Water Resources Research

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

Authors


Abstract

[1] In the Twin Cities metropolitan area (TCMA) of Minneapolis/St. Paul, Minnesota, an estimated 317,000 metric tons (t) of road salt were used annually for road deicing between 2000 and 2005. To determine the annual retention of road salt, a chloride budget was conducted for a 4150 km2 watershed encompassing the populated areas of the TCMA. In addition to inflows and outflows in the major rivers of the TCMA, multiple sources of chloride were examined, but only road salt and wastewater treatment plant (WWTP) effluents were large enough to be included in the analysis. According to the chloride budget, 235,000 t of chloride entered the TCMA annually with the Mississippi and Minnesota rivers, and 355,000 t exited through the Mississippi River. Of the 120,000 t of chloride added annually to the rivers inside the TCMA watershed boundaries, 87,000 t came from WWTPs and 33,000 t came from road salt. Of the 142,000 t of chloride applied annually 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 the TCMA watershed. Chloride budgets for 10 subwatersheds within the TCMA analyzed in a similar way, gave an average chloride retention rate of 72%. The retention is occurring in the soils, surface waters (numerous lakes, wetlands, and ponds) and in the groundwater. Chloride concentrations in many of these urban water bodies are now considerably higher than the presettlement background levels of less than 3 mg/L with concentrations as high as 2000 mg/L in shallow groundwater wells. The continued accumulation of chloride in the groundwater and surface waters is a cause for concern.

1. Introduction

[2] It has been reported that 21 million metric tons (t) of road salt were used in the United States in 2005 to improve driving safety in the winter (Data are available from United States Geological Survey at http://minerals.usgs.gov/minerals/pubs/commodity/salt/.). In the seven-county Twin Cities metropolitan area (TCMA) of Minneapolis/St. Paul, Minnesota, an estimated 317,000 t of road salt were used annually for road deicing between 2000 and 2005 [Sander et al., 2007]. Road salt (mostly NaCl) is highly soluble in water resulting in the sodium and chloride ions dissociating from one another when snowmelt occurs. Chloride and sodium ions are both transported from roads to receiving waters along three pathways: (1) a rapid runoff pathway from impervious surfaces, (2) a shallow subsurface pathway through the soil, and (3) a deeper and slower pathway through underground aquifers [Novotny et al., 1999]. All three pathways can result in the retention of sodium and chloride in the surface water and groundwater of a watershed [Ramakrishna and Viraraghocan, 2005]. In addition to the accumulation that can occur in the surficial and deep groundwater aquifers, small amounts of chloride and more so sodium could also be retained through interactions with soils and organic matter [Amrhein et al., 1992; Bastviken et al., 2006; Mason et al., 1999; Svensson et al., 2007]. The major factors influencing retention in soils and groundwater include soil permeability, vegetation cover, topography, and roadside drainage techniques [Jones and Jeffrey, 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 Health Canada, 1999; Ramakrishna and Viraraghocan, 2005]. Increased chloride concentrations decrease the biodiversity of waterways and roadside vegetation [Environment Canada Health Canada, 1999]. If chloride reaches the groundwater it can contaminate drinking water supplies [Howard and Maier, 2007]. Not only has chloride been shown to affect organisms, it can also increase the transport and bioavailability of heavy metals such as cadmium, lead, chromium, copper and even mercury in the environment, which are also harmful to biota [Novotny et al., 1999]. Other secondary consequences of road salt applications in lakes include the ability to inhibit or delay natural mixing events, limiting the oxygenation of benthic waters and sediments and facilitating the release of heavy metals, 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 groundwater and surface waters in urban environments in northern climate 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 balance studies on individual streams with watershed areas less than 450 km2 indicate that between 27% and 65% of the road salt applied was retained within the individual urban watersheds [Bubeck et al., 1971; Howard and Hayes, 1993; Huling and Hollocher, 1972; Ruth, 2003; Wulkowicz and Saleem, 1974].

