Freshwater salinization syndrome on acontinental scaleSujay S. Kaushala,1, Gene E. Likensb,c,1, Michael L. Paced, Ryan M. Utze, Shahan Haqa, Julia Gormana, and Melissa Gresea

aDepartment of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740; bCary Institute of EcosystemStudies, Millbrook, NY 12545; cDepartment of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269; dDepartment of EnvironmentalSciences, University of Virginia, Charlottesville, VA 22904; and eFalk School of Sustainability, Chatham University, Gibsonia, PA 15044

Contributed by Gene E. Likens, November 30, 2017 (sent for review June 28, 2017; reviewed by Jacqueline A. Aitkenhead-Peterson, W. Berry Lyons, Diane M.McKnight, and Matthew Miller)

Salt pollution and human-accelerated weathering are shifting thechemical composition of major ions in fresh water and increasingsalinization and alkalinization across North America. We propose aconcept, the freshwater salinization syndrome, which links saliniza-tion and alkalinization processes. This syndrome manifests as con-current trends in specific conductance, pH, alkalinity, and basecations. Although individual trends can vary in strength, changesin salinization and alkalinization have affected 37% and 90%, re-spectively, of the drainage area of the contiguous United States overthe past century. Across 232 United States Geological Survey (USGS)monitoring sites, 66% of stream and river sites showed a statisticalincrease in pH, which often began decades before acid rain regula-tions. The syndrome is most prominent in the densely populatedeastern and midwestern United States, where salinity and alkalinityhave increased most rapidly. The syndrome is caused by salt pollution(e.g., road deicers, irrigation runoff, sewage, potash), acceleratedweathering and soil cation exchange, mining and resource extraction,and the presence of easily weathered minerals used in agriculture(lime) and urbanization (concrete). Increasing salts with strong basesand carbonates elevate acid neutralizing capacity and pH, and increas-ing sodium from salt pollution eventually displaces base cations onsoil exchange sites, which further increases pH and alkalinization.Symptoms of the syndrome can include: infrastructure corrosion, con-taminant mobilization, and variations in coastal ocean acidificationcaused by increasingly alkaline river inputs. Unless regulated andmanaged, the freshwater salinization syndrome can have significantimpacts on ecosystem services such as safe drinking water, contami-nant retention, and biodiversity.

emerging contaminants | drinking water | land use | anthropocene |carbon cycle

The abundance, integrity, and distribution of Earth’s freshwater are critical for human welfare. Only 2.5% of Earth’s

water is fresh, and dissolved salts within this water determine thedegree to which it can be used for drinking, industry, agriculture,and energy production (1, 2). The primary causes of freshwatersalinization around the world are agriculture, resource extraction,and land clearing (3, 4). Relatively recently, human salt inputs areincreasingly becoming recognized as important over large geo-graphic scales (5, 6). Alkalinization has received relatively lessrecognition as a related environmental issue, but increases in al-kalinity are also generated by salt pollution, accelerated weather-ing, and microbial processes that influence water chemistry (7).Salinization and alkalinization of fresh water can be inter-connected processes that deteriorate water quality over a range ofclimates, but it is typically assumed that these processes are relatedonly in arid regions (6–9). In particular, increasing concentrations ofdissolved salts with strong bases and carbonates can increase the pHof fresh water over time, linking salinization to alkalinization.We propose a concept, the freshwater salinization syndrome,

which links salinization and alkalinization processes along hy-drologic flow paths from small watersheds to coastal waters. Thefreshwater salinization syndrome manifests to varying degrees asconcurrent trends in specific conductance, pH, alkalinity, and

base cations (i.e., sodium, calcium, magnesium, and potassium).There are at least three primary categories of salt sources drivingthe freshwater salinization syndrome: (i) anthropogenic salt inputs(e.g., road salts, sewage, brines, sodic/saline irrigation runoff);(ii) accelerated weathering of natural geologic materials bystrong acids (e.g., acid rain, fertilizers, and acid mine drainage);and (iii) human uses of easily weathered resource materials (e.g.,concrete, lime), which cause increases in salts with strong basesand carbonates (6, 9–13). Although previous work has focusedprimarily on sodium chloride as a dominant form of salt pollution,increases in different mixtures of salt ions such as sodium, bi-carbonate, magnesium, sulfate, etc., as part of the freshwater sa-linization syndrome produce differential toxicity to aquatic life(14, 15), and further supports the need for studying the dynamicsof dissolved salts holistically. Although environmental impacts ofthe freshwater salinization syndrome are still poorly understood,symptoms can include: changes in biodiversity due to osmotic stressand desiccation, corrosion of infrastructure, increased contaminantmobilization, enhanced river carbonate transport, and impacts oncoastal ocean acidification caused by increasingly alkaline river inputs.Over geologic time, salinization and alkalinization of water

have naturally influenced the distribution and abundance ofEarth’s life (16). However, rates of salinization and alkalinizationin the Anthropocene are strongly influenced by three primary

Significance

Salinization and alkalinization impact water quality, but theseprocesses have been studied separately, except in arid regions.Globally, salinization has been largely attributed to agriculture,resource extraction, and land clearing. Alkalinization has beenattributed to recovery from acidification, with less recognitionas an environmental issue. We show that salinization and al-kalinization are linked, and trends in these processes impactmost of the drainage area of the United States. Increases insalinity and alkalinity are caused by inputs of salts containingstrong bases and carbonates that originate from anthropo-genic sources and accelerated weathering. We develop a con-ceptual model unifying our understanding of salinization andalkalinization and its drivers and impacts on fresh water inNorth America over the past century.

Author contributions: S.S.K., G.E.L., and M.L.P. designed research; S.S.K., R.M.U., S.H., J.G.,and M.G. performed research; R.M.U., S.H., J.G., and M.G. contributed new reagents/analytic tools; S.S.K., R.M.U., S.H., J.G., and M.G. analyzed data; and S.S.K., G.E.L.,M.L.P., R.M.U., S.H., J.G., and M.G. wrote the paper.

Reviewers: J.A.A.-P., Texas A&M University; W.B.L., The Ohio State University; D.M.M.,University of Colorado; and M.M., USGS.

The authors declare no conflict of interest.

This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).1To whom correspondence may be addressed. Email: skaushal@umd.edu or likensg@caryinstitute.org.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1711234115/-/DCSupplemental.

