Published online 9 August 2005
Published in J Environ Qual 34:1665-1671 (2005)
DOI: 10.2134/jeq2004.0323
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Vadose Zone Processes and Chemical Transport
Degradation of Potassium Formate in the Unsaturated Zone of a Sandy Aquifer
Pasi P. Hellsténa,*,
Anna-Liisa Kivimäkia,
Ilkka T. Miettinenb,
Risto P. Mäkinena,
Jani M. Salminena and
Taina H. Nysténa
a Department of Expert Services, Hydrological Services Division, Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finland
b Department of Environment Health, National Public Health Institute, P.O. Box 95, FIN-70701 Kuopio, Finland
* Corresponding author (pasi.hellsten{at}ymparisto.fi)
Received for publication August 19, 2004.
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ABSTRACT
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This paper presents results from a lysimeter experiment on the fate of potassium formate, an alternative deicing agent. The experiment was performed through the winter and spring to identify any thermal sensitivity in the transport and biodegradation of formate in the lysimeter. Ninety-eight percent of the total quantity of formate applied was degraded while percolating through the 1.7-m-thick unsaturated sand layer within the lysimeter. Concomitantly, the bicarbonate concentration of the percolating water increased. The low concentrations of nitrogen (0.02 mg L–1) and phosphorous (<0.002 mg L–1) in the percolated water, however, potentially limited microbial activity. During the study period, 99% of the applied potassium was retained in the lysimeter, and the ion exchange between the potassium and a variety of monovalent and divalent ions was assumed to be responsible for the leaching of barium, calcium, magnesium, and sodium from the soil material. Except for manganese, the concentrations of the studied metals in the percolated water did not exceed the threshold values set for drinking water by the Council of the European Union. By contrast, the application of potassium formate had a detrimental effect on the vegetation on the lysimeter. To conclude, formate was effectively degraded in the sandy lysimeter and its application did not cause major undesirable changes in the quality of the percolating water. Further research at field scale is, however, needed for instance on the biodegradation of potassium formate and on its impacts on roadside vegetation.
Abbreviations: AOC, assimiable organic carbon • COD, chemical oxygen demand • EC, electrical conductivity • HGR, heterotrophic growth responses • ICP–AES, inductively coupled plasma atomic emission spectrometry • ICP–MS, inductively coupled plasma mass spectrometry • MAP, microbially available phosphorus • TOC, total organic carbon
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INTRODUCTION
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SODIUM CHLORIDE (NaCl) is the most commonly applied deicing chemical on the roads and highways in Finland, in other European countries, and in North America. Road salting has, however, adversely impacted ground water quality in Finland and in several other countries (e.g., Pilon and Howard, 1987; Howard, 1998; Gustafsson and Nystén, 2000; Blomqvist, 2001). In general, deicers have been reported to have three main undesirable impacts on ground water quality: (i) chemical residues, (ii) oxygen loss caused by degradation of organic chemicals, and (iii) leaching of heavy metals from soils (Dunn and Schenk, 1980; Chollar, 1984; Winters et al., 1985; Pilon and Howard, 1987; Horner, 1988; Amrhein et al., 1992, 1994; Howard and Beck, 1993; Bang and Johnston, 1998; Gilfillan, 1998; Oberts et al., 2000). High chloride concentrations in ground water also induce corrosion in pumps and pipe networks, which substantially increases the operational costs of water distribution utilities. In Finland, approximately 70% of the population relies on shallow aquifers for a supply of potable water, and 24% (528 of 2226) of Finnish ground water areas identified as being important reservoirs are traversed by roads that require deicing. Hence, deicing is a significant contributor of nonpoint-source ground water pollution and it threatens the quality of the Finnish ground water resources. It is also noteworthy that the Finnish Environment Protection Act prohibits even a negligible endangering of the quality of ground water. In addition, European Union directives require that the quality of water intended for human consumption should not be corrosive, and that any significant and continuous upward trend in the concentration of any pollutant in ground water should be identified and reversed (European Parliament, 1998, 2000).
