Chloride ion transport and mass balance in a metropolitan area using road salt

Chloride ion transport and mass balance in a metropolitan area

using road salt

Eric V. Novotny,1 Andrew R. Sander,1 Omid Mohseni,1 and Heinz G. Stefan1

Received 23 April 2009; revised 17 August 2009; accepted 3 September 2009; published 15 December 2009.

[1] In the Twin Cities metropolitan area (TCMA) of Minneapolis/St. Paul, Minnesota, anestimated 317,000 metric tons (t) of road salt were used annually for road deicing between2000 and 2005. To determine the annual retention of road salt, a chloride budget wasconducted for a 4150 km2 watershed encompassing the populated areas of the TCMA. Inaddition to inflows and outflows in the major rivers of the TCMA, multiple sources ofchloride were examined, but only road salt and wastewater treatment plant (WWTP)effluents were large enough to be included in the analysis. According to the chloridebudget, 235,000 t of chloride entered the TCMA annually with the Mississippi andMinnesota rivers, and 355,000 t exited through the Mississippi River. Of the 120,000 t ofchloride added annually to the rivers inside the TCMA watershed boundaries, 87,000 tcame from WWTPs and 33,000 t came from road salt. Of the 142,000 t of chloride appliedannually in the TCMA watershed as road salt (241,000 t NaCl), only 23% (33,000 t)were exported through the Mississippi River and 109,000 t or 77% were retained in theTCMA watershed. Chloride budgets for 10 subwatersheds within the TCMA analyzedin a similar way, gave an average chloride retention rate of 72%. The retention isoccurring in the soils, surface waters (numerous lakes, wetlands, and ponds) and in thegroundwater. Chloride concentrations in many of these urban water bodies are nowconsiderably higher than the presettlement background levels of less than 3 mg/L withconcentrations as high as 2000 mg/L in shallow groundwater wells. The continuedaccumulation of chloride in the groundwater and surface waters is a cause for concern.

Citation: Novotny, E. V., A. R. Sander, O. Mohseni, and H. G. Stefan (2009), Chloride ion transport and mass balance in a

metropolitan area using road salt, Water Resour. Res., 45, W12410, doi:10.1029/2009WR008141.

1. Introduction

[2] It has been reported that 21 million metric tons (t) ofroad salt were used in the United States in 2005 to improvedriving safety in the winter (Data are available from UnitedStates Geological Survey at http://minerals.usgs.gov/minerals/pubs/commodity/salt/.). In the seven-county Twin Cities met-ropolitan area (TCMA) of Minneapolis/St. Paul, Minnesota,an estimated 317,000 t of road salt were used annually forroad deicing between 2000 and 2005 [Sander et al., 2007].Road salt (mostly NaCl) is highly soluble in water resulting inthe sodium and chloride ions dissociating from one anotherwhen snowmelt occurs. Chloride and sodium ions are bothtransported from roads to receiving waters along three path-ways: (1) a rapid runoff pathway from impervious surfaces,(2) a shallow subsurface pathway through the soil, and (3) adeeper and slower pathway through underground aquifers[Novotny et al., 1999]. All three pathways can result in theretention of sodium and chloride in the surface water andgroundwater of a watershed [Ramakrishna and Viraraghocan,2005]. In addition to the accumulation that can occur in thesurficial and deep groundwater aquifers, small amounts of

chloride and more so sodium could also be retained throughinteractions with soils and organic matter [Amrhein et al.,1992; Bastviken et al., 2006;Mason et al., 1999; Svensson etal., 2007]. Themajor factors influencing retention in soils andgroundwater include soil permeability, vegetation cover,topography, and roadside drainage techniques [Jones andJeffrey, 1992].[3] The accumulation of sodium and chloride ions in the

environment degrades the water quality in a watershed[Jones and Jeffrey, 1992; Environment Canada HealthCanada, 1999; Ramakrishna and Viraraghocan, 2005].Increased chloride concentrations decrease the biodiversityof waterways and roadside vegetation [Environment CanadaHealth Canada, 1999]. If chloride reaches the groundwater itcan contaminate drinking water supplies [Howard andMaier,2007]. Not only has chloride been shown to affect organisms,it can also increase the transport and bioavailability of heavymetals such as cadmium, lead, chromium, copper and evenmercury in the environment, which are also harmful to biota[Novotny et al., 1999]. Other secondary consequences of roadsalt applications in lakes include the ability to inhibit or delaynatural mixing events, limiting the oxygenation of benthicwaters and sediments and facilitating the release of heavymetals, mercury and phosphorus stored in the sediments[Jones and Jeffrey, 1992].[4] Elevated and/or increasing chloride concentrations

attributable to road salt applications are present in ground-

1St. Anthony Falls Laboratory, Department of Civil Engineering,University of Minnesota, Minneapolis, Minnesota, USA.

Copyright 2009 by the American Geophysical Union.0043-1397/09/2009WR008141

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water and surface waters in urban environments in northernclimate regions [Andrews et al., 1997; Andrews et al., 2005;Bowen and Hinton, 1998; Fong, 2000; Godwin et al., 2003;Kaushal et al., 2005; Kelly, 2008; Lofgren, 2001; Marsalek,2003; Novotny et al., 2008; Thunquist, 2004]. Mass balancestudies on individual streams with watershed areas less than450 km2 indicate that between 27% and 65% of the road saltapplied was retained within the individual urban watersheds[Bubeck et al., 1971; Howard and Hayes, 1993; Hulingand Hollocher, 1972; Ruth, 2003; Wulkowicz and Saleem,1974].[5] Unlike previous studies, which analyzed small water-

sheds located in urban environments, this study examines anentire metropolitan area encompassing a surface area of4150 km2 including rural, suburban, and urban communi-ties. By examining an entire metropolitan area, a betterunderstanding of the total retention and the holistic effect ofroad salt applications on groundwater and surface waters canbe obtained. This study used data collected over an 8 yearperiod. Potential environmental damage will affect the entiremetropolitan area, and policies on road salt applicationscannot be developed by extrapolation from small subwater-sheds. Overall this study draws attention to a developingproblem that will affect around 3 million people in a4150 km2 area and provides insight for other major metro-politan areas on the affects of road salt applications.[6] The purpose of the study was to examine the spatial

and temporal chloride transport dynamics and to develop achloride budget for an entire metropolitan area. This anal-ysis was used to estimate how much of the road salt appliedannually is exported from the watershed by the MississippiRiver and how much is retained in the soils, groundwaterand surface waters.