E574–E583 | PNAS | Published online January 8, 2018 www.pnas.org/cgi/doi/10.1073/pnas.1711234115

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

factors: atmospheric deposition, geology, and land use (7). Atmo-spheric acid deposition accelerates chemical weathering over largegeographic scales due to chemical dissolution and ion exchange inrocks and soils (7, 12, 17). Accelerated weathering of geologicalsubstrates can contribute to increased concentrations of major ionsin water including bicarbonate, calcium, magnesium, and potassium(7, 12, 18–21). Finally, land-use activities enhance the anthropo-genic inputs of easily weathered materials, including agriculturallime and impervious surfaces made of concrete, which can impactchemistry of drainage waters (13, 22, 23). Road salt and brines fromfracking fluids further contribute to ion exchange in soils andsediments, and can increase salinity and alkalinity via changes inbase cations and bicarbonate concentrations (11). Mining and re-source extraction contribute to accelerated weathering of rocks andsoils and the release of major ions and salts (21, 24–26). Finally,wastewater, water reuse, and irrigation practices associated withurban and agricultural land use can increase salinity and alkalinityin fresh water (7, 10, 27). All of these factors synergistically con-tribute to the freshwater salinization syndrome. The directionalityand severity of trends for different salt ions, alkalinity, and pH areinfluenced on a regional scale by dilution capacity of rivers due towater demand, changes in river runoff due to climate variability,and sea level rise and saltwater intrusion into freshwater sectionsof rivers and estuaries (6, 28, 29). A holistic understanding ofconcurrent patterns in salinization and alkalinization on a conti-nental scale is important for identifying and prioritizing regionalmanagement needs, given that interactions between differentsalts, specific conductance, and pH can influence aquatic life,ecosystem functioning, and services (14, 15, 19, 30).In this study, we (i) quantify concurrent trends in salinization

rates, alkalinization rates, and base cation concentrations infresh water on a continental scale and (ii) present a conceptualframework, called the freshwater salinization syndrome, whichillustrates and explains how coupled salinization and alkaliniza-tion can occur. We quantified long-term trends in specific con-ductance, base cations, alkalinity, and pH in 232 stream and riversites sampled by the USGS throughout North America. Al-though these sites include both streams and rivers, we will mainlyrefer to them as rivers throughout. Watersheds drained areasprimarily in the United States, but also extended into Canada,and spanned diverse regions and climates. Watersheds alsosometimes drained major reservoirs. Surprisingly, we found thatincreasing trends in the pH of rivers occurred decades before USClean Air Act Amendments in 1990, which specifically targetedreductions in acid rain (17). After 1990, a decrease in acidicprecipitation and acid mine drainage may be expected to con-tribute to an increase in alkalinity and pH in the latter part of thetwentieth century, but such reductions in acidification would notadequately explain earlier concurrent increases in specific con-ductance, base cations, alkalinity, and pH beginning in the earlyand middle twentieth century (31). The freshwater salinizationsyndrome illustrates how anthropogenic salts and human-accelerated weathering of natural and anthropogenic sub-strates synergistically drive salinization and alkalinization dueto a plethora of salts containing strong bases and carbonateswith potential for acid neutralization. In addition, increasedsodium from salt pollution displaces other base cations on soilexchange sites, which further increases pH and contributes toalkalinization. From a biological perspective, in-stream micro-bial processes such as denitrification, sulfate reduction, etc.,can further increase pH due to biochemical reactions thatgenerate alkalinity. Finally, photosynthesis and gross primaryproduction in eutrophic fresh waters in the Anthropocene canraise pH through depletion of total dissolved carbonate relativeto equilibrium with CO2. The freshwater salinization syndromecan impact ecosystem services related to drinking water quality(6, 7, 9), economic costs related to switching water treatmentand sources (9, 10), impacts on aquatic biodiversity (8), pipe

corrosion and leaching of metals (7, 9), and shifting rates ofcoastal ocean acidification (32, 33).

ResultsA large proportion of the streams and rivers in the contiguousUnited States have increasing trends in pH and specific con-ductance impacting most of the stream flow draining humid re-gions, such as the eastern United States (Figs. 1–3; SupportingInformation). We estimate that alkalinization (significantly in-creased pH over time) and salinization (significantly increasedspecific conductance over time) have impacted 90% and 37% ofthe drainage area of the contiguous United States, respectively.Increasing trends in specific conductance were typically highestin the northeastern United States (Figs. 1 and 2), and trendstended to be negative in the arid western United States (Figs. 1and 4), which receives significantly less precipitation (SupportingInformation). Statistically increasing trends in pH were typicalthroughout all regions of the contiguous United States (Figs. 1and 3). At some sites, the pH of river water actually declinedfollowing the Clean Air Act Amendments in 1990, which spe-cifically targeted acid rain; this may have been due to a reductionin weathering rates in some watersheds (discussed further be-low). Of 232 sites, statistically increasing trends were observed at66% of sites for pH, 39% of sites for specific conductance, 34%of sites for sodium, 29% for calcium, 33% for magnesium, and36% for potassium concentrations. In contrast, there was ageographic cluster of decreasing trends in specific conductance

Fig. 1. Maps showing locations of increasing, decreasing, and/or no trendsin specific conductance and pH in stream water throughout the continentalUnited States. Streamlines represent all conterminous US rivers with meanannual discharge exceeding 20 m3/s (47).

Kaushal et al. PNAS | Published online January 8, 2018 | E575

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

SEC

OLO

GY

PNASPL

US

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

and major cations in the arid western United States (Figs. 1 and4). Thus, the fastest rates of increase for most dissolved saltsoccurred from west to east on a regional scale with the easternUnited States showing the fastest rates. For example, the fastestmedian rates of increase in specific conductance and sodiumwere in the northeastern and southeastern United States, re-spectively (Fig. 4). There were exceptions for some salts. Forexample, the fastest increase in median potassium concentra-tions was in the midwestern United States likely due to potashfrom fertilizers (Fig. 4). Interestingly, there were decreasingmedian trends for specific conductance, and sodium and potas-sium concentrations in the Pacific Northwest, Southwest, andPacific regions (Fig. 4). Decreasing trends were observed for 9%of sites for pH, 18% of sites for specific conductance, 18% ofsites for sodium, 25% for calcium, 17% for magnesium, and 21%for potassium concentrations primarily in the arid westernUnited States (e.g., west of the 100th meridian).

Across all 232 sites, there were interrelationships betweentrends in pH, specific conductance, and base cations. There wereconcurrently increasing or decreasing trends in base cations atsome sites (Fig. 5 shows examples of increasing trends, but therewere also decreasing trends primarily in the arid western UnitedStates as indicated by Figs. 1 and 4). Although the increase insodium concentrations may be expected, it was surprising thatincreases in magnesium concentrations could exceed increases incalcium concentrations at some sites (Fig. 5). Magnesium con-centrations were considerably lower than calcium concentrationsin rivers, and therefore may have been more sensitive to an-thropogenic disturbances over time at some sites (Fig. 5). Acrossall sites, variability in the rate of change of pH (whether in-creasing or decreasing) declined as the initial specific conduc-tance of stream and river water increased (Fig. 6). The mostrapid changes in long-term pH trends occurred in rivers with thelowest specific conductance (<250 μs/cm at 25 °C), whereas those

Fig. 2. Examples of increasing trends in conductancein stream water throughout the continental UnitedStates, which are characteristic of ranges in their re-spective regions. Please note that vertical axes differ.

Fig. 3. Examples of increasing trends in pH in streamwater throughout the continental United States. ThepH decreased at some sites following US Clean Air ActAmendments in 1990, which targeted reductions inacidic deposition. Please note that vertical axes differ.

E576 | www.pnas.org/cgi/doi/10.1073/pnas.1711234115 Kaushal et al.

