Rapid Wintertime Oxygen Depletion in Shallow Urban Freshwater Ponds Induced by Road De-Icing Salts: An Observational Study

Rapid Wintertime Oxygen Depletion in Shallow Urban Freshwater Ponds Induced by Road De-Icing Salts: An Observational Study | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 9 September 2025 V1 Latest version Share on Rapid Wintertime Oxygen Depletion in Shallow Urban Freshwater Ponds Induced by Road De-Icing Salts: An Observational Study Authors : Steffen Podlech Podlech 0009-0007-9557-3889 [email protected] and Casper Hornskov Hansen 0009-0008-9881-3064 Authors Info & Affiliations https://doi.org/10.22541/au.175743529.96389961/v1 Published Journal of Environmental Quality Version of record Peer review timeline 90 views 82 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract This study presents two years of high-resolution data from two urban freshwater ponds in Esbjerg, Denmark, revealing recurrent wintertime dissolved oxygen (DO) depletion linked to elevated electrical conductivity (EC) from road de-icing salt (RDS) runoff. DO levels dropped rapidly during EC spikes, often reaching hypoxic conditions, and recovered as EC returns to baseline values. The phenomenon, not yet described in existing literature, highlights the ecological sensitivity of urban freshwater systems to saline runoff. As we do not have the means, further research is needed to elucidate the underlying mechanisms. Rapid Wintertime Oxygen Depletion in Shallow Urban Freshwater Ponds Induced by Road De-Icing Salts: An Observational Study Steffen Podlech 1* and Casper Hornskov Hansen 1 1 HTX, Rybners, Spangsbjerg Møllevej 72, Esbjerg, 6700, Denmark. * Corresponding author(s). E-mail(s): [email protected] Abstract This study presents two years of high-resolution data from two urban freshwater ponds in Esbjerg, Denmark, revealing recurrent wintertime dissolved oxygen (DO) depletion linked to elevated electrical conductivity (EC) from road de-icing salt (RDS) runoff. DO levels dropped rapidly during EC spikes, often reaching hypoxic conditions, and recovered as EC returns to baseline values. The phenomenon, not yet described in existing literature, highlights the ecological sensitivity of urban freshwater systems to saline runoff. As we do not have the means, further research is needed to elucidate the underlying mechanisms. Keywords : Road de-icing salt (RDS), oxygen depletion, urban freshwater environment, ponds Introduction Urban lakes serve a pivotal role in public health as recreational sites. It is therefore essential that appropriate water monitoring of urban ponds and lakes is implemented to ensure high water quality. Monitoring water quality is an effective tool, consistently evaluating the trophic state of the aquatic system (Baumgart & Sperling, 1984, Alferes et al., 2013, Demetillo, Japitana & Taboada, 2019, Marce et al., 2016). Recent advances in sensor and communication technologies enable continuous in situ monitoring (Marce et al., 2016, Mueller et al., 2013). Urban freshwater bodies are particularly susceptible to anthropogenic stressors, notably winter road de-icing salt (RDS) runoff (Kaushal et al., 2021, 2022), which can induce density stratification and bottom-layer oxygen depletion (Ladwig et al., 2023) or exacerbating biological strain during organismal dormancy (Cañedo-Argüelles et al., 2013, 2014). Figure 1: The figure (a) shows a general overview of Denmark and the location of the city of Esbjerg (rectangle). Figure (b) illustrates Esbjerg town site including a rectangle indicating the area of interest. The lower image (c) is an enlargement of the area of interest indicating the location of Spangsbjerg Mølle lake (SL) relative to Kvaglund lake (KL). Furthermore, the location of Rybners HTX is included. In this study, data were continuously collected from May 2022 to April 2024 using automated meteorological and hydrological monitoring stations (MS) installed in two shallow urban ponds: Spangsbjerg Mill Lake (SL) (55.494502°N, 8.4488715°E) and Kvaglund Lake (KL) (55.4950327°N, 8.4682472°E), both located in Esbjerg Municipality, Denmark (figure 1). The ponds are approximately 1.5 km apart and hydrologically connected via Spangsbjerg Mill Creek. Water flows from KL to SL and ultimately discharges into the North Sea. KL has two inflows and one outflow, while SL has one inflow containing water from KL and has a single outlet. Flow rates are influenced by precipitation; however, following a week without rainfall, the in- and outflow from SL was estimated at 0.21 m³/s. It was not possible to determine the inflow or outflow rates for KL. The surface areas of SL and KL are 3690 m² and 24560 m², respectively. The mean depth of SL is 0.52 m, while that of KL is 0.98 m. These values