2. Methodology

2.1. Metropolitan Area Chloride Balance

[7] The Minneapolis/St. Paul Twin Cities metropolitan area(TCMA) is an urbanized area with a population of 2.8 million(Data are available from Metropolitan Council at http://www.metrocouncil.org/metroarea/2007PopulationEstimates.pdf.),many watercourses and 950 lakes. Located in the north centralUnited States, the TCMA experiences cold climate with anaverage annual snowfall of 1420 mm between Novemberand April (Data are available from Minnesota Climate WorkGroup at http://climate.umn.edu/doc/twin_cities/twin_cities_snow.htm.). The hydrologic drainage system of the TCMAincludes many small streams, lakes and wetlands along withan extensive storm sewer system and hundreds of detentionand infiltration basins. Under the TCMA is a system ofseveral aquifers, some of which are used for urban watersupply. Two major rivers, the Minnesota and the Mississippi,flow through the TCMA. The combined watersheds ofthese two rivers encompass 4150 km2 of the seven-countymetropolitan area, and provide the natural boundaries for acontrol volume to be used in a chloride mass balance(Figure 1).[8] The chloride ion, known to be harmful to biota [Jones

and Jeffrey, 1992; Ramakrishna and Viraraghocan, 2005]and more conservative in water than sodium [Amrhein et al.,1992; Bastviken et al., 2006; Mason et al., 1999; Svensson

et al., 2007], was chosen to develop an annual chloridemass balance (equation (1)) for the TCMA control volume.

IþMp þMnp � O ¼ S t=yrð Þ ð1Þ

Where I is the total annual inflow (t/yr) through theMississippi River at Anoka and the Minnesota River atJordan, Mp and Mnp represent the total annual mass ofchloride added inside the watershed from point sources andnonpoint sources, respectively, O is the total annual mass ofchloride exported through the Mississippi River at Hastingsand S is the annual retention of chloride (t/yr) in the TCMA.Chloride is highly soluble in water, and has been treated asconservative once it is in solution.[9] Once the water reaches the Minnesota or Mississippi

rivers storage potential is limited. It is expected that themass of chloride entering the TCMA at the inflow stationswill exit at the outflow stations. Likewise, all of thechlorides discharged directly to the rivers through pointsources are expected to reach the outflow station. However,the hydrologic transport from nonpoint sources of chlorideis unknown. Nonpoint sources of chloride can infiltrateinto soils, can accumulate in wetlands or lakes, can travelthrough storm sewers or can reach the groundwater. Onlysome of the salt applied as a nonpoint source in the TCMAis expected to reach the Mississippi River. Therefore,chloride can only be retained in the watershed if it comesfrom a nonpoint source. This assumption allows rearrange-ment of equation (1) to equation (2).

O� I�Mp ¼ mnp t=yrð Þ ð2Þ

Where mnp = Mnp � S. We will first calculate mnp, and thenS knowing Mnp. The value of mnp represents the amountof chloride from nonpoint sources that entered the riversystem and was flushed out of the watershed system at theMississippi River outflow station.

2.2. Inflows and Outflows

[10] Sodium and chloride concentrations as well as riverflow rates were measured by state and federal agencies atgauges and sampling points on the Mississippi and theMinnesota rivers (Figure 1 and Table 1). The MetropolitanCouncil Environmental Services (MCES) collected grabsamples for chemical analyses at six locations along theMississippi River and two locations on the Minnesota Rivertwo to five times per month between January 2000 andDecember 2007. For the same time period, daily average,and monthly average flow rates were obtained from theUnited States Geological Survey (USGS).[11] The watershed area upstream from the grab sampling

station at Anoka is 48,900 km2, while the watershed areaupstream from the USGS stream gauging station is49,500 km2, a difference of only 1.2%. Therefore, the flowrates and the grab sample concentrations were used togetherwithout adjustment. The USGS St. Paul gauging station islocated 38 km upstream from the outflow grab samplelocation at Hastings; the watershed areas for these twolocations are 95,300 km2 and 95,900 km2, respectively, adifference of 0.6%. The only major inflow between thesetwo points is the Metro wastewater treatment plant (WWTP).

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The river outflow rate from the TCMAwas therefore taken tobe the flow rate at St. Paul plus the outflow from the MetroWWTP. Using the daily flow data and grab sample concen-trations from 2000 to 2007, flow-weighted monthly averagechloride concentrations were calculated. Each of these con-centrations (mg/L) were multiplied by the associated meanmonthly flow rate (m3/s) to estimate the mean monthlychloride mass transport rate (t/yr).

2.3. Chloride Sources

[12] Sodium and chloride sources include natural weath-ering of minerals, natural deposition from rainfall, process-ing of agricultural products, industrial production ofchemicals and food, processing in the metal, paper, petro-leum, textile, and dying industries, water softening, androad salt applications of NaCl [Kostick, 2004]. Point sourcesof chloride in the watershed include WWTP effluents andindustrial discharges to the Mississippi or Minnesota rivers.