[5] Unlike previous studies, which analyzed small watersheds located in urban environments, this study examines an entire metropolitan area encompassing a surface area of 4150 km2 including rural, suburban, and urban communities. By examining an entire metropolitan area, a better understanding of the total retention and the holistic effect of road salt applications on groundwater and surface waters can be obtained. This study used data collected over an 8 year period. Potential environmental damage will affect the entire metropolitan area, and policies on road salt applications cannot be developed by extrapolation from small subwatersheds. Overall this study draws attention to a developing problem that will affect around 3 million people in a 4150 km2 area and provides insight for other major metropolitan 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 a chloride budget for an entire metropolitan area. This analysis was used to estimate how much of the road salt applied annually is exported from the watershed by the Mississippi River and how much is retained in the soils, groundwater and 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 central United States, the TCMA experiences cold climate with an average annual snowfall of 1420 mm between November and April (Data are available from Minnesota Climate Work Group at http://climate.umn.edu/doc/twin_cities/twin_cities_snow.htm.). The hydrologic drainage system of the TCMA includes many small streams, lakes and wetlands along with an extensive storm sewer system and hundreds of detention and infiltration basins. Under the TCMA is a system of several aquifers, some of which are used for urban water supply. Two major rivers, the Minnesota and the Mississippi, flow through the TCMA. The combined watersheds of these two rivers encompass 4150 km2 of the seven-county metropolitan area, and provide the natural boundaries for a control volume to be used in a chloride mass balance (Figure 1).

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 points listed in Table 1. Data from unlabeled chloride sampling points were used in Figure 2 but not used in the budget analysis. Center of the map is at 44.88° latitude, 93.18° longitude.

[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 chloride mass balance (equation (1)) for the TCMA control volume.

equation image

Where I is the total annual inflow (t/yr) through the Mississippi River at Anoka and the Minnesota River at Jordan, Mp and Mnp represent the total annual mass of chloride added inside the watershed from point sources and nonpoint sources, respectively, O is the total annual mass of chloride exported through the Mississippi River at Hastings and S is the annual retention of chloride (t/yr) in the TCMA. Chloride is highly soluble in water, and has been treated as conservative once it is in solution.

[9] Once the water reaches the Minnesota or Mississippi rivers storage potential is limited. It is expected that the mass of chloride entering the TCMA at the inflow stations will exit at the outflow stations. Likewise, all of the chlorides discharged directly to the rivers through point sources are expected to reach the outflow station. However, the hydrologic transport from nonpoint sources of chloride is unknown. Nonpoint sources of chloride can infiltrate into soils, can accumulate in wetlands or lakes, can travel through storm sewers or can reach the groundwater. Only some of the salt applied as a nonpoint source in the TCMA is expected to reach the Mississippi River. Therefore, chloride can only be retained in the watershed if it comes from a nonpoint source. This assumption allows rearrangement of equation (1) to equation (2).

equation image

Where mnp = Mnp − S. We will first calculate mnp, and then S knowing Mnp. The value of mnp represents the amount of chloride from nonpoint sources that entered the river system and was flushed out of the watershed system at the Mississippi River outflow station.

2.2. Inflows and Outflows

[10] Sodium and chloride concentrations as well as river flow rates were measured by state and federal agencies at gauges and sampling points on the Mississippi and the Minnesota rivers (Figure 1 and Table 1). The Metropolitan Council Environmental Services (MCES) collected grab samples for chemical analyses at six locations along the Mississippi River and two locations on the Minnesota River two to five times per month between January 2000 and December 2007. For the same time period, daily average, and monthly average flow rates were obtained from the United States Geological Survey (USGS).

Table 1. Names of Sampling Points and Data Collection Organizations for Flow Rates and Chloride Concentrations Used in the Budget Analysisa
Sample NumberNameOrganization
  • a

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

105330000 – Minnesota River at JordanU.S. Geological Survey
205288500 – Mississippi River at AnokaU.S. Geological Survey
305331000 – Mississippi River at St. PaulU.S. Geological Survey
4Upper Mississippi River Mile 871.6Metropolitan Council
5Minnesota River Mile 39.6Metropolitan Council
6Upper Mississippi River Mile 815.6Metropolitan Council
7Blue Lake WWTPMetropolitan Council
8Seneca WWTPMetropolitan Council
9Metro WWTPMetropolitan Council
10Eagle Point WWTPMetropolitan Council

[11] The watershed area upstream from the grab sampling station at Anoka is 48,900 km2, while the watershed area upstream from the USGS stream gauging station is 49,500 km2, a difference of only 1.2%. Therefore, the flow rates and the grab sample concentrations were used together without adjustment. The USGS St. Paul gauging station is located 38 km upstream from the outflow grab sample location at Hastings; the watershed areas for these two locations are 95,300 km2 and 95,900 km2, respectively, a difference of 0.6%. The only major inflow between these two points is the Metro wastewater treatment plant (WWTP). The river outflow rate from the TCMA was therefore taken to be the flow rate at St. Paul plus the outflow from the Metro WWTP. Using the daily flow data and grab sample concentrations from 2000 to 2007, flow-weighted monthly average chloride concentrations were calculated. Each of these concentrations (mg/L) were multiplied by the associated mean monthly flow rate (m3/s) to estimate the mean monthly chloride mass transport rate (t/yr).

