quantification of Groundwater-Quality Trends 39

Table 5. Arsenic trend test results for National Water Information System and Safe Drinking Water Information System data combined 

in select basins and sub-basins in Utah between 1975 and 2015.

[Red indicates significant result. Abbreviations: μg/L, micrograms per liter; —, no data; N, north; S, south; E, east; W, west; NE, northeast; NW, northwest; 

SE, southeast; SW, southwest]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(μg/L per 

year)

Tau Score Score 

variance p-value

Slope

(μg/L per 

year)

Basins

Beryl-Enterprise Area 1.00 6 9 10.089 0.15 0.60 9 28 0.133 0.15

Cache Valley –0.67 –4 9 0.308 –0.02 –0.07 –2 63 0.900 –0.01

Cedar City Valley 0.33 2 9 0.734 0.03 0.07 2 63 0.900 0.00

East Shore Area 0.83 5 8 0.149 0.01 0.67 14 43 10.048 0.02

Lower Bear River Basin — — — — — 0.43 9 44 0.230 0.13

Milford Valley 0.40 4 17 0.462 0.07 –0.21 –6 65 0.536 –0.19

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — 0.80 8 17 10.086 0.05

Parowan Valley 0.83 5 8 0.149 0.13 0.90 9 16 10.043 0.17

Salt Lake Valley 0.20 2 17 0.806 0.03 0.07 2 63 0.900 0.01

Sevier Desert –0.50 –5 16 0.312 –0.25 –0.58 –21 91 10.036 –0.24

Tooele Valley –0.50 –3 8 0.470 0.00 –0.53 –8 27 0.181 –0.01

Utah Valley — — — — — 0.76 16 43 10.023 0.06

Sub-basins

Cache Valley N 0.67 4 9 0.308 0.07 0.20 3 28 0.707 0.07

Cache Valley S –0.17 — 8 1.000 –0.02 –0.18 –5 64 0.618 –0.01

Cedar City Valley N — — — — — 0.47 7 28 0.260 0.06

Cedar City Valley S 0.00 0 9 1.000 0.00 –0.19 –4 43 0.649 –0.03

East Shore Area E 0.33 2 9 0.734 0.00 0.10 1 16 1.000 0.00

East Shore Area W — — — — — 0.73 11 28 10.060 0.48

Lower Bear River Basin E — — — — — 0.20 3 28 0.707 0.28

Lower Bear River Basin W 0.40 4 17 0.462 0.03 0.36 10 65 0.266 0.04

Salt Lake Valley NE — — — — — 0.10 1 16 1.000 0.00

Salt Lake Valley NW 1.00 6 9 10.089 0.39 0.00 0 9 1.000 –1.75

Salt Lake Valley SE — — — — — 0.80 8 17 10.086 0.01

Salt Lake Valley SW –0.40 –4 17 0.462 –0.17 –0.61 –17 64 10.046 –0.17

Utah Valley NE — — — — — 0.80 8 17 10.086 0.09

Utah Valley NW — — — — — –0.33 –2 9 0.734 –0.11

Utah Valley SE — — — — — –0.33 –5 20 0.367 0.00

Utah Valley SW 0.67 4 9 0.308 0.17 0.18 5 64 0.618 0.06

1Significant result.
40 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells Arsenic Nitrate Dissolved solids

114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109° 114° 113° 112° 111° 110° 109°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin W Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

East Shore Area E

Lower Bear River Basin E

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin W Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

East Shore Area E

Lower Bear River Basin E

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin W

Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S

Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

East Shore Area E

Lower Bear 

River Basin E

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

A. Basinwide trends B. Sub-basin trends

Major highway

Study basin boundary

Incresing trend in two tests

Increasing trend in one test

Insufficient evidence for trend

Decreasing trend in one test

Decreasing trend in two tests

EXPLANATION

Number of signifigant trends among decadal 

and sub-decadal medians using combined 

National Water Information System and 

Safe Drinking Water Information System data 

Base from U.S. Geological Survey 30-meter National 

Elevation Dataset Universal Transeverse Mercator 

projection, Zone 12, North American Datum of 1983 

(NAD 83)

0 40 80 MILES

0 40 80 KILOMETERS

N = north

E = east

S = south

W = west

Figure 17. Spatial patterns of trends in arsenic, nitrate, and dissolved solids data from the National Water Information System and the 

Safe Drinking Water Information System in select basins and sub-basins of Utah.
Results: Identification and Quantification of Groundwater-Quality Trends 41

Table 6. Nitrate trend test results for National Water Information System and Safe Drinking Water Information System data combined 

in select basins and sub-basins in Utah between 1975 and 2015.

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data; N, north; S, south; E, east; W, west; NE, northeast; NW, northwest; 

SE, southeast; SW, southwest]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Basins

Beryl-Enterprise Area –1.00 –10 17 10.027 –0.03 –0.72 –26 92 10.009 –0.03

Cache Valley 0.70 7 16 0.130 0.01 0.33 12 92 0.251 0.01

Cedar City Valley 0.20 2 17 0.806 0.00 0.11 4 92 0.754 0.00

East Shore Area 1.00 10 17 10.027 0.02 0.83 30 92 10.002 0.02

Lower Bear River Basin –0.90 –9 16 10.043 –0.005 –0.47 –17 91 10.093 –0.02

Milford Valley –0.60 –6 17 0.221 –0.03 –0.50 –18 92 10.076 –0.02

Northern Juab Valley — –10 17 10.027 –0.08 –0.67 –24 92 10.016 –0.06

Pahvant Valley 0.40 4 17 0.462 0.02 0.00 0 92 1.000 0.01

Parowan Valley –0.20 –2 17 0.806 –0.02 –0.39 –14 92 0.175 –0.02

Salt Lake Valley 1.00 10 17 10.027 0.01 0.56 20 90 10.045 0.01

Sevier Desert 0.30 3 16 0.613 0.00 0.44 16 92 0.118 0.01

Tooele Valley 0.50 5 16 0.312 0.02 0.44 16 92 0.118 0.03

Utah Valley –0.60 –6 17 0.221 –0.01 –0.50 –18 92 10.076 –0.01

Sub-basins

Cache Valley N 0.20 2 17 0.806 0.03 0.06 2 92 0.917 0.01

Cache Valley S 0.00 0 17 1.000 0.00 0.14 5 91 0.675 0.00

Cedar City Valley N –0.40 –4 17 0.462 –0.01 –0.22 –8 92 0.466 –0.01

Cedar City Valley S 0.20 2 17 0.806 0.01 0.06 2 92 0.917 0.00

East Shore Area E 1.00 10 17 10.027 0.01 0.72 26 92 10.009 0.01

East Shore Area W 0.80 8 17 10.086 0.02 0.69 25 91 10.012 0.01

Lower Bear River Basin E 0.50 5 16 0.312 0.00 0.31 11 91 0.295 0.00

Lower Bear River Basin W 0.60 6 17 0.221 0.01 0.19 7 91 0.529 0.00

Salt Lake Valley NE –0.60 –6 17 0.221 –0.02 –0.44 –16 92 0.118 –0.02

Salt Lake Valley NW — — — — — — — — — —

Salt Lake Valley SE 1.00 10 17 10.027 0.02 0.78 28 92 10.005 0.02

Salt Lake Valley SW 0.60 6 17 0.221 0.01 0.03 1 91 1.000 0.00

Utah Valley NE 0.40 4 17 0.462 0.00 0.28 10 92 0.348 0.00

Utah Valley NW — — — — — 0.33 2 9 0.734 0.04

Utah Valley SE –0.80 –8 17 10.086 –0.01 –0.33 –12 92 0.251 –0.01

Utah Valley SW 0.00 0 17 1.000 0.04 –0.21 –6 65 0.536 –0.04

1Significant value.
42 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 7. Dissolved solids trend test results for National Water Information System and Safe Drinking Water Information System data 

combined in select basins and sub-basins in Utah between 1975 and 2015.

[Red indicates significant result. Abbreviations: mg/L, milligrams per liter; —, no data; N, north; S, south; E, east; W, west; NE, northeast; NW, northwest; 

SE, southeast; SW, southwest]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Basins

Beryl-Enterprise Area –0.40 –4 17 0.462 –2 –0.28 –10 92 0.348 –1

Cache Valley 1.00 10 17 10.027 1 0.89 32 92 10.001 1

Cedar City Valley 0.60 6 17 0.221 2 0.25 9 88 0.395 3

East Shore Area 0.00 0 17 1.000 0 –0.17 –6 92 0.602 0

Lower Bear River Basin –0.20 –2 17 0.806 –1 0.00 0 92 1.000 0

Milford Valley –0.80 –8 17 10.086 –5 –0.67 –24 92 10.016 –4

Northern Juab Valley –0.20 –2 17 0.806 –3 –0.43 –12 65 0.174 –6

Pahvant Valley –0.40 –4 17 0.462 –2 –0.39 –14 92 0.175 –4

Parowan Valley –0.40 –4 17 0.462 –1 –0.17 –6 92 0.602 –1

Salt Lake Valley 0.60 6 17 0.221 3 0.44 16 92 0.118 3

Sevier Desert 0.20 2 17 0.806 1 0.22 8 92 0.466 1

Tooele Valley 0.40 4 17 0.462 2 0.11 4 92 0.754 3

Utah Valley –0.40 –4 17 0.462 –1 –0.22 –8 92 0.466 0

Sub-basins

Cache Valley N –0.10 — 16 1.000 0 –0.21 –6 65 0.536 –1

Cache Valley S 0.80 8 17 10.086 1 0.83 30 92 10.002 1

Cedar City Valley N 0.00 0 17 1.000 0 –0.17 –6 92 0.602 –1

Cedar City Valley S 0.60 6 17 0.221 4 0.56 20 92 10.048 5

East Shore Area E 0.40 4 17 0.462 0 0.25 9 91 0.402 0

East Shore Area W –1.00 –10 17 10.027 –3 –0.56 –20 92 10.048 –3

Lower Bear River Basin E 0.40 4 17 0.462 1 0.28 10 92 0.348 1

Lower Bear River Basin W –0.40 –4 17 0.462 –1 –0.11 –4 92 0.754 –2

Salt Lake Valley NE 1.00 10 17 10.027 4 0.67 24 92 10.016 4

Salt Lake Valley NW –0.40 –4 17 0.462 –34 –0.20 –3 28 0.707 –11

Salt Lake Valley SE 0.80 8 17 10.086 4 0.78 28 92 10.005 3

Salt Lake Valley SW –0.60 –6 17 0.221 –2 –0.22 –8 92 0.466 –4

Utah Valley NE 0.40 4 17 0.462 1 0.22 8 92 0.466 1

Utah Valley NW — — — — — –0.67 –4 9 0.308 –18

Utah Valley SE –1.00 –10 17 10.027 –1 –0.50 –18 92 10.076 –1

Utah Valley SW 0.80 8 17 0.086 7 0.21 6 65 0.536 5

1Significant value.
Results: Identification and Quantification of Groundwater-Quality Trends 43

