Project for schools yielded interesting results—and effective systems.
By Gary Shawver, MGWC
Some of my most interesting and gratifying projects I ever worked on were high-production geothermal pump and re-inject projects.
The concept fell into my lap while working on a contract with the school district in Cedar Rapids, Iowa—and after discussion with lawyers, department of natural resources personnel, nearby residents, and others, it became a method used many times by our company.
The first job called for us to drill three large-diameter (10-inch) production wells (approximately 600 gpm each) for Cedar Rapids Kennedy High School. The wells were part of a three-phase project to convert the school from a conventional heating and cooling system to a geothermal “pump and dump” system.
We had drilled similar production wells at three other Cedar Rapids schools. All were limestone production wells finished at a depth of roughly 500 to 600 feet, cased and pressure grouted with neat cement grout to a depth of about 150 feet.
After we were awarded the Cedar Rapids Kennedy project, a group of citizens voiced concern about the groundwater being used for geothermal and dumped into the Cedar River which flowed through Cedar Rapids. This groundwater is high quality and many communities use it for their drinking water. Concern focused on most of the wells drilled pumping 500 to 600 gpm and the schools using two of these wells at any one time during peak heating season.
An oversight committee for Cedar Rapids Schools decided to hold a public hearing. An overview was provided of why the school district was using groundwater for their geothermal rather than closed, vertical loop systems.
The hearing also featured the engineering firm overseeing the project giving an in-depth overview of the geology of the area and the difficulty of getting loop systems into a broken-limestone formation. The Iowa Department of Natural Resources, which issued the withdrawal and discharge permits, and members of the Iowa Geological Survey also testified to the availability of the groundwater as well as what effects, if any, the discharge water might have on local streams.
When the presentation was complete, the IDNR stated it had no problem issuing water use permits to withdraw water from the aquifer, as well as issuing permits for the discharge of the water into the local Cedar River.
After that we continued our work on the three wells and were in the final stages of test pumping the wells when the engineer overseeing the project showed up on the school campus one day with a letter from the IDNR. The IDNR had backtracked on the discharge permits for the wells.
The reasoning was the city of Cedar Rapids had not fully completed its stormwater discharge plan and until it did so, no further discharge permits were to be issued. The engineer asked, “What do we do now?”
Adding a Testing Well
I recommended the engineer first contact an attorney specializing in dealing with DNR issues and try to get a stay on the order not to discharge until such time as alternate means could be determined to handle the discharge water. This was mid-June and the wells needed to be online and operating by the start of the school year in late August.
I next suggested trying to consider taking one of the three wells that were being drilled for supply and turn it into an injection well and do some testing. Only two of the three wells were going to be needed to supply the water for the first phase of the Kennedy HVAC conversion, with supplemental wells being drilled in later phases of the conversion.
Members of the engineering firm met with the attorney I recommended and he handled getting a temporary stay order. In the interim, we set up a plan of action to test injection water from one production well back into one of the other production wells intended originally for supply. We had never done something like this before. Since we had already test pumped the wells to determine production, we used the well that was the most prolific for the injection well, reasoning it would accept the water better if it was the most productive.
The wells were about 600 feet apart, which certainly helped (more on this later). To do the testing, we ran 600 feet of 6-inch-diameter flat hose across the campus from one of the two other supply wells and into the designated injection well. Then approximately 140 feet of 6-inch-diameter steel column pipe was installed into the injection well to facilitate placing the water below the static water level.
We conducted the test injection for 24 hours at the maximum production rate of 500 gpm. When we started the test, we started out at a rate of 300 gpm and after monitoring the “cone of inversion” in the injection well, upped the flow rate to 400 gpm and ultimately 500 gpm after three hours of pumping.
After a 24-hour period, the supply well had a static level of about 22 feet below grade. The original static level was approximately 120 feet below grade before the testing started. This means the water rose in the injection well nearly 100 feet after 24 hours of pumping at a rate of 500 gpm.
As time has passed, there have been no issues with thermal contamination from the injection wells to the supply wells.
After the testing was complete, we decided to drill three more wells on the campus as phases two and three were implemented. These subsequent phases were due to the fact the whole school was not converted to geothermal at one time due to its size. The other two phases were done in subsequent years.
The other three wells were to be one supply well to replace the one which was used for injection in phase one and two more injection wells. This totals three supply wells and three injection wells, with only two production wells being used at any one time.
The final design was to allow all discharge water from the three supply wells to “run wild” to whatever injection well would accept the water. All wells were fitted with transducers to monitor the water levels in all wells.
Additionally, the discharge wells were fitted with Monitor flowing well pitless units to prevent any of the water from the injection wells overflowing the top of the well. The pressure was constantly monitored on the discharge lines as were the transducers for the water level in all three wells.
Since that time the Cedar Rapids, Waterloo, Cedar Falls, and Davenport school districts in Iowa have used pump and re-inject systems for HVAC conversion in their existing and new schools when groundwater proved available and of high quality (low minerals and solids).
As time has passed, there have been no issues with thermal contamination from the injection wells to the supply wells. Temperatures have stayed consistently close to the original groundwater temperature, which ranges from 52° to 55°F in all those areas.
Cedar Rapids Kennedy was the first school system in the state to use pump and re-inject. After our initial testing, I sat down with the engineer on the project and laid out some suggested separation distances between supply and injection wells.
Our basic rule of thumb was to have a minimum of 300 feet of separation on any well up to 300 gpm. Over 300 gpm, we recommended another 100 feet of separation for every increase in 100 gpm production. So, in essence, if a well is to pump 800 gpm, we want the injection well 800 feet from the supply well. We have never had a thermal contamination issue between supply and injection wells on all the projects with this rule.
Not all injection wells have flowing well pitless units. We test inject all production wells to all injection wells for a 24-hour period and do a drawdown test on the supply and then a cone of inversion on the injection wells. If the injection wells show little rise during testing, we don’t use flowing well pitless units.
All wells, however, have transducers that give a full-time readout to the central control. To my knowledge none of the injection wells have had to be rehabilitated. Many of the original supply and injection wells, however, were acidized to increase the production and subsequent ability to receive water. Most all of the acidizings significantly increased the supply and receptability of the injection water.
There are many other factors we established and employed when these projects were complete. Some projects had wells more prolific than others. We’ve also used these types of systems for heat treat and heat dissipation in tire manufacturing plants.
Working on these projects was phenomenally interesting.
Gary Shawver, MGWC, is president of Shawver Well Co. Inc. in Fredericksburg, Iowa. He has been in the water well industry for 40 years and is a Master Groundwater Contractor. He has served as president of the Iowa Water Well Association, the Iowa Groundwater Association, and most recently served on the NGWA Board of Directors. Shawver is semi-retired, having sold his business to his employees. He contributes to NGWA’s member e-publication and can be reached at firstname.lastname@example.org.