2.3. Chloride Sources

[12] Sodium and chloride sources include natural weathering of minerals, natural deposition from rainfall, processing of agricultural products, industrial production of chemicals and food, processing in the metal, paper, petroleum, textile, and dying industries, water softening, and road salt applications of NaCl [Kostick, 2004]. Point sources of chloride in the watershed include WWTP effluents and industrial discharges to the Mississippi or Minnesota rivers. Most industrial sources of chloride are connected to the WWTPs, and are included in the point source discharges from the WWTPs. Nonpoint sources of chloride are natural sources (atmospheric deposition, weathering), rural household septic systems, fertilizer applications, and snowmelt runoff 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) and industrial NaCl uses. Chloride concentrations in effluent grab samples were measured by the Metropolitan Council (MCES) from June 2007 to June 2008 in two week intervals at the four major wastewater treatment plants within the TCMA watershed. Locations of the WWTPs are shown in Figure 1 and are listed in Table 1. The annual average chloride concentrations and the annual average flow rates determined from daily WWTP effluent flow data were used to determine the mean annual rates (t/yr) of chloride input to the rivers from the four WWTPs. The majority of industries and 2.4 million of the 2.8 million people living in the TCMA contribute wastewater through sanitary sewers to the four wastewater treatment plants discharging within the boundaries of the TCMA watershed. The majority of the other 400,000 people are either located outside the boundaries of the watershed or contribute to one of the three other wastewater treatment plants that do not discharge within the control volume. Combined sewers in the TCMA have been reduced to a minimum. Therefore, all household and industrial sources of chloride within the control volume boundaries in the TCMA are included in the effluent values from the 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 geological formations, and wet deposition from ocean evaporation [Jackson and Jobbagy, 2005]. Mineral salt deposits and geological sources of chloride are negligible in the TCMA. The annual average concentration of chloride in rainwater was measured at a site in the northern part of the TCMA from 2000 to 2007 to be 0.07 mg/L (Data are available from National Atmospheric Deposition Program at http://nadp.sws.uiuc.edu/sites/siteinfo.asp?net=NTN&id=MN01.). This value combined with an annual average rainfall of 747 mm was 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 TCMA were using a septic system (Data are available from Metropolitan Council, Estimate number of septic systems by community for 1997, at http://www.metrocouncil.org/environment/Watershed/bmp/septic_sys.htm.). Of those 70,000 household around 60% were located outside of the watershed boundaries. This population estimation was used to 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 percentage was calculated using land use data from 2005 provided by the Metropolitan Council. For the production of corn 36 kg/ha of potassium chloride are required according to the Minnesota Department of Agriculture [Wilson, 2007]. A value of 49.9 kg/ha per year was used in a study of fertilizer and manure 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 within the seven-county TCMA by government agencies and commercial/private users was estimated to be 317,000 t/yr [Sander et al., 2007]; 241,000 or 76% came from public applications (i.e., city, county and state agencies), the other 24% or 76,000 t was estimated to come from commercial/private applications. The commercial percentage provided by the American Salt Institute for the years 2005 and 2006 was based on market share information for road salt purchases. Commercial/private applications were determined to be the amount of bulk salt purchased by nongovernment agencies plus the amount of package deicing salt purchased by both homeowners and commercial applicators.

[18] The TCMA watershed boundaries do not coincide with the political TCMA boundaries (Figure 1). For that reason the amount of road salt applied within the political seven-county TCMA had to be adjusted using road kilometers. City, county and state road data were obtained from the GIS database of the Metropolitan Council. Road lengths were divided by government entity into total kilometers of city roads for each city, of county roads for each county and state road. Fractions (percent) of road lengths inside the watershed versus the total length inside a municipality, county, or state jurisdiction were multiplied by the total mass of road salt applied by each individual agency to estimate the total mass of salt applied in the watershed. The total mass of road salt applied by private and commercial uses on parking lots, sidewalks etc. was added to the public road salt applications by using a value equal to 24% of the total salt applications.

2.4. Chloride Balances in Subwatersheds

[19] The chloride budget study for the entire TCMA was supplemented by a chloride budget study for 10 subwatersheds of small streams located entirely within the TCMA. This study was done to determine if salt retention rates obtained at a geographic scale of less than on tenth the TCMA were comparable to those obtained for the entire TCMA.