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

with the highest salinity and buffering capacity had relativelyminor or insignificant rates of change in pH (Fig. 6). Thresholdsin the initial specific conductance of rivers and buffering influ-enced whether there were significant long-term increasing ordecreasing pH trends. Rates of change in both specific conduc-tance and alkalinity were positively and linearly related to ratesof change in sodium concentrations across all sites (Fig. 6).

DiscussionBased on long-term river chemistry data, positive trends in sa-linization and alkalinization of fresh water were observed acrosslarge geographic areas of North America, which include water-sheds draining the United States and Canada. Such trends werecommon in humid regions of the eastern United States, whereacidic precipitation and human population density are thegreatest (6, 7, 17). Our analysis demonstrates that salinizationand alkalinization of inland waters are not restricted to arid re-gions as once assumed, and it suggests that these processes arecoupled throughout many humid regions in the United States.Human activities have increased salt inputs and acceleratedchemical weathering and cation exchange in watersheds, whichcontribute to dissolved salts containing strong bases and alkalimetals and alkaline earth metals (Na+, Ca2+, Mg2+, K+) and

carbonates (7, 13, 23). Geochemical and biogeochemical rela-tionships between these salts and acid-neutralizing capacity leadto interrelated increases in specific conductance, alkalinity, basecations, and pH (6, 7, 34, 35). Our results suggest that thesecoupled changes in specific conductance, major ions, and pH re-lated to the freshwater salinization syndrome have influenced thewater quality of most of the stream flow in the eastern UnitedStates (Fig. 1; Supporting Information). The causes and conse-quences of this freshwater salinization syndrome have implicationsfor ecosystem services of water including availability of high-quality drinking water, scaling and corrosion of pipes, toxicity ofmetals and ammonia, and buffering of coastal ocean acidification.

Variations in the Freshwater Salinization Syndrome Across a ContinentalScale. As mentioned earlier, regional patterns in the freshwatersalinization syndrome are influenced by geographic variations inatmospheric deposition, geology, and land use (35–38). It is wellknown that there are regional differences in sodic and saline watersbased on pH, electrical conductivity, and sodium absorption ratiothroughout North America. Typically, these regional patterns rep-resent: (i) fresh water with low salinity/alkalinity in humid regionsdraining bedrock resistant to weathering; (ii) fresh water withmoderate salinity/alkalinity in humid regions draining bedrockprone to weathering; and (iii) fresh water with high salinity/alka-linity in arid regions prone to high evaporation rates. However,regional differences in salinization and alkalinization rates andchanges in major ions over time on a continental scale are lessknown, and we discuss patterns and processes of the freshwatersalinization syndrome below.In the northeastern United States, watersheds draining humid,

developed land cover with low to moderate salinity/alkalinity typ-ically had the fastest increasing trends in salinization and alkalin-ization (Fig. 1). These watersheds also had the fastest median ratesof increasing sodium concentrations. In general, this region rep-resented the fastest increases in specific conductance, major cat-ions, and pH due to anthropogenic salts and human-acceleratedweathering (6, 7). Regional recovery from acid rain and acid minedrainage can contribute to increases in pH in the latter part of therecord for some rivers, but do not adequately explain concurrentand widespread increases in specific conductance and base cationsduring the early and middle twentieth century before the Clean AirAct (31). Road salt is a dominant source of salinization of freshwater in colder, humid regions of the northeastern United States,and has contributed to long-term, increasing trends in sodium andchloride in surface water and ground water (5, 6, 34, 39). Althoughless considered, road salts can also influence pH and long-termalkalinization of fresh water, and this is discussed further below ina subsequent section regarding fast weathering (23).Despite no or minimal use of road salts in the region, the fresh-

water salinization syndrome also impacts the southeastern UnitedStates, which suggests additional mechanisms. The freshwater sali-nization syndrome is also strongly influenced by the chemicalweathering of soils, bedrock, and easily weathered anthropogenicsubstrates (e.g., agricultural lime, urban impervious surfaces) acrossall regions (13, 18, 22, 23). It is well known that alkalinity, basecations, and pH in rivers of humid regions are related to underlyinglithology, land cover/land use, and weathering rates (7, 37). Thesoutheastern United States has elevated rates of acid deposition andurban impervious surfaces similar to the northeastern United States(17, 38), which further contribute to the freshwater salinizationsyndrome along with salt pollution and accelerated weathering.Agricultural activities further contribute to the freshwater sa-

linization syndrome in the midwestern United States. The fastestmedian rates of increasing potassium concentrations were ob-served in midwestern rivers. This increase was likely due topotassium in potash in agricultural fertilizers, and the variabilityin rates was likely due to K limitation in response to excessnitrogen and phosphorus from fertilizers (40). Agricultural liming

Specific Conductance

Sodium

Potassium

ry)mc/Sµ(

-1(m

g /L

) yr-1

(mg

/L) y

r- 1

Fig. 4. Differences in trends in specific conductance and sodium and potassiumconcentrations in stream and river sites across different regions of the UnitedStates. The center vertical lines of the box and whisker plots indicate the medianof the sample. The length of each whisker shows the range within which thecentral 50% of the values fall. Box edges indicate the first and third quartiles.Outliers were excluded and were primarily in the southwestern US region.

Kaushal et al. PNAS | Published online January 8, 2018 | E577

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

SEC

OLO

GY

PNASPL

US

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

to improve crop yields can also contribute to long-term alka-linization and increased dissolved salts in rivers of the mid-western United States (13). Finally, fertilizer-enhanced nitrificationcan also synergistically accelerate chemical weathering of cationsfrom bedrock and soils via leaching and enhanced ion exchange

reactions (17, 38, 41, 42). Thus, agricultural activities in com-bination with high levels of road salt pollution from impervioussurfaces and human-accelerated weathering can further con-tribute to the freshwater salinization syndrome in the mid-western United States.

0

1

2

3

4

5

6

7

8

10

20

30

40

50

60

70

80

90

100

1977

1981

1984

1988

1991

1994

1998

2001

2005

2008

Patuxent River, Maryland

0

5

10

15

20

25

0

50

100

150

200

250

300

1962

1967

1972

1977

1983

1988

1993

1 998

2003

2009

Saddle River, New Jersey

0

1

2

3

4

0

20

40

60

80

100

120

140

1978

1982

1985

198 9

1 992

1996

1 99 9

2003

2 007

2 010

Blackstone River,Rhode Island

0

20

40

60

80

100

120

140

0

100

200

300

400

500

600

700

800

900

1000

1957

1963

1969

1975

1981

198 6

1992

199 8

200 4

2 010

Canadian River, Texas

0

20

40

60

80

100

120

140

0

20

40

60

80

100

120

140

1959

1964

1970

1976

1982

1987

1993

1 99 9

2005

2 010

Sheyenne River,North Dakota

0

1

2

3

4

5

6

7

8

0

10

20

30

40

50

60

70

80

1974

1978

1983

1987

1991

1996

2 00 0

2 00 4

2009

0

0.5

1

1.5

2

2.5

3

3.5

0

5

10

15

20

25

30

35

1954

1959

1965

1970

1975

198 0

1 98 5

1991

1996

2 001

Neuse River, North Carolina

0

1

2

3

4

5

6

7

8

9

10

0

5

10

15

20

25

30

35

40

45

50

1969

1972

1975

1979

1982

1985

1989

1992

1995

199 8

Tombigbee River, Alabama

Ca

Na

Mg

0

2

4

6

8

10

12

14

16

18

0

20

40

60

80

100

120

140

16019

62

1967

1 97 2

197 7

198 3

1988

1993

1998

2 003

2 009

Passaic River, New Jersey

Fig. 5. Examples of increasing trends in base cations (sodium, calcium, and magnesium) in stream water throughout the continental United States. Timeseries were smoothed as moving averages over every three data points/observations. Please note that vertical axes differ.

