Alkalinity of the San Diego Water Supply: Some Basic Information

Alkalinity is a concept that people run across more often in everyday life than you might think. If you’ve ever kept an aquarium, gone through the trouble of maintaining a pool in your backyard (and to those of you who do: kudos, I’m a chemist by education and pool chemistry is still time consuming enough that I’m happy to pay someone else to worry about it!) or spent time trying to figure out a way to get rid of that chalky white stuff that might build up around your kitchen tap or shower head have firsthand experience with this. Simply put, alkalinity deals with two concepts: the amount of calcium carbonate in the water which is where the chalky white rings that make all your faucets stop working comes from, and how well the water resists acidification.

Water that begins to exceed 120 parts per million (a unit of measurement meaning 1 molecule of calcium carbonate per selection of one million molecules that is 

also approximated as milligrams per liter) is what is referred to as hard water. The highly technical term for water with exceptionally high concentrations of calcium carbonate, ‘very hard water’ runs anywhere from 240 to over 1000ppm. One of the interesting things about hard water, unlike many other environmental problems, is that as a health concern it is actually relatively innocuous. It will do a number on your house’s pipes but not your pipes as it were. Really, the only ‘harm’ it can do is to your palette as a number of people find the taste of find the taste of unfiltered mineral water (not the bottles you pay an exorbitant amount of money for at the grocery store) rather unpleasant. Which is why San Diego like most cities, though granted more for the longevity of its pipes than our personal taste, measures and treats its water for calcium carbonate.

The table below contains the most recent data on Alkalinity and Water Hardness running through the city’s three primary water treatment facilities in 2016.

(The chart might be a little tricky to read due to some formatting disagreements so a link to the full report is included here, we recommend checking out page 15: https://www.sandiego.gov/sites/default/files/2016_annual_drinking_water_quality_report.pdf)

Undergraduate students at the University of San Diego recently took measurements including the Tijuana River, Tecolote Creek and Mission Bay, the samples ranging between 30 and 400ppm placing the average of the extremes pretty much right in the range of what the city’s reporting reveals. Fortunately for us, the treatment plants do a pretty good job of keeping enough calcium out of the water so that we don’t have to worry about water softeners (shout out to my fellow Midwesterners who know what those are and how noisy they can be).

BUT WAIT, ISN’T THIS AN ENVIRONMENTAL BLOG?

Well since you insist yes it is. Alkalinity in water can do two things and like most things in nature it’s something of a balancing act. Sufficiently alkaline water can resist pH changes which is good for animals that live there. Acidified rain or runoff can lower the pH of even large bodies of water and if the pH changes too quickly and can kill even species that ordinarily could accommodate large pH ranges.

However, as the above diagram indicates, carbonate salts that contribute to the water’s alkalinity can also bind to carbon dioxide to form carbonic acid. This reduces the amount of carbon dioxide available in the water and as it turns out, underwater plant life needs carbon dioxide too. Water with exceptionally high alkalinity can kill off plant life by depriving them of carbon dioxide. By virtue of volume this normally isn’t a problem for large bodies of water, not to mention the trend is towards acidification so generally speaking a little more limestone leaching into water in many places (including San Diego if it were present it larger quantities) would not be unwelcome. For the time being, it remains more of a problem for those creating simulated environments like aquarists.

So, unlike our last conversation, there is much less doom and gloom to report. The controls in place already work well for our water systems, and given prevailing trends surface water alkalinity is rarely an issue. If anything, artificially increasing the alkalinity of some surface water might be a solution for acidification, but that is a conversation for another time.

By Shearing Volkir

References:

San Diego Water Annual Drinking Water Quality Report.  Report. City of San Diego Public Utilities Deparment.

De Haan, David et al. USD environmental chemistry lab notebook

Photo Credits: Ocean acidification and faucet images taken from non-copyrighted public domains. Alkalinity and water hardness table provided by San Diego Public Utilities 2016 Annual Drinking Water Quality Report.

Disclaimer: The opinions represented herein belong solely to the author and are not necessarily representative of the University of San Diego or the USD Department of Chemistry and Biochemistry.  

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