were derived from multiple depth measurements and surface hight measurements of the surroundings at each pond, forming the basis for three-dimensional bathymetric models providing the surface water catchment and main flow pattern for each pond (based on topography). Esbjerg municipality employs a separated sewer system, ensuring that only surface runoff enters the local aquatic environment, with no contribution from domestic or industrial wastewater. This study presents observational data offering novel insights into transient dissolved oxygen (DO) depletion events occurring during winter in the presence of RDS, measured as electric conductivity (EC). The findings highlight the need for further investigation into the mechanisms by which RDS influences oxygen dynamics in urban freshwater systems. Methodology Each MS applied to SL and KL was equipped with the following instrumentation: Meteorological Sensors: • Young probe for relative humidity and air temperature (Model 41382LC2) (both SL and KL) • Young Wind Monitor for wind speed and direction (Model MA05106) (both SL and KL) • PRONAMATIC Rain-O-Matic professional rain gauge for precipitation (both SL and KL) Hydrological Sensors: Water temperature measurement at Aquaread probe depth with PT-100 sensors (SL at 0,75 m below surface; KL at 1,15 m below surface) Water Quality Probes: SL: Aquaread AP-5000 probe with optical DO and EC sensors KL: Aquaread AP-7000 probe with optical DO and EC sensors Solar Radiation: KL: Kipp & Zonen SMP3 pyranometer recording incoming solar radiation All sensors recorded data at 10-minute intervals from May 2022 to April 2024, using a DataTaker DT84 datalogger. Road de-icing salt (RDS) data were kindly provided by Esbjerg Municipality and represent average values derived from the total mass of RDS applied, distance travelled, and spread width. Results Measurements of air temperature, EC, and DO for both SL and KL are presented in figure 2 left and right, respectively. Air temperature data exhibit a clear annual cycle, with summer peaks exceeding 25 °C and winter lows falling below −5°C over the two-year observation period (figure 2 a & c). EC remained stable at approximately 500 µS/cm (0,33 g/L) during spring, summer, and autumn, but showed marked increases during winter, reaching up to 8000 µS/cm (5,12 g/L) in SL and 6000 µS/cm (3,9 g/L) in KL, coinciding with the application of RDS. The conversion between EC and total dissolved solids (TDS) in g/L following (Rusydi, 2018). Figure 2: Air temperature and electric conductivity (EC) for SL for the period May 2022 to April 2024 is illustrated in (a) as well as 8 events of significant increase in EC during winter marked with SL 1 to SL 8. DO and EC measured in SL for the same period is illustrated in (b). Air temperature and EC for KL for the period May 2022 to April 2024 is illustrated in (c), as well as 7 events of significant increase in EC during winter marked with KL 1 to KL 7. DO and EC measured for KL for the same period is illustrated in (d). SL experienced summer hypoxic conditions beginning on 8 th of July 2022, while both ponds exhibited similar events starting on 23 rd of June 2023, with intermittent recovery (figure 2 b & d). These DO depletion events occur gradually (over ca. 10 days) also showing a pronounced diurnal cycle during the time of decrease in DO. Nevertheless, summer DO depletion events are commonly caused by metabolic activity and are not further described here, as the main focus in this study is on the temporal winter hypoxia events. Distinct EC spikes during winter are clearly identifiable in both ponds and are marked in figures 2 a & c. Given the consistent pattern of abrupt EC increase followed by abrupt decline in DO, the analysis focuses on four representative events: SL 2 and KL 2 in November / December 2022, and SL 6 and KL 5 in November / December 2023. However, all other marked events show similar patterns concerning EC and DO. Baseline values for EC and DO were calculated from Figure 2 b & d, with EC summer stability reflecting natural background salinity due to the absence of external salt input. Average baseline EC values, calculated for SL and KL between the 1 st of May 2023 and the 1 st of September 2023, are 467 µS/cm and 483 µS/cm for SL and KL, respectively. Baseline DO values for SL and KL, calculated from average measurements between the 1 st of April 2023 and the 1 st of June 2023, are 7,03 mg/L and 9,84 mg/L, respectively. During this period, spring mixing occurred in both ponds, where the highest DO values are recorded at the respective sensor depth. Figure 3: Events SL 2 (left) and KL 2 (right), previously introduced in figure 2 a and 2 c, are here presented in greater detail with supplementary data. Subfigures (a) illustrate the timing and quantity of RDS applied to roads and pathways in the catchment area of SL and KL, respectively. Subfigures (b) display the recorded air temperature, while subfigures (c) show the water temperature at the depth corresponding to the DO and EC sensors. Subfigures (d) present wind speed measurements at the respective MS. Subfigures (e) depict precipitation recorded at SL and KL. Subfigure (ee) shows incoming solar radiation at KL; this parameter was not measured at SL. Finally, subfigures (f) display the measured EC and DO values for SL and KL. Figure 3 (left) presents multiple parameters from event SL 2 (figure 2 a). Figure 3 b (left) illustrates five intervals of sub-zero air temperatures beginning on 7 th of December 2022. Prior to and during these cold periods, precautionary RDS application was recorded (figure 3 a (left)). A precipitation event on 9 th of December 2022 (figure 3 c (left)) facilitated the transport of dissolved RDS into SL, resulting in a sharp EC increase to 8000 µS/cm (Figure 3 f (left)) and a simultaneous DO drop below 1 mg/L. A brief recovery on the 10 th of December 2022 is likely attributable to increased inflow and turbulence upstream, enhancing aeration due to precipitation (figure 3 e (left)). Wind speeds reaching 6 m/s (figure 3 d (left)) may have further contributed to this temporary rise in DO. However, from the 11 th to the 15 th of December 2022, DO levels in SL fell to 0 mg/L. Subsequent precipitation reduced EC and temporarily elevated DO. As temperatures again dropped below freezing on the 15 th of December 2022, further RDS application was observed (figure 3 a & b (left)), leading to another EC increase and DO decline to 0 mg/L, persisting for four additional days. In total, SL experienced seven days of complete oxygen depletion, interrupted only briefly on the 14 th of December 2022. DO levels returned to baseline values due to precipitation, which introduced oxygen-rich, low-salinity water (figure 3 e (left)). Between the 19 th to 20 th and the 24 th to 25 th of December 2022, further EC increases were observed due to additional RDS application, as air temperatures drop to sub-zero (figure 3 a & b (left)), resulting again in temporary DO depletion (figure 3 f (left)). After the 26 th December 2022, both EC and DO return to baseline values, as air temperatures rise above 0°C, which negated the need for further RDS application (figure 3 f (left)). Water temperatures in SL at the depths of the DO and EC sensors were closely linked to precipitation and air temperature (figure 3 e (left)), typically ranging between 4 to 6°C during hypoxic episodes (figure 3 e (left)). Figure 3 (right) presents event KL 2 (figure 2 c), which shows slight and steady increase in EC from the 7 th of December 2022 and hence a more gradual DO decline over four days, likely due to KL’s larger volume and differing catchment characteristics. Nonetheless, KL also experienced oxygen depletion from the 10 th to the 26 th of December 2022, with a brief recovery on the 21 st of December 2022, likely to precipitation and elevated wind speeds (figure 3 d & e (right)), which introduced oxygenated surface runoff. DO levels fell during this period below 2 mg/L, fluctuating slightly except on the 21 st of December 2022, and coincided with elevated EC values (4000–6000 µS/cm), directly linked to RDS application prior to sub-zero air temperatures (figure 3 a & b (right)). After the 26 th of December 2022, EC and DO values gradually return to baseline levels (figure 3 f (right)). Solar radiation during this period of DO deletion in KL was minimal due to winter solstice and cloud cover (figure 3 ee (right)), suggesting that photosynthetic activity had negligible influence on DO levels. Instead, precipitation and wind-induced mixing appear to be the dominant factors affecting oxygen dynamics during winter. The absence of a diurnal signal in the DO data during rapid depletion events suggests a chemical process rather than metabolic activity as underlying cause. Figure 4: Events SL 6 (left) and KL 5 (right), previously introduced in figure 2 a and 2 c, are here presented in greater detail with supplementary data. Subfigures (a) illustrate the timing and quantity of (RDS) applied to roads and pathways in the catchment area of SL and KL, respectively. Subfigures (b) display the recorded air temperature, while subfigures (c) show the water temperature at the depth corresponding to the DO and EC sensors. Subfigures (d) present wind speed measurements at the respective MS. Subfigures (e) depict precipitation recorded at SL and KL. Subfigure (ee) shows incoming solar radiation at KL; this parameter was not measured at SL. Finally, subfigures (f) display the measured EC and DO values for