[20] The analysis of the subwatersheds was based on equation (1). The inflow (I) for the subwatersheds was based on the estimated chloride concentration in the stream if road salt were not applied in the watershed. This value is different from the overall TCMA study and was defined as the background concentration in the stream. The background concentration was estimated from a linear relationship between the annual average chloride concentrations in the streams versus the mass of chloride from road salt applied per ha per year within the watersheds. The background concentration of chloride was found by extrapolating the linear relationship to a chloride application rate of zero. The background concentration was multiplied by the annual average discharge (flow) from the watershed to obtain the inflow loading (I).

[21] The only internal chloride source (M) in the watershed was from road salt applications. No WWTP effluents or other chloride sources besides road salts were being applied in the subwatersheds. The mass of road salt applied to the roads was determined using the methods described for the TCMA watershed analysis explained above. The percentage of the watershed covered by impervious surfaces and the total watershed areas were found using GIS data from the University of Minnesota Remote Sensing and Geospatial Analysis 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 and grab sample data, collected by the Metropolitan Council Environmental Services (MCES) two to five times per month between 1 January 2000 and 31 December 2007, at the outflow from each subwatershed were used.

3. Results and Discussion

3.1. Inflows and Outflows of Chloride in the Major Rivers

[23] The hydrologic transport of sodium and chloride in the TCMA was inferred from the concentrations in its major rivers. Average annual concentrations measured in these rivers show significant changes with distance through the TCMA (Figure 2). Increased Na+ and Cl concentrations in the Mississippi River were most pronounced downstream from the confluence with the Minnesota River and the Metropolitan WWTP where mean annual chloride concentrations increased from 16 mg/L to 33 mg/L. The Minnesota River arrived in the TCMA with higher concentrations than the Mississippi River. Discharges into the Minnesota River include effluents from the Blue Lake and the Seneca WWTPs as well as surface runoff from populated areas, resulting in a mean annual chloride concentration increase from ∼30 mg/L to ∼42 mg/L. The St. Croix River is outside the TCMA and carries much lower chloride concentrations (∼5 mg/L) because the watershed is largely undeveloped. Inflow from the St. Croix River caused a decrease in chloride concentrations in the Mississippi River downstream from the TCMA. In all three rivers sodium and chloride follow similar concentrations distributions with distance.

Figure 2.

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

[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 grab samples showed only small variations for a particular month with the exception of January, February and November. In the Minnesota River inflow to the TCMA at Jordan, mean monthly chloride concentrations ranged from 20 to 40 mg/L (Figure 3). Flow-weighted mean monthly concentrations were highest between December and February and lowest from March to August. Variability between individual grab samples for a given month were highest in February.

Figure 3.

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

[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 and March, and the lows between April to July. Individual grab sample concentrations varied the most in March, and the least in June.

[26] Estimates were made for the total mass of chloride passing through the inflow and outflow observation points in Figure 3 for every month. The annual mass (rates) of chloride entering the TCMA by the Minnesota and Mississippi River inflows were determined to be 119,000 and 116,000 t/yr, respectively. The mass (rate) of chloride flowing out of the TCMA watershed with the Mississippi River was found to be 355,000 t/yr. Roughly 50% more chloride was found to be exiting the watershed than was entering with the Mississippi and 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 treatment plants (Figure 4). Effluent chloride concentrations increased during the winter months at the Metro WWTP. Domestic and industrial waste loads to the wastewater treatment system were expected to remain fairly constant throughout the year, as was shown for the other three WWTPs. Therefore, the increase at Metro during the winter was attributed to the addition of road salt to the system through car washes, a small number of combined sewers and possible seepage into the sanitary sewer system. To avoid double counting road salt inputs, the average Cl concentration between June and November was used for the entire year in the annual chloride budget. Although the other three WWTPs did not display a significant concentration increase in winter, the same procedure was used for consistency. The (June 2007 to November 2007) average chloride concentrations in the WWTP effluents and the annual average effluent flow rates are shown in Table 2. The associated mass (rate) of chloride entering the river system from the four WWTPs including domestic and industrial wastewater, but excluding chloride from road salt applications, was estimated to be 87,000 t/yr.

Figure 4.

Grab sample chloride concentrations from the effluents of the four WWTPs. Dates are month/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 of WWTPChloride (mg/L)Flow (m3/s)Mass (t/yr)
  • a

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

Metro2278.6362,000
Blue Lake3871.1814,000
Seneca2801.019,000
Eagle Point3480.182,000
Total 11.0087,000

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 concentrations of chloride in precipitation in the TCMA, the chloride contribution from natural sources was estimated to be around 220 t/yr.

