Water Table depth below Surface (Depth to Water: DTW)
To measure the water table using this technique, the top seal of the well must be lifted slightly along with the pipes and moved to the side (with the pump pipe still passing through it), and the drilled depth of the well must be known. The well must be turned off and let rest for several hours so that the water table will reach a static level. If you do not know the flow rate of your well, it should be shut off over night and the measurement made in the morning. The tube can be installed the day before measurement. A hand calculator may come in handy for the calculation.
The kit consists of:
- a reel of Teflon tubing of 300 feet with a weighted rod attached to the bottom to help the tube drop down the well. The tube is marked in 50 ft intervals
- a metal T attached to the high end with an air valve in the cross of the T. It is taped to the reel to avoid flopping and getting in the way when lowering/raising the tubing.
- a battery driven air pump with included digital pressure meter
- a battery pack with 12v receptacle for the air pump (it is a car starting battery set). The battery is charged when the kit is requested, but it can be plugged into its own charger overnight to make sure.
With the well seal moved aside, lower the tubing carefully into the well avoiding contact with the casing edge. If you feel some resistance, lift the tube gently a foot or two and then let it drop (bobbing). Knowing the drilled well depth, lower the tube until it will go no further. Try the bobbing a few times to make sure. Also note the number of 50 ft marks that have been observed to help confirm that you are at or near the required depth. Note the height of the casing above the ground when estimating the bottom using the markers. Over time some wells may have silted in at the bottom, so the depth might be several feet shorter than the original drilled depth.
Attach the air pump to the T valve tightly, and then plug the air pump into the battery. You should note the pressure display will light up showing 0 psi. Have a notepad ready to record the display reading(s) .
Press the red on/off button on the pump to turn it on. Watch the pressure increase as air is pumped down the tube. When the air pressure is enough to cause air to bubble out of the tube bottom, the display will reach a stable number. The pressure display will show some variation….eg + – 0.1psi at this point. The time to reach this state will be several minutes. A 150 ft well will be about 2 min. Time can vary with the type of air pump and the charge on the battery. Record the pressure when the pump is still running.
Shut off the pump. You may wish to repeat the measurement just to confirm the reading.
Measurement-take the pressure reading and multiply by 2.31. This will give you the calculated height of the water column from the bottom. You will then subtract the water column height from the drilled depth of the well. The difference is the depth the water table is from the surface. [eg for a 150 ft drilled well and a calculated water column of 100 ft the DTW will be 150-100= 50 ft.]
If your well is deeper than 200 ft, the air pump will take longer to reach the stable pressure reading or may not be able reach that point. In this case, lower the tubing to some depth using the 50 ft markers on the tubing (say 200 ft) and then make sure the tube can’t drop any further during the test. You would then perform the measurement and record the calculated water column height, which in this example would be the column above 200ft. So if your well was say 270 ft deep and you measured a water column of 80 ft, then the actual water column would be 80 ft plus the 70 ft the tube bottom is above the well bottom or 150 ft. The DTW would then be 270-150= 120 ft. This method may require a few tries to get the tube down into the water.
Draw-down is the amount the static water level in the well drops to when pumping. The draw-down will vary with the seasons as the aquifer changes, the pumping flow rate set, and the length of pumping time. There might be two draw-down levels. One level (the real draw-down) results from continuous pumping until a stable level is reached. This would mean a match of pumping volume and inflow. In the dry season however it is possible for the level to reach the level of the pump intake and take in air at which point, the pump controller should sense an over speed pulse and shut the well down for a pre-set resting time. The amount of resting required depends on the recovery time (see next heading).
Another type of draw-down is one that relates to the pumping time of your well which might be significantly shorter than that of the real draw-down. I’ll call this the operating draw-down. This can happen if you have a storage tank or pressure tank of a size that would be less than the volume of the water column in the well (6 inches is normal well diameter, so pi r squared times the depth of the static water column) and probably a good inflow to the well. The pump would be shut off by the level controller in a storage tank, or the pressure limit in a boost pump/tank. This level could be higher than the real draw-down and gives you some safety factor from over pumping.
From either the real or operating draw-down level and the static DTW you can use the monitoring kit to establish recovery times. You will have to experiment. I suggest starting with 15 min intervals so see if there is change. You will know the static DTW so can see how fast or slow the well is recovering. Some wells will recover to 90% in15 min, and others might be hours.
The air tube system can have small air leaks that would cause the reading to drop after the air pump is shut off at the stable reading. With no leaks, you should be able to watch the water level recover on the air pump display. Practically, I always release air from the tube and drop the air pressure to near zero, then start the air pump again at the predetermined interval.
Knowing the recovery time (summer and winter) helps you schedule the shut off resting period when wishing to measure the static DTW. It also can help avoiding over pumping by having a flow control valve in the pumping line and slowing the pumping flow to match your known safe draw-down. Knowing the recovery time from the real and operating draw-down will provide some ability to set a safe draw-down.
By looking at the drilling log for your well you can get some information about the levels that inflow occurs. This depends a lot on the drillers record keeping. Knowing the inflow levels and static DTW will give you some insight about when to be concerned. Building up a database takes several years and you will find that measurements are not required for several months in the winter. You will just get to know what the lowest DTW’s are and when the water table starts to drop (usually within a week or two of when the rainfall drops very low or stops). In the summer you might want to check the depths weekly for the first year. I set one month as the normal interval and go to two weeks in the dry season if the water table has dropped to/below recorded lows. You will also find out how fast the recharge happens and how closely it relates to the rainfall pattern
Installing a meter on your pump output is probably a good idea. You can see changes as you adopt conservation strategies and see what your normal highs and lows are…and when.
Data can be collected about the aquifer if several of your neighbours also get involved in well monitoring and metering. Nearby wells may not react the same at all. If they do, that might show some stronger coupling below the surface and might indicate potential interference. Water chemistry analysis right after full re-charge and at the seasonal low (Aug-Sept) provides other insights about well casing integrity or the well being under the influence of surface water. Remember, a lot of the island has only a few inches of soil and once saturated, water can freely enter fractured rock aquifer.
This is a living document. Comments and suggestions are welcome. The air tube technique is only a start because it is relatively inexpensive. A wetted tape is more accurate and there are submersible data loggers that can be downloaded to a computer. Anything that is put down the well routinely can be a problem by catching on the pump wires, so in the long term a conduit advised. All of these cost more. If you are doing more than getting a dry and wet season static level, you should consider installing your own tube down the well on a permanent basis. Sharing the air pump and battery pack with a neighbour would help reduce the cost.