This important step should be a key aspect of any treatment operation.
By Michael J. Schnieders, PG, PH-GW
And, in a basic sense, it does. Leaving the well idle or just evacuating the bare minimum does aid in preserving water quantity within the source aquifer, but at what cost?
In the most basic aspect of its design, a well serves as a transfer pipe. This pipe allows flow of groundwater to the surface for a variety of uses. When a well sits idle, flow into and out of the borehole ceases. In halting this operation, the water quality begins to change. We call this the “concentrating effect”—the water begins to see increases in the amount of dissolved solids, the microbial populations, and the sediment load.
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With regards to water chemistry, the changes that occur when a well is idle can be dramatic. The water can become saturated with respect to mineral forming ions, increasing the potential for scale formation. The water can change from oxidative to reducing, altering the basic characteristics of the water, impacting the microbial community as well as the water quality. The idle state can also impact the corrosion potential of the water, increasing the mobilization of iron and other ions.
Changes in the microbial community during this time can be quite dramatic too. It is not uncommon to see a minimum increase of twofold with regards to the total microbial population. With an increase in the population, the potential for biofilm development increases. Additionally, the biodiversity and maturity of the population can be impacted, often manifesting as an increase in the anaerobic population. Increases in anaerobic activity often impact water quality including increases in the presence of sulfate reducing bacteria as well as environmental coliforms. These changes can impact both aesthetic water quality as well as the human health aspect.
Physical sediment, mobilized into and out of the well during active pumping, can become problematic during idle periods. With the halting of flow, the physical material settles in the lower extension of the filter pack and well. While restarting operation of the well can bring about the flushing of this material, a small percentage is often left behind. Accumulating overtime in the lower reaches of the well, the sediments often lead to mechanical fouling as well as begin to impact flow and the geochemical and microbiological environments.
Often, when a well has sat idle, the initial discharge can exhibit discoloration, turbidity, or foul odors, indicators that the aforementioned concentrating is occurring. Flushing the well should aid in reducing these impacts. As the effectiveness of flushing reduces, the need for cleaning, disinfection, or redevelopment becomes prevalent.
When performing maintenance, disinfection, or more invasive rehabilitation, we are often challenged with the handling of fluids while preserving the water resource. We seek to preserve the resource during restoration efforts and desire to limit the need for unnecessary pumping. However, the need for fully evacuating the well during these operations is essential, helping to improve treatment efforts while reducing the likelihood of future fouling.
In the basic sense, anything introduced into the well should be removed. As an industry, we utilize a variety of chemicals and materials for treatment. Chlorine, acid, dispersants, surfactants, and compressed gasses are placed into the well to target problems downhole. Once the targeted problems have been addressed, the material should be removed to reduce the potential for re-formation of the problem—be in mineral scale, biomass, or sediment. Neutralization through dilution, time, or introduction of a secondary chemistry, should occur at the surface, away from the well environment. Leaving this material downhole, over time, is counterproductive and will serve to further damage the well and aquifer.
Guidelines for monitoring discharge from the well as an indication that evacuation is complete vary widely, depending on the means of treatment used. As water wells are very dynamic entities, we recommended using multiple parameters as a means of guiding your decision.
First, you should have a baseline of a few simple parameters of the water quality before beginning work. Although not a complete list, the minimum parameters you should follow include: pH, total dissolved solids (TDS) or conductivity, and visual turbidity. Collecting a sample in a clear plastic or glass bottle during evacuation can allow you to evacuate these parameters and quickly deduce if additional pumping is needed. These three tests can be conducted quickly, alerting you to the need for further evacuation. In deeper completions or well designs with multiple screened zones, it can be important to pump and evaluate multiple levels to insure complete evacuation.
During disinfection, typically an oxidative chemical such as chlorine or peroxide is utilized. With chlorine, you can monitor the concentration with test strips, hand held meters, or color scale tests, as well as the odor.
With heavily impacted wells, the differences during evacuation can be quite dramatic. In wells lightly experiencing fouling or undergoing pump maintenance, the variations can be subtle yet remain important. Regardless, the need to fully evacuate the disrupted material and introduced chemicals is very important and should be a key aspect of any treatment operation.
Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders was the 2017 NGWA McEllhiney Lecturer in Water Well Technology. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He can be reached at firstname.lastname@example.org.