To meet the needs of ground water protection and the traffic safety, and to mitigate other negative effects of deicing, we have to find alternative solutions to chloride salt–based deicing. Therefore, organic deicers, such as acetates and formates, have been proposed. The use of these compounds is currently limited to deicing airport runways, and also roads and bridges plus other essential civil infrastructure systems that are sensitive to corrosion. One of the suggested alternative deicers is potassium formate (KCOOH), which is the potassium salt of formic acid, and which has obvious advantages. The following have already been reported: (i) the biological oxygen demand of formate is lower than that of acetates or urea, (ii) the degradation of formate occurs also at low temperatures, (iii) formate induces less leaching of trace metals from roadsides than do sodium chloride and acetates (Roseth and Bjornstad, 1998; Hellstén and Nystén, 2003), and (iv) the taste level of potassium formate in ground water is lower than that of conventional road salt and potassium acetate (CH3COOK) (Kivelä et al., 2003). In our earlier sand and gravel column experiments (Hellstén and Nystén, 2001, 2003; Hellstén et al., 2002), potassium formate was found to be the least harmful deicing chemical with respect to ground water quality when compared to sodium chloride, calcium chloride (CaCl2), magnesium chloride (MgCl2), calcium magnesium acetate [Ca3Mg7(CH3COO)20], and potassium acetate.
From the Finnish point of view, the migration and fate of organic deicers into the shallow aquifers common in Finland is of particular concern. Unfortunately, information on the behavior of organic deicers in the environment is still generally scarce. Also, it is possible that standard tests may not identify the actual rate of biodegradation in soil and ground water. While our earlier sand and gravel column experiments provided valuable insights into the behavior of potassium formate and other deicers in soil, they did not adequately simulate the role of the potentially crucial environmental factors such as snow cover, ice, soil frost, precipitation, and temperature. It is clear, therefore, that a greater understanding of the processes of the degradation and transport of formate in the unsaturated zone during cold periods is urgently needed if potassium formate is to be introduced into the environment as a result of road deicing.
The objective of this research was to study the migration and degradation of potassium formate in the unsaturated zone of a lysimeter in a sandy aquifer in real winter and spring conditions.
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MATERIALS AND METHODS
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The lysimeter experiment was performed in southwestern Finland at the Oripää lysimeter station (60°55' N, 22°44' E) from December 2001 to July 2002. The station is situated in a natural pine forest on the Oripää esker, which is composed of well-graded sand and gravel.
The two lysimeters—one constructed in 1972, the other in 1974—at the Oripää station measured 1.7 m deep and had a collection chamber at the bottom (Fig. 1)
. One of the lysimeters was applied with potassium formate, while the parallel lysimeter was used as a control. Precipitation was recorded on a daily basis and the percolation rates in the lysimeter were recorded by a hydraulic pressure datalogger (GWM-Systems, Kuopio, Finland). Soil moisture content was measured at 10-cm intervals by time domain reflectometry (Model 1502B; Tektronix, Beaverton, OR) and a neutron probe (NEA, Copenhagen, Denmark), and the residual evapotranspiration was determined by mass-balance calculations. The soil type in the container and the vegetative cover, which included moss (Dicranum polysetum, Pleurozium schreberi), heather [Calluna vulgaris (L.) Hull], lingonberry (Vaccinium vitis-idaea L.), young birches (Betula spp.), and fireweed (Epilobium angustifolium L.) were similar to those prevailing in the surrounding natural pine forest. Because the lysimeter barrels have been in place for 30 years, the stratification and the podzol zones were nearly identical to the natural soil in the area. The annual uncorrected precipitation and infiltration at the lysimeter station ranged from 500 to 780 mm and from 320 to 540 mm per year, respectively (Mäkinen and Peltonen, 2001).