SL and KL. Figure 4 (left and right) presents SL 6 and KL 5. In SL, EC increases to ca. 1000 µS/cm on the 25 th and 27 th of November 2023 correspond with sharp DO declines (figure 4 f (left)), following precipitation that dissolved and mobilised RDS applied on the 24 th of November 2023 (figure 4 a (left)). On the 5 th of December 2023, in SL, EC rose to ca. 1000 µS/cm, accompanied by a DO drop (figure 4 f (left)). EC peaked at 7000 µS/cm on the 7 th of December 2023, with DO levels remaining low and variable. These fluctuations may be attributed to sustained precipitation (6 th to 8 th of December2023) and elevated wind speeds (6 th and 9 th of December 2023) (figures 4 e & f (left)). After the 9 th of December 2023, EC and DO return to baseline values, although subsequent variations could not be directly linked to municipal applied RDS (figure 4 a (left)), possibly due to unrecorded local applications near residential or industrial areas within the catchment area. In KL, EC gradually increased from the 17 th to the 27 th of November 2023, while DO steadily declined until 24 th November 2023 (figure 4 f (right)). On the 24 th November 2023, DO abruptly dropped to 0 mg/L. Despite wind speeds of 6–8 m/s and solar radiation reaching 500 W/m² as well as slight precipitation (figures 4 d, e & ee (right)), DO remained near zero in KL. Recovery began after the 4 th of December 2023 as EC approached 500 µS/cm (figure 4 f (right)). A subsequent DO decline on the 6 th of December 2023 coincided with precipitation and EC rising to 1000 µS/cm (figures 4 a, e & f (right)). Hereafter, both EC and DO approach baseline levels (figure 4 f (right)). Discussion The two-year dataset obtained from the two locations at SL and KL reveals multiple instances of DO depletion occurring between November and February in 2022-2023 and 2023-2024. These events consistently coincide with elevated electrical conductivity (EC), suggesting a strong link to RDS input. The observed pattern—where EC levels increases coinciding with a decline in DO level, supports the hypothesis that salinity fluctuations play a critical role in modulating oxygen availability in these urban freshwater systems. Although this phenomenon has not yet been described in existing literature, preliminary indications are found in the works of Berger, Frör & Schäfer 2019 and Bernhardt et al. (2018). Given the current limitations in analytical instrumentation and mechanistic understanding, further research is required to elucidate the processes underlying these DO depletion events, which this communication will encourage. Furthermore, all DO depletion events occur rapidly (within hours) and no diurnal signal is visible in the DO data during the events, strongly suggesting a chemical process as underlying mechanism rather than metabolic activity. Conclusion This study demonstrates that road de-icing salts (RDS) can induce rapid and severe DO depletion in urban freshwater ponds during winter. The observed interaction between elevated EC levels triggering depletion of DO (hypoxic conditions) in the two observed ponds, SL and KL, highlights the ecological vulnerability of these systems to saline runoff, particularly during periods of low temperature. These observations underscore the need for further research into the mechanisms driving wintertime DO depletion events. Publishing these comprehensive data to a broader audience may stimulate research initiatives aimed at elucidating the currently unknown relationship between RDS and DO dynamics during winter. References Alferes, J., Tik, S., Copp, J., & Vanrolleghem, P. A. (2013). Advanced monitoring of water systems using in situ measurement stations: Data validation and fault detection. 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IOP Conference Series: Earth and Environmental Science , 118 (1). https://doi.org/10.1088/1755-1315/118/1/012019 Information & Authors Information Version history V1 Version 1 09 September 2025 Peer review timeline Published Journal of Environmental Quality Version of Record 19 Mar 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords oxygen depletion ponds road de-icing salt (rds) urban fresh water environment Authors Affiliations Steffen Podlech Podlech 0009-0007-9557-3889 [email protected] Rybners View all articles by this author Casper Hornskov Hansen 0009-0008-9881-3064 Rybners View all articles by this author Metrics & Citations Metrics Article Usage 90 views 82 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Steffen Podlech Podlech, Casper Hornskov Hansen. Rapid Wintertime Oxygen Depletion in Shallow Urban Freshwater Ponds Induced by Road De-Icing Salts: An Observational Study. Authorea . 09 September 2025. 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