Most industrial sources of chloride are connected to theWWTPs, and are included in the point source dischargesfrom the WWTPs. Nonpoint sources of chloride are naturalsources (atmospheric deposition, weathering), rural house-hold septic systems, fertilizer applications, and snowmeltrunoff containing road salt.2.3.1. Point Sources: Wastewater Treatment Plants[13] Effluents from WWTPs are a significant source of

chloride from domestic (foods and water softening) andindustrial NaCl uses. Chloride concentrations in effluentgrab samples were measured by the Metropolitan Council(MCES) from June 2007 to June 2008 in two week intervalsat the four major wastewater treatment plants within theTCMA watershed. Locations of the WWTPs are shown inFigure 1 and are listed in Table 1. The annual averagechloride concentrations and the annual average flow ratesdetermined from daily WWTP effluent flow data were usedto determine the mean annual rates (t/yr) of chloride input to

Figure 1. Watershed boundaries (thick gray lines) of the Twin Cities metropolitan area and major rivers(Mississippi, Minnesota, and St. Croix). Numbers identify each of the data collection or sampling pointslisted in Table 1. Data from unlabeled chloride sampling points were used in Figure 2 but not used in thebudget analysis. Center of the map is at 44.88� latitude, 93.18� longitude.

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the rivers from the four WWTPs. The majority of industriesand 2.4 million of the 2.8 million people living in theTCMA contribute wastewater through sanitary sewers tothe four wastewater treatment plants discharging within theboundaries of the TCMA watershed. The majority of theother 400,000 people are either located outside the bound-aries of the watershed or contribute to one of the three otherwastewater treatment plants that do not discharge within thecontrol volume. Combined sewers in the TCMA have beenreduced to a minimum. Therefore, all household and indus-trial sources of chloride within the control volume bound-aries in the TCMA are included in the effluent values fromthe four major wastewater treatment plants.2.3.2. Nonpoint Sources: Natural Sources[14] Natural sources of chloride and sodium, in general,

include mineral salt deposits, weathering of geologicalformations, and wet deposition from ocean evaporation[Jackson and Jobbagy, 2005]. Mineral salt deposits andgeological sources of chloride are negligible in the TCMA.The annual average concentration of chloride in rainwaterwas measured at a site in the northern part of the TCMAfrom 2000 to 2007 to be 0.07 mg/L (Data are available fromNational Atmospheric Deposition Program at http://nadp.sws.uiuc.edu/sites/siteinfo.asp?net=NTN&id=MN01.). Thisvalue combined with an annual average rainfall of 747 mmwas used to estimate natural source loads of chloride.2.3.3. Septic Systems of Rural Households[15] In 1997 around 70,000 households in the seven-

county TCMAwere using a septic system (Data are availablefromMetropolitanCouncil, Estimate number of septic systemsby community for 1997, at http://www.metrocouncil.org/environment/Watershed/bmp/septic_sys.htm.). Of those70,000 household around 60% were located outside of thewatershed boundaries. This population estimation was usedto determined chloride loads from private septic systems.2.3.4. Agricultural Sources of Chloride[16] Farming in the outskirts of the TCMA is limited to

19% (77,000 ha) of the total watershed area. This percentagewas calculated using land use data from 2005 provided by theMetropolitan Council. For the production of corn 36 kg/ha ofpotassium chloride are required according to the Minnesota

Department of Agriculture [Wilson, 2007]. A value of49.9 kg/ha per year was used in a study of fertilizer andmanure contributions to streams in Sweden [Thunquist, 2004].2.3.5. Road Salt[17] The total mass of road salt (NaCl) applied annually

(2000 to 2005 average) to roads and parking lots withinthe seven-county TCMA by government agencies andcommercial/private users was estimated to be 317,000 t/yr[Sander et al., 2007]; 241,000 or 76% came from publicapplications (i.e., city, county and state agencies), the other24% or 76,000 t was estimated to come from commercial/private applications. The commercial percentage provided bythe American Salt Institute for the years 2005 and 2006 wasbased on market share information for road salt purchases.Commercial/private applications were determined to be theamount of bulk salt purchased by nongovernment agenciesplus the amount of package deicing salt purchased by bothhomeowners and commercial applicators.[18] The TCMA watershed boundaries do not coincide

with the political TCMA boundaries (Figure 1). For thatreason the amount of road salt applied within the politicalseven-county TCMA had to be adjusted using road kilo-meters. City, county and state road data were obtained fromthe GIS database of the Metropolitan Council. Road lengthswere divided by government entity into total kilometers ofcity roads for each city, of county roads for each county andstate road. Fractions (percent) of road lengths inside thewatershed versus the total length inside a municipality,county, or state jurisdiction were multiplied by the totalmass of road salt applied by each individual agency toestimate the total mass of salt applied in the watershed. Thetotal mass of road salt applied by private and commercialuses on parking lots, sidewalks etc. was added to the publicroad salt applications by using a value equal to 24% of thetotal salt applications.

2.4. Chloride Balances in Subwatersheds

[19] The chloride budget study for the entire TCMA wassupplemented by a chloride budget study for 10 sub-watersheds of small streams located entirely within theTCMA. This study was done to determine if salt retentionrates obtained at a geographic scale of less than on tenth theTCMA were comparable to those obtained for the entireTCMA.[20] The analysis of the subwatersheds was based on

equation (1). The inflow (I) for the subwatersheds wasbased on the estimated chloride concentration in the streamif road salt were not applied in the watershed. This value isdifferent from the overall TCMA study and was defined asthe background concentration in the stream. The back-ground concentration was estimated from a linear relation-ship between the annual average chloride concentrations inthe streams versus the mass of chloride from road saltapplied per ha per year within the watersheds. The back-ground concentration of chloride was found by extrapolat-ing the linear relationship to a chloride application rate ofzero. The background concentration was multiplied by theannual average discharge (flow) from the watershed toobtain the inflow loading (I).[21] The only internal chloride source (M) in the water-

shed was from road salt applications. No WWTP effluentsor other chloride sources besides road salts were being

Table 1. Names of Sampling Points and Data Collection

Organizations for Flow Rates and Chloride Concentrations Used

in the Budget Analysisa

SampleNumber Name Organization

1 05330000 – Minnesota Riverat Jordan

U.S. Geological Survey

2 05288500 – Mississippi Riverat Anoka


3 05331000 – Mississippi Riverat St. Paul


4 Upper Mississippi RiverMile 871.6

Metropolitan Council

5 Minnesota River Mile 39.6 Metropolitan Council6 Upper Mississippi River

Mile 815.6Metropolitan Council

7 Blue Lake WWTP Metropolitan Council8 Seneca WWTP Metropolitan Council9 Metro WWTP Metropolitan Council10 Eagle Point WWTP Metropolitan Council

aLocations are shown in Figure 1. WWTP, wastewater treatment plant.

