The water released through automatic hydrostat valves attached to the well points under the main outlet sumps is as clear as spring water. Most lay people expect it to be dirty because of its source in the ground. While this may be true shortly after construction, or during construction, the gravel sub-base and soil functions similar to that of a gravity filter. I’m sure this information can be corroborated by Andy, Bob and Scot, all of whom have emptied and cleaned pools.
Sometimes pools are stained due to construction dirt in the below grade pipes which can be washed into the pool if the tank is being filled for the first time through the surge tank and subsequently through the main drain lines. For this reason pools are often filled through the gutters (main drained closed and gutter pipes open) so the water overflows the gutter lip and runs down the walls. When the pumps start, the dirt in the suction pipes is drawn into the filters. Another advantage to this type of filling is that it can warm the water, flowing down the walls, several degrees inside a warm natatorium and, inversely, cool the walls and floor gradually, thereby reducing the variable between the incoming water and the warm pool shell. As most of you are aware, cold water on a warm shell can result in thermal shock and the cracking of the pool tank.
Another misconception by many is that ground water must be at or below the main outlet elevation to avoid foating the pool tank. Field experience, and the laws of physics, proves that this is not the case due to the ballast weight of the concrete pool shell. During the early years of my pool management company, I personally emptied several hundred pools. In a number of cases the automatic hydrostats opened and ground water ran into the pool for as long as thirty minutes while the submersible sump pumps were extracting the water out of the outlet basins onto the deck. The pool at Country Hill (which was located at the top of a hill) had so much ground water one year, due to rains, that water started squirting up to 6 inches high through the expansion joint at the 5’ break when there was still several feet of water in the deep end. The hydrostats were plugged rather than fitted with check valves. There was a case at a swim club in Collinsville which was down in a small gorge with a clubhouse and tennis courts approximately thirty to forty feet above. When one of the pool cleaning crew unthreaded the brass hydrostat plug in the main drain, it shot over the three meter diving board. For that reason everyone knew to stand aside and not be over a hydrostat when the plug was removed.
Hopefully this specific background information will be helpful should you have an occasion to discuss the subjects with clients or operators.