E578 | www.pnas.org/cgi/doi/10.1073/pnas.1711234115 Kaushal et al.

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

Interestingly, specific conductance and base cation concentra-tions have decreased in some streams and rivers throughout theNorthwest, Pacific, and Southwest (particularly in arid regionswith high baseline salinity and alkalinity in surface waters). De-spite decreasing trends in specific conductance, there were stilllong-term increasing trends in pH in fresh water throughout thewestern United States. Previous work has also documented de-creasing trends in total dissolved solids and increasing trends inpH in the southwestern United States (35, 43). Concentrations ofalkalinity and major ions have decreased throughout the westernUnited States due to a combination of dilution and retention ofweathering products behind dams in reservoirs (e.g., calcite pre-cipitation) (31). For example, decreasing trends in total dissolvedsolids have been attributable to complex water diversions (mostlyfor agriculture) and decreased irrigation return flows of salts toriver systems (35). Efficiency of agricultural and irrigation prac-tices has also increased over time, which could also decrease saltaccumulation rates in some drainages (44, 45).Furthermore, return flow from urban wastewater has increased

while return flow from irrigation has decreased in some areasthroughout the western United States (46, 47). Although urbanwastewater contains dissolved salts, its salinity can be significantlylower than irrigation return flow and contribute to dilution overtime (46). Wastewater discharges represent a significant fractionof stream flow and nutrients in some rivers of the western UnitedStates (47, 48). Regional wastewater treatment has improved withreductions in organic carbon and oxygen deficit, which has con-tributed to long-term increases in pH in western rivers (46, 47).Nutrients in wastewater can increase pH of stream water furtherby stimulating primary production in the receiving streams (48,49). Long-term effects of dilution and retention of weatheringproducts due to combined effects of hydrologic modifications,land-use change, and urban wastewater treatment on decreasingsymptoms of the freshwater salinization syndrome warrant furtherinvestigation throughout the western United States.

Acid Rain and Road Salt Pollution Increases Ion Exchange: FastWeathering. From a regional and continental perspective, a his-torical legacy of acid deposition still exerts a major influence onthe acid/alkaline chemistry of fresh water throughout much of theUnited States, despite regulations reducing acid deposition (7, 17,38). Long-term monitoring shows that acid rain accelerateschemical weathering via ion exchange and leaching processes suchas displacement of base cations by hydrogen ions and forms ofnitrogen (17, 20, 38). Increased base cations transported fromwatersheds can eventually contribute to increased acid bufferingcapacity and long-term increases in pH of fresh water (50). Aciddeposition has decreased during recent decades in some regionsdue to regulations, and atmospheric inputs of acidity can beneutralized; both factors contribute to increasing acid neutralizingcapacity and elevated pH in some rivers (38, 51). However, de-clining acid deposition can also decrease weathering rates, andthis decrease can actually lead to decreases in pH at other sites.For example, we observed a decline in pH at some sites followingClean Air Act Amendments in 1990, which may have been due todecreased weathering rates and a corresponding decrease in bi-carbonate alkalinity from weathering of rocks and soils (7). Long-term effects of acid deposition may be particularly evident inlarger watersheds with longer hydrologic residence times due topotential storage and release of weathering products in soils andground water (7). Accelerated cation exchange from acid rain maybe particularly important in soils that already have high basecation saturation (50). Furthermore, acid rain may interact withother anthropogenic salts (e.g., road salt) to accelerate chemicalweathering and base cation exchange in watersheds (46).Similar to acid deposition, road salts can significantly mobilize

transport of base cations from soil exchange sites to streams viaion exchange (41, 52, 53). For example, road salts increase

-1.5E-08

-1E-08

-5E-09

0

5E-09

1E-08

1.5E-08

2E-08

0 5000 10000 15000 20000

Theil Sen Intercept of Specific Conductance (µS/cm)

Changes in acidity are buffered as salt concentra ons increase

Thei

lSen

Slop

e[H

+ ] (m

olL-1

y-1)

Alkalinizaon

Acidificaon

-200

-150

-100

-50

0

50

100

-50 -40 -30 -20 -10 0 10 20

Theil Sen Slope [Na+] (mg L-1 y-1)

Thei

lSen

Slop

e Sp

ecifi

c Co

nduc

tanc

e (µ

S cm

-1y-1

)

R2 = 0.90

-12

-10

-8

-6

-4

-2

0

2

4

6

-50 -40 -30 -20 -10 0 10 20

Theil Sen Slope [Na+] (mg L-1 y-1)

R2 = 0.32

Thei

lSen

Slop

e[A

lkal

inity

](m

g L-1

y-1)

Fig. 6. Rates of change in acidity of stream water (Theil–Sen slopes) arerelated to initial specific conductance of stream water (y intercept of slopes)throughout the continental United States. Rates of change in sodium con-centrations (Theil–Sen slopes) over time are also strongly related to rates ofchanges in specific conductance (Theil–Sen slopes) over time. Ratesof change in sodium concentrations (Theil–Sen slopes) are related to rates ofchange in alkalinity concentrations (Theil–Sen slopes) except for 6 outof 232 sites with the most extreme rates of change in sodium concentrations(open circles).

Kaushal et al. PNAS | Published online January 8, 2018 | E579

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

SEC

OLO

GY

PNASPL

US

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

leaching of calcium and magnesium from soil cation exchangesites (53), which are primarily responsible for maintaining soilaggregate structure. Cation exchange driven by sodium in roadsalt can release up to 40% to 80% of calcium and magnesiumfrom soil exchange sites to ground water and streams (52, 53).Other related work has documented regionally increasing trendsin base cations in lakes affected by road salt applications (41).Similarly, there can be long-term increases in pH in lakes due toincreased acid neutralizing capacity and base cation inputs (50,54). Road salts accumulate in soils and ground water contrib-uting to enhanced cation exchange and mobilization of majorions over time (6, 23). Therefore, acid rain, road salt, and otherforms of salt pollution (such as sodic/saline irrigation runoff) cansynergistically contribute to the freshwater salinization syndromeover decades by enhancing fast weathering and ion exchange.