Dissolved solids trend results were similar to or smaller 

in magnitude than trends identified elsewhere using similar 

methods. Significant increases in dissolved solids median 

concentrations ranged from 1 to 7 mg/L per year and decreases 

in concentrations ranged from –1 to –5 mg/L per year. The 

rates of change were within the ranges of increases and 

decreases calculated in a range of well networks representing 

a range of land-use and principal aquifers across the U.S., 

where between 1988 and 2010 dissolved-solids concentrations 

increased between 1.3 and 33 mg/L per year and decreased 

between –1.7 and –7.5 mg/L per year (Lindsey and Rupert, 

2012). For a comparison between trends from this study and 

in the southwestern U.S., a statistically significant increasing 

trend of 4.4 mg/L per year was identified in alluvial aquifers 

in the Upper Colorado River Basin, and no significant trends 

were identified in the Nevada Basin and Range basin-fill 

aquifers or the Rio Grande Aquifer System (Lindsey and 

Rupert, 2012). The period of record varies among all these 

studies, but the comparison is meant to give some general 

context for the slope of the trend line.

Trends in Arsenic, Nitrate, and Dissolved 

Solids from Safe Drinking Water Information 

System Data

Evidence for trends in arsenic, nitrate, and 

dissolved-solids concentrations was identified using SDWIS 

data, which represents water from public-supply wells prior 

to any treatment. Increases in median arsenic concentrations 

between 0.06 and 0.1 μg/L per year were identified in in Cedar 

City Valley and Utah Valley (table 8; fig. 18). In Utah Valley, 

an increasing trend was identified in the northeast sub-basin. 

An increase of 0.16 μg/L per year also was identified in 

the Lower Bear River Basin West. In the Salt Lake Valley, 

median concentrations increased 0.02 μg/L per year in the 

Southeast sub-basin and decreased –0.17 μg/L per year in the 

Southwest sub-basin.

Increases in median nitrate concentrations between 0.01 

and 0.06 mg/L per year were identified in the Beryl-Enterprise 

Area, East Shore Area, Salt Lake Valley, and Sevier Desert 

(table 9; fig. 18). In the East Shore Area, the western sub-basin 

had an increasing trend. In the Salt Lake Valley, the Southeast 

sub-basin had an increasing trend, whereas the Northeast 

sub-basin had a decreasing trend. Decreasing trends between 

–0.04 and –0.11 mg/L per year were identified in Northern 

Juab Valley and Pahvant Valley.

Increases in median dissolved-solids concentrations 

between 0.4 and 5 mg/L per year were identified in Cache 

Valley, East Shore Area, Milford Valley, and Sevier Desert 

(table 10; fig. 18). The Cache Valley South sub-basin had 

an increasing trend. The East Shore Area East sub-basin 

had increasing trends, whereas the western sub-basin 

had a decreasing trend. In the Salt Lake Valley and Utah 

Valley, no overall basin trends were identified. However, 

an increasing trend was identified in the Salt Lake Valley 

Southeast sub-basin, consistent with findings by Thiros and 

Spangler (2010). A decreasing trend was identified in Utah 

Valley Southeast. Decreases in median dissolved-solids 

concentrations between –16 and –19 mg/L per year were 

identified in Northern Juab Valley.

Increasing trends are more commonly identified in 

SDWIS data than combined NWIS and SDWIS data, 

particularly for nitrate and dissolved solids. Assuming that 

SDWIS data represent deeper wells and that increased 

concentrations are due to human impacts on groundwater, 

these results indicate that the deeper aquifers within study 

basins have been impacted by human activities. Generally, 

shallower aquifers are more susceptible to human activity at 

land surface, so changes to the deeper aquifers indicate that 

impacts are substantial.
44 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 8. Arsenic trend test results for Safe Drinking Water Information System data in select basins and sub-basins in Utah between 

1975 and 2015.

[Red indicates significant result. Abbreviations: μg/L, micrograms per liter; —, no data; N, north; S, south; E, east; W, west; NE, northeast; SE, southeast; 

SW, southwest; NW, northwest]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(μg/L per 

year)

Tau Score Score 

variance p-value

Slope

(μg/L per 

year)

Basins

Beryl-Enterprise Area 0.67 4 9 0.308 0.11 0.20 2 17 0.806 0.03

Cache Valley –0.83 –5 8 0.149 –0.02 –0.43 –12 63 0.167 –0.03

Cedar City Valley 1.00 6 9 10.089 0.10 0.54 15 64 10.081 0.06

East Shore Area 0.33 2 9 0.734 0.00 0.10 1 16 1.000 0.00

Lower Bear River Basin — — — — — 0.60 9 28 0.133 0.23

Milford Valley 0.40 4 17 0.462 0.10 0.05 1 44 1.000 0.08

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.20 2 17 0.806 0.03 0.07 2 65 0.902 0.02

Sevier Desert –0.70 –7 16 0.130 –0.21 –0.17 –6 92 0.602 –0.07

Tooele Valley — — — — — –0.67 –4 9 0.308 –0.01

Utah Valley — — — — — 0.71 15 42 10.031 0.06

Sub-basins

Cache Valley N 0.17 1 8 1.000 0.04 0.20 3 28 0.707 0.01

Cache Valley S –0.33 –2 9 0.734 –0.03 –0.32 –9 64 0.319 –0.02

Cedar City Valley N — — — — — 0.47 7 28 0.260 0.06

Cedar City Valley S 0.33 2 9 0.734 0.06 0.00 0 25 1.000 0.00

East Shore Area E 0.33 2 9 0.734 0.00 0.50 3 8 0.470 0.01

East Shore Area W — — — — — 0.30 3 16 0.613 0.05

Lower Bear River Basin E — — — — — 0.53 8 27 0.181 0.35

Lower Bear River Basin W 0.60 6 17 0.221 0.11 0.64 18 65 10.035 0.16

Salt Lake Valley NE — — — — — 0.30 3 16 0.613 0.02

Salt Lake Valley SE — — — — — 0.90 9 16 10.043 0.02

Salt Lake Valley SW –0.40 –4 17 0.462 –0.23 –0.57 –16 65 10.063 –0.17

Utah Valley NE — — — — — 0.80 8 17 10.086 0.11

Utah Valley NW — — — — — — — — — —

Utah Valley SE — — — — — –0.40 –6 25 0.314 –0.01

Utah Valley SW 0.67 4 9 0.308 0.24 0.47 7 28 0.260 0.10

1Significant value.
Results: Identification and Quantification of Groundwater-Quality Trends 45

Arsenic Nitrate Dissolved solids 42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

42°

41°

40°

39°

38°

37°

Lower Bear River Basin E

114° 113° 112° 111° 110° 109° 114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109° 114° 113° 112° 111° 110° 109°

114° 113° 112° 111° 110° 109° 114° 113° 112° 111° 110° 109°

A. Basinwide trends B. Sub-basin trends

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin W Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

East Shore Area E

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin Cache Valley

East Shore Area

Salt Lake Valley Tooele Valley

Utah Valley

Sevier Desert Northern Juab Valley

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley

Beryl-Enterprise 

Area

Lower Bear 

River Basin W

Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

Lower Bear 

River Basin W

Cache Valley N

East Shore Area W

Salt Lake Valley SE Tooele Valley

Utah Valley SW

Northern Juab Valley

Sevier Desert

Pahvant Valley Milford 

Valley

Parowan Valley

Cedar City Valley S Beryl-Enterprise 

Area

Utah Valley NW

Utah Valley NE

Utah Valley SE

Cedar City Valley N

East Shore Area E

East Shore Area E

Cache Valley S

Salt Lake Valley NE

Salt Lake Valley SW

Salt Lake Valley NW

Lower Bear River Basin E

Lower Bear River Basin E

Major highway

Study basin boundary

EXPLANATION

Number of signifigant trends among decadal 

and sub-decadal medians using Safe Drinking

Water Information System data 

Incresing trend in two tests

Increasing trend in one test

Insufficient evidence for trend

Decreasing trend in one test

Decreasing trend in two tests

Base from U.S. Geological Survey 30-meter National 

Elevation Dataset Universal Transeverse Mercator 

projection, Zone 12, North American Datum of 1983 

(NAD 83)

0 40 80 MILES

0 40 80 KILOMETERS

N = north

E = east

S = south

W = west

Figure 18. Spatial patterns of trends in arsenic, nitrate, and dissolved solids data from the Safe Drinking Water Information System in 

select basins and sub-basins of Utah.
46 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 9. Nitrate trend test results for Safe Drinking Water Information System data in select basins and sub-basins in Utah between 

1975 and 2015.