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applied in the subwatersheds. The mass of road salt appliedto the roads was determined using the methods described forthe TCMA watershed analysis explained above. The per-centage of the watershed covered by impervious surfacesand the total watershed areas were found using GIS data fromthe University of Minnesota Remote Sensing and GeospatialAnalysis Laboratory (http://land.umn.edu/index.html).[22] To calculate the annual export rate of chloride mass

(O) exiting each subwatershed, daily average flow data andgrab sample data, collected by the Metropolitan CouncilEnvironmental Services (MCES) two to five times per monthbetween 1 January 2000 and 31 December 2007, at theoutflow from each subwatershed were used.

3. Results and Discussion

3.1. Inflows and Outflows of Chloridein the Major Rivers

[23] The hydrologic transport of sodium and chloride inthe TCMAwas inferred from the concentrations in its majorrivers. Average annual concentrations measured in theserivers show significant changes with distance through theTCMA (Figure 2). Increased Na+ and Cl� concentrations inthe Mississippi River were most pronounced downstreamfrom the confluence with the Minnesota River and theMetropolitan WWTP where mean annual chloride concen-trations increased from 16 mg/L to 33 mg/L. The MinnesotaRiver arrived in the TCMAwith higher concentrations thanthe Mississippi River. Discharges into the Minnesota Riverinclude effluents from the Blue Lake and the SenecaWWTPs as well as surface runoff from populated areas,resulting in a mean annual chloride concentration increasefrom �30 mg/L to �42 mg/L. The St. Croix River isoutside the TCMA and carries much lower chloride con-centrations (�5 mg/L) because the watershed is largelyundeveloped. Inflow from the St. Croix River caused adecrease in chloride concentrations in the Mississippi Riverdownstream from the TCMA. In all three rivers sodium andchloride follow similar concentrations distributions withdistance.[24] Where the Mississippi enters the TCMA at Anoka

mean monthly flow-weighted chloride concentrations were

between �15 and �20 mg/L (Figure 3). Individual grabsamples showed only small variations for a particular monthwith the exception of January, February and November. Inthe Minnesota River inflow to the TCMA at Jordan, meanmonthly chloride concentrations ranged from 20 to 40 mg/L(Figure 3). Flow-weighted mean monthly concentrationswere highest between December and February and lowestfrom March to August. Variability between individual grabsamples for a given month were highest in February.[25] At the Mississippi River outflow station in Hastings

mean monthly concentrations ranged between �20 and�50 mg/L with a strong seasonal variation (Figure 3).The high concentrations occurred between January andMarch, and the lows between April to July. Individualgrab sample concentrations varied the most in March, andthe least in June.[26] Estimates were made for the total mass of chloride

passing through the inflow and outflow observation pointsin Figure 3 for every month. The annual mass (rates) ofchloride entering the TCMAby theMinnesota andMississippiRiver inflowswere determined to be 119,000 and 116,000 t/yr,respectively. The mass (rate) of chloride flowing out of theTCMAwatershed with the Mississippi River was found to be355,000 t/yr. Roughly 50% more chloride was found to beexiting the watershed than was entering with the Mississippiand Minnesota rivers combined.

3.2. Metropolitan Area Chloride Sources

3.2.1. Point Sources[27] The only point source of chloride in the TCMA is the

wastewater discharged from four wastewater treatmentplants (Figure 4). Effluent chloride concentrations increasedduring the winter months at the Metro WWTP. Domesticand industrial waste loads to the wastewater treatmentsystem were expected to remain fairly constant throughoutthe year, as was shown for the other three WWTPs. There-fore, the increase at Metro during the winter was attributedto the addition of road salt to the system through car washes,a small number of combined sewers and possible seepageinto the sanitary sewer system. To avoid double countingroad salt inputs, the average Cl� concentration betweenJune and November was used for the entire year in the

Figure 2. Median chloride concentrations of sodium and chloride in grab samples (2000–2007) fromthe two major rivers of the Twin Cities metropolitan area. Arrows denote locations of wastewatertreatment plant (WWTP) discharges or river junctions.


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annual chloride budget. Although the other three WWTPsdid not display a significant concentration increase inwinter, the same procedure was used for consistency. The(June 2007 to November 2007) average chloride concen-trations in the WWTP effluents and the annual averageeffluent flow rates are shown in Table 2. The associatedmass (rate) of chloride entering the river system from thefour WWTPs including domestic and industrial wastewater,but excluding chloride from road salt applications, wasestimated to be 87,000 t/yr.3.2.2. Nonpoint Sources[28] Nonpoint chloride loads from natural sources, septic

systems, agricultural sources and road salt were evaluated.Using recorded rainfall amounts and measured concentra-tions of chloride in precipitation in the TCMA, the chloridecontribution from natural sources was estimated to bearound 220 t/yr.[29] The majority of people within the TCMA watershed