The Freshwater Salinization Syndrome: A Conceptual Model. Thefreshwater salinization syndrome can be influenced by cumula-tive weathering reactions across hydrologic flow paths extendingfrom soils, ground water, and headwater streams to larger rivers.Although the freshwater salinization syndrome originates inheadwaters, the terminus along rivers and receiving waters is stillpoorly understood, but may actually extend into tidal waters (Fig.7) (55–57). For example, there have been long-term increases inalkalinity in some tidal waters of tributaries in the ChesapeakeBay and more recent increases in dissolved inorganic carbon(primarily bicarbonate) in the tidal Hudson River estuary (Fig.7). Changes in weathering and cation exchange may explain al-kalinization in some tidal waters of the Chesapeake Bay (7, 24),whereas increasing dissolved inorganic carbon in the HudsonRiver estuary may be due to a combination of changes in primaryproduction, land development, changes in salt pollution, and/orresponses to changes in acid rain. In fact, a wide array of po-tential mechanisms has been proposed to explain recent obser-vations of increased river alkalinization (7, 13, 31). However,there are currently no conceptual models linking salinization andalkalinization directly from headwaters to receiving waters andexplicitly recognizing the synergistic importance of atmosphericinputs, geochemical processes, and anthropogenic salt pollution.Such conceptual models are needed to drive our understanding,research, and management of these complicated interactions.The freshwater salinization syndrome recognizes that acid

deposition, anthropogenic salts, and fertilizers have differentialimpacts on weathering of watersheds draining different litholo-gies (51, 54, 58) (Fig. 8). Along drainage networks, weatheringproducts accumulate in ground and surface waters across hy-drologic flow paths due to acid neutralization and ion exchangereactions (18, 51, 58) (Fig. 8). Buffering capacity increases asbase cation concentrations increase (23). Calcium, magnesium,potassium, and sodium are highly soluble and increase in con-centrations downstream relative to other less soluble elementsthat may be adsorbed onto soils and sediments (18, 58) (Fig. 8).The pH in streams increases along hydrologic flow paths due tothe accumulation of weathering products and increased bufferingcapacity (51, 54, 58) (Fig. 8). Thus, differences in lithology andsurface and subsurface groundwater flow paths along thedrainage network influence the potential for acidification vs.alkalinization of fresh water (38). For example, headwaters areprone to acidification depending on underlying lithology (sedi-mentary vs. crystalline) (50, 51). Larger rivers are prone to al-kalinization downstream due to cumulative effects of weatheringreactions and biological processes, which can also increase al-kalinity and pH along flow paths (e.g., primary production andanaerobic metabolism) (23, 49, 50) (Fig. 8).In addition to human-accelerated weathering, anthropogenic

salts and fertilizers have greatly contributed to salinization andalkalinization, and their relative importance can accumulatealong increasing stream order (Fig. 8). There are inputs of dif-

ferent mixtures of salts due to urbanization, agriculture, mining,and resource extraction (6, 8) (Fig. 8). Densities of urban andagricultural land within a watershed can be strong predictors ofbase cations and pH in streams and rivers (22, 23, 54, 59). Inresidential and urban areas with colder climates, road deicersincluding sodium chloride, calcium chloride, and magnesiumchloride are an important source of salinization in the UnitedStates and globally (6, 34). Fertilizers can stimulate aquaticprimary production, which increases pH and alkalinity (49, 50,54). Anaerobic metabolism and decomposition can further in-crease dissolved inorganic carbon production, alkalinity, and pHin urbanized and agricultural watersheds (22, 23, 50). Agriculturecan contribute significant bicarbonate and base cations, includingcalcium, magnesium, and potassium, from liming, potash, andfertilizer applications (in addition to those ions generated fromagriculturally enhanced weathering) (13, 60). Fertilizers canstimulate nitrification and accelerated weathering of agriculturalsoils, leading to further increases in concentrations of base cationsin water (20, 42). Mine drainage can produce strong acids fromsulfide oxidation, which contributes to accelerated weathering andincreased concentrations of base cations downstream (13, 25).Brines from energy extraction activities, including hydrofracking,may also further contribute to salinization and alkalinization offresh water (11). Overall, anthropogenic salt inputs can interactwith and accelerate chemical weathering processes throughout theentire drainage network (Fig. 8). The net result of acceleratedweathering and cation exchange and salt pollution is to producethe freshwater salinization syndrome on a continental scale with

0

4

8

12

16

20

0

25

50

75

100

125

150

Nov-84

Dec-88

Jan-93

Feb-97

Apr-01

May-05

Jun-09

Aug-13

Sep-17

Tidal Susquehanna RiverTidal Potomac RiverTidal Hudson River

Varia�ons in Alkaliniza�on of Tidal Waters

Diss

olve

d In

orga

nic

Carb

on(m

g/L)

Tota

l Alk

alin

ity (m

g/L)

Fig. 7. Examples of significant increasing trends (P < 0.05) in alkalinity anddissolved inorganic carbon in tidal waters of the US East Coast. Time serieswere smoothed as moving averages over every three data points/observa-tions. Alkalinity concentrations are shown for the tidal Potomac and Sus-quehanna Rivers, and dissolved inorganic carbon concentrations are shownfor the tidal Hudson River. Please note that vertical axes differ in scale.

E580 | www.pnas.org/cgi/doi/10.1073/pnas.1711234115 Kaushal et al.

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

Time

Conductance

pH

Cations

SEDIMENTARY LITHOLOGY

Time

Conductance

Cations

pH

Acidic

Basic

Acidic

Basic

Acidic

Basic

CCRYSRR TALLINETT LLITHOLOGY

Time

Total Dissolved Solids

Speci c Conductance

pH

Soil Cation Depletion

ATMOSPHERIC INPUTSWeathering Agents (H

2SO

4 , HNO

3 , H

2CO

3)

Cation Deposition

INCREASED HUMAN INPUTS

Road Salts

Agricultural Lime

Wastewater

Fracking Brines

Mine Drainage

GEOLOGIC POOLSRocks

Soils

Sediments

THE FRESHWATER

SALINIZATION SYNDROME

Cation Exchange

Mineral Dissolution

Na+, Ca2+, Mg2+,

K+, HCO3

-

Na+, Ca2+, Mg2+,

K+, HCO3

-

RIVER OUTPUTS

Drinking Water Contamination

Mobilization of Metals & Nutrients

Decreased CO2 Sequestration

HUMAN-ACCELERATED WEATHERING

BIOLOGICAL ALKALINIZATION

Primary Production

Microbial Metabolism

( SO4

reduction, e.g.

Hydrologic

Saltwater Intrusion

Fig. 8. A conceptual model of the freshwater salinization syndrome, illustrating potential drivers and changes in increased salinization and alkalinization offresh water. At least three sets of process contribute to the freshwater salinization syndrome, and they are listed in a hypothetical order from upstream todownstream including: (i) accelerated weathering in headwaters and throughout the drainage network; (ii) human salt inputs from developed landscapes;and (iii) increased biological alkalinization in larger-order streams with increased light and nutrient availability. Crystalline and sedimentary lithology resultsin different starting points in headwaters based on the potential for chemical weathering. Weathering process such as dissolution and cation exchange in soilsand sediments generate alkalinity, bicarbonate, and base cations along drainage networks (accelerated weathering is represented by the dashed lineextending throughout the drainage network). The relative importance of accelerated weathering may change with increasing disturbances along streamorder, as salt and nutrient pollution increases further downstream and/or in response to hydrologic modifications. The terminus of the freshwater salinizationsyndrome varies based on climate, land use and management, saltwater intrusion, and underlying geology but can potentially extend into tidal waters.

