Environ. Sci. Technol. 2009, 43, 40–46
Aquatic Toxicity of Airfield-Pavement Deicer Materials and Implications for Airport Runoff S T E V E N R . C O R S I , * ,† S T E V E N W . G E I S , ‡ GEORGE BOWMAN,‡ GREG G. FAILEY,§ AND TROY D. RUTTER† U.S. Geological Survey Wisconsin Water Science Center, Middleton, Wisconsin 53562, Wisconsin State Laboratory of Hygiene, Madison, Wisconsin 53718, and General Mitchell International Airport, Milwaukee, Wisconsin 53207
Received June 26, 2008. Revised manuscript received October 6, 2008. Accepted October 31, 2008.
Concentrations of airfield-pavement deicer materials (PDM) in a study of airport runoff often exceeded levels of concern regarding aquatic toxicity. Toxicity tests on Vibrio fischeri, Pimephales promelas, Ceriodaphnia dubia, and Pseudokirchneriella subcapitata (commonly known as Selenastrum capricornutum) were performed with potassium acetate (KAc) PDM, sodium formate (Na-For) PDM, and with freezingpoint depressants (K-Ac and Na-For). Results indicate that toxicity in PDM is driven by the freezing-point depressants in all tests except the Vibrio fisheri test for Na-For PDM which is influenced by an additive. Acute toxicity end points for different organisms ranged from 298 to 6560 mg/L (as acetate) for K-Ac PDM and from 1780 to 4130 mg/L (as formate) for NaFor PDM. Chronic toxicity end points ranged from 19.9 to 336 mg/L (as acetate) for K-Ac PDM and from 584 to 1670 mg/L (as formate) for Na-For PDM. Sample results from outfalls at General Mitchell International Airport in Milwaukee, WI (GMIA) indicated that 40% of samples had concentrations greater than the aquatic-life benchmark for K-Ac PDM. K-Ac has replaced urea during the 1990s as the most widely used PDM at GMIA and in the United States. Results of ammonia samples from airport outfalls during periods when urea-based PDM was used at GMIA indicated that 41% of samples had concentrations exceeding the U.S. Environmental Protection Agency (USEPA) 1-h water-quality criterion. The USEPA 1-h water-quality criterion for chloride was exceeded in 68% of samples collected in the receiving stream, a result of road-salt runoff from urban influence near the airport. Results demonstrate that PDM must be considered to comprehensively evaluate the impact of chemical deicers on aquatic toxicity in water containing airport runoff.
Introduction During periods of freezing precipitation, airports must clear snow and ice from runways, taxiways, and other paved surfaces to continue operations. Snow plows and rotary snow brooms are used to remove loose snow and ice from * Corresponding author phone: (608) 821-3835; fax: (608) 8213817; email:
[email protected]. † U.S. Geological Survey. ‡ Wisconsin State Laboratory of Hygiene. § General Mitchell International Airport. 40
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pavement, but when physical removal is not sufficient, the use of chemical pavement deicing and anti-icing material (PDM) is necessary. The primary impact to aquatic systems resulting from airport runoff containing PDM includes potential depressed oxygen due to elevated biochemical oxygen demand (BOD) and aquatic toxicity (1, 2). PDM consists primarily of freezing-point depressants (FPD) and water but also contains lesser concentrations of various additives such as corrosion inhibitors and anticaking chemicals. Current FPDs used in liquid PDM include potassium acetate- (K-Ac), potassium formate-, propylene glycol-, and ethylene glycol-based fluids. FPDs used in solid PDM include sodium acetate and sodium formate (Na-For). Urea PDM formulations are available in liquid form mixed with ethylene or propylene glycol and in solid (granular) form. In addition to chemical PDM, sand is used for added surface friction at many airports. Direct application of liquid PDM helps to melt existing snow and ice and reduce adhesion to pavement, thereby enhancing effectiveness of physical removal systems. Prewetting of pavement surfaces with liquid PDM is conducted in advance of some freezing precipitation events to prevent adhesion to pavement, facilitating physical removal of snow and ice. Solid PDM is applied directly to penetrate existing ice, followed by application of liquid PDM to reduce adhesion of ice to paved surfaces. In addition, ice and packed snow warrant application of a mixture of sand and solid PDM to improve traction. Application rates of these chemical deicers are recommended by PDM manufacturers, with the greatest application rates recommended at intersections and high traffic areas. Exact application methods can vary depending on the airport and environmental conditions. One commonly used system to reduce total usage is to apply PDM across the entire width of runways but only down the center of