How to find a leak in your rain cistern
Updated: Feb 8
Have an old cistern? Suspecting that it may be leaking somewhere? While there are no easy methods to finding leaks, here are some tricks we've learned over the years:
1) Start by ensuring that the leak is coming from the cistern and not, say, your plumbing or an underground water line. First, if you have a pressure gauge inside your house on your pressure tank, watch it. Without any water running inside your home, is your pressure gauge dropping? If so, that indicates that there's a leak in your water line. In order to narrow in on where the leak is coming from, close off your whole house shut-off valve (usually found directly after your pressure tank). This will isolate your system so that you're reading the pressure between your cistern and the shut-off valve. Is the pressure gauge still dropping? If so, your leak is between the cistern and the shut off valve (and, most likely, it is a leak in the water line inside your cistern... perhaps a faulty foot valve, if you're using a jet pump). If the pressure is not still dropping, then the leak is after(or downstream from) your shut off valve (most likely, you have a leaky toilet).
2) If your pressure gauge does not drop when you look at it, then, before you go to bed, shut off the power to your pump and take a measurement of your cistern water level. Try to be as exact as possible. Write down the measurement. Then, in the morning when you wake up, take a measurement again in your cistern and see if it dropped. If the measurement dropped, you likely have a leak in your cistern tank.
There are a few different ways to try to track down a leak. If you have a concrete tank, then chances are that you're due to coat the tank with a potable-grade waterproofing cement, like MasterSeal 581 (PLEASE take precautions when coating or parging the tank yourself to ensure proper ventilation and respiratory protection). But, you may be able to "patch" the cistern with hydraulic cement by tracing the leak one of three ways:
1) The waiting game: Don't use the water for several days and see where it stops dropping down. That would be the location of your leak.
2) Food coloring: Get inside the tank when it has water in it, and inject a brightly-colored food coloring via a syringe into the water along the perimeter of the tank. With a flashlight and a lotof patience, see if you can tell where the food coloring moves. If it sucks into a wall, that's an indication of a leak.
3) Pump down: My favorite method is to pump down the tank all the way and, while it's pumping down, get into the cistern. A lot of times (especially with cisterns made of concrete block), the leak points will send water back into the tank once you've pumped it out. In other words, you'll see a stream a water or a steadily running drip where you have compromises in your tank wall, because water that previously had been flowing out into the ground will now come back into the tank once you remove the water, since the empty void of the tank will now be the path of least resistance. This method works quite well, but the downside is that you have to drain out your precious water supply.
I hope this helps provide some guidance in what can be a frustrating and trying process. Remember that, when in doubt, re-coat/parge the tank or install a cistern liner!