[29] The majority of people within the TCMA watershed boundaries are connected to one of seven WWTPs by sanitary sewers and a majority of the households using a private septic system are located outside the watershed boundaries. If as many as 60,000 people, i.e., the number of people connected to the Eagle Point WWTP, were using septic systems within watershed boundaries the 2000 t of chloride discharged annually from this WWTP would be a reasonable 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 designated agricultural land were used to grow corn, the value would be 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 the 317,000 t of road salt applied annually in the seven-county metropolitan area was applied within the watershed boundaries shown in Figure 1. This translates to a nonpoint source input of 142,000 t/yr of chloride from road salt applications. It was determined that natural deposition, agricultural inputs and septic sewer systems would only contribute an additional 1–3%, depending on the calculation method, to the total chloride load (from point and nonpoint sources). Therefore the loads from these sources were neglected, leaving road salt applications as the only significant nonpoint source of chloride within the watersheds boundaries.

3.3. Metropolitan Area Chloride Balance Calculation

[32] The individual chloride balance components (Mississippi River inflow, Minnesota River inflow, Mississippi River outflow, road salt application, WWTP effluents) presented in the previous sections were combined in a chloride mass balance using equation (1) on a monthly timescale.

[33] Mean monthly mass fluxes (t/month) of chloride entering or exiting the TCMA watershed from the two major rivers and the four WWTPs are illustrated in Figure 5a. Dashed lines in Figure 5a give cumulative contributions by month from WWTP effluents, the Minnesota River and the Mississippi River up to the total “Inflow + WWTP” curve. The “Outflow” curve is from the Mississippi River outflow station in Hastings. Figure 5a is a graphical representation of equation (2), showing the difference between the mass of chloride exiting the watershed (O) and the sum of the amount entering the watershed (I) and the amount added by point sources (Mp). Figure 5b represents the amount of chloride added to the river system from nonpoint sources (mnp in equation (2)). Since road salt is the only significant nonpoint source of chloride inside the watershed Figure 5b also represents the monthly mass of chloride from road salt applications exported by the Mississippi River from the TCMA watershed. The monthly nonpoint source chloride contribution to the Mississippi River outflow was highest during the winter months when road salt was being applied to the roads. The high December to April values can be interpreted as the direct impact of road salt applications and snowmelt water runoff through systems of storm sewers and small streams to the big rivers. Delays occur because winter is a low flow season, and only when snowmelt sets in does the routing processes accelerate. The small contributions in late summer can be attributed to the flushing of chloride from lakes and wetlands [Novotny et al., 2008] or to the delayed transport to the rivers through interflow or groundwater flow [Novotny et al., 1999].

Figure 5.

(a) Monthly chloride fluxes (t/yr) from point sources entering or exiting the Twin Cities metropolitan area watershed. Shaded areas between dashed lines give monthly contributions from four WWTP 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” and the “Inflow + WWTP” (solid lines) from Figure 5a. By virtue of the mass balance in equation (2), the plotted values give the monthly amounts of nonpoint source chloride exported by the river as well as the uncertainties in the mass balance.

[34] In March the difference between the mass of chloride exported and the mass imported had a maximum. Road salt applied in the TCMA watershed accounted for 34% of the chloride passing the Mississippi River outflow station in March, WWTPs contributed 19%, and the inflows from the Mississippi and Minnesota rivers into the TCMA watershed provided 47% (Figure 5a). The total amount of chloride exported 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 control volume (the TCMA watershed) was found to be 33,000 t/yr. It had previously been estimated that 142,000 t of chloride/yr were applied as road salt to the TCMA watershed area. If 33,000 of the 142,000 t were carried away by the Mississippi then 109,000 t or 77% of the chloride applied annually had to stay behind in the TCMA watershed system (S in equations (1) and (2)).

3.4. Subwatershed Chloride Balance Calculation

[35] The average annual chloride concentrations in the 10 small streams within the larger TCMA watershed ranged from 37 mg/L in Carver Creek to 185 mg/L in Shingle Creek; annual average flow rates were from 0.093 m3/s in Riley Creek to 1.685 m3/s in Minnehaha Creek (Table 3). The total watershed area, the percentage covered by impervious surfaces and the mass of road salt applied annually were also determined (Table 3). Chloride application rates in the 10 subwatershed ranged from 0.08 to 0.82 t/ha per year.