taxiways. Fate and transport of spent PDM around the airfield is affected by such factors as management of deicer-contaminated stormwater, plowing to snowbanks, wind drift, seepage through pavement joints, seepage into pervious areas, tracking by aircraft and ground-support vehicles, drainage into receiving surface- and groundwater systems, and eventual degradation. Because PDM is applied across a large area throughout airports, containment of PDM into a deicermanagement system is difficult and costly. For this reason, many deicer-runoff-management systems in place at airports focus on aircraft deicing and anti-icing application areas. Although some applied PDM that is not collected through runoff management degrades near the point of application (3), the remainder is presumably discharged to soils, groundwater, and surface water systems near airports. Field studies of airport deicer impact on aquatic systems have primarily been focused on aircraft deicers rather than PDM. The limited field data that are available on environmental impact of PDM has primarily focused on urea deicers because of the dominance of urea in this market before the mid-1990s. Although urea is still used at many airports, its usage has decreased while acetate- and formate-based PDM have gained popularity. Environmental impact was a factor in this change due to the BOD of urea and aquatic toxicity of ammonia, a degradation product of urea (1). Studies of urea-based PDM in airport runoff at two different airports concluded that use of urea-based deicers had adverse impacts on receiving water. Researchers from the United Kingdom reported a mean concentration of 106 mg/L ammonia in airport surface runoff during seven “freezing” sampling periods as a result of urea degradation with resulting toxic effects on tested organisms in the 10.1021/es8017732 CCC: $40.75
2009 American Chemical Society
Published on Web 12/03/2008
receiving stream (4). Concentrations of ammonia-nitrogen as high as 53.3 mg/L from breakdown of urea and BOD5 concentrations as high as 942 mg/L from urea and glycols were reported in airport-runoff samples from an airport in western Pennsylvania (5). In receiving streams, ammonianitrogen concentrations were detected up to 5.62 mg/L, and BOD concentrations were detected up to 355 mg/L. Resulting conditions included dense biological growth on the streambed due to stimulation by the organic-waste load from deicers and a stressed community of aquatic organisms that were dominated by pollution-tolerant species. In comparison to USEPA water-quality criteria (6), the above-cited literature indicates that ammonia toxicity could sometimes be of concern with regard to aquatic toxicity in runoff from airports where urea deicers are in use. Information regarding measured levels of acetate- and formate-based deicers in airport runoff and subsequent field evaluation of environmental impact could not be found for review. Water from airport-deicer-recovery storage facilities was sampled by USEPA in 1999 that included one sampling period at each of four individual airports. Samples were collected from paved airport surfaces during deicing activities to be treated or recycled for glycol content (1). This sampling was focused more toward ethylene glycol- and propylene glycol-based aircraft deicing fluids than PDM; however, potassium was detected at up to approximately 60 mg/L in these samples (1). Acute aquatic toxicity end points (EC50) for K-Ac have been reported for Daphnia similis between 1050 and 1210 mg/L, and chronic effects of reduced reproduction in C. dubia were reported at 600 mg/L, which was the lowest concentration tested (7). The range of acute and chronic end points for formulated K-Ac roadway deicer products from two previous studies ranged from less than 25 mg/L for the three-brood C. dubia IC25 to 3535 mg/L for the 96-h IC50 for Pseudokirchneriella subcapitata (referred to as Selenastrum capricornutum) (8, 9). Other available PDM products such as glycol-based fluids are known to have impacts on BOD, but it is not possible to distinguish glycol from these PDM materials from glycol originating from aircraft deicers in airport-runoff samples. Information regarding glycol-based PDM in airport runoff and field evaluation of environmental impact due to PDM as opposed to aircraft deicers could not be found for review. Because a change in PDM used at airports occurred in the mid-1990s and environmental impacts of PDM products currently used at airports have not been thoroughly evaluated, this study was undertaken to better understand the potential toxicity levels of PDM, to investigate the source of observed toxicity, and to provide a comparison of toxicity benchmark concentrations to concentrations measured in airport runoff.
