Storage Tanks for Low Flow Wells (or Rain Water)
Every kid who made it through the third grade in school knows how to do word problems, or at least should. I used to love word problems. I realized, even then, that they were an adult way to show us kids that math is practical. Well it doesn’t get more practical than the problems described below, and the word problems that result are simple. Getting an adult to understand it and look beyond the $$$ is the hard part. Kids get it. The money thing is usually the adult’s problem.
In the Lowcountry, we cannot always get a lot of water from a shallow well. The Deep wells are generally too salty to use for irrigation. What should you do? I am positive that if some oil tycoon was posed with that question, he would drill sideways until he stole all of his neighbors water! But since that is illegal, we won’t consider that option.
Does a low flow well mean that it will fail? To answer these questions, we will look at “out of the box” solutions that we have done in years past and today, in order to meet the current water demands of irrigation and drinking water.
If you recall another blog on “How much water are you able to get us” you will remember that most folks want to water directly out of the well and into the irrigation supply line. Certainly it would be wonderful if we could get 20 gallons per minute and just water everything zone after zone. But it does not always work that way.
For years, irrigation contractors have pumped out of ponds, because it seemingly was an endless amount of water. When the droughts came, and the ponds ran low, a lightbulb went off! Eureka! We will drill a well, and then pump it into the pond. Guess what? It worked.
My Grandfather used to tell the pond digging contractor to show him where he would be digging the pond. He would back the rig into the center of where the pond would be dug. Drill a 40 or 50 ft shallow well (assuming there was water there) and then tell them to dig around the well. After pond was dug, it takes a while to fill (several days sometimes). A man was lowered into the pond (sometimes on the trackhoe bucket) and would cut the casing off below the water table, which essentially created a flowing well that would keep the pond fed and the levels at the shallow water table, which back then may have been 2 ft or less. Now it is 10 ft or more. New rules that the health department enacted in the 80’s prevents well drillers from doing that little trick, but it worked back in those days. Mostly we drill a well, install a pump, get electricity to it and pump into the pond. These were the first irrigation cisterns dating back a hundred years. Cisterns for drinking water, usually rain collection, however gross we may think that is, was commonplace in downtown Charleston 200 years ago. The rain water was fairly pure as it fell from the sky. How you collected and stored it was the determination of how pure it stayed. Municipal distribution was not very commonplace back in those days and certainly where they had it, it was not reliable or safe usually. This made the Cistern popular.
Today we have our own “tricks”. We may drill a shallow well where the quality is exceptional, but the flow is terrible, but consistent. You may be able to water directly with 20 gpm, but with 3 gpm you cannot practically water directly onto the grass. Drip systems will work, but who drip irrigates their grass? A more practical solution exists. It is an atmospheric storage tank (or Cistern). These tanks come in all sizes and shapes, underground or above ground, but the principles are the same. Low flow well pump is installed. A float activated solenoid (24 volts) is installed. Pressure lines are installed to holding tank. Holding tank may be configured in many ways, but they all must have a float inside, and sometimes two floats. A safety float will hang on the bottom, and when tank level drops, the bottom float will “break” the 24 volt irrigation common (much like an old school rain sensor stopped panel). A re-pressurization pump is installed in the tank (usually submersible, but sometimes above ground centrifugal). This pump will directly drive the irrigation. The old days of Cistern and hand pumping or gravity feeding is a thing of the past. Out West where things are much more dry, this is old technology. SC has always been lucky with our fresh water supply, but as development spreads and drought increases, we must look at these solutions. Keep in mind that these solutions require some maintenance and monitoring to ensure that the well still pumps the same each year, and that the floats are still engaging and the solenoids still working.
So to recap that last paragraph, the low flow shallow well pump will turn on when the top float drops. The float will engage a low voltage solenoid to turn on the water to the tank automatically. The pump in the bottom will only engage if a pressure sensor or a relay tells it to come on and start supplying water from the tank. Once it gets too low, it will engage the bottom float and cut off the irrigation. We usually time the amount of water in the tank, and the fill going into the tank, and then space the zones out to allow for fill. It is best not to have to engage the bottom float, and all you need to set up the zones is a stop watch, a five gallon bucket, a calculator, a pencil and paper.