Table 3. Small Stream Watershed Information and Road Salt Applications Rates Within Each Watersheda
Creek NameArea (ha)Impervious Surface (%)Annual Average Flow (m3/s)Annual Average [Cl] (mg/L)Total Cl Applied (t/yr)Cl Applied per Area (t/yr ha)Mass Cl Exported (t/yr)Cl Retained (%)
  • a

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

Bassett11,100340.971388,1000.733,60056
Battle3,000320.221472,4000.8090063
Bluff2,300110.11656000.2620067
Carver21,60040.98371,8000.0860067
Credit River13,30090.50441,8000.1440078
Fish1,300270.091006000.4620067
Minnehaha46,100151.696817,7000.382,60085
Nine Mile9,900290.71744,7000.471,20074
Riley3,400180.11526000.1810083
Shingle10,800350.491857,0000.652,60063
Total    45,300 12,50072

[36] A background concentration of 18.6 mg/L (±34.9 mg/L at the 95% confidence interval) was determined using a linear regression analysis between annual average Cl concentrations in the stream (mg/L) and total Cl applied (t/yr) per watershed area (Figure 6, R2 = 0.79). The background concentration of 18.6 mg/L matched with concentrations in the Mississippi River before it entered the TCMA. Retention of chloride in each individual subwatershed was calculated to be from 55% to 83% (Table 3). The total combined mass of chloride applied in the 10 subwatersheds was 45,300 t/yr, which represents 32% of the amount applied in the entire TCMA watershed. The total amount of chloride exported from the 10 subwatersheds was estimated at 12,500 t/yr. This means that only 28% of the salt applied was exported from the 10 subwatersheds each year, and therefore 72% was retained. Because of the large confidence interval around the background concentrations an analysis was conducted by setting the background concentration to 0 mg/L. This represents a scenario where all of the chloride exiting the watershed in a given year is from road salt applications during that year. This analysis resulted in the lowest possible retention rate. If the background concentration was set to 0 mg/L the amount of road salt exiting the watershed would be raised to 15,900 t/yr reducing the retention rate to 65%. The retention estimates of 72% and 65% are lower than the 77% value obtained for the entire TCMA watershed, but comparable.

Figure 6.

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

3.5. Sodium Retention

[37] The retention of chloride from road salt applications in a watershed encompassing the Twin Cities metropolitan area was determined to be around 77%. While an analysis was not conducted on the other ion in rock salt, sodium, because of its slightly less conservative behavior a value equal to or higher than 77% would be expected. Sodium interacts more readily with soils through ion exchange allowing for the possibility that higher amounts could be stored in the soil column [Amrhein et al., 1992; Mason et al., 1999].

3.6. Sensitivity of the Results

[38] The assessment of chloride (road salt) retention was made with the best information available, however assumptions, which had to be made, do influence the results obtained. The sensitivity of the findings to three assumptions and procedures used was therefore investigated: the sampling frequency of the grab samples at the inflow and outflow stations, the method of calculation for chloride export from the watershed, and the estimation method for the nonpoint sources of chloride.

3.6.1. Sampling Frequency

[39] An analysis was conducted to determine if continuous monitoring or more frequent sampling was needed to capture chloride concentrations in snowmelt events of short duration at the inflow and out flow stations on the Mississippi and Minnesota rivers [Ruth, 2003]. To test the sensitivity to sampling frequency, records of daily average specific conductance in the Mississippi River near Hastings (outflow station) were used from a continuous monitoring station maintained by the Metropolitan Council (Figure 7a). Although a direct relationship between chloride and specific conductance could not be obtained, because of the dampening of the chloride signal in relation to other ions from the high flow rates, snowmelt events could be clearly detected by fluctuations in specific conductance during the winter. A Comparison was conducted between the continuous time series of daily specific conductance values and the specific conductance and chloride values recorded on the grab sample dates. This analysis provided evidence that a suitable representation of the chloride dynamics was obtained with the biweekly sampling frequency used (Figure 7a). Furthermore, the cumulative distributions of specific conductance values obtained from the continuous daily time series and the values on the days when chloride grab samples were taken were virtually identical (Figure 7b). It was concluded that the sampling frequency used was adequate to estimate the annual load of Cl (salt) exiting or entering the TCMA watershed over the study period.

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 grab samples (solid circles). Data are from the Mississippi River outflow station in Hastings. (b) Cumulative normalized distribution functions of daily specific conductance values. Specific conductance values only for days when a chloride grab sample was taken (solid line) and all daily values from Figure 7a (dashed line).