Methods Airport Operations. PDM were applied as needed at General Mitchell International Airport (GMIA) in Milwaukee, WI, to clear airport surfaces of snow and ice on 79 ha of pavement, including 5390 m of runway and 22 ha of terminal ramp area. Urea PDM was applied at GMIA through the middle of the 1998-99 deicing season, at which time its use was phased out and it was replaced by acetate- and formate-based PDM. K-Ac (liquid) and Na-For (solid) PDM are the two PDMs used at GMIA since 1998. Although PDM usage at GMIA has not historically been tracked, deliveries of K-Ac PDM to this airport for the 2005-06 season totaled 888 000 L and deliveries for the 2006-07 season totaled 788 000 L. Formulated K-Ac and Na-For PDM samples were collected on site directly from storage at GMIA in March 2005. Potassium formate (99.5%) and sodium acetate (99.8%) standards were purchased from Fischer Scientific, Fair Lawn, NJ. Surface-Water Sampling. Water samples were collected at four sites near GMIA from 1996 through 2006. A 2.31-km2
headwaters region of Wilson Park Creek was sampled to provide water quality data for this section of the stream, which primarily drains an urban area (residential and commercial) in addition to a small portion of runway and a U.S. National Guard facility at GMIA (Figure 1). There is potential for a small amount of airport (and deicer) runoff to reach the upstream reference site, but most deicer runoff enters Wilson Park Creek downstream from this site. The two airport outfall sites that contribute flow to Wilson Park Creek were sampled to characterize runoff from the airport. The primary outfall site (Figure 1) includes flow from the upstream site combined with flow from storm sewers that drain the terminal area as well as some taxiways and runways. The secondary outfall (Figure 1) drains a small area of the airport where most air-cargo activities take place. This outfall enters Holmes Ave. Creek, which subsequently enters Wilson Park Creek approximately 0.8 km downstream from the primary outfall. The drainage area of the primary outfall is 5.83 km2 (3.52 km2 within the airport), and the drainage area of the secondary outfall is 0.08 km2. The fourth sampling site, referred to as the “receiving stream site”, is 5.54 km downstream from the airport, and is used to characterize water-quality effects in the receiving stream. The stream between the airport and the monitoring site alternates between a concrete-lined and an earthen channel bottom. The drainage area of Wilson Park Creek at this point is 14 km2. Immediately beyond this site, the stream drains into the Kinnickinnic River, which flows for 4.33 km to the Kinnickinnic River Estuary and eventually to Milwaukee Harbor. Average flow at the four monitoring locations from December through March is 17 L/s at the upstream site, 62 L/s at the primary outfall, 1.3 L/s at the secondary outfall, and 309 L/s at the receiving stream site. Airport personnel were contacted to determine when significant deicing was occurring and, thus, when samples should be collected. Precipitation data were collected by the National Weather Service on airport property. Surface-waterquality sampling was conducted during 49 cold-weather periods, including 15 dry-weather periods and 34 runoff periods with freezing precipitation that required application of deicers. Urea-based PDM was in use during 30 of these 49 sampling periods, and acetate- and formate-based PDM was in use for 19 of them. Dry-weather samples were collected as grab samples by submerging the sample bottles directly into the water, immediately followed by appropriate preservation through acidification or filtration. For sampling during runoff periods, specific details of the sampling protocol used to collect and process water samples are outlined in Corsi et al. (10). Briefly, flow-weighted composite samples were collected at each site using refrigerated automatic samplers and Teflon-lined polyethylene sample tubing (model 3700R, Isco Industries, Lincoln, NE). Samples were subsequently split for separate chemical analyses, iced, and delivered to the laboratory within 24 h. Constituents reported in this paper include acetate, formate, and potassium. Constituents referenced in the Supporting Information for this paper include sodium, ammonia-nitrogen, propylene glycol, ethylene glycol, and chloride. Stream-water level was measured every 5 min during periods of increased runoff and every hour during other periods using bubbler-type pressure transducers (Sutron Accubar, Sutron Corporation, Sterling, VA). Flow