FAQ - Groundwater Quantity

Not really, although it’s a lovely vision. Groundwater is hosted in both unconfined shallow aquifers and confined bedrock aquifers. The distribution of these water-bearing units in the subsurface is directly related to the geologic history of the region. And frequently, the geology is quite complicated. You might have an unconfined aquifer inset into a bedrock aquifer such that some wells yield 500 gallons per minute (gpm), but a well a mile away only yields 5 gpm. Even within a rock unit, there can be a lot of variation in rock type throughout the thickness of the unit. This means you could have mudstone in one area, but move over 100 feet and have a nice sandstone aquifer. Also, the thickness of various aquifer units can vary quite a bit regionally. This makes it really tricky to understand where your groundwater is and how much you have.

The geology that hosts our groundwater systems is quite complicated. Sometimes you are indeed drawing from the same aquifer as your neighbor. If their well is significantly deeper than yours, and they draw on it, they can produce a cone of depression that will pull the water table down below the bottom of your well. However, sometimes two wells that are close together are actually in different aquifers, either because the rock type has changed between wells or because one well has been drilled much deeper into a different aquifer unit that is separated from shallower aquifers by an aquitard (an impermeable rock layer that acts as a barrier to flow). If you notice your well’s behavior changes significantly when your neighbor is using their well, that can be a sign that they are connected. It’s a good idea to try to understand the local hydrogeology to determine if you’re in the same or different aquifer units. Sometimes wells go dry because the local aquifer they were drawing from has simply become too depleted to use: this may not necessarily be because of a nearby well being used.

We use either a measuring tape or a pressure transducer (also called a data logger). If we’re measuring the depth to the water in a well with a tape, we use a graduated steel tape that is thin enough to slip past pumps, tubing, safety lines, sucker rods and other things that might be in your well. If the well is empty, we can also use an e-tape that has a probe on the end of it. This type of measurement provides a spot reading of the depth to water at that well. If this measurement is taken twice a year (summer and winter) over several years, we can see a pattern in the behavior of a local water table over time. Pressure transducers are small instruments that are installed in the well and can be programmed to take a reading every day. These provide a much more detailed history of the water table over time.

We can use two different data sets to help determine if your local water table can recover if there’s significant precipitation in your area: static water levels and tritium isotopes. By measuring static water levels over several years, we can gain some understanding of how your local water table behaves. If it is constantly declining, even when you get a good rain or snow year, that’s a sign that precipitation is either taking a long time to percolate down to the water table or it is not reaching the water table at all. Tritium, an isotope of hydrogen, was released in large quantities during atomic bomb testing in the 1950s. So if there is measurable tritium in your groundwater, it indicates that post-1950s water is making its way into the aquifer system. However, tritium does not indicate the quantity of recharge, just that there is some entering your local water table.