Concrete cisterns are, in our experience, the best type of tank for underground drinking water systems. Rainwater tends to have a lower pH (acidic), while concrete has a higher pH (alkaline). As rainwater fills a concrete tank, the concrete actually helps neutralize the water and balance the pH — if not slightly alkalinize it. After the cistern has gone through a few empty–full cycles, a litmus test of your cistern water will typically reveal a pH of 6–8, the optimal range for a good drinking water system.
While concrete cisterns help neutralize acidic rainwater over time, initially the residual concrete dust from the manufacturing and casting process will elevate the water’s pH to high alkaline levels (10–11). This is completely normal. For new concrete cisterns, however, precautions should be taken to bring the water down to the 7–9 range before drinking. There are two approaches:
1) Flush the tank. Haul in a load of water from a local water hauler (in Ohio, a list of haulers can be obtained through your County’s Department of Health or from the Ohio Department of Health Office of Environmental Health). Use that water to rinse the walls of the tank and remove as much concrete dust residue as possible. Do not drink this water — pump it out after a few days or use it for non-potable applications (flushing toilets, showering, irrigation, etc.). It may occasionally take two or three full empty–fill cycles before the water inside the tank reaches a safe pH level of 9 or below.
2) Pressure wash or scrub. Once the tank has been installed, use a pressure washer with a 55-gallon drum of clean water and a cup of unscented chlorine bleach to wash the walls of the tank. You can also use a scrub brush with a water/chlorine mix and hand-scrub the walls. This removes the majority of concrete dust from the side walls. Then pump out the water from the bottom of the tank when possible (you may need to add more water to be able to pump the dirty water out). If pumping isn’t feasible after washing, haul in a load of water but treat it as non-potable — use that first load for non-potable uses only.
These precautions may sound intimidating, but the work on the front end is minimal compared to the lifetime of fresh, clean water you’ll receive from a concrete cistern. As always, thanks for reading.
