I am trying to install a submersible pump in an old drilled well that has not been in use for forty years. It is 120 feet deep and the water level is twenty feet from the top. I understand that most wells have a well screen and the pump should be installed about ten feet above the screen. My problem is that I don't know how deep the well screen is and therefore how deep to place the pump. How could I find the depth of the top of the screen? Alternatively, what would be a good guess as to how deep I should place the pump? I don't know how much water the well will produce. The pump is 1/2 HP. I will use it for occasional outside watering, probably about five gallons per minute for a couple of hours at a time.

Thanks for any information.