Environ. Sci. Technol. 2005, 39, 5095-5100
Use of Potassium Formate in Road Winter Deicing Can Reduce Groundwater Deterioration
TABLE 1. Annual Amounts of Chloride Salts (NaCl and CaCl2) (106 kg) Used in Road Winter Maintenance in Japan, North America, and 12 European Countries Practicing Significant Winter Road Deicing
P A S I P . H E L L S T EÄ N , J A N I M . S A L M I N E N , * KIRSTEN S. JØRGENSEN, AND T A I N A H . N Y S T EÄ N Finnish Environment Institute, P.O. Box 140, FI-00251 Helsinki, Finland
We present here an aquifer scale study on the fate of potassium formate, an alternative, weakly corrosive deicing agent in soil and subsurfaces. Potassium formate was used to deice a stretch of a highway in Finland. The fate of the formate was examined by monitoring the groundwater chemistry in the underlying aquifer of which a conceptual model was constructed. In addition, we determined aerobic and anaerobic biodegradation rates of formate at low temperatures (-2 to +6 °C) in soil microcosms. Our results show that the formate did not enter the saturated zone through the thin vadose zone; thus, no undesirable changes in the groundwater chemistry were observed. Furthermore, the conceptual model explained the distribution of chloride in the aquifer used in deicing for the past 30 years. We recorded mineralization potential up to 97% and up to 17% within 24 h under aerobic and anaerobic conditions, respectively, in the soil and subsurface samples obtained from the site. This demonstrates that biodegradation in the topsoil layers was responsible for the removal of the formate. We conclude that the use of potassium formate can potentially help diminish the negative impacts of road winter deicing on groundwater without jeopardizing traffic safety.
Introduction Sodium chloride (NaCl) has been extensively used as a deicing agent in winter road maintenance for over 50 years in the northern hemisphere of the world. In recent years, the annual amount of chloride salts used in Japan, the U.S., and Canada and in those European countries practicing significant road winter deicing has ranged from approximately 50 000 tons to 15 million tons (Table 1). The use of NaCl has, however, major disadvantages in terms of (i) groundwater quality (1, 2), (ii) drinking water quality (1, 3), (iii) soil quality (4), (iv) vegetation (5), and (v) corrosion both of vehicles and of infrastructure such as bridges (3, 6, 7). In Finland, chloride concentrations in many aquifers affected by road deicing have steadily increased since the early 1970s (8), which has led to a corresponding increase in the corrosiveness of the typically very soft groundwater found in Finland. Similarly, Pilon and Howard (9) demonstrated an accumulation of deicing chemicals in subsurface waters in Toronto and Environment Canada (10) published an extensive review demonstrating a remarkable effect of the inorganic deicing chemicals on various environmental matrixes in Canada. * Corresponding author phone: (358) 9 40 300 800; fax: (358) 9 40 300 890; e-mail:
[email protected]. 10.1021/es0482738 CCC: $30.25 Published on Web 05/28/2005
2005 American Chemical Society
a
country
NaCl and CaCl2 used (106 kg year-1)
Japan (20)a Austria (20, 21) Belgium (20) Denmark (21) Estonia (20) Finland (20, 21) Germany (20, 21) Italy (21) The Netherlands (21) Norway (20) Sweden (20, 21) Switzerland (21) UK (21) Canada (20) U.S.A. (22)
92 150 140 400 50 100 300 500 350 70 180 130 2000 5000 15000
The references are indicated.
While the overall environmental and economic consequences of road winter deicing using NaCl are currently not fully recognized, the European Union has directed that any significant and sustained upward trend in the concentration of any pollutant in groundwater should be identified and reversed by the year 2015 (11). Furthermore, under Sections 64(a) and 64(b) of the Canadian Environmental Protection Act, all road deicing chemicals containing inorganic chloride salts are defined as toxic (10). Thus, it is becoming more widely accepted that while road safety shall not be jeopardized, actions have to be taken to reduce the disadvantages caused by the extensive use of chloride salts in winter road maintenance. Consequently, a more carefully reasoned and cautious use of chloride salt-based deicers has been advised. In addition, alternative deicing agents, such as calcium magnesium acetate, potassium acetate, sodium formate, and potassium formate, have been proposed (5, 7, 12-15). No aquifer scale studies on the applicability, fate, and groundwater effects of potassium formate or other alternative deicers have been carried out. However, previous studies have shown that sodium formate is weakly corrosive (15) and formate salts are thus applied to deice airport runways in, for example, Scandinavia. Furthermore, our earlier pilot scale study using sand and gravel columns showed that potassium formate is advantageous over potassium acetate and calcium magnesium acetate in terms of oxygen consumption and heavy metal leachability in soil (16). It should also be noted that there is no drinking water guideline limit, for example, in the European Union and in Canada (11, 17), regarding the effects of potassium on human health, as there is for sodium. In this paper, we report the results of a field experiment that was performed at an aquifer scale at the Kauriansalmi aquifer in Finland during the winter of 2002-2003. Potassium formate was the sole deicing agent used, with a total of 4100 kg of the chemical being applied to deice a stretch of highway. The fate of potassium formate in the subsurface and its effect on groundwater chemistry were together monitored in the groundwater wells installed in the area. Also, a conceptual model of the aquifer was constructed based on drilling and geophysical data to elucidate the groundwater flow conditions and the potential formate transportation in the saturated zone. In addition, the aerobic and anaerobic