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; N, north; S, south; E, east; W, west; NE, northeast; SE, southeast; SW, southwest; 

NW, northwest; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Basins

Beryl-Enterprise Area 0.67 4 9 0.308 0.05 0.64 18 65 10.035 0.06

Cache Valley 0.60 6 17 0.221 0.02 0.28 10 92 0.348 0.01

Cedar City Valley 0.40 4 17 0.462 0.00 0.33 12 92 0.251 0.01

East Shore Area 0.80 8 17 10.086 0.01 0.56 20 90 10.045 0.01

Lower Bear River Basin –0.20 –2 17 0.806 0.00 –0.03 — 91 1.000 0.00

Milford Valley 0.67 4 9 0.308 0.01 0.36 10 63 0.258 0.01

Northern Juab Valley –0.80 –8 17 10.086 –0.04 –0.44 –16 92 0.118 –0.04

Pahvant Valley –0.80 –8 17 10.086 –0.11 –0.58 –21 91 10.036 –0.10

Parowan Valley 0.67 4 9 0.308 0.01 0.33 2 9 0.734 0.01

Salt Lake Valley 1.00 10 17 10.027 0.01 0.72 26 92 10.009 0.01

Sevier Desert 0.67 4 9 0.308 0.01 0.79 22 65 10.009 0.01

Tooele Valley 0.60 6 17 0.221 0.00 0.39 14 90 0.171 0.01

Utah Valley –0.20 –2 17 0.806 0.00 –0.11 –4 92 0.754 0.00

Sub-basins

Cache Valley N 0.20 2 17 0.806 0.03 0.14 5 91 0.675 0.02

Cache Valley S 0.00 0 17 1.000 0.00 0.22 8 92 0.466 0.01

Cedar City Valley N –0.40 –4 17 0.462 –0.01 –0.25 –9 91 0.402 –0.01

Cedar City Valley S 0.33 2 9 0.734 0.01 0.43 12 65 0.174 0.01

East Shore Area E 0.40 4 17 0.462 0.01 0.44 16 92 0.118 0.01

East Shore Area W 0.80 8 17 10.086 0.01 0.69 25 91 10.012 0.01

Lower Bear River Basin E 0.40 4 17 0.462 0.01 0.47 17 91 10.093 0.01

Lower Bear River Basin W 0.80 8 17 10.086 0.01 0.14 4 65 0.711 0.01

Salt Lake Valley NE –0.80 –8 17 10.086 –0.01 –0.36 –13 91 0.208 –0.02

Salt Lake Valley SE 1.00 10 17 10.027 0.02 0.78 28 92 10.005 0.02

Salt Lake Valley SW 0.20 2 17 0.806 0.00 0.33 12 92 0.251 0.01

Utah Valley NE 0.60 6 17 0.221 0.01 0.31 11 91 0.295 0.00

Utah Valley NW — — — — — 0.33 2 9 0.734 0.05

Utah Valley SE –0.60 –6 17 0.221 0.00 –0.39 –14 92 0.175 0.00

Utah Valley SW — — — — — 0.20 2 17 0.806 0.00

1Significant value.
Results: Identification and Quantification of Groundwater-Quality Trends 47

Table 10. Dissolved solids trend test results for Safe Drinking Water Information System data in select basins and sub-basins in Utah 

between 1975 and 2015.

[Red indicates significant result. Abbreviations: mg/L, milligrams per liter; —, no data; N, north; S, south; E, east; W, west; NE, northeast; SE, southeast; 

SW, southwest; NW, northwest]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Basins

Beryl-Enterprise Area 0.33 2 9 0.734 1 0.29 6 43 0.448 1

Cache Valley 0.40 4 17 0.462 0 0.50 18 92 10.076 1

Cedar City Valley 0.60 6 17 0.221 3 0.33 12 92 0.251 2

East Shore Area 0.70 7 16 0.130 1 0.57 16 61 10.054 0.4

Lower Bear River Basin 0.00 0 17 1.000 0 0.17 6 92 0.602 1

Milford Valley 1.00 10 17 10.027 5 0.61 22 92 10.029 5

Northern Juab Valley –0.80 –8 17 10.086 –16 –0.79 –22 65 10.009 –19

Pahvant Valley 0.60 6 17 0.221 8 0.40 4 17 0.462 1

Parowan Valley — — — — — 0.33 2 9 0.734 3

Salt Lake Valley 0.60 6 17 0.221 2 0.17 6 92 0.602 2

Sevier Desert 1.00 10 17 10.027 2 0.50 14 65 0.108 2

Tooele Valley 0.00 0 17 1.000 0 0.00 0 65 1.000 0

Utah Valley –0.30 –3 16 0.613 0 –0.08 –3 91 0.834 –0

Sub-basins

Cache Valley N –0.20 –2 17 0.806 0 –0.29 –8 65 0.386 –1

Cache Valley S 0.40 4 17 0.462 1 0.56 20 92 10.048 1

Cedar City Valley N 0.40 4 17 0.462 1 0.17 6 92 0.602 2

Cedar City Valley S 0.33 2 9 0.734 5 0.29 6 43 0.448 4

East Shore Area E 0.80 8 17 10.086 1 0.79 22 65 10.009 1

East Shore Area W –0.60 –6 17 0.221 –2 –0.57 –16 65 10.063 –3

Lower Bear River Basin E 0.60 6 17 0.221 1 0.33 12 92 0.251 1

Lower Bear River Basin W 0.00 0 17 1.000 –1 0.05 1 44 1.000 1

Salt Lake Valley NE 0.60 6 17 0.221 3 0.39 14 92 0.175 2

Salt Lake Valley SE 0.80 8 17 0.086 3 0.83 30 92 10.002 3

Salt Lake Valley SW –0.60 –6 17 0.221 –7 –0.39 –14 92 0.175 –7

Utah Valley NE 0.20 2 17 10.806 0 0.00 0 65 1.000 0

Utah Valley NW — — — — — — — — — —

Utah Valley SE –0.60 –6 17 0.221 –1 –0.47 –17 91 10.093 –2

Utah Valley SW 0.00 0 9 1.000 –1 — — — — —

1Significant value.
48 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Linking Trends to Land-Use Change

Broad patterns in land use and land-use change, and 

related demographic and water-use patterns can be associated 

with water-quality changes. Although arsenic in groundwater 

is primarily naturally sourced, humans may influence aquifer 

geochemical conditions that mediate arsenic concentrations 

through activities that impact redox conditions and pH 

(Bexfield and others, 2011). Increasing trends also could 

be caused by the addition of deeper wells tapping into 

older groundwater that has had more time to interact with 

arsenic-bearing rocks in response to growing water demand. 

Humans can more directly influence arsenic, nitrate, and 

dissolved solids in groundwater by controlling their sources 

and practices that mediate loading (water-use practices such 

as artificial recharge, groundwater pumping, and well depths; 

Bexfield and others, 2011). Trends in nitrate and dissolved 

solids indicate that humans, through a range of activities, have 

impacted groundwater quality over time.

Generally, arsenic trends were not directly linked to 

land-use change taking place on land immediately surrounding 

wells. There were not enough data in many basins to do a 

trend analysis for some land-use change categories. There 

were fewer data from a fewer number of wells and from a 

shorter period of time available for each land-use change 

category in each basin (table 11). The median arsenic 

concentration over time in each basin for each land-use change 

category is shown in figure 19.

Basinwide, no significant arsenic trends were identified 

for wells experiencing any land-use change except a 

decreasing trend was identified at wells in the Salt Lake Valley 

where the land use changed from low use to urban (table 12). 

This decreasing trend may be explained by an increase over 

time in deeper wells (or samples from deeper wells) seeking 

cleaner water in response to the increased development or 

urbanization in areas where groundwater has lower arsenic 

concentrations. Trends were evaluated in shallow wells (depth 

less than 200 feet) to test this explanation; however, there 

were not enough data to identify significant trends in shallow 

wells experiencing a transition from low use to urban land. In 

the shallow wells where land use did not change, a significant 

decreasing trend was identified in the Salt Lake Valley and a 

significant increasing trend was identified in Utah Valley.

Generally, nitrate trends were not linked to land-use 

change at wells. There were insufficient data in many basins 

for many land-use change categories to do a trend test 

(table 13). The median concentration over time in each basin 

for each land-use change category is shown in figure 20.

Nitrate trends were associated with land-use changes at 

wells in a few basins (table 14). For example, significant 

increasing trends were identified in the Cache Valley wells 

where land had changed from urban to production, presumably 

resulting from increased fertilizer application associated 

with agricultural production. However, in Cedar City, a 

significant positive trend in nitrate was identified among wells 

experiencing a transition from production to urban. This trend 

may be related to the timing and nature of the transition to 

urban land. Nitrate may have accumulated in aquifers from a 

history of production (fertilizer and manure associated with 

agriculture and livestock), leading to a positive trend that 

has been augmented by widespread use of septic systems 

accompanying development. Construction of sewer systems in 

and around Enoch began in 1994, although many households 

use septic systems as their primary means of wastewater 

disposal (Lowe and Wallace, 2001). Cedar City Valley also 

has naturally high nitrate concentrations (Lowe and Wallace, 

2001). The percentage of land in each basin that has been 

converted from urban to production also is very low and so 

the results should be interpreted with caution. Significant 

increasing and decreasing nitrate trends were identified for 

wells where land use did not change.

When considering broader land-use change across a basin 

and the impacts on groundwater-quality trends, the results 

showing nitrate increases in more urban basins including 

the Salt Lake Valley and East Shore Area and decreases 

in other basins with more agricultural production may be 

counterintuitive. However, it is possible that the impacts 

of urbanization may have substantial effects on nitrate in 

groundwater through activities such as overfertilization of 

urban vegetation (for example, lawns and golf courses) or 

additional sources of nitrate including vehicles and industrial 

processes. In a nationwide study of decadal-scale changes 

in groundwater quality, Lindsey and Rupert (2012) reported 

a higher percentage of significant increases in nitrate 

concentrations in urban areas than agricultural areas, although 

they also reported large increases in nitrate concentrations 

in agricultural areas. Although agricultural activities are 

generally considered more important sources of nitrogen to 

hydrologic systems, the impacts of urban activities can be 

substantial as well. Further, if nitrate loading from agriculture 

has not changed substantially, nitrate concentrations would not 

be impacted.

Generally, dissolved solids trends were not linked to 

land-use change at wells. There were insufficient data in many 

basins for many land-use change categories to do a trend test 

(table 15). Among the different land-use change categories, 

the “no change” category has the most wells and samples. The 

median dissolved-solids concentration over time in each basin 

for each land-use change category is shown in figure 21.
Results: Identification and Quantification of Groundwater-Quality Trends 49

Table 11. Number of wells; period of record; number of arsenic measurements; and minimum, maximum, and median arsenic 

concentration in each basin for each land-use change category.