boundaries are connected to one of seven WWTPs bysanitary sewers and a majority of the households using aprivate septic system are located outside the watershedboundaries. If as many as 60,000 people, i.e., the numberof people connected to the Eagle Point WWTP, were usingseptic systems within watershed boundaries the 2000 t ofchloride discharged annually from this WWTP would be areasonable estimate of chloride releases from septic tanks(Table 2).[30] For agricultural loads, using a value of 49.9 kg/ha of

chloride from fertilizer and manure results in only 3800 t of

chloride added to the watershed. If all of the designatedagricultural land were used to grow corn, the value wouldbe 2800 t/yr of chloride.[31] The largest nonpoint source of chloride in the TCMA

was road salt. It was determined that 241,000 t of the317,000 t of road salt applied annually in the seven-countymetropolitan area was applied within the watershed bound-aries shown in Figure 1. This translates to a nonpoint sourceinput of 142,000 t/yr of chloride from road salt applications.It was determined that natural deposition, agricultural inputsand septic sewer systems would only contribute an additional1–3%, depending on the calculation method, to the totalchloride load (from point and nonpoint sources). Thereforethe loads from these sources were neglected, leaving road saltapplications as the only significant nonpoint source ofchloride within the watersheds boundaries.

3.3. Metropolitan Area Chloride Balance Calculation

[32] The individual chloride balance components(Mississippi River inflow, Minnesota River inflow, Missis-sippi River outflow, road salt application, WWTP effluents)presented in the previous sections were combined in achloride mass balance using equation (1) on a monthlytimescale.[33] Mean monthly mass fluxes (t/month) of chloride

entering or exiting the TCMAwatershed from the two majorrivers and the four WWTPs are illustrated in Figure 5a.Dashed lines in Figure 5a give cumulative contributions bymonth from WWTP effluents, the Minnesota River and the

Figure 3. Flow-weighted average monthly chloride concentrations and flow rates (2000–2007) inmajor rivers at the inflow and outflow points of the Twin Cities metropolitan area watershed.

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Mississippi River up to the total ‘‘Inflow + WWTP’’ curve.The ‘‘Outflow’’ curve is from the Mississippi River outflowstation in Hastings. Figure 5a is a graphical representation ofequation (2), showing the difference between the mass ofchloride exiting the watershed (O) and the sum of the amountentering the watershed (I) and the amount added by pointsources (Mp). Figure 5b represents the amount of chlorideadded to the river system from nonpoint sources (mnp inequation (2)). Since road salt is the only significant nonpointsource of chloride inside the watershed Figure 5b alsorepresents the monthly mass of chloride from road saltapplications exported by the Mississippi River from theTCMA watershed. The monthly nonpoint source chloridecontribution to the Mississippi River outflow was highestduring the winter months when road salt was being applied tothe roads. The high December to April values can beinterpreted as the direct impact of road salt applications andsnowmelt water runoff through systems of storm sewers andsmall streams to the big rivers. Delays occur because winter isa low flow season, and only when snowmelt sets in does therouting processes accelerate. The small contributions in latesummer can be attributed to the flushing of chloride fromlakes and wetlands [Novotny et al., 2008] or to the delayedtransport to the rivers through interflow or groundwater flow[Novotny et al., 1999].[34] In March the difference between the mass of chloride

exported and the mass imported had a maximum. Road saltapplied in the TCMA watershed accounted for 34% of thechloride passing the Mississippi River outflow station inMarch, WWTPs contributed 19%, and the inflows from theMississippi and Minnesota rivers into the TCMAwatershedprovided 47% (Figure 5a). The total amount of chlorideexported from the system during this month was 13,000 t(Figure 5b). By adding the values for all months in Figure 5b,

the annual mass of chloride from road salt exiting the controlvolume (the TCMAwatershed) was found to be 33,000 t/yr. Ithad previously been estimated that 142,000 t of chloride/yrwere applied as road salt to the TCMA watershed area. If33,000 of the 142,000 t were carried away by the Mississippithen 109,000 t or 77% of the chloride applied annually hadto stay behind in the TCMA watershed system (S inequations (1) and (2)).

3.4. Subwatershed Chloride Balance Calculation

[35] The average annual chloride concentrations in the10 small streams within the larger TCMAwatershed rangedfrom 37 mg/L in Carver Creek to 185 mg/L in ShingleCreek; annual average flow rates were from 0.093 m3/s inRiley Creek to 1.685 m3/s in Minnehaha Creek (Table 3).The total watershed area, the percentage covered by imper-vious surfaces and the mass of road salt applied annuallywere also determined (Table 3). Chloride application ratesin the 10 subwatershed ranged from 0.08 to 0.82 t/ha peryear.

Figure 4. Grab sample chloride concentrations from the effluents of the four WWTPs. Dates aremonth/day/year.

Table 2. Estimates of Average Chloride Concentrations, Flow

Rates, and Total Mass of Cl� in Effluents From Major WWTPs in

the TCMA Each Yeara

Name ofWWTP Chloride (mg/L) Flow (m3/s) Mass (t/yr)

Metro 227 8.63 62,000Blue Lake 387 1.18 14,000Seneca 280 1.01 9,000Eagle Point 348 0.18 2,000Total 11.00 87,000

aAverage chloride concentrations are flow weighted and for the periodJune 2006 to June 2007 excluding the winter months. Average flow ratesare from 2000 to 2007. TCMA, Twin Cities metropolitan area.


