Chloride

Chloride (Cl ^- ) is the ionized form of chlorine (Cl), and frequently bonds with other ions to form salts. It is the most common ion found in seawater (Hong et al., 2023). However, in freshwaters, elevated levels of this ion can be detrimental. It often can harm freshwater bodies by increasing the salt content of the water, also known as salinization. In urban areas, salinization of freshwater is often the result of the application of deicing road salts in winter months. The amount of deicing salt application is only increasing in the US. The United States Geological Survey (USGS) estimates that, today, the usage of deicing salts has increased three times that of 1970 (Water Resources Mission Area, 2019). This follows similar trends supported by other studies conducted in recent years (Dugan et al., 2017). Philadelphia itself is no stranger to deicing salts. The Pennsylvania Department of Transportation (PennDOT) estimated that in 2021 road salt was applied to over 1,600 miles of road in Philadelphia. That winter alone saw the usage of more than 16,500 tons of salt (PennDOT, 2023). It’s conditions like these that have made chloride an important water quality parameter in the Wissahickon and surrounding watersheds.

Chloride can primarily be introduced into urban waters, such as the Wissahickon, through runoff of road salt (NaCl). However, there are several other ways it can enter the water as well.

• Wastewater Treatment Plants (Overbo et al., 2021)
• Sewage effluent (Dodds & Whiles, 2020)

• Agricultural runoff (Dodds & Whiles, 2020)
• Road salt application (Thunqvist, 2004)

The sources above are further additions to overall total salt, and thus chloride, load in the Wissahickon. These sources, although present, are less impactful than road salt application for the Wissahickon. Several other correlations exist with regards to levels of chloride in water bodies and external factors. If a water body is closer to the road, then it has been found to possess higher levels of chloride (Thunqvist, 2004). This results from there being a more direct path for these salts and other contaminants to reach the water more readily with little filtration. Other factors such as the sales of deicer salt and the paving of roads have been found to have positive correlations with the chloride levels in the water (Kaushal et al., 2008; Robinson et al., 2012).

Salinization of drinking waters can impact the quality of the drinking water and the resulting high levels has been found to affect those with low sodium diets and people with high blood pressure (EPA, 2020). In Philadelphia, a study of three municipalities during the winter of 2018-19 found concentrations of sodium to be 1-6.4x greater than what the EPA recommends for individuals restricted to low sodium diets (Cruz et al., 2022).

Salinization changes the ionic composition of fresh water, and many organisms often cannot cope with high levels of salinity. This is due to the importance of osmoregulation for these organisms. Osmoregulation is the ability for organisms to regulate the level of ions in their body compared to ions outside of their body. Increasing salt concentration in the water increases the ions outside of their body and disturbs their regulatory patters. With higher levels outside of their body, these organisms cannot effectively regulate how much goes into their body (Greenwald, 1982). This ion regulation is important to the survival of aquatic organisms and tipping this equilibrium out of balance can have deadly effects for those aquatic organisms (Iglesias, 2020). As noted in previous sections, the death of many organisms can lead to further imbalance in the ecosystem. This can be shown as the collapse of the food web or a lack of energy transfer between different trophic levels. This collapse can further lead to losses of natural habitat and subsequent ecosystem services that we humans could have gained from that habitat (Iglesias, 2020).

Road salt runoff into freshwater has been documented as a cause of corrosion in drinking water infrastructure (Pieper et al., 2018). The perpetrators of this corrosion are the same ones that affect aquatic wildlife: ions. Salts are often found in water in the form of electrolytes. These electrolytes are ion conductors, or they want and take electrons. They take electrons from metals such as iron or steel. When these electrons are taken it results in what is more commonly known as rusting (Corbett, 2021). The rusting of these metals, which infrastructure is made of, can cause structural weakening and damage. This leads to both economic and physical damage. In fact, the application of road salt has been found to have corrosive effects on cars, bridges, and other infrastructure that results in $5 billion in annual repairs within the United States alone (EPA, 2020).

For the following analysis the chloride levels of water samples were measured and reported. The baseline for comparison for chloride was 230 mg/L. This wasvthe  recommended level of chloride  for ambient waters defined by the EPA.

Main Takeaways from Chloride Levels by Sampling Site

• Only eight of the samples exceeded the limit of 250 mg/L, a sharp decline from the nitrogen and phosphorous analysis
• The samples taken in February were consistently the highest levels by a significant margin
• The maximum and minimum levels of chloride in the samples were much more drastic than those of phosphorous and nitrogen

Main Takeaways from Chloride Time Slider

• Although the highest levels tended to be towards the main body of Wissahickon Creek, the levels were far more evenly distributed throughout the watershed
• The highest levels, represented by the largest circles, arise during the month of February each year
  • this building on the analysis of the data by site earlier in the StoryMap