Kaushal et al. PNAS | Published online January 8, 2018 | E581

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

SEC

OLO

GY

PNASPL

US

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

regional- and watershed-scale variations related to atmosphericdeposition, land and water use, and underlying geology.

Environmental Implications of the Freshwater Salinization Syndrome.The freshwater salinization syndrome currently poses direct andindirect concerns for ecosystem services and may influence rates ofcoastal ocean acidification. Elevated pH can contribute to ammo-nia toxicity and mortality for aquatic organisms (49, 60). Differentsalts influence toxicity to aquatic organisms leading to recent callsfor regulating salt composition and concentrations in fresh water bygovernment agencies (8). Elevated pH and base cations, such ascalcium and magnesium, may reduce the bioavailability and toxicityof trace metals (60), but salinization may also enhance ion ex-change and mobilization of trace metals from soils to streams (61).Increased sodium concentrations decrease soil aggregate stabilityand soil fertility through leaching of calcium and magnesium (62).Salinization from road salts can increase leaching and mobilizationof carbon, nitrogen, and phosphorus from soils and sediments tostreams (63). However, detailed work has also shown that alka-linization or rather sodium absorption ratio or exchangeable so-dium percentage is responsible for increasing dissolved organiccarbon and not salinization (64). Salinization also may directly in-fluence the quality of different fractions of organic matter releasedfrom soils to streams (65). Elevated alkalinity can also stimulateproduction of nitrate by microbial nitrification, and salinization canalter the urban evolution of ecosystems and aquatic communities(22). Increased temperatures in rivers throughout the United States(66) may further influence weathering and other geochemical re-actions, and interact with impacts of freshwater salinization.Furthermore, increased pH, alkalinity, and salinity influence the

amount and quality of inorganic and organic carbon transportedby rivers, which can influence aquatic species and rates of coastalocean acidification (32, 33, 56, 57). Changes in the pH and al-kalinization of rivers also influence rates of CO2 evasion, whichare important for global carbon budgets (67). From a coastalperspective, increased alkalinity can contribute to increased shellthickness of calcareous organisms such as found in zebra musselsin the Hudson River estuary (57), where there has been an in-crease in alkalinity over the past decade (Fig. 8). Estuarine watersof the Chesapeake Bay also have long-term positive trends in pHand alkalinity near tributaries (Fig. 8) (56). Overall, the effects ofthe freshwater salinization syndrome on the riverine carbon cycleand variations in coastal ocean acidification due to riverine inputswarrant further investigation (12, 32, 33).The freshwater salinization syndrome can increase risks to the

safety of drinking water and infrastructure. Elevated salt levels indrinking water can contribute to hypertension in people onsodium-restricted diets and is of concern to people requiringkidney dialysis (9). Salinization and alkalinization influence thecorrosivity of water, and this can affect leaching of metals frompipes carrying drinking water (9, 68). Salinization increases cor-rosion of transportation infrastructure with United States eco-nomic costs estimated in the billions of dollars (69). Givenincreasing impacts on ecosystems and human welfare, increasedsalinization and alkalinization of fresh water is now a pervasivewater quality issue, which may require aggressive management inboth arid and humid climates across latitudes.

MethodsLong-term time series of specific conductance, pH, alkalinity, and base cationconcentrations (calcium,magnesium, potassium, and sodium) were analyzed from232 different stream and river sites located throughout the continental United

States (7). Some rivers were sampled at multiple locations. All records werecompiled from water quality measurements made by the USGS. Time series in-cluded in our analyses were based on at least 25 y of data and more than156 observations for at least one parameter. We retained all series for pH, alka-linity, and specific conductance with less than six consecutive years of missing dataand a period of monitoring until at least the year 2000. Some datasets had longertemporal discontinuities for base cations (e.g., Fig. 4). Time series for all param-eters included in our analysis ranged from 25 to almost 80 y in length. Furtherinformation on related methods for analyses can be found elsewhere (7) and inSupporting Information. All data are publicly available through the USGSwebsite.

Theil–Sen slopes were estimated to characterize average rates of change inspecific conductance, pH, and base cation concentrations per year at each site.The Theil–Sen regression estimates the slope of change in a parameter acrosstime by calculating the median slope of all pairwise points in a series (70),providing robust slope estimates despite missing data, nonnormal distributions,and occasional outliers (7). We considered slopes to be statistically significant ifP values, estimated via nonparametric Wilcoxon tests as applied to the Theil–Sen slope (71), were <0.05. Because of the large number of sites that hadirregular sampling periods, seasonal average values were used to detect long-term trends. Seasonal averages were defined as winter (December January,February), spring (March, April, May), summer (June, July, August), and fall(September, October, November). A bootstrap resampling procedure was usedto generate confidence intervals and estimate P values. Theil–Sen slopes werecalculated using R package mblm (R 3.2.1, mblm version 0.12). One limitation tothe approach used here is that concentration data were not flow normalized.Flow normalization describes the changing state of the system over time byremoving the influence of variations in concentration caused by variations indischarge. Discharge can vary considerably over time at the study sites and caninfluence trends. One way to deal with this would be to test for trends indischarge. Higher discharge would cause dilution reducing salinity and alka-linity. For example, the Hudson River showed an increase in dissolved inorganiccarbon concentrations, which could be due to lower discharge, but the oppo-site was actually the case—discharge has increased. Therefore, our analysispresents estimates of what was actually observed at the sites over time (and theinterrelationships between those trends). This approach canmake teasing apartland use and lithology more complicated for some individual sites.

We applied an interpolation and precipitation-weighting procedure to de-termine the total drainage area and amount of surfacewater subjected to eitherdecreasing or increasing slopes at the continental scale (Supporting In-formation). The estimated slope of all areas within the continental UnitedStates was determined using ordinary kriging (72) to interpolate within the232 analyzed sites to generate a raster of Theil–Sen slope estimates for theentire continental United States. Kriging was achieved using a spherical semi-variogrammodel with a 12-point, variable distance search radius and an outputcell resolution of 10 km2. The kriging approach represents an oversimplificationof watershed processes in that broad, 2D interpolation will neglect entities thataffect water quality such as point sources. However, our goal was to broadlyillustrate and estimate trends occurring at the continental scale, thus our in-terpolated estimates are meant to reflect patterns associated with large-scalegeologic attributes, land-cover patterns, and climatic variables but should notbe used as precise estimates for a specific waterway. We also determined thevolume of water for every 1 km2 pixel in the same area by subtracting theestimated total mean annual evapotranspiration (73) from mean annual pre-cipitation (74) (Supporting Information). The two rasters were merged to acommon spatial scale (10 km2), which allowed for subsequent calculation of thetotal amount of water that becomes groundwater and surface runoff subjectedto increasing or decreasing alkalization (Supporting Information).

