I’m a member of the Chili Line Forum that is managed by Ernie Barney. Years ago I posed the question on why some water towers have what appears to be lime/calcium (limescale) build up on the sides of the tank. I should have also asked why do some towers appear to have black mold on the sides. I had also failed to mention that the tannin in the redwood would also cause staining. Fortunately one of the members of the forum brought that to my attention. Another member speculated that maybe additives were added to the water as a means to deal with the highly mineralized water. I posted a version of the following on the forum site:
In most cases the white staining on the exterior sides of wooden (most often redwood) towers is calcium carbonate (CaCO3). It is a common substance found in rocks as the minerals calcite and aragonite. They both are a type of sedimentary rock consisting mainly of calcium carbonate. Calcium carbonate is created when calcium ions in hard water react with carbonate ions to create limescale.
Towers that get their water directly from a river or a shallow spring have a better chance of having more black mold on the surface of the tank and less limescale. Towers that get its water from a well or aquifer have a greater likelihood of having considerable limescale buildup. Usually, the deeper the well the greater the mineral content, therefore the greater amount of limescale.
I am sure that you have all seen photos of towers with limescale shaped like a series of triangles pointing up with mold on the outer edges. Lime kills mold but the process is not rapid and equilibrium can be reached on the outer edges. There will always be more limescale or mold buildup at the base of the tank than towards the top. We can’t say that a triangular pattern is the norm.
In my most humble opinion if a model railroad is to achieve a reasonable degree realism an appropriate amount of weathering must be employed. I believe that if more modelers understood why/how water tower tanks weathered they would be more willing to weather their tanks and do it correctly.
As I said above, a member of the forum pointed out that staining on wood towers could also have been caused by the tannin in the redwood. Tannin stains tends to be more gray than black and the staining is more indiscriminant. You will note that on my yellow water tower the staining is more indicative of tannin induced stains rather than limescale or mold. The red tower has more limescale and a little mold. The unpainted gray tower has some limescale and tannin. The gray is caused by the natural aging and weathering of redwood. Don’t ask why I weathered the three towers the way I did. I have no idea.
I’ve often wondered about water treatment. For those of us that have ridden the Cumbers & Toltec, the Durango & Silverton and other such scenic railroads we have witnessed boiler blow-downs to remove sediment. While I am clearly not an expert with how railroads dealt with highly mineralized water I do have some appreciation for and experience with water treatment. That said, I do not believe that chemicals (additives) were added to water tower tanks directly. As we know, the point of a water-treatment plant is to reduce boiler maintenance cost and ultimately increase the life of the boiler/tubes. It is my understanding that on narrow gauge and branch line railroads water treatment facilities were nearly nonexistent. Starting in the mid 1940s some narrow gauge and branch line railroads would put demineralizing chemicals directly into the tender. Prior to that, I believe that the only viable option for dealing with highly mineralized water was for the railroad to require a higher frequency of boiler blow-downs and washes (flushing of the tubes).