Samples from the percolated water were taken 12 times during the period from 19 Dec. 2001 to 1 July 2002. Samples for metal analyses were filtered through a 0.45-µm filter, and fixed with 0.5 mL of 65% HNO3. The samples were also analyzed for electrical conductivity (EC), pH, formate (HCOO–), HCO–3, CO2, SO2–4, total organic carbon (TOC), chemical oxygen demand (COD), total N, NO2+3–N, NH4–N, total P, PO4–P, sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), barium (Ba), manganese (Mn), cadmium (Cd), chromium (Cr), copper (Cu), ferrous iron (Fe), nickel (Ni), lead (Pb), and zinc (Zn). In addition, assimilable organic carbon (AOC), microbially available phosphorus (MAP), and heterotrophic growth responses (HGR) were analyzed from water samples obtained in January through to April. All samples were analyzed according to the procedures and standard methods listed in Table 1.
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Table 1. Analytical methods applied to the water samples, and the quality of percolated water from the Oripää lysimeter in the period of 19 Dec. 2001 to 1 July 2002.
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A total potassium formate loading of 3.4 kg m–2 was used and it was applied by sprinkler irrigation over the surface of the snow-covered lysimeter in five stages between 19 Dec. 2001 and 4 Mar. 2002 (Table 2). In this way, the experiment aimed to simulate occasional snowmelt periods and the springtime snowmelt peak and the subsequent transporting of high loads of deicing compounds into the defrosted soil. The total amount of potassium formate applied was chosen based on the highest load of road salt (NaCl) used per year on four-lane highways in southern Finland (20000 kg km–1) (Gustafsson, 2000). Assuming that the chemical used for deicing disperses at a distance of 4 m from the road, the total amount of the deicer input per year into the ground is 2.5 kg m–2. However, since no experience of the use of potassium formate in road deicing has been gained so far, the quantity of potassium formate applied in this experiment was estimated based on the assumption that the volume of liquid potassium formate required for deicing might be somewhat higher than that of granular sodium chloride.
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Table 2. Timetable of formate application (0.68 kg m–2 applied in each case), precipitation, temperature, and snow cover at the Oripää Station.
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RESULTS
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From 19 Dec. 2001 to 1 July 2002, 225 mm m–2 of water was collected in the lysimeter, which represented 53% of the total corrected precipitation. The average soil moisture content was 12.6 vol. %, and the change of soil water storage during the experiment was 3.3 vol. %. Evapotranspiration was estimated to be 35% of precipitation. This agreed with the data collected from 1973 to 2000 (Mäkinen, unpublished data). During the study, air temperature at the Oripää lysimeter station ranged from –20.5 to +18.8°C, whereas the temperature of the percolated water ranged from +3.5 to +8.5°C (Fig. 2) .
The mean concentration of the formate as it entered the soil was 2730 mg L–1 and the mean infiltration rate was 1.2 L m–2 d–1 (max. 6.2 L m–2 d–1). The highest concentration (350 mg L–1) of formate in the percolated water was detected in March (Fig. 3)
. The total input and output of formate were 1820 and 38 g m–2, respectively. Thus the total recovery of formate in the percolated water was 2% of the total input. The disappearance of formate was accompanied with formation of carbon dioxide and bicarbonate in the percolating water (Fig. 3). In addition, we also observed an increase in EC, COD, pH, and SO2–4, while TOC of the percolated water decreased during the test period (Fig. 3). The concentrations of nutrients (NO3–N and PO4–P) in the percolated water were below 0.5 mg L–1. The recovery of potassium was approximately 1% of the total application from 19 December to 1 May. The concentrations of Cd, Cr, Cu, Fe, Ni, Pb, and Zn were low throughout the experiment (Table 1). The concentrations of Ba, Ca, Mg, Na, and Zn increased after application of potassium formate (Fig. 4)
. Moreover, the concentration of Mn increased from 23 µg L–1 (minimum) to 665 µg L–1 (maximum).
The AOC concentrations in samples taken between 14 January and 5 February ranged from 3 to 19 µg AOC-C L–1 (Table 1). Concurrent with the peak in formate concentration, the maximum concentration of assimilable organic carbon (2290 µg AOC-C L–1) was recorded on 20 February. This was followed by a significant decrease in April, down to a concentration of 246 µg AOC-C L–1. The concentration of microbially available phosphorus was below the detection limit of 0.08 µg MAP L–1 in nearly all the samples throughout the monitoring period (Table 1). The highest heterotrophic growth (546400 CFU mL–1) was recorded on 14 January.