was computed using a log-log relation between water level and flow (11). Chemical Analysis. All chemical analyses were conducted at the Wisconsin State Laboratory of Hygiene. Acetate and formate were analyzed using methods as defined in the manual for the DIONEX AS15 separator column (DIONEX, Sunnyvale, CA). Instrument conditions are as follows: direct injection of 10 µL of water, use of a DIONEX AS15 separator VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Location of field study area and monitoring locations. column and AG15 guard column, gradient separation using potassium hydroxide eluent delivered using a DIONEX EG40 eluent generator, and suppressed conductivity detection using electrolytic suppressor. Reporting levels were determined as the concentrations below which spike recoveries exceed 25% uncertainty (5 mg/L for acetate and 2.5 mg/L for formate). Standardized methods used for analysis of potassium, sodium, propylene glycol, ammonia-nitrogen, and chloride and resulting quality-control data for chemical analyses are summarized in the Supporting Information (Table S1). Toxicity Tests. Static renewal toxicity tests were conducted on formulated K-Ac- and Na-For-based PDM products and pure samples of the respective FPDs K-Ac and Na-For. These tests were conducted between March 2005 and September 2006 for determination of acute- and chronic-end point concentrations. Test organisms used in acute tests included the marine bacterium Vibrio fischeri used in the Microtox test (15-min EC50), Pimephales promelas or fathead minnow (96-h LC50), and Ceriodaphnia dubia (48-h LC50). Test 42
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organisms used in chronic tests included Pimephales promelas (7-d IC25), Ceriodaphnia dubia (IC25), and the green alga Pseudokirchneriella subcapitata (commonly known as Selenastrum capricornutum) (96-h IC25). Initial range-finding tests were conducted to approximate the appropriate toxic concentration for each test organism. PDM concentrations are reported as nominal (concentrations were calculated from dilutions). All tests were conducted in accordance to standard US EPA protocol (12, 13). Details of the toxicity-testing methods have been previously reported (14) and are included in the Supporting Information. Toxicity tests were conducted at the Wisconsin State Laboratory of Hygiene in Madison, Wisconsin.
Results and Discussion Toxicity of PDM Formulations and Freezing-Point Depressants. Test results indicate that toxicity end points for formulated K-Ac and Na-For PDM are similar to those of the FPD contained in the PDM (Table 1). This result indicates that toxicity of PDM is driven primarily by the acetate- and
TABLE 1. Toxicity-Test Results for Formulated Pavement Deicers and Freezing-Point Depressants (all units are expressed in mg/L) acute-end point concentration (95% confidence interval) test material
Microtox (EC50)
potassium acetate deicer 6560 (as acetate)a,b (3990-10800) potassium acetate 6440 (as acetate)a (5900-7220) sodium formate deicer 1780 (as formate)c (1450-2140) sodium formate 23000 (as formate) (22300-23600)
chronic-end point concentraion (95% confidence interval)
P. promelas (LC50) C. dubia (LC50) P. promelas (IC25) C. dubia (IC25) P. subcapitatad (IC25) 298
421
336
54.5
19.9
(262-340)
(355-499)
(173-496)
(41.3-121)
(17.8-26.0)
421
313
324
43.0
28.6
(361-499)
(259-379)
(172-444)
(30.8-78.9)
(28.2-29.1)
4130
1860
1200
584
1670
(3700-4620)
(1600-2170)
(856-1790)
(506-707)
(1390-1,820)
2300
1400
1190
713
2300
(1980-2660)
(1140-1730)
(452-1330)
(624-775)
(2090-2490)
a
Multiply by 0.66 for toxicity end point expressed as potassium concentration. b Multiply by 3.32 for toxicity end point expressed as original K-Ac deicer product. c Multiply by 1.54 for toxicity end point expressed as original Na-For deicer product. d Commonly known as Selenastrum capricornutum.
formate- based FPDs rather than additives. The Microtox test result from the Na-For PDM is an exception to this pattern because it resulted in a lower end point concentration from the formulated deicer than from the FPD. This result indicates increased sensitivity of the organism due to an additive component, such as a corrosion inhibitor or anticaking agent, in the Na-For PDM. End point values in Table 1 are presented as concentration of acetate or formate rather than concentration of the entire material tested to provide direct comparison to parameters monitored in airport runoff. Product literature indicates that the K-Ac-based PDM is 50% K-Ac, 50% water, and
