I got to wondering if it would be practical to add salt to the water to lower its freezing point.

You can dissolve 35.7g (or .610 mol) NaCl in 100mL water at room temp. This would translate to 6.10 molal (.610mol * 1000mL H2O/1 kg H2O * 1/100mL H2O).

This would depress the freezing point as follows:

T_f = K_f * b * i
T_f = 1.853 * 6.1 * 2
T_f = 22.6 C

So this heavily saturated solution would freeze around -23C or -10F.

What about the cost? In bulk, salt is about a dollar a pound, so 357g would cost about 80 cents. But this is way more expensive than clean water in the US: 80 cents can buy you close to 2000L of water.

I would say that this isn't remotely practical, even though we wouldn't necessarily have to totally saturate the water with salt. However, I didn't even consider the fact that the salt would probably cause a lot of damage to the firefighters' equipment through corrosion. Perhaps there is a cheaper ionic compound with a better Van't Hoff factor out there, but we'll probably just have to deal with the ice. It was a fun little exercise anyhow.