Measuring groundwater levels in a well
“You can’t manage what you can’t measure” they say, but how about when what you’re measuring is groundwater- buried far beneath your feet? Determining groundwater levels can be tricky without specialized instruments, but in this article we’ll describe a simple system you can implement at home for measuring water levels in a well, using only basic materials, a pressure gauge, and a simple formula to accurately measure changing water levels throughout the year.
Most domestic wells in our area consist of a well casing pipe (6 in) extending deep into the earth, with the submersible pump pipe (1.25 inch) and cable enclosed within. Often there is enough extra space within the casing to run an extra tube, which we will use to fill with pressurized air for our groundwater measurement. In this example project we used 3/4 inch PEX tubing, a type of flexible polyethylene pipe approved for domestic water, which comes in a roll so you don’t need to couple or glue sections together. If for whatever reason you are already needing to pull your pump, then that would be the time to implement this additional measuring system, rolling out and attaching the air tube to the pumpline as it is reinstalled down into the well. Though it may also be possible to install a rigid tube down into the well without pulling the pump.
The tubing is pre-measured and cut at exactly 90 ft. long and then secured to the pumpline with zipties to ensure it stays stretched out and doesn’t kink. The bottom end of the tube is open, 90 feet below grade within the well, and the top of the tube is connected to transitional fittings 3/4 x 1/2 push-to-fit to a multiport air pressure fitting, equipped with a permanent air pressure gauge, and a shrader type tire fill valve. Some of the tube at the bottom is already filled with water, and some near the top is filled with air, @ zero pressure (0 PSI).
Connect a bike pump to the fill valve and start pumping! Each PSI (pounds per square inch) you pump into the tube will push the water in the tube down exactly 2.31 feet. You will see the pressure gauge keep rising, until it reaches a point where it will remain and cease to rise, no matter how much you keep pumping. At this point the tube is completely filled with air, and any extra air you pump in will simply come out the bottom and bubble out of the well.
Our measurement was 19 PSI. 19 x 2.31 =43.89 feet. We know that the total length of our tube was 90 feet, so we subtract 43.89 from 90 and get 46.1 feet water level! Taking into account the height of the risers and associated fittings, it looks like groundwater level is at 45 ft below grade.
If the 2.31 constant looks familiar, you may have seen it in your irrigation design course as it relates to natural pressure changes that occur in pipelines due to elevation (0.433 PSI per ft of elevation). 0.433 pressure gain over 2.31 ft would equal a pressure gain of exactly 1 PSI.
What was of most interest to us was understanding the relative changes in water level that occur during heavy irrigation season, vs winter rainy season. Interestingly enough, we found that the measured water level in the well does not change substantially during even the driest part of year, despite the extraction of tens of thousands of gallons during the 7 month dry season to irrigate approximately 2 acres of intensive food production. We took this as confirmation of the success of a number of groundwater recharge provisions we employ including reengaging floodplains, maximizing retention and minimizing velocity as we slow, spread, and sink all available stormwater that flows through the site.