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[36] A background concentration of 18.6 mg/L(±34.9 mg/L at the 95% confidence interval) was deter-mined using a linear regression analysis between annualaverage Cl� concentrations in the stream (mg/L) and totalCl� applied (t/yr) per watershed area (Figure 6, R2 = 0.79).The background concentration of 18.6 mg/L matched withconcentrations in the Mississippi River before it entered theTCMA. Retention of chloride in each individual sub-watershed was calculated to be from 55% to 83% (Table 3).The total combined mass of chloride applied in the 10subwatersheds was 45,300 t/yr, which represents 32% ofthe amount applied in the entire TCMAwatershed. The totalamount of chloride exported from the 10 subwatersheds wasestimated at 12,500 t/yr. This means that only 28% of the saltapplied was exported from the 10 subwatersheds each year,and therefore 72% was retained. Because of the large confi-dence interval around the background concentrations ananalysis was conducted by setting the background concen-tration to 0 mg/L. This represents a scenario where all of the

chloride exiting thewatershed in a given year is from road saltapplications during that year. This analysis resulted in thelowest possible retention rate. If the background concentra-tion was set to 0 mg/L the amount of road salt exiting thewatershed would be raised to 15,900 t/yr reducing theretention rate to 65%. The retention estimates of 72% and65% are lower than the 77% value obtained for the entireTCMAwatershed, but comparable.

3.5. Sodium Retention

[37] The retention of chloride from road salt applicationsin a watershed encompassing the Twin Cities metropolitanarea was determined to be around 77%. While an analysiswas not conducted on the other ion in rock salt, sodium,because of its slightly less conservative behavior a valueequal to or higher than 77% would be expected. Sodiuminteracts more readily with soils through ion exchangeallowing for the possibility that higher amounts could be

Figure 5. (a) Monthly chloride fluxes (t/yr) from point sources entering or exiting the Twin Citiesmetropolitan area watershed. Shaded areas between dashed lines give monthly contributions from fourWWTP effluents, the Minnesota River, and the Mississippi. Values are additive up to the total ‘‘Inflow +WWTP’’ curve. The ‘‘Outflow’’ curve is from the Mississippi River outflow station in Hastings,downstream from the Twin Cities metropolitan area. (b) Monthly differences between the ‘‘Outflow’’ andthe ‘‘Inflow + WWTP’’ (solid lines) from Figure 5a. By virtue of the mass balance in equation (2), theplotted values give the monthly amounts of nonpoint source chloride exported by the river as well as theuncertainties in the mass balance.

Table 3. Small Stream Watershed Information and Road Salt Applications Rates Within Each Watersheda

Creek NameArea(ha)

ImperviousSurface(%)

AnnualAverage Flow

(m3/s)Annual Average[Cl�] (mg/L)

Total Cl�

Applied(t/yr)

Cl� Appliedper Area(t/yr ha)

Mass Cl� Exported(t/yr)

Cl� Retained(%)

Bassett 11,100 34 0.97 138 8,100 0.73 3,600 56Battle 3,000 32 0.22 147 2,400 0.80 900 63Bluff 2,300 11 0.11 65 600 0.26 200 67Carver 21,600 4 0.98 37 1,800 0.08 600 67Credit River 13,300 9 0.50 44 1,800 0.14 400 78Fish 1,300 27 0.09 100 600 0.46 200 67Minnehaha 46,100 15 1.69 68 17,700 0.38 2,600 85Nine Mile 9,900 29 0.71 74 4,700 0.47 1,200 74Riley 3,400 18 0.11 52 600 0.18 100 83Shingle 10,800 35 0.49 185 7,000 0.65 2,600 63Total 45,300 12,500 72

aAverage concentrations and average flow rates are from 2000 to 2007. Mass (rate) of chloride exported from each stream watershed in t/yr and as apercentage of the chloride applied as road salt in the watershed.

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stored in the soil column [Amrhein et al., 1992; Mason etal., 1999].

3.6. Sensitivity of the Results

[38] The assessment of chloride (road salt) retention wasmade with the best information available, however assump-tions, which had to be made, do influence the resultsobtained. The sensitivity of the findings to three assump-tions and procedures used was therefore investigated: thesampling frequency of the grab samples at the inflow andoutflow stations, the method of calculation for chlorideexport from the watershed, and the estimation method forthe nonpoint sources of chloride.3.6.1. Sampling Frequency[39] An analysis was conducted to determine if continu-

ous monitoring or more frequent sampling was needed tocapture chloride concentrations in snowmelt events of shortduration at the inflow and out flow stations on the Mis-sissippi and Minnesota rivers [Ruth, 2003]. To test thesensitivity to sampling frequency, records of daily averagespecific conductance in the Mississippi River near Hastings(outflow station) were used from a continuous monitoringstation maintained by the Metropolitan Council (Figure 7a).Although a direct relationship between chloride and specificconductance could not be obtained, because of the damp-ening of the chloride signal in relation to other ions from thehigh flow rates, snowmelt events could be clearly detectedby fluctuations in specific conductance during the winter. AComparison was conducted between the continuous timeseries of daily specific conductance values and the specificconductance and chloride values recorded on the grabsample dates. This analysis provided evidence that a suit-able representation of the chloride dynamics was obtainedwith the biweekly sampling frequency used (Figure 7a).Furthermore, the cumulative distributions of specific con-ductance values obtained from the continuous daily timeseries and the values on the days when chloride grabsamples were taken were virtually identical (Figure 7b). Itwas concluded that the sampling frequency used was

adequate to estimate the annual load of Cl� (salt) exitingor entering the TCMA watershed over the study period.3.6.2. Chloride Export[40] Other pathways of chloride export such as airborne

transport of salt particles, or chlorination of natural organicmatter were not analyzed. Salt particles can become air-borne behind vehicles and during strong winds, howeverthey are typically deposited within 100 m of the road[Blomqvist and Johansson, 1999]. Cl� has also been foundto interact with natural organic matter forming chlorinatedorganic matter when transported through soils [Bastviken etal., 2007]. It is therefore possible that some of the Cl�

from road salt applied in the TCMA was exported in theMississippi River while attached to organic matter and isthus not accounted for. Our budget analysis did not includeairborne or organic matter transport mechanisms. If incor-rect, this assumption would cause an overestimation ofchloride retention in the watershed. If as much as 10% ofthe road salt applied in the TCMA were exported byalternative and not included transport mechanisms, the totalexport would rise to 47,800 t/yr from 33,000 t/yr, and theroad salt retention estimate for the TCMAwould be loweredto 64%.3.6.3. Nonpoint Chloride Sources in the TCMA[41] Only road salt was considered as a significant

nonpoint source of chloride in the watershed. Other sources,such as natural deposition, septic tank seepage from resi-dences not connected to a WWTP or fertilizer applications,were assumed negligible. If an additional 3% (4500 t) ofchloride were added to the system by outside processes, theestimated Cl� retention in the watershed would increaseslightly.[42] The 24% commercial road salt application rate was

taken from market share information provided by theAmerican Salt Institute for the entire United. While thisvalue is based on national statistics it is the best estimate fora large metropolitan area for a number of reasons. Thediversity in area including suburban, urban and rural land