[μg/L, micrograms per liter; —, no data]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(μg/L)

Maximum 

concentration 

(μg/L)

Median 

concentration 

(μg/L)

All wells

Production to low use

Beryl-Enterprise Area 1 2013 2013 1 0 0 2.8 2.8 —

Milford Valley 1 2011 2011 1 0 0 3.8 3.8 —

Sevier Desert 1 1979 1986 3 0 0 590 730 610

Urban to production

Cache Valley 1 1989 2013 7 3 43 0.5 5 0.9

East Shore Area 1 1991 1991 1 1 100 5 5 —

Salt Lake Valley 3 1993 2009 10 0 0 5 21 14

Tooele Valley 1 2000 2000 1 0 0 1 1 —

Utah Valley 1 2003 2003 1 0 0 0.4 0.4 —

No change

Beryl-Enterprise Area 21 1978 2015 81 10 12 0.04 95.7 4

Cache Valley 60 1975 2015 234 88 38 0.02 42.4 0.9

Cedar City Valley 40 1978 2015 144 38 26 0.1 15.7 2

East Shore Area 128 1976 2015 482 275 57 0.1 50 0.7

Lower Bear River Basin 77 1978 2015 342 108 32 0.1 106 2

Milford Valley 42 1978 2015 175 6 3 1 39 6.6

Northern Juab Valley 19 1978 2015 59 27 46 0.23 10 0.7

Pahvant Valley 59 1978 2015 111 17 15 0.21 19 2

Parowan Valley 15 1978 2015 48 6 13 0.5 11.3 3.8

Salt Lake Valley 332 1975 2015 1,443 421 29 0.005 360 1.7

Sevier Desert 74 1978 2015 209 20 10 0.08 730 7.5

Tooele Valley 101 1977 2015 348 80 23 0.005 206 1.6

Utah Valley 137 1977 2015 519 224 43 0.1 53 1

Production to urban

Cache Valley 9 1979 2013 32 20 63 0.46 10 0.6

Cedar City Valley 1 1997 2014 6 4 67 0.5 5 —

East Shore Area 21 1978 2013 61 30 49 0.5 44 1

Lower Bear River Basin 1 1998 1998 1 0 0 95 95 —

Salt Lake Valley 51 1977 2015 206 54 26 0.3 99 4.4

Sevier Desert 1 1978 2008 11 0 0 10 28 12.2

Tooele Valley 7 1981 2013 24 4 17 0.6 7 2

Utah Valley 26 1978 2015 81 35 43 0.1 72.9 1

Low use to production

Beryl-Enterprise Area 1 1987 2013 8 5 63 1 10 1

Cache Valley 4 1978 2015 20 5 25 0.5 17.3 5.6

Northern Juab Valley 2 2005 2012 4 0 0 0.19 1.3 —

Pahvant Valley 2 1985 2015 4 0 0 4 6.7 5.9

Parowan Valley 2 2007 2013 5 0 0 2.3 6 2.4

Salt Lake Valley 8 1978 2015 76 9 12 0.005 275 7

Sevier Desert 1 1980 2015 6 0 0 1.8 3 1.9
50 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 11. Number of wells; period of record; number of arsenic measurements; and minimum, maximum, and median arsenic 

concentration in each basin for each land-use change category.—Continued

[μg/L, micrograms per liter; —, no data]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(μg/L)

Maximum 

concentration 

(μg/L)

Median 

concentration 

(μg/L)

All wells—Continued

Low use to production—Continued

Tooele Valley 1 1997 2013 5 1 20 1 5 1.4

Utah Valley 10 1998 2015 22 1 5 0.5 18 2.6

Low use to urban

Cedar City Valley 17 1979 2015 62 28 45 0.5 10 2

Lower Bear River Basin 2 1978 2010 13 8 62 0.5 7 0.6

Salt Lake Valley 18 1980 2015 79 15 19 0.5 155 6.5

Sevier Desert 1 2009 2013 2 0 0 12.4 14.3 —

Tooele Valley 15 1978 2013 43 23 53 0.2 10 0.6

Utah Valley 21 1978 2013 81 26 32 0.5 50 2

Wells less than 200 feet deep

Urban to production

Salt Lake Valley 2 1998 2008 3 0 0 5 11.3 5.1

No change

Beryl-Enterprise Area 2 2005 2011 9 0 0 0.05 9.5 3.3

Cache Valley 9 1983 2015 26 9 35 0.55 13.1 0.7

Cedar City Valley 1 2007 2007 1 0 0 0.57 0.57 —

East Shore Area 10 1978 2015 33 15 45 0.5 22.8 1.3

Lower Bear River Basin 9 1985 2014 22 7 32 0.5 10 1.7

Milford Valley 2 2012 2015 2 0 0 2.9 20.5 —

Pahvant Valley 14 1985 2015 24 1 4 1 19 8.1

Parowan Valley 1 2005 2014 4 0 0 5.3 6.6 5.8

Salt Lake Valley 45 1980 2015 88 19 22 0.5 360 2.1

Sevier Desert 17 1979 2015 22 0 0 3.9 700 7.1

Tooele Valley 20 1991 2014 37 2 5 0.3 5.8 1.7

Utah Valley 19 1978 2015 35 1 3 0.5 12 1.8

Production to urban

East Shore Area 1 1981 2008 8 8 100 0.5 5 —

Lower Bear River Basin 1 1998 1998 1 0 0 95 95 —

Salt Lake Valley 9 1981 2008 35 3 9 1 99 21

Tooele Valley 1 2000 2003 4 0 0 4.9 7 5.6

Utah Valley 1 2014 2014 1 0 0 5.9 5.9 —

Low use to production

Cache Valley 1 2005 2015 10 0 0 5.6 8.2 6

Salt Lake Valley 4 1978 2015 18 2 11 1 275 6.4

Utah Valley 1 2008 2013 3 0 0 1 1.1 1

Low use to urban

Salt Lake Valley 3 1991 2008 6 0 0 5 155 11.1

Tooele Valley 6 2001 2001 6 0 0 0.2 1.8 0.8

Utah Valley 4 1981 2005 12 7 58 0.5 5 0.6
Results: Identification and Quantification of Groundwater-Quality Trends 51

2

4

6

2

4

6

0.8

1.0

1.2

1.4

1.6

2

4

6

0

5

10

15

20

1

2

3

4

5

6

0.70

0.71

0.72

0.73

0.74

0.75

4

8

12

2

4

6

8

4

6

8

10

1

2

3

4

5

1

2

3

4

0

1

2

3

2

4

6

8

2

4

6

0

5

10

15

20

2

4

6

0

5

10

15

20

25

2.5

5.0

7.5

10.0

0.6

0.7

0.8

0.9

4

8

12

16

2

4

6

8

5

10

15

20

5

10

15

1

2

3

4

0

1

2

3

Low use to urban, shallow wells

Production to urban, shallow wells

Low use to urban, all wells

No change, shallow wells

Low use to production, shallow wells

No change, all wells

Low use to production, all wells

Production to low use, shallow wells

Urban to production, all wells

Production to low use, all wells

A. Decadal medians B. Sub-decadal medians

Arsenic median concentration, in milligrams per literArsenic median concentration, in milligrams per liter

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Dessert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

1970 1980 1990 2000 2010

Decade

1970 1980 1990 2000 2010

Decade EXPLANATION

Figure 19. Decadal and sub-decadal median arsenic concentration in select A, basins and B, sub-basins by land-use 

change category in Utah.
52 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 12. Trend test results for arsenic in basins for each land-use change category.

[Red indicates significant result. Abbrebiations: μg/L, micrograms per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Tau Score Score variance p-value

Slope

(μg/L per 

year)

All wells

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area — — — — — 0.33 2 9 0.734 0.47

Cache Valley –0.83 –5 8 0.149 –0.03 –0.18 –5 62 0.610 –0.02

Cedar City Valley 0.00 0 9 1.000 0.02 –0.21 –6 65 0.536 –0.02

East Shore Area — — — — — 0.33 5 26 0.436 0.02

Lower Bear River Basin 0.67 4 9 0.308 0.17 0.43 9 44 0.230 0.16

Milford Valley 0.40 4 17 0.462 0.07 –0.21 –6 65 0.536 –0.22

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — 0.40 4 17 0.462 0.01

Parowan Valley 0.83 5 8 0.149 0.13 0.70 7 16 0.130 0.09

Salt Lake Valley 0.00 0 9 1.000 0.02 0.05 1 44 1.000 0.01
Results: Identification and Quantification of Groundwater-Quality Trends 53

Table 12. Trend test results for arsenic in basins for each land-use change category.—Continued

[Red indicates significant result. Abbrebiations: μg/L, micrograms per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Tau Score Score variance p-value

Slope

(μg/L per 

year)

All wells—Continued

No change—Continued

Sevier Desert –0.20 –2 17 0.806 –0.13 –0.44 –16 90 0.114 –0.14

Tooele Valley 0.33 2 9 0.734 0.02 0.14 4 63 0.706 0.01

Utah Valley — — — — — 0.60 9 28 0.133 0.02

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — –0.83 –5 8 0.149 –0.04

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — –0.17 — 8 1.000 0.00

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — –0.27 –4 27 0.566 –0.03

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — 0.20 2 17 0.806 0.03

Low use to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — –0.17 — 8 1.000 –0.03

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — –0.33 –2 9 0.734 –0.06

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —
54 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 12. Trend test results for arsenic in basins for each land-use change category.—Continued

[Red indicates significant result. Abbrebiations: μg/L, micrograms per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Tau Score Score variance p-value

Slope

(μg/L per 

year)

All wells—Continued

Low use to urban—Continued

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — –1.00 –10 17 10.027 –0.20

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — –0.20 –2 17 0.806 –0.05

Wells less than 200 feet deep

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area — — — — — — — — — —
Results: Identification and Quantification of Groundwater-Quality Trends 55

Table 12. Trend test results for arsenic in basins for each land-use change category.—Continued

[Red indicates significant result. Abbrebiations: μg/L, micrograms per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Wells less than 200 feet deep—Continued

No change—Continued

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area 0.33 2 9 0.734 0.01 –0.10 — 16 1.000 0.00

Lower Bear River Basin –0.67 –4 9 0.308 –0.08 –0.67 –4 9 0.308 –0.08

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley –0.67 –4 7 0.245 –0.12 –0.67 –10 27 10.085 –0.29

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — 1.00 6 9 10.089 0.06

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0.00 0 9 1.000 0.03

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0.30 3 16 0.613 3.58

Sevier Desert — — — — — — — — — —
56 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 13. Number of wells; period of record; number of nitrate measurements; and minimum, maximum, and median nitrate 

concentration in each basin for each land-use change category.