Verify gallons per minute with a stopwatch and the bucket. One minute to fill is 5gpm. It helps to have gallon measurements on side of bucket, but a good estimate is ok too. Always use a spigot nearest the pump. A spigot (faucet, hose bib…etc) should pass 10 gallons per minute if pump/well is capable. A 5/8″ short hose is best to use to test. Custom variations will work as long as it is according to these instructions. We always recommend running the pump wide open for 10 minutes to get the water pumping from well, not the pressure tank. We then will test it twice. Record results. Go eat a fantastic lunch (at least an hour) while pump is running, and then test it again. This will give the best results for a real world pumping test. If the well still pumps the same flow it did before lunch, then the well is consistent. If it flows substantially less, then the water supply is in question. We test like this on every well. Some of you may have noticed I have put on weight…I always say my weight is proportional to the number of new Mexican restaurants in our area!
Once you arrive with reliable figures, then you must sit down and do a story problem. We are going to do a whirlwind tour of elementary school. Here it is:
If John drills a water well, and it only produces three gallons per minute, and he pumps the water into a 1000 gallon tank, how long will it take to fill?
Answer: 333.33 minutes or @ 5hours and 30 minutes
Ok…that was soooo first grade!…moving on to third grade…
John has an irrigation system. He waters it with the well/storage tank listed above. In his yard, it is divided into zones. He has 10 zones. Each zone uses 10 gpm. How long can each zone run and water entire yard without stopping?
Answer: 10 minutes…100 gallons per zone times 10 zones…
Ok, now we are moving into the fifth grade with this one…
If well pump fills tank while zones are running, how much water will be put into tank from well until the last zone finishes watering.
Answer: 300 gallons…This water prevents pump from running dry. Ever seen water leaving a bathtub as it nears the bottom? It creates a little tornado effect. It is the same with a pump, and cavitation can occur, which is not good. Could you calculate the use of this water to get more watering done? Yes, but don’t be greedy.
Congratulations, you just graduated from elementary math…now on to middle school algebra.. just kidding… I flunked algebra cause of that blonde sitting in front of me, so we will just move right along with some simple math…
Let us say that your space is limited, and you only have room for a 400 gallon in-ground tank. Same 3 gpm low flow well. You have six zones each at 15 gpm (this figure is higher, because it makes the irrigation solenoid efficient with more heads per zone). Irrigation should be done every third day between the hours of 12 midnight and 6 am. What this means is that you have three days to water all of the yard at night, which is best due to evaporation and absorption. Trust me the grass knows best! You generally have a Program A, B, and C. The restriction here is the need or desire to water only during the cool of the night. You are only restricted to this with grass watering. Drip can be watered anytime of the day with little consequence. And even if you watered around sundown, it wouldn’t hurt…it rains in Florida all the time during the day, and frankly I have even seen it with the sun shining! Grasses are very tolerant when it comes to moisture as long as you don’t over water. Then you open yourself up for molds and diseases that will damage it.
Each zone will be watered for 20 minutes (300 gallons per zone). A three hour recovery will be programmed between zones. Program A will have zone 1 and 2 running on the first night. At midnight the zone 1 runs and after refill zone 2 kicks on at 3:20 am and waters. It refills and waits. The following evening Program B will have zone 3 and 4 running. This cycle repeats itself over and over. The whole time you will be using nothing but pure ground water that otherwise could not be utilized.
Lessons learned:
Simple math pays off…Mexican food does not, so excuse me while I get a Tums!
If you have serious questions concerning installing one of these systems, no questions are too dumb. We have probably been asked before. Costs associated with these are higher than just a well/pump installation, obviously, but the payoff is measured in years and the numbers don’t lie. Let me sit down and show you how we can make this work. Many customers have thanked me for taking the time, and purchased one and never had any issues.
Jody