Salt in our Fresh Waters
Examining the increasing levels of salt in freshwater bodies across North America, and how this issue might be addressed.
ABSTRACT:
There has been a significant rise in salt levels in freshwater bodies across the United States, Canada, Europe, and elsewhere. Researchers have found that chloride concentrations in these freshwater bodies have not only increased over the last two hundred years, but that this trend is accelerating year of year.
Though current chloride concentrations are mostly benign, these trends warrant preventative action on the part of local and federal governments globally in order to preserve the integrity and usability of freshwater ecosystems.
The research additionally examines potential causes of increased salinity, predictors of future salinity increase, and possible solutions to this problem. The overall findings seem to suggest that even a modest reduction in road salt usage in some of the largest contributing communities could be enough to slow the rate of salinization, and allow the lakes to return to their historic levels of chloride per liter of water.
There’s a beautiful region of North America that’s peppered with freshwater lakes: ranging from Hudson Bay, Canada down to the southernmost point of Michigan and beyond, from the forests of Minnesota to the coastal regions of Maine and New York. Thanks to the likes of the Mississippi, Ohio, and other such rivers throughout the continent, this bounty of freshwater is shared far beyond their shores. These lakes are the remnants of the last ice age and glacial recession, and have been enjoyed by humans across the continent for countless generations. The nature of these lakes has been taken for granted for so long that few can imagine a future where this isn’t the case. Unfortunately, given recent trends, that reality becomes more likely year after year.
These lakes are largely the byproduct of the last ice age when glaciers dominated the northern half of the continent. As they came and went, these massive walls of ice gouged the Earth, leaving craters that would later become the lakes we know today. Some are small, measuring in at just a few square acres in total size, while others are so staggeringly large that they’re often referred to as inland seas. In terms of size, shape, biodiversity, resident ecosystems and more, these lakes and rivers are incredibly varied. But, they do share an important and worrying similarity: all across the region, the relative salinity in these lakes is on the rise.
Millions of years ago, the Great Lakes region was covered by an ancient ocean. As that ocean gradually subsided it left behind massive deposits of salt that lie dormant below much of the region to this day. In fact, there are mines currently operating in Detroit, Michigan that are harvesting these ancient deposits. By and large, the existence of subterranean salt doesn’t pose much of an issue; in fact, many living above these deposits may never know that they exist.
Yet in some cases, lake bottoms are deep enough that their lowest points can mingle with the salt, gradually creating a briny solution that’s diffused upwards. This interaction also happens with subterranean watersheds, which then brings more of this salt to aboveground lakes and rivers. Atmospheric deposition also introduces small amounts of salt via precipitation. However, these processes have been ongoing for thousands of years and have largely reached an equilibrium across the various bodies of water. At least in the Great Lakes, the historic level of chloride measured was approximately 1 milligram per liter of water.
Were this natural equilibrium still in place, there’d be no reason to be alarmed. It’s intense and expensive work to desalinate water, and the ecosystems and societies that interact with these lakes have evolved under these conditions. The amount of salt winding up in freshwater only becomes a problem once it starts accumulating and rising above this natural level.
Using historical data stretching back over 200 years, researchers have found that there has been a fifteen-fold increase in chloride levels. Furthermore, the rate that this level is rising has been accelerating in recent decades, with a majority of that increase occurring since the 1970’s. Across a variety of studies, it’s shown that bodies of water throughout the United States, Canada and even in Europe are becoming increasingly salty. There has also been a great deal of research into why this is happening.
Beginning in the 20th century, it became common practice in the United States and elsewhere to use salt to help minimize the impact of ice on public roadways. While undeniably effective for road safety, the rising salinity in freshwater environments has been a major unintended consequence. Estimates attribute at least 50% of the increased salinity to salt used for deicing. There are other ways that humans can introduce salt into freshwater, such as industrial runoff or ground wells being dug too deep into their aquifer and extracting salt from underground deposits. However, researchers argue that road salt is the most pressing concern.
Two of the strongest predictors of increased lake salinization are proximity of roads to the lakeshore and the density of roads relative to permeable land. Research shows that if even 1% of land within 500m (about a third of a mile) of lakeshore is covered in roads, it’s likely that salinization will increase. This applies to thousands of lakes across the United States and Canada, and so should be taken seriously.
Roads increase salt runoff directly by requiring deicing, and indirectly by preventing the natural absorption of precipitation. Since asphalt and other common materials used in road construction don’t allow for permeation, the saltwater created from melting ice rushes downhill and collects in bodies of water. All of these factors have contributed to rising levels of chloride in fresh water across the continent. In Lake Michigan specifically, this level has gone from about 1 milligram per liter of water to roughly 15 milligrams.
To be clear, this is still a far cry from the salinity levels of our oceans which are closer to 35 grams of chloride per liter of water. But even the increase seen thus far is cause for alarm. Given the rate that these chloride levels have been rising, in conjunction with the fact that this rate has been accelerating in recent years, it may be only a few decades until this freshwater isn’t so fresh. It only takes around 250 milligrams per liter for water to take on a distinctly salty taste and becomes unhealthy to drink, and researchers say that if current trends persist it’s possible that threshold is met within five decades. Well before that threshold, however, it begins causing damage to freshwater ecosystems.
Fortunately, some governments have already begun to act. It has been known for decades that chloride levels in freshwater bodies have been on the rise. Governments understand the risk that this poses to local ecosystems, residential and commercial use, as well as the expense of trying to tackle this problem if it becomes more severe. As such, research is being conducted on how to reduce the amount of salt that’s being added unnaturally through means such as road salting and other methods of polluting.
It’s important to keep in mind that due to natural processes, salinity levels will never be zero. A little salt isn’t a bad thing, but as the relative level increases it can cause numerous issues. Furthermore, the rate that humans have been increasing the chloride level in freshwater isn’t unmanageable. Research shows that roughly half of the extra salt being dumped into these reservoirs comes from road deicing. Again referencing Lake Michigan, it’s been found that roughly 70% of the salt being added comes from just 5 of its 300 tributaries. If the communities responsible for adding these excessive levels of salt to these tributary rivers could reduce their usage by even a modest amount, it could be enough to get the lakes back to a manageable level.
In cold climates, proper road deicing is of utmost importance; even a small buildup of ice on roadways can be life threatening. The solution doesn’t have to be no salt, but alternatives should be explored to use salt more efficiently and supplement when possible. Michigan State University has been at the forefront of this issue, both in terms of researching the rising chloride levels and coming up with solutions to address it. Currently across their campuses they use a salty brine instead of traditional rock salt, which both reduces the amount of salt used and applies it more evenly. By using this method they’re helping to reduce the amount of salt that runs off into the Grand and Red Cedar rivers, and thus into Lake Michigan.
The case of rising levels of salt in our freshwater is a testament to environmental monitoring and protection. Fresh water is critical to the survival of life on Earth, and civilizations rise and fall depending on their access to it. At this point governments are aware of the problem and are beginning to take action to nip it in the bud. It’s quite possible that several decades from now it has become a nonissue that fades into the background, never causing concern for the average citizen. However, if it goes unaddressed the threat remains. It’s important that we have researchers and government agencies putting their time and effort towards protecting that which we hold most dear and remains so critical to our wellbeing as a species and society.
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