[mg/L, miligrams per liter; —, no data]

Basin Number 

of wells

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells

Production to low use

Beryl-Enterprise Area 1 2013 2013 1 0 0 1.41 1.41 —

Cedar City Valley 2 1999 1999 2 0 0 0.25 0.39 —

Milford Valley 2 1975 2011 3 0 0 0.77 1.3 1.1

Parowan Valley 1 2013 2013 1 0 0 1.55 1.55 —

Sevier Desert 1 1981 1981 1 0 0 0.02 0.02 —

Tooele Valley 1 1999 1999 4 0 0 0.92 1.71 1.1

Urban to production

Cache Valley 2 1989 2015 30 0 0 0.77 8.84 5.8

East Shore Area 3 1980 1991 3 0 0 0.6 1.5 0.73

Salt Lake Valley 3 1993 2009 62 1 2 0.2 7.6 4.2

Tooele Valley 1 2000 2000 1 0 0 0.83 0.83 —

Utah Valley 2 1981 2003 2 0 0 2.45 2.5 —

Table 12. Trend test results for arsenic in basins for each land-use change category.—Continued

[Red indicates significant result. Abbrebiations: μg/L, micrograms per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Tau Score Score variance p-value

Slope

(μg/L per 

year)

Wells less than 200 feet deep—Continued

Low use to production—Continued

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

1Significant value.
Results: Identification and Quantification of Groundwater-Quality Trends 57

Table 13. Number of wells; period of record; number of nitrate measurements; and minimum, maximum, and median nitrate 

concentration in each basin for each land-use change category.—Continued

[mg/L, miligrams per liter; —, no data]

Basin

Number 

of we

lls

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells—Continued

No change

Beryl-Enterprise Area 33 1975 2015 288 2 1 0.04 10 1.9

Cache Valley 79 1975 2015 636 34 5 0.01 18.85 1.3

Cedar City Valley 71 1975 2015 434 12 3 0.02 13.3 0.9

East Shore Area 175 1975 2015 1,521 182 12 0.01 18 1.1

Lower Bear River Basin 93 1975 2015 764 43 6 0.001 27.9 1

Milford Valley 53 1975 2015 374 23 6 0.01 40.3 0.75

Northern Juab Valley 38 1975 2015 195 1 1 0.01 42 3.15

Pahvant Valley 75 1975 2015 354 4 1 0.02 43.3 3.2

Parowan Valley 41 1975 2015 148 20 14 0.01 6.38 1.01

Salt Lake Valley 396 1975 2015 3,240 230 7 0.01 86 1.34

Sevier Desert 83 1975 2015 379 78 21 1.00E–06 22 0.36

Tooele Valley 188 1975 2015 841 12 1 0.02 36.9 1.83

Utah Valley 200 1975 2015 1,732 128 7 9.00E–04 46 0.85

Production to urban

Cache Valley 11 1977 2015 110 3 3 0.05 8.9 0.8

Cedar City Valley 4 1995 2015 22 0 0 0.93 4.83 3.66

East Shore Area 33 1975 2015 206 29 14 0.01 3 0.45

Lower Bear River Basin 1 1998 1998 1 1 100 0.05 0.05 —

Salt Lake Valley 63 1976 2015 370 7 2 0.03 25 2.42

Sevier Desert 1 1978 2011 16 0 0 0.04 1.1 0.3

Tooele Valley 8 1981 2015 66 0 0 0.3 6.36 2.3

Utah Valley 53 1977 2015 315 12 4 0.01 15.4 1.63

Low use to production

Beryl-Enterprise Area 1 1987 2015 17 0 0 2.06 5.8 3.81

Cache Valley 4 1976 2015 41 4 10 0.04 4.26 0.25

Cedar City Valley 3 1999 2000 3 0 0 0.521 3.28 2.45

Northern Juab Valley 2 1975 2012 13 0 0 1.68 6.4 5.3

Pahvant Valley 3 1979 2015 9 0 0 4.6 16 6.1

Parowan Valley 2 1979 2013 11 0 0 1.7 2.21 2

Salt Lake Valley 6 1978 2015 110 3 3 0.1 2.2 1

Sevier Desert 1 1976 2015 13 0 0 6 16 11.1

Tooele Valley 7 1994 2013 15 0 0 0.2 3.5 0.4

Utah Valley 15 1975 2015 49 2 4 0.1 32.5 1.6

Low use to urban

Cache Valley 1 1991 1991 1 0 0 0.01 0.01 —

Cedar City Valley 27 1977 2015 259 3 1 0.035 19.5 1.1

East Shore Area 1 1984 1984 1 1 100 0.1 0.1 —

Lower Bear River Basin 2 1978 2015 44 1 2 0.01 3.5 1.65

Salt Lake Valley 18 1976 2015 152 21 14 0.01 9.16 1.3
58 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 13. Number of wells; period of record; number of nitrate measurements; and minimum, maximum, and median nitrate 

concentration in each basin for each land-use change category.—Continued

[mg/L, miligrams per liter; —, no data]

Basin

Number 

of we

lls

Starting 

year

Ending 

year

Number

of 

samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells—Continued

Low use to urban—Continued

Sevier Desert 1 1998 2013 3 1 33 0.1 0.3 —

Tooele Valley 18 1977 2015 105 0 0 0.2 4.6 0.9

Utah Valley 27 1976 2015 246 11 4 0.01 23.14 0.4

Wells less than 200 feet deep

Production to low use

Tooele Valley 1 1999 1999 4 0 0 0.92 1.71 1.1

Urban to production

Salt Lake Valley 2 1998 2008 3 0 0 2.74 3.47 3.06

No change

Beryl-Enterprise Area 5 1975 2011 67 0 0 0.04 10 2.1

Cache Valley 13 1977 2015 55 6 11 0.02 6.66 0.9

Cedar City Valley 8 1977 2013 13 0 0 0.15 5.46 1.04

East Shore Area 14 1978 2014 55 10 18 0.01 18 0.7

Lower Bear River Basin 12 1979 2015 72 11 15 0.01 27.9 0.7

Milford Valley 3 1975 2015 16 0 0 0.597 5.69 1.1

Northern Juab Valley 6 1976 1998 16 0 0 1.1 9.3 2

Pahvant Valley 15 1975 2015 59 2 3 0.1 9.1 2.42

Parowan Valley 1 1986 2014 9 0 0 0.567 2.4 1.82

Salt Lake Valley 65 1976 2015 221 22 10 0.01 86 1.32

Sevier Desert 20 1977 2015 30 0 0 0.01 4.8 0.65

Tooele Valley 49 1979 2015 87 3 3 0.02 31 1.4

Utah Valley 30 1978 2015 61 18 30 0.02 6.19 0.8

Production to urban

East Shore Area 4 1978 2015 32 2 6 0.1 2.6 0.53

Lower Bear River Basin 1 1998 1998 1 1 100 0.05 0.05 —

Salt Lake Valley 14 1977 2008 43 1 2 0.1 25 1.22

Tooele Valley 1 1999 2003 5 0 0 4.29 4.93 4.46

Utah Valley 5 1980 1981 5 0 0 0.14 3.1 1.5

Low use to production

Cache Valley 1 1979 2015 19 1 5 0.1 0.26 0.133

Cedar City Valley 1 1999 1999 1 0 0 3.28 3.28 —

Salt Lake Valley 3 1978 2014 40 1 3 0.1 1.8 0.95

Tooele Valley 3 1994 1994 3 0 0 0.54 3.5 2.2

Utah Valley 1 2008 2013 3 0 0 1.88 1.94 1.88

Low use to urban

Cedar City Valley 1 1999 1999 1 0 0 8.98 8.98 —

Salt Lake Valley 2 1993 2008 15 0 0 0.355 2 0.5

Tooele Valley 6 2001 2001 6 0 0 0.68 1.03 0.83

Utah Valley 7 1981 2008 31 6 19 0.05 1.8 0.2
Results: Identification and Quantification of Groundwater-Quality Trends 59

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

Low use to urban, shallow wells

Production to urban, shallow wells

Low use to urban, all wells

Production to urban, all wells

No change, shallow wells

Low use to production, shallow wells

No change, all wells

Low use to production, all wells

Production to low use, shallow wells

Urban to production, all wells

Production to low use, all wells

A. Decadal medians B. Sub-decadal medians

Nitrate median concentration, in milligrams per literNitrate median concentration, in milligrams per liter

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

1970 1980 1990 2000 2010

Decade

1970 1980 1990 2000 2010

Decade EXPLANATION

Figure 20. Decadal and sub-decadal median nitrate concentration in select A, basins and B, sub-basins by land-use change 

category in Utah.
60 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 14. Trend test results for nitrate in basins for each land-use change category.

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — 0.8 8 17 10.086 0.19

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0 0 9 1.000 –0.01

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area –1.00 –10 17 10.027 –0.03 –0.72 –26 92 10.009 –0.03

Cache Valley 0.90 9 16 10.043 0.01 0.50 18 92 10.076 0.02

Cedar City Valley 0.20 2 17 0.806 0.00 0.19 7 91 0.529 0.00

East Shore Area 1.00 10 17 10.027 0.02 0.83 30 92 10.002 0.01

Lower Bear River Basin –0.80 –8 17 10.086 –0.01 –0.47 –17 91 10.093 –0.01

Milford Valley –0.70 –7 16 0.130 –0.03 –0.50 –18 92 10.076 –0.03

Northern Juab Valley –1.00 –10 17 10.027 –0.05 –0.58 –21 91 10.036 –0.05

Pahvant Valley 0.20 2 17 0.806 0.01 0.06 2 92 0.917 0.01

Parowan Valley –0.40 –4 17 0.462 –0.02 –0.50 –18 92 10.076 –0.02

Salt Lake Valley 1.00 10 17 10.027 0.01 0.64 23 91 10.021 0.01
Results: Identification and Quantification of Groundwater-Quality Trends 61

Table 14. Trend test results for nitrate in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells—Continued

No change—Continued

Sevier Desert 0.20 2 17 0.806 0.00 0.31 11 91 0.295 0.01

Tooele Valley 0.60 6 17 0.221 0.02 0.50 18 92 10.076 0.03

Utah Valley 0.00 0 17 1.000 0.00 –0.17 –6 90 0.598 0.00

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley –0.20 –2 17 0.806 –0.04 0.08 3 91 0.834 0.00

Cedar City Valley — — — — — 1.00 6 9 10.089 0.10

East Shore Area 0.60 6 17 0.221 0.01 0.44 16 92 0.118 0.01

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley –0.20 –2 17 0.806 –0.02 –0.11 –4 92 0.754 –0.01

Sevier Desert — — — — — — — — — —

Tooele Valley –0.33 –2 9 0.734 –0.01 –0.20 –3 28 0.707 –0.02

Utah Valley 0.20 2 17 0.806 0.01 –0.28 –10 92 0.348 –0.02

Low use to production

Beryl-Enterprise Area — — — — — –0.33 –2 9 0.734 –0.03

Cache Valley –0.80 –8 17 10.086 –0.01 –0.43 –9 44 0.230 –0.01

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley –1.00 –6 9 10.089 –0.02 — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.00 0 9 1.000 0.00 0.14 3 44 0.764 0.01