At the end of the experiment, the pH of the topsoil (0–20 cm) was higher in the test lysimeter than in the control lysimeter (Table 3). Consequently, almost all of the perennial plants, for example, moss, heather, and lingonberry, present on the surface of the lysimeter died. Only the young birches and fireweed remained alive at the end of the experiment.
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DISCUSSION
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During the two first months of the experiment, infiltration was minimal due to the soil in the lysimeter being frozen (Fig. 3). However, a rapid thaw of the soil frost in the middle of February (Fig. 2) allowed increased percolation of melt water containing a high concentration of formate through the sandy lysimeter. This pulse of melt water led to a high concentration of formate in the outlet water at the end of February and at the beginning of March (Fig. 3). However, a decreasing infiltration rate coupled with elevated temperatures in April and May (Fig. 2) resulted in the formate being removed effectively from the lysimeter. This removal of formate coincided with the appearance of CO2 and HCO–3 in the percolated water suggesting that the formate had been microbially degraded. This observation agrees with our earlier study (Hellstén and Nystén, 2003), where we demonstrated an effective removal of formate in sand and gravel columns at +3 to +6°C after a three-week lag phase. Furthermore, Hellstén et al. (2005) recorded rapid biodegradation of formate even at sub-zero temperatures in sandy soil and subsurface samples obtained from a field site where potassium formate was used in highway winter deicing. Interestingly, they observed no lag phase in the initiation of the biodegradation and concluded that rapid biodegradation prevented the introduction of formate into the ground water through the thin unsaturated zone. The contradiction between the findings obtained in these above-mentioned studies might be explained by the fact that in the lysimeter study, unusually high concentrations of formate were applied, whereas in the field-scale study the dilution of formate is likely to occur before its introduction into the ground.
In this study we also wanted to estimate the effects of potassium formate application on microbial growth: formate may act as an easily assimilable organic compound in ground water, which might increase microbial growth and activity in the network of water distribution pipelines (LeChevallier et al., 1991; Van Der Kooij, 1992). We found that the AOC concentrations in the percolating water were low (<20 µg L–1) during the winter period until we recorded a peak value of 2290 µg AOC-C L–1 on 20 February. The peak coincided with a surge of melt water containing a high formate concentration (240 mg L–1). However, the increase in formate and AOC concentration in the percolating water did not appear to affect the growth of heterotrophic microorganisms. However, this does not necessarily mean that the application of potassium formate had not resulted in bacterial growth, as it is well-known that only 0.1 to 10% of soil microorganisms are culturable (Amann et al., 1995). In addition to the availability of organic carbon, nutrients, and especially phosphorus, may regulate microbial growth in ground waters (Miettinen et al., 1996, 1997; Sathasivan and Ohgaki, 1999). We recorded concentrations of microbially available phosphorus (MAP) below 0.08 µg MAP L–1 in nearly all the samples taken from the percolated water. Low phosphorous concentrations might partly explain why there was no correlation between formate concentration and microbial growth. In other words, it is possible that bacterial growth and biodegradation were seriously limited by the low phosphorus levels.
Potassium formate increased the pH of soil and percolated water, which also agrees with our previous sand and gravel column experiments (Hellstén and Nystén, 2003). Throughout the experiment, the concentrations of metals such as Cr, Cd, Cu, Ni, Pb, Zn, and Fe remained low and did not exceed the threshold values set for drinking water by the Council of the European Union. However, the introduction of potassium into the lysimeter and the subsequent ion exchange reactions between potassium and a variety of monovalent and divalent ions in the silicate mineral particles were the likely reasons causing the leaching of Ba, Ca, Mg, and Na from the soil (Fig. 4). The increase of Mn concentration above the threshold value of 50 µg L–1 (European Parliament, 1998) may reflect consumption of oxygen arising from microbial activity and the subsequent reduction of manganese in anoxic conditions: according to Smith et al. (2001) and Madigan et al. (2000), formate could serve as a potential electron donor for subsurface denitrification, manganese, iron, and sulfate reduction, and methanogenesis. Our observation agrees with that of French et al. (2001), who reported an increase in manganese concentrations in the soil water of a lysimeter subjected to potassium acetate and propylene glycol loading. They also concluded that reduction rather than ion exchange was the mechanism responsible for the increased manganese concentrations in the soil water.