3.6.2. Chloride Export

[40] Other pathways of chloride export such as airborne transport of salt particles, or chlorination of natural organic matter were not analyzed. Salt particles can become airborne behind vehicles and during strong winds, however they are typically deposited within 100 m of the road [Blomqvist and Johansson, 1999]. Cl has also been found to interact with natural organic matter forming chlorinated organic matter when transported through soils [Bastviken et al., 2007]. It is therefore possible that some of the Cl from road salt applied in the TCMA was exported in the Mississippi River while attached to organic matter and is thus not accounted for. Our budget analysis did not include airborne or organic matter transport mechanisms. If incorrect, this assumption would cause an overestimation of chloride retention in the watershed. If as much as 10% of the road salt applied in the TCMA were exported by alternative and not included transport mechanisms, the total export would rise to 47,800 t/yr from 33,000 t/yr, and the road salt retention estimate for the TCMA would be lowered to 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 residences not connected to a WWTP or fertilizer applications, were assumed negligible. If an additional 3% (4500 t) of chloride were added to the system by outside processes, the estimated Cl retention in the watershed would increase slightly.

[42] The 24% commercial road salt application rate was taken from market share information provided by the American Salt Institute for the entire United. While this value is based on national statistics it is the best estimate for a large metropolitan area for a number of reasons. The diversity in area including suburban, urban and rural land uses and the size of the watershed studied allow for a comparison with the national scale. National trends in municipal salt purchases match Minnesota trends in rock salt purchases [Sander et al., 2007]. Finally, obtaining information from commercial road salt appliers and estimating application rates or areas where road salt was applied commercially is very difficult. The results from this analysis would likely include more error than accurate national sales data. If the 24% commercial road salt application rate were lowered to 10%, a value reported in a Canadian study [Environment Canada Health Canada, 1999], the chloride contribution from road salt applications would be reduced to 122,000 t/yr, the water-borne export rate from the TCMA would be raised to 27%, and the retention rate in the watershed would drop to 73%.

[43] If all effects reducing the retention rate were combined (commercial application rate reduced from 24 to 10% of total application rate; increased export of 10% of the applied amount) the application rate would be lowered to 122,000 t/yr, and the export rate would be raised to 33,000 + 12,200 = 45,200 t/yr. The total export rate would then be 37% and the retention rate 63%. In other words, even with extremely favorable assumptions for road salt flushing from the TCMA the estimated retention rate remains high.

3.7. Comparison to Other Metropolitan Areas

[44] Other studies have shown significant retention of chlorides in urban watersheds, but the estimated retention percentages vary significantly. Most studies were conducted in small, urbanized watersheds ranging in size from 104 to 435 km2. Retention percentages were found to be 55% in a stream watershed in the greater Toronto, Canada area [Howard and Hayes, 1993], 50–65% in Helsinki, Finland [Ruth, 2003], 28–45% in Chicago, Illinois [Wulkowicz and Saleem, 1974], 59% in Rochester, New York [Bubeck et al., 1971], and 35% in Boston, Massachusetts [Huling and Hollocher, 1972].

[45] In Toronto, Ontario, Canada, the accumulation of salt in the watershed due to deicing practices has seriously compromised the shallow aquifers [Howard and Maier, 2007]. In Waterloo, Ontario chloride concentrations in the aquifers had not reached equilibrium after 57 years of road salt applications. It was estimated that on the order of 100 years will be required to reach equilibrium concentrations under current conditions [Bester et al., 2006]. In shallow aquifers near Chicago, Illinois, chloride concentrations have increased since 1960; 24% of the wells studied in the 1990s had concentrations above 100 mg/L, when median values before 1960 were less than 10 mg/L, and 15% of the wells had rate increases greater than 4 mg/L per year [Kelly, 2008]. The accumulation of salt in shallow groundwater also affects base flow concentrations in streams [Marsalek, 2003]. Baseline salinity in urban streams and streams near roadways have been increasing in the northeastern 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 TCMA Watershed

[46] Evidence of significant chloride retention within the TCMA watershed is provided in streams lakes and aquifers. Four streams (Minnehaha Creek, Battle Creek, Shingle Creek and Nine Mile Creek) in the TCMA are on the MPCA's 2008 list of “impaired waters” for chloride (Data are available 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 and other small streams of the TCMA during the winter months (Figure 8).

Figure 8.