biodegradation and mineralization rates of formate at low temperatures were VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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equipped with 100 and 200 MHz antennas. Then, using the Surfer-software, the geological data collected were employed to build a three-dimensional conceptual model of the aquifer. Application of Potassium Formate at the Study Site. Potassium formate was applied during the period from October 6, 2002 to April 6, 2003 for 60 days. In total, 6.1 m3 of a solution containing 50% (v/v) of potassium formate (free of additives) in water was distributed on a 0.76 km stretch of the road transecting the Kauriansalmi aquifer. The total amount of potassium formate applied (4100 kg) corresponds to 5.4 kg of potassium formate per road meter. The regional road administration of southeastern Finland and a local subcontractor were together responsible for the application of potassium formate, the road winter maintenance, and the friction testing of the road pavement at the study site. Groundwater Sampling, Monitoring, and Analyses. Groundwater samples were obtained from the eight monitoring wells (M1-M8), a municipal groundwater production well (M-OT), and two privately owned concrete ring wells (M-SE and M-KS) located at the study site (Figure 1). Both wells M-SE and M-KS were 120 cm in diameter and were 4.2 and 4.9 m deep, respectively. From all wells, hydraulic heads were monitored during 2002 and 2003 using an electronic water level meter (Solinst Water Level Meter Model 101). Prior to groundwater sampling, multi-level measurements at 1 m intervals were performed using a field meter (YSI 556 MPS) to measure groundwater temperature, electrical conductivity, pH, O2 concentration, and redox potential. Thereafter, the monitoring wells were purged for 30 min, and the groundwater samples were obtained with a foot valve pump (Waterra, PowerPack PP1), equipped with a gasoline driven engine (Honda GX). From the private concrete ring wells, groundwater samples were obtained with a bailer sampler. From the production well, groundwater samples were taken from an outlet pipe. In the laboratory, the groundwater samples were analyzed for chloride, total organic carbon (TOC), alkalinity (HCO3-), iron, potassium, sodium, and calcium using standard methods. In addition, the formate concentration in the samples was determined by using a high performance liquid chromatograph (Waters 501) equipped with an ion exclusion column (Waters IC-Pak 7 µm, 7.8 × 150 mm) and a photodiode array detector (Waters 996) with a quantification wavelength of 210 nm. The eluent used was 10 mN H2SO4 with a flow rate of 1.0 mL min-1. Soil Samples. Soil samples were obtained with a spade from various depths (Table 2) at points B, C, and E (Figure 1). The samples were sieved (8 mm) and analyzed for water and organic matter content as previously described (18). For the determination of total nitrogen and phosphorus, the samples were sieved (2 mm), dried at 30 °C, and digested (2020 Digestor, Foss Tecator) with concentrated sulfuric acid. Then, the total phosphorus and total nitrogen contents were determined spectrophotometrically (Hitachi U-2000) and titrimetrically (Kjeltec system 1026 Distilling Unit, Tecator Inc.), respectively, using standard methods. Aerobic Mineralization of 14C-Formate in Soil Samples. 14C-Formate mineralization under aerobic conditions was
FIGURE 1. Groundwater monitoring wells (M1-M8, M-OT, M-KS, and M-SE) and soil and subsurface sampling points (B, C, and E) at the Kauriansalmi study site. Highway 13 and the local gravel road are indicated by an arrow, and the study area is encircled with a gray line. obtained in laboratory microcosms by using soil and subsurface samples obtained from the study site.
Experimental Procedures Site Description. The Kauriansalmi aquifer is located in southeastern Finland (N 68° 6′, E 35° 22′). The aquifer is small with a total area of 0.28 km2 and recharge area of approximately 0.2 km2 and consists of sandy and gravelly deposits with occasional thin layers of silt. The elevation of the bedrock is from 0 to 15 m from the ground surface, and the groundwater table lies between 1 and 5 m below the ground surface. The hydraulic gradient of the aquifer and the hydraulic conductivity of the overburden are high, thereby enabling the detection of the possible changes in the groundwater chemistry caused by the utilization of potassium formate without a major time lag. Moreover, the study area is transected by Highway 13 (Figure 1), and deicing using NaCl has been performed at the site for approximately 30 years prior to this study. Also, CaCl2 is applied for dust binding during the spring and summer seasons (1200 kg km-1) on the gravel road on the western part of the study area (Figure 1). Characterization of the Subsurface Geology, Bedrock Topography, and Hydrogeology. To determine the soil properties, groundwater chemistry, and stratigraphy of the aquifer, bore holes were first drilled by an engine-driven drill (Gerox, Unimoc) down to the bedrock surface. Eight groundwater-monitoring wells (PEH with i.d./o.d. of 51/63 mm) were then installed in the drilling holes. The well screen extended from the surface of the bedrock up to the groundwater level to ensure that changes in the geochemistry of the relatively shallow saturated zone (range: 2-13 m) would be observed. To estimate the depth of the surface of the bedrock, ground penetrating radar soundings were performed by RAMAC/GPR ground radar (Malå GeoScience)
TABLE 2. Organic Matter Content, Total Nitrogen (Ntot), Total Phosphorus (Ptot), and Aerobic and Anaerobic Biodegradation Rates of Added Formate in Soil and Subsurface Samples Obtained from the Sampling Points B, C, and E at the Kauriansalmi Study Sitea sampling point and depth
Ntot g kg-1 dwt
Ptot g kg-1 dwt
organic matter content % of dry weight
B 70-80b cm C 5-15c cm C 50-60b cm E 100-110b cm
