Chloride Balance in Freshwater System of a Highly Anthropized Subalpine Area: Load and Source Quantification Through a Watershed Approach

Abstract

Recent studies have highlighted an increase of chloride in many lakes worldwide, with negative effects on chemical and physical properties of inland waters and freshwater biota. In this study, we assessed the long-term trend of chloride in Lake Iseo, located in the midlatitudes, and analyzed its relationship with discharge data of the inflows and hydrological and hydroclimatic changes. Additionally, we performed a specific annual survey, collecting water samples at a biweekly frequency in tributaries, calculating chloride load, and comparing the result with the chloride increase observed in lacustrine water. The combined use of Geographic Information System (GIS) and multivariate analyses allowed identification of possible sources of chloride and their seasonal dynamic within the mixed-land-use watershed: impervious surface and deicing salt, population and sewage effluents, livestock, and rainwater. Their potential contribution to total mass balance was assessed, cross-checking the results between chloride source evaluation and load estimation. Chloride load from wastewater treatment plants played an important role, albeit we highlighted that the main source was the road deicing salt, with a primarily winter contribution. The increased anthropization in the watershed was likely the main cause of the chloride enhancement. This study demonstrates that the problem of salinization can also affect lakes located in the midlatitude areas and provides a reproducible method for the identification and quantification of the different sources. Our results will help to better understand the potential sources of salinity in rivers that may reveal processes controlling the salinization of freshwater systems, with implications for future management practices.

Salinization of Alpine rivers during winter months

“…Besides this summer-winter pattern, our temporal data indicates that the baseline of specific conductance significantly increases over time, aligning with the global trends in freshwater ecosystems (e.g., Kaushal et al 2005Kaushal et al , 2018Estévez et al 2019;Le et al 2019). Although agriculture (Williams 2001;Cañedo-Argüelles et al 2013) and resource extraction (Palmer et al 2010;Cañedo-Argüelles et al 2012;Vidic et al 2013) are probably the main drivers of salinization worldwide, this study provides further evidence that it is very likely that the main salt input for freshwater ecosystems in mountainous areas comes from the roads or other urban structures (Swinton et al 2015;Corsi et al 2015;Hintz and Relyea 2017;Nava et al 2020). The observed levels and seasonal changes were relatively small compared with other studied aquatic systems outside the Alps (Kaushal et al 2005;Corsi et al 2010).…”

“…For example, in the Alpine region of Tyrol, Austria, an average of 30,000 tons of salt (sodium chloride) are applied annually to approximately 2240 km of roads (Amt der Tiroler Landesregierung, press release). During periods of snow-removal from the roads (which is partly poured into rivers and stored next to them), and especially during melting and precipitation events, these salts might enter the rivers, as it has been shown for a northern Italian catchment (Nava et al 2020) and North American (Crowther and Hynes 1977;Dugan et al 2020) and Scandinavian streams (Ruth 2003). In all the cases, seasonal salinity cycles were synchronized with road salt applications.…”

“…5 Season-specific (summer in red and winter in blue) dischargeconductance plots in all study sites (site A-site D) for one hydrological year (05.2018-04.2019) with indication of the quality of the relationship (R 2 ) and the correlation (Pearson correlation). Since conductance is usually negatively related to discharge (see also Brown et al (2006) for mountain streams), dissimilar correlations in summer and in winter (and late winter) indicate additional influences on ion balance, apart from discharge variabilities Italian catchment (Nava et al 2020)). However, our results call for an implementation of monitoring programs in rural and urban Alpine catchments, since the observed fluctuations might stress the biota in these rivers, which presumably are adapted to low and relatively constant conductivities (Niedrist and Füreder 2016).…”

The effects of roadways on lakes and ponds: a systematic review and assessment of knowledge gaps

“…Increases in dissolved salts, salt ions, and conductivity have been linked to the increasing application of road salts, road density, road proximity/distance, runoff systems, and % impervious surface in the surrounding area (Rosenberry et al 1999;Novotny et al 2008;Kohli et al 2017;Bunbury et al 2020). Identifying the linkage between increasing conductivity and dissolved salt concentrations in lentic environments with road salt application in the surrounding area has been facilitated by long-term monitoring of water quality (Molot and Dillon 2008;Yao et al 2016;Scott et al 2019;Nava et al 2020) and analysis of sediment cores (Siver et al 1999; Environ. Rev.…”

Spatio‐temporal distribution of atmospheric chloride deposition over a small island

“…Understanding chloride deposition holds significant importance for a range of practical applications. Given its stability, conservative nature, and high solubility, chloride serves as a valuable tracer in hydrological investigations, facilitating the comprehension of water sources, movement and solute transport in the environment (Alcalá & Custodio, 2014; Crosbie et al, 2018; Feth, 1981; Kirchner et al, 2010; Nava et al, 2020; Neal & Kirchner, 2000). Although long‐term chloride data sets in rivers and groundwater are commonly available to support catchment‐scale tracer studies, long‐term deposition data sets are rare, especially those representing spatial variability over catchments.…”