In this study, the death of the perennial plants on the lysimeter area was presumably caused by the extremely high concentration and alkaline nature of the potassium formate (pH 10.6–11.4) applied to the soil surface. When potassium formate is applied in road winter deicing, dilution will likely occur as discussed above and the effects on the vegetation might not be as dramatic as recorded in this study. However, this study does clearly demonstrate that the effects on vegetation of the application of potassium formate require more research. This is further underlined by the fact that elevated levels of potassium content in the topsoil may also affect the diversity of the vegetation present, if potassium formate is applied for several years. In addition to this, Sieghardt et al. (1998) found that high potassium concentrations in soil may also inhibit the uptake of calcium and magnesium by plant roots. Moreover, Joutti et al. (2003) showed that the toxicity of the alternative deicers used in their study varied greatly, and that calcium-magnesium-acetate, potassium acetate, and potassium formate, all organic deicers, were more toxic to the test plants, namely onion (Allium cepa L.) and duckweed (Lemna spp.), than were the inorganic salts they tested.
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CONCLUSIONS
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In this lysimeter study we found that formate was effectively removed (98%) in a sandy lysimeter after a cold winter period. The disappearance of formate was accompanied by the formation of carbon dioxide and bicarbonate in the percolating water indicating biodegradation of formate. Potassium seemed to be adsorbed (99%) in the soil of lysimeter and the ion exchange between potassium and a variety of monovalent and divalent ions in mineral particles in the aquifer material was assumed to be responsible for the leaching of barium, calcium, magnesium, and sodium from the soil material.
If potassium formate is applied as a deicer for several years, potassium may leach into the ground water. In this study, the direct application of alkaline potassium formate at high concentrations (50 wt. %) destroyed most of the plants present on the soil surface of the lysimeter. If potassium formate is used for road deicing, impacts on the roadside vegetation are likely to be less critical, however, because of the dilution effect of melt water and precipitation. Nevertheless, further research at field scale is still needed for instance on the impacts of potassium formate on vegetation, road surfaces, and leaching of heavy metals from soil before allowing the widespread use of formate over Finnish shallow sandy aquifers.
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RELATED STUDIES
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The experiments at the aquifer scale were started in 2002 at a field site in southeastern Finland where potassium formate is used in highway winter road deicing (Hellstén et al., 2005). In the course of this study, the volume of potassium formate required per kilometer to achieve the optimal deicing result will be determined. Since 2002, the fate and effects of potassium formate have been monitored at the site by analyzing the chemical and microbiological quality of the ground water. Furthermore, the aerobic and anaerobic biodegradation of formate at low temperatures in the soil and subsurface samples obtained from the site have been studied (Hellstén et al., 2005). Studies on the adsorption of potassium in soil and on the effect of high potassium load on cation exchange capacity at the site are also underway. Moreover, the long-term impact of formate on the vegetation of the roadsides and the effects of formate on road surfaces will be monitored at the Kauriansalmi site until 2009. It is hoped that the results of this project can be applied in countries where hydrogeological conditions are similar to those experienced in Finland.
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ACKNOWLEDGMENTS
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The authors gratefully acknowledge the contributions of Shayne Giles at the Department of Civil Engineering at the University of Waterloo, Kirsti Korkka-Niemi at the University of Turku, and Kirsti Granlund and Kirsten Jørgensen from the Finnish Environment Institute, all of whom provided extensive editorial comments on this paper. We also wish to acknowledge the Finnish Road Administration, the Civil Aviation Administration of Finland, and Kemira Corporation for their financial support.
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ABSTRACT
INTRODUCTION