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley –0.40 –4 17 0.462 –0.07 –0.36 –10 65 0.266 –0.04

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley –0.60 –6 17 0.221 –0.01 –0.43 –12 65 0.174 –0.01

East Shore Area — — — — — — — — — —

Lower Bear River Basin –1.00 –6 9 10.089 –0.01 –0.33 –5 28 0.452 0.00

Milford Valley — — — — — — — — — —
62 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 14. Trend test results for nitrate in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells—Continued

Low use to urban—Continued

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.33 2 9 0.734 0.00 –0.14 –3 44 0.764 –0.01

Sevier Desert — — — — — — — — — —

Tooele Valley 0.00 0 17 1.000 0.00 –0.19 –7 91 0.529 –0.01

Utah Valley –0.30 –3 16 0.613 –0.02 –0.39 –14 92 0.175 –0.02

Wells less than 200 feet deep

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area –0.80 –8 17 10.086 –0.03 –0.86 –24 65 10.004 –0.04
Results: Identification and Quantification of Groundwater-Quality Trends 63

Table 14. Trend test results for nitrate in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Wells less than 200 feet deep—Continued

No change—Continued

Cache Valley 0.17 1 8 1.000 0.00 0.27 4 27 0.566 0.01

Cedar City Valley — — — — — — — — — —

East Shore Area –0.33 –2 9 0.734 0.00 –0.14 –3 44 0.764 0.00

Lower Bear River Basin 1.00 6 9 10.089 0.03 0.81 17 44 10.016 0.03

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — 0.33 2 9 0.734 0.09

Pahvant Valley 0.00 0 17 1.000 0.00 0.00 0 43 1.000 0.00

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.80 8 17 10.086 0.06 0.64 18 65 10.035 0.07

Sevier Desert — — — — — — — — — —

Tooele Valley 0.33 2 9 0.734 0.02 0.00 0 17 1.000 0.02

Utah Valley –0.67 –4 9 0.308 –0.01 –0.60 –9 28 0.133 –0.02

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area 0.67 4 9 0.308 0.01 0.60 9 28 0.133 0.01

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0.00 0 17 1.000 0.00

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley –1.00 –6 9 10.089 –0.004 — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.33 2 9 0.734 0.02 0.33 2 9 0.734 0.03

Sevier Desert — — — — — — — — — —
64 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 14. Trend test results for nitrate in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Wells less than 200 feet deep—Continued

Low use to production—Continued

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — –0.83 –5 8 0.149 –0.03

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — –1.00 –6 9 10.089 –0.002

1Significant value.

Table 15. Number of wells; period of record; number of dissolved solids measurements; and minimum, maximum, and median 

dissolved-solids concentration in each basin for each land-use change category.

[mg/L, miligrams per liter; —, no data; NA, not applicable]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells

Production to low use

Beryl-Enterprise Area 1 2013 2013 1 0 0 281 281 —

Cedar City Valley 2 1999 1999 2 0 0 343 352 —

Milford Valley 2 1975 2011 3 0 0 436 3,320 3,230

Parowan Valley 1 2013 2013 1 0 0 267 267 —

Sevier Desert 2 1980 1986 3 0 0 378 2,200 1,840

Tooele Valley 3 1978 1999 6 0 0 674 3,360 957

Urban to production

Cache Valley 2 1989 2013 10 0 0 304 448 358

East Shore Area 7 1980 1991 9 0 0 242 790 515

Salt Lake Valley 4 1980 2009 12 0 0 1,065 1,600 1,320

Tooele Valley 1 2000 2000 1 0 0 438 438 —

Utah Valley 2 1981 2003 2 0 0 260 353 —
Results: Identification and Quantification of Groundwater-Quality Trends 65

Table 15. Number of wells; period of record; number of dissolved solids measurements; and minimum, maximum, and median 

dissolved-solids concentration in each basin for each land-use change category.—Continued

[mg/L, miligrams per liter; —, no data; NA, not applicable]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells—Continued

No change

Beryl-Enterprise Area 34 1975 2015 253 0 0 125 1,950 406

Cache Valley 73 1975 2015 273 0 0 150 1,986 290

Cedar City Valley 69 1975 2015 288 0 0 110 3,070 404

East Shore Area 204 1975 2015 835 0 0 28 4,000 300

Lower Bear River Basin 92 1975 2015 397 0 0 88 2,360 338

Milford Valley 55 1975 2015 367 0 0 156 10,200 456

Northern Juab Valley 38 1975 2015 151 0 0 18 2,940 810

Pahvant Valley 76 1975 2015 330 0 0 10 6,520 961

Parowan Valley 43 1975 2015 122 0 0 135 672 312

Salt Lake Valley 413 1975 2015 2,230 0 0 10 20,900 450

Sevier Desert 92 1975 2015 259 0 0 162 24,300 344

Tooele Valley 206 1975 2015 553 0 0 143 17,000 658

Utah Valley 211 1975 2015 791 0 0 55 2,560 313

Production to urban

Cache Valley 9 1977 2015 41 0 0 174 535 286

Cedar City Valley 5 1977 2014 10 0 0 285 1,460 880

East Shore Area 36 1975 2013 113 0 0 112 2,460 258

Lower Bear River Basin 1 1998 1998 1 0 0 906 906 —

Salt Lake Valley 64 1976 2015 280 0 0 130 8,550 794

Sevier Desert 1 1978 2008 8 0 0 214 262 224

Tooele Valley 9 1978 2013 44 0 0 234 5,080 848

Utah Valley 55 1977 2015 142 0 0 131 1,390 312

Low use to production

Beryl-Enterprise Area 1 1987 2013 8 0 0 336 723 430

Cache Valley 4 1977 2015 28 0 0 218 504 258

Cedar City Valley 3 1999 2000 3 0 0 369 1,790 761

Northern Juab Valley 2 1975 2012 14 0 0 299 746 698

Pahvant Valley 3 1979 2015 10 0 0 673 3,140 851

Parowan Valley 2 1979 2013 11 0 0 268 333 278

Salt Lake Valley 9 1978 2015 97 0 0 394 1,150 704

Sevier Desert 1 1976 2015 16 0 0 421 629 555

Tooele Valley 7 1978 2013 10 0 0 300 2,100 358

Utah Valley 14 1975 2015 41 0 0 206 1,230 581

Low use to urban

Cedar City Valley 25 1977 2015 83 0 0 112 2,510 584

East Shore Area 1 1984 1984 1 0 0 2,960 2,960 —

Lower Bear River Basin 2 1978 2010 16 0 0 212 588 256

Salt Lake Valley 21 1976 2015 100 0 0 269 9,290 620
66 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 15. Number of wells; period of record; number of dissolved solids measurements; and minimum, maximum, and median 

dissolved-solids concentration in each basin for each land-use change category.—Continued

[mg/L, miligrams per liter; —, no data; NA, not applicable]

Basin

Number 

of

wells

Starting 

year

Ending 

year

Number

of samples

Number of 

censored 

samples

Percent 

censored

Minimum 

concentration 

(mg/L)

Maximum 

concentration 

(mg/L)

Median 

concentration 

(mg/L)

All wells—Continued

Low use to urban—Continued

Tooele Valley 20 1977 2013 64 0 0 196 6,460 393

Utah Valley 27 1976 2013 107 0 0 96 1,290 278

Wells less than 200 feet deep

Production to low use

Tooele Valley 1 1999 1999 4 0 0 674 1,120 803

Urban to production

Salt Lake Valley 2 1998 2008 3 0 0 1,140 1,240 1,230

No change

Beryl-Enterprise Area 5 1975 2011 68 0 0 288 1,160 628

Cache Valley 13 1977 2015 36 0 0 162 539 311

Cedar City Valley 8 1977 2013 13 0 0 248 1,350 363

East Shore Area 17 1978 2015 44 0 0 152 4,000 267

Lower Bear River Basin 12 1977 2014 45 0 0 119 1,630 224

Milford Valley 3 1975 2015 19 0 0 376 1,080 551

Northern Juab Valley 6 1976 1998 18 0 0 399 1,070 629

Pahvant Valley 15 1975 2015 60 0 0 426 6,050 3,550

Parowan Valley 1 1986 2014 10 0 0 248 363 304

Salt Lake Valley 64 1976 2015 164 0 0 57 8,970 699

Sevier Desert 20 1977 2015 32 0 0 246 24,300 377

Tooele Valley 45 1978 2014 71 0 0 264 5,010 945

Utah Valley 33 1978 2015 71 0 0 91 2,560 326

Production to urban

East Shore Area 4 1978 2008 12 0 0 112 613 172

Lower Bear River Basin 1 1998 1998 1 0 0 906 906 NA

Salt Lake Valley 14 1977 2008 49 0 0 390 2,630 528

Tooele Valley 2 1978 2003 7 0 0 557 1,070 963

Utah Valley 7 1980 2014 7 0 0 230 1,390 341

Low use to production

Cache Valley 1 1979 2015 19 0 0 218 266 257

Cedar City Valley 1 1999 1999 1 0 0 761 761 NA

Salt Lake Valley 4 1978 2013 31 0 0 415 1,150 560

Tooele Valley 1 1978 1978 1 0 0 2,100 2,100 NA

Utah Valley 1 2008 2013 3 0 0 206 218 208

Low use to urban

Cedar City Valley 1 1999 1999 1 0 0 735 735 NA

Salt Lake Valley 3 1991 2008 8 0 0 652 4,060 843

Tooele Valley 6 2001 2001 6 0 0 350 1,050 726

Utah Valley 7 1979 2005 29 0 0 114 479 232
Results: Identification and Quantification of Groundwater-Quality Trends 67

Low use to urban, shallow wells

Production to urban, shallow wells

Low use to urban, all wells

Production to urban, all wells

No change, shallow wells

Low use to production, shallow wells

No change, all wells

Low use to production, all wells

Production to low use, shallow wells

Urban to production, all wells

A. Decadal medians B. Sub-decadal medians

Dissolved solids median concentration, in milligrams per literDissolved solids median concentration, in milligrams per liter

1970 1980 1990 2000 2010

Decade

1970 1980 1990 2000 2010

EXPLANATION Decade

250

300

350

200

250

300

350

400

300

500

700

500

1,000

1,500

1,000

2,000

3,000

200

400

600

800

500

600

700

800

900

1,000

500

1,000

1,500

1,000

2,000

3,000

4,000

5,000

400

500

600

700

400

500

600

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Lower

Bear

River

Basin

Cache

Valley

East

Shore

Area

Tooele

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Salt

Lake

Valley

Utah

Valley

Northern

Juab

Valley

Sevier

Desert

Pahvant

Valley

Sevier

Desert

Pahvant

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

Milford

Valley

BerylEnterprise

Area

Parowan

Valley

Cedar

City

Valley

290

300

310

320

330

400

600

800

200

300

400

200

250

300

350

250

500

750

1,000

1,250

500

1,000

1,500

1,000

2,000

3,000

200

400

600

800

1,000

500

600

700

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900

1,000

400

600

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3,000

4,000

5,000

400

500

600

700

400

500

600

290

300

310

320

330

340

400

600

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Figure 21. Decadal and sub-decadal median dissolved-solids concentration in select A, basins and B, sub-basins by 

land-use change category in Utah.
68 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Dissolved solids trends were generally not associated 

with land-use change at a well except in a few areas (table 16). 