Figure 6. Plot of average annual chloride concentrations versus amount of chloride applied perwatershed area for the 10 subwatershed streams. Intercept was used as background concentration in thesubwatershed chloride budget.


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uses and the size of the watershed studied allow for acomparison with the national scale. National trends inmunicipal salt purchases match Minnesota trends in rocksalt purchases [Sander et al., 2007]. Finally, obtaininginformation from commercial road salt appliers and esti-mating application rates or areas where road salt wasapplied commercially is very difficult. The results from thisanalysis would likely include more error than accuratenational sales data. If the 24% commercial road salt appli-cation rate were lowered to 10%, a value reported in aCanadian study [Environment Canada Health Canada,1999], the chloride contribution from road salt applicationswould be reduced to 122,000 t/yr, the water-borne exportrate from the TCMA would be raised to 27%, and theretention rate in the watershed would drop to 73%.[43] If all effects reducing the retention rate were com-

bined (commercial application rate reduced from 24 to 10%of total application rate; increased export of 10% of theapplied amount) the application rate would be lowered to122,000 t/yr, and the export rate would be raised to33,000 + 12,200 = 45,200 t/yr. The total export rate wouldthen be 37% and the retention rate 63%. In other words, evenwith extremely favorable assumptions for road salt flushingfrom the TCMA the estimated retention rate remains high.

3.7. Comparison to Other Metropolitan Areas

[44] Other studies have shown significant retention ofchlorides in urban watersheds, but the estimated retentionpercentages vary significantly. Most studies were conductedin small, urbanized watersheds ranging in size from 104 to435 km2. Retention percentages were found to be 55% in astream watershed in the greater Toronto, Canada area[Howard and Hayes, 1993], 50–65% in Helsinki, Finland[Ruth, 2003], 28–45% in Chicago, Illinois [Wulkowicz andSaleem, 1974], 59% in Rochester, New York [Bubeck et al.,1971], and 35% in Boston, Massachusetts [Huling andHollocher, 1972].

[45] In Toronto, Ontario, Canada, the accumulation of saltin the watershed due to deicing practices has seriouslycompromised the shallow aquifers [Howard and Maier,2007]. In Waterloo, Ontario chloride concentrations in theaquifers had not reached equilibrium after 57 years ofroad salt applications. It was estimated that on the orderof 100 years will be required to reach equilibrium concen-trations under current conditions [Bester et al., 2006]. Inshallow aquifers near Chicago, Illinois, chloride concen-trations have increased since 1960; 24% of the wells studiedin the 1990s had concentrations above 100 mg/L, whenmedian values before 1960 were less than 10 mg/L, and15% of the wells had rate increases greater than 4 mg/L peryear [Kelly, 2008]. The accumulation of salt in shallowgroundwater also affects base flow concentrations instreams [Marsalek, 2003]. Baseline salinity in urban streamsand streams near roadways have been increasing in thenortheastern part of the United States [Godwin et al., 2003;Kaushal et al., 2005], in the Greater Toronto Area, Canada[Bowen and Hinton, 1998] and in Sweden [Lofgren, 2001;Thunquist, 2004].

3.8. Chloride Retention in the TCMAWatershed

[46] Evidence of significant chloride retention within theTCMAwatershed is provided in streams lakes and aquifers.Four streams (Minnehaha Creek, Battle Creek, ShingleCreek andNineMile Creek) in the TCMA are on theMPCA’s2008 list of ‘‘impaired waters’’ for chloride (Data are avail-able from Minnesota Pollution Control Agency at http://www.pca.state.mn.us/water/tmdl/tmdl-303dlist.html.).Pulses of very high chloride concentrations occur in these andother small streams of the TCMA during the winter months(Figure 8).[47] Volume-weighted average chloride concentrations in

38 lakes of the TCMA have increased from 1984 to 2005 byan average of 1.5 mg/L per year (range of 0.1 to 15 mg/Lper year), following a pattern similar to the mass of road salt

Figure 7. (a) Daily averages of specific conductance from 15 min continuous monitoring (solid line),days when chloride grab samples were taken (solid triangles), and chloride concentrations from grabsamples (solid circles). Data are from the Mississippi River outflow station in Hastings. (b) Cumulativenormalized distribution functions of daily specific conductance values. Specific conductance values onlyfor days when a chloride grab sample was taken (solid line) and all daily values from Figure 7a (dashedline).

