Why This Is Essential
Knowing the specific geology of your own well can offer you a glimpse into the potential sources of contamination, on top of just how much water that your well could be capable of pumping. It goes into the science of why certain wells can run out of their water when others still have plenty to offer. It can explain why some wells happen to be more susceptible to contamination as compared to others.
About The Hydrologic Cycle
Water is always moving. It moves through many processes. These include but are not limited to transpiration, percolation, plant uptake, infiltration, runoff, evaporation, and of course precipitation. The water which you drink today has probably been through countless trips through this hydrologic cycle in the past. It’s been on different continents, it’s spent time in the oceans, and it’s been drunk by thousands if not millions of others in the past. It’s an ever-changing and yet continuous cycle.
The water that’s in today’s ground infiltrated there from the surface. What didn’t get used by plants or just held in the topsoil wound up migrating down through the entire soil zone into the water table where it turned into groundwater. When it’s groundwater, it’s held on fractures, pores, and other open spaces between rock and soil particles. The water table is a level where pore spaces are saturated or totally filled with water. However, things don’t stop there. Since water flows downhill, any pressure from up above it will push water through the ground towards areas that have lower pressure. This happens both vertically as well as horizontally. It might keep migrating downward through a number of various geologic units, and it can also move horizontally through just one geologic material until it reaches a point of discharge. Throughout the hydrologic cycle, a point of discharge typically proves to be any low point in the landscape where water might discharge to an ocean, stream, or lake. Here, the cycle begins again. Sometimes, it is possible for water to spend centuries to even millennia in the ground prior to finding its way back up to the surface.
The recharge of groundwater is usually a local process, although this is not always the case.
First of all, rainfall will infiltrate into the ground, making its way into the below geologic formations. There, it gets stored in pore spaces that are between the soil materials, be they unconsolidated sands, clays, or silts; it can also get stored in open crevices or fractures in the bedrock, which is a consolidated material. Your well will tap into such geologic units, letting you pump water up to your home. In some other areas, the recharge isn’t quite so local. In some of the deeper aquifers, the area which recharges the aquifer might be a considerable distance away from the well you have. The distance is sometimes a mile, but it can also sometimes be many multiple hundreds of miles.
Aquifers are geologic units. They’re usually consolidated bedrock or unconsolidated gravel and sand. They’re able to offer serviceable quantities of water to your well. This, of course, means that there needs to be enough water available in that aquifer in order for the well to pump. This is known as porosity. Also, the water needs to be able to flow through an aquifer with enough ease that the well can be supplied even when it is getting pumped. This is known as permeability. The specific geologic units which slow down the movements of water are known as aquitards or confining layers. One single location can have multiple aquitards and aquifers at the same time.
Porosity is a measurement of the volume of pore space per unit relative to volumes of geologic units. So, when an aquifer has a porosity of 25 percent, then it means that a quarter of the overall space that it occupies is a void space, filled with either water or air. Porosity is something that can vary quite greatly. For instance, an aquifer of sand and gravel might have a porosity range of 20 percent to 30 percent. Clay sometimes has even higher porosity, getting towards 50 percent. Bedrock is something that can be quite creviced and fractured, meaning its overall porosity is quite moderate, although it can also be quite solid throughout, which means its porosity is just a few percentage points If you have a well in a low-porosity geologic unit, then you might only have a limited availability of water. In a lot of bedrock aquifers, when a well doesn’t go through a multitude or just large crevices and fractures, then the water availability might just be limited to several gallons a minute, or even less.
How easily water might flow through any aquifer can also determine the capability of well providing water. This is known as the permeability of the aquifer. Unconsolidated but large-grained deposits such as gravel and sand usually have high levels of permeability which means that water is able to flow easily between the grains towards a well. However, as the particles become smaller, then it will get harder and harder for water to actually move between them, which brings the permeability down. Really small particles such as silts and clays might have a high porosity, as much as 50 percent water by volume. On the other hand, since it’s so difficult for water to move around these particles towards a well, then geologic units which are made up of silt or clay are typically designated as aquitards. Permeability proves essential since a well typically draws its water from a portion of the aquifer that is around that well. When water isn’t able to get freely to the well, then it can limit the rate of pumping. Pumping at too great a rate will draw down well water to the actual pump intake, and that can interrupt the water supply.
In most bedrock aquifers, the majority of the water gets stored in crevices and fractures. in order to be an actual aquifer, bedrock needs to have enough crevices and fracturing, on top of connections in between such openings, in order to supply enough water to a well. This kind of rock isn’t very porous itself, and it won’t hold a lot of water. Sandstone is an exception to this, as it not only features crevices and fractures, but will also hold water right in the actual rock when it’s not totally cemented. Gravels and sands are able to let water move through them at rates as high as 3,000 daily feet, although certain clays might restrict water movement nearly entirely.
This isn’t to suggest that water moves through every aquifer that quickly, but it is an upper limit. High permeability can also mean that a well is pumped at higher rates with not as much drawdown, which is the well water level getting lowered, as compared to a well that’s in a lower-permeability aquifer.
The aquifer from which you get your water, as well as how that particular aquifer gets recharged, can tell you much regarding your water supply. As with other natural resources, any groundwater which is available to you is based primarily on the local conditions, specifically your location’s geography. In certain cases, you could actually have choices if there is more than one aquifer available. However, for many, they might only get one single choice. For other well owners, they might not even have a genuine aquifer to provide them an available water supply. There might not be any gravel or sand aquifers, and there might not be reliable bedrock aquifers either. It might be that deeper geologic unit groundwater just isn’t potable, meaning it’s not safe to drink. One instance of this might be a bedrock aquifer that has saltwater. In such cases, a shallow bored or dug well might be the only option for groundwater. Such wells are typically intended to be just storage reservoirs, as they collect water from the near-surface infiltration and/or any thin lenses of available sand in an area which otherwise has primarily silt- or clay-based geology.
If you want to learn more about the specific aquifer supplying your well, then you should consult any agencies that conduct geologic mapping projects and maintain well logs in your state. In most states, that will be something like the state’s Department of Natural Resources, your state’s Department of Health, or your state’s specific Geological Survey. In some cases, there might even be overlap among multiple agencies. In other states, the responsibility might be handled by a different agency. For instance, in Texas, it’s the purview of the Water Development Board. You should be able to find all the contacts you could ever need just by calling your municipal Cooperative Extension service or health department. Also just use Google with the search string “[my state name] well logs” in order to find any agencies that either store the well logs in your area or just have them available. Your state has put many years into mapping its aquifers in order to have a better understanding of just how much water could be available, on top of what potential water-quality issues there could be. It can be worth your time finding out who in your area or state has the power and knowledge to give you assistance. Hopefully, once you find them, they’ll be happy to help you. If you have any issues finding the proper agency, make sure to let us know about it, so we can try and help out.
Your driller and well log are likely the best sources of information regarding your own well. Your state agency or driller can explain the local geology to you. If you don’t actually have your log, then have your state agency help you find it, should it be on file. You might also contact your driller to ask for a copy. If you wind up contacting your state to look for the log, then you’ll need to know several things. The first is a legal description of the location of the well, the second is the latitude and longitude, and the third is the depth of the well. If your well has no log, or it’s old, then you can still use nearby logs to find out local geology information. If you don’t know a lot about your well, then you might want to have a professional pump installer or driller to visit the property to get such information. When you don’t know where your pump is set, where your well water comes from, or your well depth, then you are missing information that you might require if you have an issue with your well or water supply.