Significant increases in dissolved solids concentrations were 

identified in the Cache Valley wells experiencing a transition 

from production to urban land, and in the Salt Lake Valley 

wells experiencing a transition from low use to production. 

Wells in the Salt Lake Valley experiencing a transition 

from low use to urban had a significant decreasing trend, 

although this was not observed in the shallow subset of wells 

due to insufficient data and so it may be associated with an 

increase in deeper, cleaner wells to supply urban needs. In a 

nationwide study of decadal-scale changes in groundwater 

quality, Lindsey and Rupert (2012) reported more significant 

increases in dissolved-solids concentrations in urban areas 

than agricultural areas. Significant decreasing trends also 

were identified in the Northern Juab Valley wells associated 

with a transition from low use to production. However, these 

results represent two wells, which are likely not representative 

of more widespread water-quality conditions. Significant 

decreasing trends in Milford Valley and increasing trends 

in the Salt Lake Valley were identified among wells where 

land use did not change. Among shallow wells where land 

use did not change, increasing trends were identified in 

Beryl-Enterprise Area and Tooele Valley, and a decreasing 

trend was identified in Utah Valley.

These results highlight the complexity of the relationship 

between land use and arsenic, nitrate, and dissolvedsolids concentrations, and trends that depend on a range of 

conditions at various spatial and temporal scales. Geologic 

and geochemical conditions are the most important factors 

affecting arsenic concentrations in groundwater (Bexfield 

and others, 2011). Groundwater redox condition, fertilizer 

application rates, and irrigation practices (Paul and 

others, 2007) likely all contribute to differences in nitrate 

concentrations among basins and over time. Fertilizer 

application rate and sprinkler irrigation have been reported 

to correlate positively with elevated nitrate concentrations, 

whereas reducing geochemical conditions have been reported 

to correlate negatively with elevated nitrate concentrations 

because of denitrification (Paul and others, 2007). Many of 

the processes that influence nitrate in groundwater apply 

to dissolved solids as well, although there are additional 

processes that control dissolved solids in groundwater. 

Recharge of surface water containing high dissolved-solids 

concentrations can increase groundwater concentrations. 

Surface water can have elevated dissolved solids due to runoff, 

wastewater discharge, spills, or mining and forestry activities. 

Groundwater interaction with aquifer material can result 

in increased dissolved-solids concentrations. For example, 

concentrations increase along flow paths in Utah Valley and 

Salt Lake Valley (Anning and others, 2007).

Although arsenic, nitrate, and dissolved solids trends 

were not generally associated with land-use changes at 

wells, land use and other human activities are still important 

drivers of groundwater conditions. Basinwide trends have 

been detected, despite a relatively small amount of basinwide 

land-use change. The indirect connection between water 

quality and land use at wells relates more to the nuanced 

effects of human activities, which can occur at different spatial 

and temporal scales, and effects at wells can be affected by 

travel time lags. Further, land use may not have to change 

for activity on the land to create impacts to groundwater. 

For example, building development can increase in an urban 

area, which may increase dissolved solids in urban runoff that 

eventually impacts groundwater. The land-use category did 

not change, but the activity may still impact groundwater. The 

data available on land-use change does not necessarily capture 

the distinctions of increased development or population 

density either.
Results: Identification and Quantification of Groundwater-Quality Trends 69

Table 16. Trend test results for dissolved solids in basins for each land-use change category.

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area –0.20 –2 17 0.806 –1 –0.11 –4 92 0.754 –2

Cache Valley 0.20 2 17 0.806 0 0.00 0 92 1.000 0

Cedar City Valley 0.00 0 17 1.000 –1 0.33 12 92 0.251 3

East Shore Area 0.30 3 16 0.613 0 0.08 3 91 0.834 0

Lower Bear River Basin –0.40 –4 17 0.462 –1 –0.11 –4 92 0.754 –1

Milford Valley –0.80 –8 17 10.086 –5 –0.67 –24 92 10.016 –4

Northern Juab Valley –0.60 –6 17 0.221 –5 –0.50 –14 65 0.108 –10

Pahvant Valley –0.40 –4 17 0.462 –3 –0.44 –16 92 0.118 –7

Parowan Valley –0.40 –4 17 0.462 0 –0.06 –2 92 0.917 0

Salt Lake Valley 0.80 8 17 10.086 3 0.44 16 92 0.118 3
70 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 16. Trend test results for dissolved solids in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells—Continued

No change—Continued

Sevier Desert –0.40 –4 17 0.462 –1 0.28 10 92 0.348 1

Tooele Valley 0.40 4 17 0.462 1 0.17 6 92 0.602 2

Utah Valley –0.20 –2 17 0.806 0 –0.08 –3 91 0.834 0

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley 0.80 8 17 10.086 4 0.71 15 44 10.035 4

Cedar City Valley — — — — — — — — — —

East Shore Area –0.40 –4 17 0.462 –5 –0.14 –4 65 0.711 –2

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley –0.20 –2 17 0.806 –1 –0.14 –4 65 0.711 –4

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley 0.00 0 17 1.000 0 –0.21 –6 65 0.536 0

Low use to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley 0.60 6 17 0.221 0 0.00 0 17 1.000 0

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — –6 9 10.089 –2 — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley 1.00 6 9 10.089 8 0.71 15 44 10.035 7

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley 0.00 0 17 1.000 0 0.07 2 65 0.902 1

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley –0.20 –2 17 0.806 –2 0.14 3 44 0.764 2

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —
Results: Identification and Quantification of Groundwater-Quality Trends 71

Table 16. Trend test results for dissolved solids in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

All wells—Continued

Low use to urban—Continued

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley –0.80 –8 17 10.086 –13 –0.57 –16 65 10.063 –12

Sevier Desert — — — — — — — — — —

Tooele Valley –0.33 –2 9 0.734 –3 –0.14 –3 44 0.764 –9

Utah Valley 0.20 2 17 0.806 1 0.07 2 65 0.902 0

Wells less than 200 feet deep

Production to low use

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Urban to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

No change

Beryl-Enterprise Area 0.80 8 17 10.086 1 0.36 10 65 0.266 1
72 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Table 16. Trend test results for dissolved solids in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Wells less than 200 feet deep—Continued

No change—Continued

Cache Valley 0.83 5 8 0.149 3 0.60 6 17 0.221 4

Cedar City Valley — — — — — — — — — —

East Shore Area 0.40 4 17 0.462 3 0.04 1 64 1.000 0

Lower Bear River Basin –0.20 –2 17 0.806 0 –0.21 –6 65 0.536 –1

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — –0.67 –4 9 0.308 –24

Pahvant Valley 0.20 2 17 0.806 59 0.05 1 44 1.000 14

Parowan Valley — — — — — — — — — —

Salt Lake Valley 0.60 6 17 0.221 5 0.36 10 65 0.266 6

Sevier Desert 0.00 0 9 1.000 3 0.00 0 9 1.000 3

Tooele Valley 0.67 4 9 0.308 17 0.87 13 28 10.024 26

Utah Valley –0.67 –4 9 0.308 –1.00 –0.62 –13 44 10.072 —

Production to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0.00 0 17 1.000 –1

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to production

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley –0.17 –1.00 8 1.000 0 — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — 0.30 3 16 0.613 4

Sevier Desert — — — — — — — — — —
Results: Identification and Quantification of Groundwater-Quality Trends 73

Trends Across Analytes and Land-Use Change

Trends across multiple analytes can indicate basinwide 

changes to the hydrologic system. In comparing the total 

number of trends (using decadal and sub-decadal medians 

from combined NWIS and SDWIS data) in each basin across 

analytes, the water quality in several basins has changed 

more (fig. 22A). Basins with three significant trends across 

all analytes include the Beryl-Enterprise Area, East Shore 

Area, and Milford Valley. In the Beryl-Enterprise Area, 

concentrations of arsenic increased, whereas concentrations 

of nitrate decreased. In the East Shore Area, concentrations of 

arsenic and nitrate increased. In Milford Valley, concentrations 

of nitrate and dissolved solids decreased. Cache Valley, Lower 

Bear River Basin, Northern Juab Valley, and Salt Lake Valley 

each had two significant trends, although they were for a 

single analyte in each basin. This gives increased confidence 

in the trend result for the particular analyte but does not 

indicate that trends in other constituents are linked. Utah 

Valley had two trends across two analytes. The other basins 

had between zero and one significant arsenic trend.

The East Shore Area, Cache Valley, and Salt Lake 

Valley had the most increasing trends. The number of 

increasing trends in a basin can be related to basinwide 

land use and land-use change patterns, although land-use 

change is relatively limited. Change occurring in more than 

20 percent of a basin area only occurred in the Salt Lake 

Valley (25 percent), Utah Valley (24 percent), and East Shore 

Area (20 percent). In basins with the most land-use change, 

the highest percentage of land changed from production to 

urban. The total number of increasing trends were correlated 

with transitions from production to urban land (Pearson 

correlation coefficient equals 0.77, p-value equals 0.04). 

However, the limited area of land-use change in many basins 

reduces confidence in results. Some basins that experienced 

the most increasing trends such as the East Shore Area and 

Salt Lake Valley also are where most of the state’s population 

lives and where much of the population growth has occurred. 

These basins also had substantial areas of agriculture, which 

may account for the increasing nitrate trends in these basins. 

The absence of trends in some analytes in some basins may 

be related to the small amount of land-use change in those 

basins. For example, there was no nitrate trend in Cache 

Valley, where about 4 percent of the land had been converted 

from production to urban and another 4 percent had been 

converted from low use to production, and effectively replaced 

the production land lost to urban land. Land use, land-use 

change, and population all influence water-use practices as 

well, which can impact water and solute movement through 

the subsurface.