ACKNOWLEDGMENTS. This was work primarily supported by National ScienceFoundation Grant EAR 1521224 with partial support from Grants DEB 1027188and CBET 105850. Long-term data were provided by the USGS. Data from theHudson River were provided by the Hudson River group at the Cary Institute ofEcosystem Studies and supported by National Science Foundation Grant DEB-1119739 and the Hudson River Foundation. Data from the tidal Potomac Riverand the tidal Susquehanna River were provided by the Chesapeake BayProgram and Maryland Department of Natural Resources.

1. Jackson RB, et al. (2001) Water in a changing world. Ecol Appl 11:1027–1045.2. Oki T, Kanae S (2006) Global hydrological cycles and world water resources. Science

313:1068–1072.3. WilliamsWD (2001) Anthropogenic salinization of inland waters.Hydrobiologia 466:329–337.4. Cañedo-Argüelles M, et al. (2013) Salinisation of rivers: An urgent ecological issue.

Environ Pollut 173:157–167.

5. Dugan HA, et al. (2017) Salting our freshwater lakes. Proc Natl Acad Sci USA 114:4453–4458.

6. Kaushal SS, et al. (2005) Increased salinization of fresh water in the northeasternUnited States. Proc Natl Acad Sci USA 102:13517–13520.

7. Kaushal SS, et al. (2013) Increased river alkalinization in the Eastern U.S. Environ SciTechnol 47:10302–10311.

E582 | www.pnas.org/cgi/doi/10.1073/pnas.1711234115 Kaushal et al.

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020

8. Cañedo-Argüelles M, et al. (2016) WATER. Saving freshwater from salts. Science 351:914–916.

9. Kaushal SS (2016) Increased salinization decreases safe drinking water. Environ SciTechnol 50:2765–2766.

10. Rengasamy P (2006) World salinization with emphasis on Australia. J Exp Bot 57:1017–1023.

11. Vidic RD, Brantley SL, Vandenbossche JM, Yoxtheimer D, Abad JD (2013) Impact ofshale gas development on regional water quality. Science 340:1235009.

12. Guo J, Wang F, Vogt RD, Zhang Y, Liu CQ (2015) Anthropogenically enhancedchemical weathering and carbon evasion in the Yangtze Basin. Sci Rep 5:11941.

13. Raymond PA, Oh N-H, Turner RE, Broussard W (2008) Anthropogenically enhancedfluxes of water and carbon from the Mississippi River. Nature 451:449–452.

14. Kunz JL, et al. (2013) Use of reconstituted waters to evaluate effects of elevatedmajor ions associated with mountaintop coal mining on freshwater invertebrates.Environ Toxicol Chem 32:2826–2835.

15. Hintz WD, Relyea RA (2017) Impacts of road deicing salts on the early-life growth anddevelopment of a stream salmonid: Salt type matters. Environ Pollut 223:409–415.

16. Kempe S, Kazmierczak J (2002) Biogenesis and early life on Earth and Europa: Favoredby an alkaline ocean? Astrobiology 2:123–130.

17. Likens GE, Driscoll CT, Buso DC (1996) Long-term effects of acid rain: Response andrecovery of a forest ecosystem. Science 272:244–246.

18. Meybeck M (1987) Global chemical-weathering of surficial rocks estimated from riverdissolved loads. Am J Sci 287:401–428.

19. Cañedo-Argüelles M, et al. (2017) Effects of potash mining on river ecosystems: Anexperimental study. Environ Pollut 224:759–770.

20. Aquilina L, et al. (2012) Long-term effects of high nitrogen loads on cation and car-bon riverine export in agricultural catchments. Environ Sci Technol 46:9447–9455.

21. Palmer MA, et al. (2010) Science and regulation. Mountaintop mining consequences.Science 327:148–149.

22. Kaushal SS, McDowell WH, Wollheim WM (2014) Tracking evolution of urban bio-geochemical cycles: Past, present, and future. Biogeochemistry 121:1–21.

23. Kaushal SS, et al. (2017) Human-accelerated weathering increases salinization, majorions, and alkalinization in fresh water across land use. Appl Geochem 83:121–135.

24. Raymond PA, Oh N-H (2009) Long term changes of chemical weathering products inrivers heavily impacted from acid mine drainage: Insights on the impact of coalmining on regional and global carbon and sulfur budgets. Earth Planet Sci Lett 284:50–56.

25. Lindberg TT, et al. (2011) Cumulative impacts of mountaintop mining on an Appa-lachian watershed. Proc Natl Acad Sci USA 108:20929–20934.

26. McKnight DM, Bencala KE (1990) The chemistry of iron, aluminum, and dissolvedorganic material in three acidic, metal-enriched, mountain streams, as controlled bywatershed and in-stream processes. Water Resour Res 26:3087–3100.

27. Steele MK, Aitkenhead-Peterson JA (2011) Long-term sodium and chloride surfacewater exports from the Dallas/Fort Worth region. Sci Total Environ 409:3021–3032.

28. Jeppesen E, et al. (2015) Ecological impacts of global warming and water abstractionon lakes and reservoirs due to changes in water level and related changes in salinity.Hydrobiologia 750:201–227.

29. Mahmuduzzaman M, Ahmed ZU, Nuruzzaman AKM, Ahmed FRS (2014) Causes ofsalinity intrusion in coastal belt of Bangladesh. Int J Plant Res 4:8–13.

30. Cormier SM, Suter GW, 2nd, Zheng L, Pond GJ (2013) Assessing causation of the ex-tirpation of streammacroinvertebrates by a mixture of ions. Environ Toxicol Chem 32:277–287.

31. Stets EG, Kelly VJ, Crawford CG (2014) Long-term trends in alkalinity in large rivers ofthe conterminous US in relation to acidification, agriculture, and hydrologic modifi-cation. Sci Total Environ 488-489:280–289.

32. Mackenzie FT, Andersson AJ, Arvidson RS, Guidry MW, Lerman A (2011) Land-seacarbon and nutrient fluxes and coastal ocean CO2 exchange and acidification: Past,present, and future. Appl Geochem 26:S298–S302.

33. Pfister CA, et al. (2014) Detecting the unexpected: A research framework for oceanacidification. Environ Sci Technol 48:9982–9994.

34. Corsi SR, Graczyk DJ, Geis SW, Booth NL, Richards KD (2010) A fresh look at road salt:Aquatic toxicity and water-quality impacts on local, regional, and national scales.Environ Sci Technol 44:7376–7382.

35. Anning DW, Flynn ME (2014) Dissolved-solids sources, loads, yields, and concentra-tions in streams of the conterminous United States. US Geol Surv Sci Investig Rep,10.3133/sir20145012.

36. Gibbs RJ (1970) Mechanisms controlling world water chemistry. Science 170:1088–1090.

37. Thornton GJP, Dise NB (1998) The influence of catchment characteristics, agriculturalactivities and atmospheric deposition on the chemistry of small streams in the EnglishLake District. Sci Total Environ 216:63–75.