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

[47] Volume-weighted average chloride concentrations in 38 lakes of the TCMA have increased from 1984 to 2005 by an average of 1.5 mg/L per year (range of 0.1 to 15 mg/L per year), following a pattern similar to the mass of road salt purchased by the state of Minnesota over that same time period [Novotny et al., 2008]. Median concentrations, in the surface waters of the 38 lakes from 2001 to 2005, of 87 mg/L (range 31 to 505 mg/L) [Novotny et al., 2008] were much higher than the estimated presettlement concentrations of 3 mg/L [Ramstack et al., 2004], In urban lakes of the TCMA chloride concentrations tend to fluctuate seasonally, with maximum in February or March and minimum in October or November. In the long-term, a mean annual equilibrium concentration is reached when all of the chloride added to a lake during the winter season is flushed out during the summer and fall season. There was an indication that smaller lakes with high summer flushing rates have already reached equilibrium, while larger lakes and lakes with low flushing rates can be expected to have rising mean annual chloride concentrations for years to come, if current salt application rates continue. If salt applications were stopped completely, the recovery of many urban lakes would take from 10 to 30 years (E. V. Novotny and H. G. Stefan, Model of present and future chloride concentrations in lakes of the Twin Cities metropolitan 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 substantially with land use. Median chloride values of 46 mg/L were found in urban areas, 17 mg/L in agricultural areas, and 1.2 mg/L in forested area [Fong, 2000]; 3% of the water samples taken from wells in the TCMA were found to exceed the USEPA secondary chloride standard of 250 mg/L [Fong, 2000]. In a cross section of the surficial aquifer in a northwestern suburb of Minneapolis directly down gradient from a high-traffic roadway, chloride concentrations ranged from 200 mg/L at the water table 3 m below the soil surface, to 590 mg/L at a depth of 13.5 m below the soil surface [Andrews et al., 2005]. Concentrations of 380–470 mg/L were also measured down gradient from a major Interstate Highway (I-94) in summer and late fall pointing toward a long-term storage of road salt in the surficial aquifer [Andrews et al., 2005]. Data collected by the Minnesota Pollution Control Agency (MPCA) throughout the seven-county TCMA showed that the highest chloride concentrations in groundwater, up to 2000 mg/L, were found in shallow wells (Figure 9).

Figure 9.

Chloride concentration in wells located throughout the Twin Cities metropolitan area in 2004 and 2005. Information was reported by the Minnesota Pollution Control Agency.

[49] Elevated chloride concentrations in aquifers may be delayed because of storage in the subsurface [Bester et al., 2006; Kelly et al., 2008]. Shallow wells respond first resulting in some to have already reached concentrations above state standards. The storage potential of the TCMA aquifer system is very large. If road salt applications were completely stopped today, chloride concentrations in deep wells may continue to increases for many years until subsurface saline transport has reached equilibrium [Bester et al., 2006; Kelly, 2008]. Residence times in the aquifers can be high ranging from tens to hundreds of years in the upper and surficial aquifers to thousands of years in the deep aquifers. Residence times in lakes are smaller, on the order of 3 to 14 years, but still provide a means of chloride storage (Novotny and Stefan, submitted manuscript, 2009). The TCMA has hundreds of lakes and wetlands, and hundreds 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 into these systems. Although very useful in storm water management, these practices could be adding to the accumulation of road salt in the watershed by promoting retention of surface runoff and/or infiltration of the contaminated water into the groundwater.

4. Summary and Conclusions

[50] Road salt is used to increase driving safety in the Twin Cities metropolitan area (TCMA) of Minnesota. A chloride budget for the TCMA watershed (Figure 1) with data from 2000 to 2007 revealed the final destinations of the road salt after it was dissolved in the snowmelt water. The TCMA watershed analyzed covered an area of 4150 km2 with a population of 2.8 million. According to the annual chloride budget 235,000 t of chloride entered the TCMA annually in the Mississippi and Minnesota rivers, and 355,000 t exited through the Mississippi River resulting in 120,000 t being added to the Mississippi and Minnesota rivers as they traveled through the TCMA. Of the 120,000 t of chloride added annually 87,000 t came from the four WWTPs (point sources) and 33,000 t came from road salt (nonpoint source). Of the 142,000 t of chloride applied annually in the TCMA as road salt, only 23% (33,000 t) were exported through the Mississippi River and 109,000 t or 77% were retained in the TCMA watershed. Chloride budgets for 10 subwatersheds within the TCMA analyzed in a 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's 2008 list of Cl impaired waters, 38 lakes in the TCMA had a rising mean annual Cl concentration over a 22 year period [Novotny et al., 2008], and elevated Cl concentrations in groundwater have been measured [Andrews et al., 2005; Fong, 2000]. Chloride retention in urban areas where road salt (NaCl) is applied should cause much concern. Mitigation measures, best management practices (BMPs) for road salt application and alternatives to NaCl need to be examined.

Acknowledgments

[52] We acknowledge and thank the following individuals 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 for this research; the Technical Advisory Panel, lead by Wayne Sandberg of Washington County, for input and suggestions to our research; Karen Jensen of the Metropolitan Council Environmental Services, the Metropolitan 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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