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purchased by the state of Minnesota over that same timeperiod [Novotny et al., 2008]. Median concentrations, in thesurface waters of the 38 lakes from 2001 to 2005, of 87 mg/L (range 31 to 505 mg/L) [Novotny et al., 2008] were muchhigher than the estimated presettlement concentrations of3 mg/L [Ramstack et al., 2004], In urban lakes of the TCMAchloride concentrations tend to fluctuate seasonally, withmaximum in February or March and minimum in Octoberor November. In the long-term, a mean annual equilibriumconcentration is reached when all of the chloride added to alake during the winter season is flushed out during thesummer and fall season. There was an indication that smallerlakes with high summer flushing rates have already reachedequilibrium, while larger lakes and lakes with low flushingrates can be expected to have rising mean annual chlorideconcentrations for years to come, if current salt applicationrates continue. If salt applications were stopped completely,the recovery of many urban lakes would take from 10 to30 years (E. V. Novotny and H. G. Stefan, Model of presentand future chloride concentrations in lakes of the Twin Citiesmetropolitan area, Minnesota, submitted to Journal of Air,Water and Soil Pollution, 2009).[48] Chloride concentrations in surficial sand and gravel

aquifers throughout the state of Minnesota vary substantial-ly with land use. Median chloride values of 46 mg/L werefound in urban areas, 17 mg/L in agricultural areas, and1.2 mg/L in forested area [Fong, 2000]; 3% of the watersamples taken from wells in the TCMA were found toexceed the USEPA secondary chloride standard of 250 mg/L [Fong, 2000]. In a cross section of the surficial aquifer ina northwestern suburb of Minneapolis directly down gradi-ent from a high-traffic roadway, chloride concentrationsranged from 200 mg/L at the water table 3 m below thesoil surface, to 590 mg/L at a depth of 13.5 m below the soilsurface [Andrews et al., 2005]. Concentrations of 380–470 mg/L were also measured down gradient from a majorInterstate Highway (I-94) in summer and late fall pointingtoward a long-term storage of road salt in the surficialaquifer [Andrews et al., 2005]. Data collected by theMinnesota Pollution Control Agency (MPCA) throughoutthe seven-county TCMA showed that the highest chlorideconcentrations in groundwater, up to 2000 mg/L, werefound in shallow wells (Figure 9).

[49] Elevated chloride concentrations in aquifers may bedelayed because of storage in the subsurface [Bester et al.,2006; Kelly et al., 2008]. Shallow wells respond firstresulting in some to have already reached concentrationsabove state standards. The storage potential of the TCMAaquifer system is very large. If road salt applications werecompletely stopped today, chloride concentrations in deepwells may continue to increases for many years untilsubsurface saline transport has reached equilibrium [Besteret al., 2006; Kelly, 2008]. Residence times in the aquiferscan be high ranging from tens to hundreds of years inthe upper and surficial aquifers to thousands of years in thedeep aquifers. Residence times in lakes are smaller, on theorder of 3 to 14 years, but still provide a means of chloridestorage (Novotny and Stefan, submitted manuscript, 2009).The TCMA has hundreds of lakes and wetlands, andhundreds of man-made detention and infiltration basins.Policy has been to delay runoff from rainfall and snowmelt,and to increase infiltration by routing storm sewers intothese systems. Although very useful in storm water man-agement, these practices could be adding to the accumula-tion of road salt in the watershed by promoting retention of

Figure 9. Chloride concentration in wells located through-out the Twin Cities metropolitan area in 2004 and 2005.Information was reported by the Minnesota PollutionControl Agency.

Figure 8. Chloride concentrations in two streams of the Twin Cities metropolitan area from grabsamples obtained by the Metropolitan Council Environmental Services. Both streams are on the 2008 listof chloride impaired waters (Data are available from Minnesota Pollution Control Agency at http://www.pca.state.mn.us/water/tmdl/tmdl-303dlist.html.).


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surface runoff and/or infiltration of the contaminated waterinto the groundwater.

4. Summary and Conclusions

[50] Road salt is used to increase driving safety in theTwin Cities metropolitan area (TCMA) of Minnesota. Achloride budget for the TCMA watershed (Figure 1) withdata from 2000 to 2007 revealed the final destinations of theroad salt after it was dissolved in the snowmelt water. TheTCMA watershed analyzed covered an area of 4150 km2

with a population of 2.8 million. According to the annualchloride budget 235,000 t of chloride entered the TCMAannually in the Mississippi and Minnesota rivers, and355,000 t exited through the Mississippi River resulting in120,000 t being added to the Mississippi and Minnesotarivers as they traveled through the TCMA. Of the 120,000 tof chloride added annually 87,000 t came from the fourWWTPs (point sources) and 33,000 t came from road salt(nonpoint source). Of the 142,000 t of chloride appliedannually in the TCMA as road salt, only 23% (33,000 t)were exported through the Mississippi River and 109,000 tor 77% were retained in the TCMA watershed. Chloridebudgets for 10 subwatersheds within the TCMA analyzed ina similar way, gave a retention rate of 72% for road salt.[51] Evidence of chloride retention is widespread in the

TCMA. Four streams in the TCMA are on the MPCA’s2008 list of Cl� impaired waters, 38 lakes in the TCMA hada rising mean annual Cl� concentration over a 22 yearperiod [Novotny et al., 2008], and elevated Cl� concen-trations in groundwater have been measured [Andrews etal., 2005; Fong, 2000]. Chloride retention in urban areaswhere road salt (NaCl) is applied should cause muchconcern. Mitigation measures, best management practices(BMPs) for road salt application and alternatives to NaClneed to be examined.

[52] Acknowledgments. We acknowledge and thank the followingindividuals and institutions: the Minnesota Local Road Research Board(LRRB) in cooperation with the Minnesota Department of Transportation(Mn/DOT) and the University of Minnesota for providing the funding forthis research; the Technical Advisory Panel, lead by Wayne Sandberg ofWashington County, for input and suggestions to our research; KarenJensen of the Metropolitan Council Environmental Services, the Metro-politan Council (MCES), and the United States Geological Survey (USGS)for providing data used in this study; numerous individuals in cities,counties, and Mn/DOT for providing data on road salt applications.

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��O. Mohseni, E. V. Novotny, A. R. Sander, and H. G. Stefan, St. Anthony

Falls Laboratory, Department of Civil Engineering, University of Minnesota,2 Third Ave. SE, Minneapolis, MN 55414, USA. ([email protected])


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Chloride ion transport and mass balance in a metropolitan area using road salt

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