38. Likens GE (2013) Biogeochemistry of a Forested Ecosystem (Springer, New York),3rd Ed.

39. Dailey KR, Welch KA, Lyons WB (2014) Evaluating the influence of road salt on waterquality of Ohio rivers overt time. Appl Geochem 47:25–35.

40. Tripler CE, Kaushal SS, Likens GE, Walter MT (2006) Patterns in potassium dynamics inforest ecosystems. Ecol Lett 9:451–466.

41. Rosfjord CH, et al. (2007) Anthropogenically driven changes in chloride complicateinterpretation of base cation trends in lakes recovering from acidic deposition.Environ Sci Technol 41:7688–7693.

42. Fortner SK, et al. (2012) Silicate weathering and CO2 consumption within agriculturallandscapes, the Ohio-Tennessee River Basin, USA. Biogeosciences 9:941–955.

43. Lettenmaier DP, Hooper ER, Wagoner C, Faris KB (1991) Trends in stream quality inthe continental United States, 1978-1987. Water Resour Res 27:327–339.

44. Miller MP, Buto SG, Lambert PM, Rumsey CA (2017) Enhanced and updated spatiallyreferenced statistical assessment of dissolved-solids load sources and transport instreams of the Upper Colorado River Basin (USGS, Reston, VA), Scientific Investiga-tions Report 2017-5009.

45. Anning DW (2011) Modeled sources, transport, and accumulation of dissolved solidsin water resources of the Southwestern United States. J Am Water Resour Assoc 47:1087–1109.

46. Passell HD, Dahm CN, Bedrick EJ (2004) Hydrological and geochemical trends andpatterns in the Upper Rio Grande, 1975 to 1999. JAWRA 40:111–127.

47. Passell HD, Dahm CN, Bedrick EJ (2005) Nutrient and organic carbon trends andpatterns in the upper Rio Grande, 1975-1999. Sci Total Environ 345:239–260.

48. Kaushal SS, Lewis WM, Jr, McCutchan JH, Jr (2006) Land use change and nitrogenenrichment of a Rocky Mountain watershed. Ecol Appl 16:299–312.

49. O’Brien WJ, Noyelles F (1972) Photosynthetically elevated pH as a factor in zoo-plankton mortality in nutrient enriched ponds. Ecology 53:605–614.

50. Kilham P (1982) Acid precipitation–Its role in the alkalinization of a lake in Michigan.Limnol Oceanogr 27:856–867.

51. Johnson NM (1979) Acid rain: Neutralization within the Hubbard Brook ecosystemand regional implications. Science 204:497–499.

52. Ostendorf DW, Xing B, Kallergis N (2009) Cation exchange in a glacial till drumlin at aroad salt storage facility. J Contam Hydrol 106:118–130.

53. Shanley JB (1994) Effects of ion-exchange on stream solute fluxes in a basin receivinghighway deicing salts. J Environ Qual 23:977–986.

54. Whitmore TJ, et al. (2006) Inadvertent alkalinization of a Florida lake caused by in-creased ionic and nutrient loading to its watershed. J Paleolimnol 36:353–370.

55. Muller JD, Schneider B, Rehder G (2016) Long-term alkalinity trends in the Baltic Seaand their implications for CO2 induced acidification. Limnol Oceanogr 61:1984–2002.

56. Prasad MBK, Kaushal SS, Murtugudde R (2013) Long-term pCO2 dynamics in rivers inthe Chesapeake Bay watershed. Appl Geochem 31:209–215.

57. Natesan S, Strayer DL (2016) Long-term increases in shell thickness of zebra mussels(Dreissena polymorpha) in the Hudson River. Fundam Appl Limnol 188:245–248.

58. Chiarenzelli J, Lock R, Cady C, Bregani A, Whitney B (2012) Variation in river multi-element chemistry related to bedrock buffering: An example from the Adirondackregion of northern New York, USA. Environ Earth Sci 67:189–204.

59. Zampella RA, Procopio NA, Lathrop RG, Dow CL (2007) Relationship of land-use/land-cover patterns and surface-water quality in the Mullica River Basin. J Am WaterResour Assoc 43:594–604.

60. Boyd CE, Tucker CS, Somridhivej B (2016) Alkalinity and hardness: Critical but elusiveconcepts in aquaculture. J World Aquacult Soc 47:6–41.

61. Lofgren S (2001) The chemical effects of deicing salt on soil and stream water of fivecatchments in southeast Sweden. Water Air Soil Pollut 130:863–868.

62. Rosenberry DO, et al. (1999) Movement of road salt to a small New Hampshire lake.Water Air Soil Pollut 109:179–206.

63. Duan S, Kaushal SS (2015) Salinization alters fluxes of bioreactive elements fromstream ecosystems across land use. Biogeosciences 12:7331–7347.

64. Steele MK, Aitkenhead-Peterson JA (2013) Salt impacts on organic carbon and ni-trogen leaching from senesced vegetation. Biogeochemistry 112:245–259.

65. Gabor RS, et al. (2015) Influence of leaching solution and catchment location on thefluorescence of water-soluble organic matter. Environ Sci Technol 49:4425–4432.

66. Kaushal SS, Likens GE, Jaworski N, Pace ML (2010) Rising stream and river tempera-tures in the United States. Front Ecol Environ 8:461–466.

67. Raymond PA, et al. (2013) Global carbon dioxide emissions from inland waters.Nature 503:355–359.

68. Stets EG, Lee CJ, Lytle DA, Schock MR (2017) Increasing chloride in rivers of theconterminous U.S. and linkages to potential corrosivity and lead action level ex-ceedances in drinking water. Sci Total Environ, 10.1016/j.scitotenv.2017.07.119.

69. Koch GH, Brongers MH, Thompson MG, Virmani YP, Payer JH (2002) Corrosion costand preventative strategies in the United States (Federal Highway Administration,Washington, DC), Report FHWA-RD-01-156.

70. US Geological Survey (2013) National hydrography geodatabase: The nationalmap viewer available on the World Wide Web. Available at https://nhd.usgs.gov/NHD_High_Resolution.html. Accessed December 1, 2016.

71. Siegel AF (1982) Robust regression using repeated medians. Biometrika 69:242–244.72. Oliver MA (1990) Kriging: A method of interpolation for geographical information

systems. Int J Geogr Inf Syst 4:313–332.73. Drapek RJ, Kim JB, Neilson RP (2015) The dynamic general vegetation model

MC1 over the United States and Canada at a 5-arc-minute resolution: Model inputsand outputs (US Department of Agriculture Forest Service Pacific Northwest ResearchStation, Portland, OR), General Technical Report PNW-GTR-904.

74. Daly C, et al. (2008) Physiographically-sensitive mapping of temperature and pre-cipitation across the conterminous United States. Int J Climatol 28:2031–2064.

Kaushal et al. PNAS | Published online January 8, 2018 | E583

EART

H,A

TMOSP

HER

IC,

ANDPL

ANET

ARY

SCIENCE

SEC

OLO

GY

PNASPL

US

Dow

nloa

ded

by g

uest

on

Sep

tem

ber

1, 2

020