They used an even easier method than what's described in the other comments: the salt comes from the initial construction of the SPR caverns. When they first b

They used an even easier method than what's described in the other comments: the salt comes from the initial construction of the SPR caverns. When they first built the SPR, they filled it with water to wash away any easily-dissolvable salt features as well as any impurities inside the cavern. The result of filling the cavern and letting it sit for a while is saturated brine: eventually all the salt that can be dissolved is dissolved. Then they pumped that brine to aboveground brine ponds. Those brine ponds contain exactly the amount of salt water needed to completely fill the cavern with saturated brine. So future drawdowns just use the same water that has already been saturated. Some of the water will evaporate on the surface, leaving behind salt crystals at the edges of the brine pond, but the crucial thing to remember is the quantity of salt in the newly created brine pond is equal to the amount of salt needed to generate enough saturated brine to completely fill the cavern. Because that's where it came from in the first place. So if you need to replenish it with more water, you just pump in fresh water, it dissolves the extra salt in the brine pond, filter out any surface guck, and then pump it back into the cabin. Periodically you might want to add a little extra salt if say the cavern expands or wind erosion carries salt away from the dry brine pond bed. But this quantity is much less, it's the delta in cavern size, not the full initial size of the cavern.