Table 16. Trend test results for dissolved solids in basins for each land-use change category.—Continued

[Red indicates significant result. Abbreviations: mg/L, miligrams per liter; —, no data]

Basin

Decadal medians Sub-decadal medians

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Tau Score Score 

variance p-value

Slope

(mg/L per 

year)

Wells less than 200 feet deep—Continued

Low use to production—Continued

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — — — — — —

Low use to urban

Beryl-Enterprise Area — — — — — — — — — —

Cache Valley — — — — — — — — — —

Cedar City Valley — — — — — — — — — —

East Shore Area — — — — — — — — — —

Lower Bear River Basin — — — — — — — — — —

Milford Valley — — — — — — — — — —

Northern Juab Valley — — — — — — — — — —

Pahvant Valley — — — — — — — — — —

Parowan Valley — — — — — — — — — —

Salt Lake Valley — — — — — — — — — —

Sevier Desert — — — — — — — — — —

Tooele Valley — — — — — — — — — —

Utah Valley — — — — — –0.17 –1.00 8 1.000 0

1Significant value.
74 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

−2

−1

0

1

2

Number and direction of trends Number and direction of trends

−2

−1

0

1

2

BerylEnterprise

Area

Lower

Bear River

Basin

Northern

Juab

Valley

Salt Lake

Valley

Utah

Valley

Pahvant

Valley

Parowan

Valley

Sevier

Desert

Cedar City

Valley

Tooele

Valley

Tooele

Valley

Parowan

Valley

Utah

Valley

Cache

Valley

Sevier

Desert

Lower Bear 

River

Basin

BerylEnterprise 

Area

Salt Lake 

Valley

Pahvant 

Valley

Milford 

Valley

Cedar City 

Valley

Northern 

Juab

Valley

East

Shore

Area

East

Shore

Area

Milford

Valley

Cache

Valley

A. Combined National Water Information System and Safe Drinking Water Information System data

B. Safe Drinking Water Information System data

Arsenic trends Dissolved solids trends

EXPLANATION

Nitrate trends

Figure 22. Number and direction of trends for each analyte in each basin for the A, National Water Information System 

and Safe Drinking Water Information System data combined and the B, Safe Drinking Water Information System data.
Summary 75

The activity within a basin appears to determine the 

number and direction of trends more than geographic 

location, which favors human drivers of trends over natural 

drivers. Basins proximal to each other with similar geologic 

conditions, such as the East Shore Area and Lower Bear 

River Basin had substantially different trend behavior. These 

basins have similar geologic histories and climate conditions 

(Bjorklund and McGreevy, 1974; Clark and others, 1990). 

Both basins lie adjacent to the Great Salt Lake and formed 

through normal faulting along the Wasatch Fault. Subsequent 

erosion of the uplifted mountains deposited sediment 

in the basins and rising and falling of Lake Bonneville 

further modified sediment deposition and erosion as well as 

groundwater quality. These basins have a similar temperate 

and arid climate and similar amounts of precipitations 

and temperatures. Precipitation in both basins increases 

significantly in the mountains, which feeds streams and 

groundwater recharge. Despite these similarities, increasing 

trends in arsenic and nitrate occurred in the East Shore Area, 

whereas nitrate decreased in the Lower Bear River Basin. 

Some of these patterns may be explained by population 

and land use. The East Shore Area spans Davis and Weber 

Counties, which had a combined population in 2010 of more 

than 500,000 people, whereas Box Elder County had almost 

50,000 people, of which the Lower Bear River Basin is less 

than one-fourth of the area (U.S. Census Bureau, 2019). The 

rate of population growth is estimated to be greater in Weber 

and Davis counties (10 and 15 percent, respectively) compared 

to Box Elder County (10 percent) from 2010 to 2018 

(U.S. Census Bureau, 2019). The East Shore Area had more 

urban area, whereas the Lower Bear River Basin had more 

production (agricultural) land in 1974 and 2012 (Falcone, 

2015). The East Shore Area also experienced land-use change 

across a greater area (20 percent) than the Lower Bear River 

Basin (5 percent). Further, the East Shore Area had more 

land converted from production to urban land over this 

period (13 percent compared to 1 percent in the Lower Bear 

River Basin).

In comparing the total number of trends using decadal 

and sub-decadal medians from SDWIS data in each basin 

across analytes, the water quality in several basins changed the 

most in the East Shore Area and Northern Juab Valley (three 

trends in each basin, fig. 22B). There were more increasing 

trends than decreasing trends for all analytes, and increasing 

trends among SDWIS data were more common than among 

NWIS and SDWIS data combined. Only Northern Juab Valley 

and Pahvant Valley had any decreasing trends in data from 

public-supply wells.

Summary

The U.S. Geological Survey, in cooperation with 

the Utah Department of Environmental Quality, Division 

of Water Quality, studied trends in arsenic, nitrate, and 

dissolved-solids concentrations in basins throughout Utah 

that have experienced substantial groundwater development. 

The significance and magnitude of decadal and sub-decadal 

(5-year) scale trends was determined using data from the 

National Water Information System (NWIS) and Safe 

Drinking Water Information System (SDWIS) datasets 

combined, and from the SDWIS dataset independently. 

Spatial variation in temporal trends and the relationship to 

land-use change were evaluated. Additionally, spatial patterns 

in concentrations and regulatory exceedances of arsenic, 

nitrate, dissolved solids, and other inorganic contaminants 

were assessed.

Data stored in the NWIS and SDWIS databases represent 

water samples taken at different kinds of wells; SDWIS data 

represent drinking water (before treatment) and NWIS data 

represent water used for a broader range of purposes. Trends 

in each basin were tested using SDWIS data separately to 

identify changes in water that will eventually be used for 

drinking water. However, combining the datasets increased 

the number of samples for trend analysis and captured a 

more complete picture of the overall water-quality conditions 

within a basin. Decadal and sub-decadal medians were 

calculated to increase the number of medians available for 

analysis. Although this more frequent calculation often 

provided enough medians for trend analysis, it also introduced 

increased variability in median concentrations over time 

that could obscure trend identification, particularly with the 

Mann-Kendall trend test, which identifies monotonic changes.
76 Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells

Changes in decadal and sub-decadal median arsenic, 

nitrate, and dissolved-solids concentrations over time occurred 

throughout the basins in this study. Significant trends in 

arsenic were identified in the Beryl-Enterprise Area, East 

Shore Area, Utah Valley, Pahvant Valley, Parowan Valley, and 

Sevier Desert. Rates of median-concentration change ranged 

between decreases of –0.24 microgram per liter (ug/L) per 

year and increases of 0.48 ug/L per year across basins and 

sub-basins. Significant nitrate trends were identified in the 

Beryl-Enterprise Area, East Shore Area, Milford Valley, Lower 

Bear River Basin, Northern Juab Valley, Salt Lake Valley, and 

Utah Valley. Rates of median-concentration change ranged 

between decreases of –0.08 milligrams per liter (mg/L) per 

year and increases of 0.02 mg/L per year across basins and 

sub-basins. More basins had decreasing trends than increasing 

trends in nitrate. Significant trends in dissolved solids were 

identified in Milford Valley, Cache Valley, and parts of the 

East Shore Area, Salt Lake Valley, and Utah Valley. Rates 

of median-concentration change ranged between decreases 

of –5 mg/L per year and increases of 7 mg/L per year across 

basins and sub-basins. Changes within sub-basins can drive 

or be obscured by inclusion of data from a larger basin. The 

rates of change for nitrate and dissolved solids were below 

or similar to rates of change observed nationwide and in the 

southwestern United States. The similarity between rates of 

change in Utah and Central Valley, California, is noteworthy 

in that nitrogen fertilizer application rates and population were 

substantially higher in the Central Valley.

Public-supply wells experienced a number of increasing 

trends, particularly for nitrate and dissolved solids. Many 

of the basins experienced trends in similar direction for 

nitrate and dissolved solids. The Salt Lake Valley Southeast 

experienced increases in arsenic, nitrate, and dissolved 

solids. Increasing trends were more common among data 

from public-supply wells than among data from all well 

types combined.

Broad land-use change, as well as population growth, 

was associated with water-quality changes over time, and 

land-use change at wells was more loosely associated with 

trends. However, this was in part affected by a lack of data 

from wells experiencing different kinds of land-use change. 

Information about land-use change provided insight into 

drivers of water-quality changes. Land-use changes directly 

at wells were only one component of the range of factors 

that impacted water quality at a well, including land and 

water use over a larger area surrounding and up-gradient 

from the well, rates and direction of groundwater movement, 

and geologic and hydrologic conditions. The controls on 

groundwater quality were complex and included spatial 

and temporal variability in the local hydrology, land use, 

and other human activities. Increasing trends identified in 

this report occurred in areas that had experienced land-use 

change, population growth and associated development, and 

substantial groundwater use. Basins where concentrations of 

arsenic, nitrate, or dissolved-solids concentrations increased 

represent areas of potential concern, whereas basins where 

concentrations decreased represent areas where improvements 

occurred. Human activity has impacted groundwater quality 

in Utah, and may continue to do so as the state’s population 

continues to grow.
References Cited 77

References Cited

Adams, W.J., DeForest, D.K., Tear, L.M., Payne, K., and 

Brix, K.V., 2015, Long-term monitoring of arsenic, 

copper, selenium, and other elements in Great Salt Lake 

(Utah, USA) surface water, brine shrimp, and brine flies: 

Environmental Monitoring and Assessment, v. 187, no. 118, 

13 p., https://doi.org/10.1007/s10661-014-4231-6.

Ahamed, S., Sengupta, M.K., Mukherjee, A., Hossain, M.A., 

Das, B., Nayak, B., Pal, A., Mukherjee, S.C., 

Pati, S., Dutta, R.N., Chatterjee, G., Mukherjee, A., 

Srivastava, R., and Chakraborti, D., 2006, Arsenic 

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

 Director, Utah Water Science Center 

U.S. Geological Survey

2329 West Orton Circle

Salt Lake City, Utah 84119-2047

801-908-5000

https://ut.water.usgs.gov

Publishing support provided by the U.S. Geological Survey 

Science Publishing Network, Sacramento Publishing Service Center
Miller—Quantifying Trends in Arsenic, Nitrate, and Dissolved Solids from Selected Wells—Scientific Investigation Report 2020–5047

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205047