When conducting maintenance on a well, it is vital the disrupted material and any introduced materials be fully evacuated from the well.
By Michael Schnieders, PG, PH-GW
Remove the pump and column pipe, pull the screen and casing, evacuate the filter pack and what do you have? Yes, a hole in the ground, but in the chemical and biological sense, you have a mixing bowl.
From day one, each borehole serves as a great collector. During construction, a variety of materials are introduced into or mixed within the well: drilling fluids, smeared clays, intermixed formations, steel, heat from friction, and energy, just to name a few.
Development is designed to remove these materials and restore some level of normalcy to the well and to the flow patterns. Even when executed correctly, some level of interplay remains.
Now, let’s look at daily operation of the well. In a perfect well system, the material coming into the well promptly exits, right along with the produced water. However, as we all know, water wells are far from perfect.
Login with your NGWA member account to read the full article.
Opportunities for Fouling
Dissolution that occurs within the aquifer mobilizes cations and anions into the well. This natural occurrence can yield a mixture of various chemical profiles, even from the same aquifer. This results in what we call chemical congestion.
Chemical congestion can lead to higher levels of conductivity and total dissolved solids, impacting such things as the corrosion potential or the saturation index. The saturation index is the point at which the water becomes saturated with respect to a certain ion and compound. The most common we see is the saturation of calcium and the increased formation of calcium carbonate. Additionally, the precipitation potential is aided by oxidation that may occur during normal operation of the well.
In addition to the increase of chemical constituents, microbiology native to the supporting aquifer can be impacted as well. The borehole offers a much larger area for communities to develop, and with screens, casing, and filter pack, a substantial amount of surface area becomes available for colony and biofilm development.
Biofilm, a natural expression of bacteria, is a sticky exopolymer that is perfect substance to entrap migrating sediments and entrain precipitating minerals, compounding the natural fouling potential of a well system.
Sediment, either as fine silt, clay, or sand, often migrates towards the well. Over time, mechanical fouling can develop within the near-well aquifer interface zone, the filter pack, and entrapped within biofilm in the well.
When the well sits idle or fouling begins to develop, the natural evacuation of the chemical, biological, and physical congestion that occurs, is halted. Cycling of the well will often see some evacuation of this material, possibly resulting in a surge of turbidity, discoloration, or odor, but as the fouling mechanisms compound, the evacuation becomes less efficient. This often results in a need for mitigation in either the form of disinfection, rehabilitation, or re-development.
When we introduce equipment or chemicals into the well to remediate the fouling that has developed, we are compounding that same congestion that exists. Ideally, we are doing so in a proactive manner, but we must take careful steps to limit problems from occurring.
Brushing, swabbing, surging, jetting, and even blasting are mechanical methods of well treatment. These methods are designed to physically disrupt accumulations of scale, biomass, or sediment within the well that have developed on the casing, screen, and within the filter pack.
The key to successful employment of these methods is to select the right method, use it correctly, and evacuate the material removed. As a rule of thumb, we recommend collecting a sample and evaluating it for visual turbidity and sediment, evacuating until the sample is absent of these and has returned to a normal background level of conductivity.
Cleaning and disinfection utilizing chemical means is a further complication. Typically, the selected solutions are highly concentrated, introducing various ions and compounds into the well depending on the problem identified and the chemical selected. The activity of these chemicals on the fouling mechanisms generally results in a dramatic spike in the amount of dissolved solids downhole.For example, during acid-based cleaning of a well, the normal groundwater conductivity of 450 µS/cm can increase to as high as 65,000 µS/cm, reflecting the introduction of chemicals and resulting dissolution of the mineral scale and assimilated particulate. As part of the NSF Standard 60 approval, chemicals introduced into a potable well are to be flushed out prior to the well system being returned to active use as a public water supply.
Protect the Future
When conducting maintenance on a well, it is vital the disrupted material and any introduced materials be fully evacuated from the well. Why is it important that we remove this material? Simply put, leaving the disrupted materials and introduced chemicals behind leaves the building blocks for future fouling.
By leaving the dissolved material behind in the same environment, we continue the chemical congestion that led to the formation of solids, just enhancing the process and reducing the time for the return of fouling.
With regards to biofouling, the need to flush the disrupted organisms and organic matter is even greater, preventing the killed microorganisms from becoming a food source for new growth and propagation. The introduction of acids, oxidizers, polymers, and other chemicals is designed for a targeted timeframe, and even when inhibited, leaving them downhole can have detrimental effects on the well structure or aquifer.
As part of our general recommendations, we recommend water well system professionals and well owners conducting maintenance actively evacuate the well until the produced water is free of visual turbidity, discoloration, or odor, and that the water has returned to a normal, background level of pH and conductivity. This may require some additional time, but it is an important step in successful well maintenance.
Michael Schnieders, PG, PH-GW, is a professional geologist currently serving as the principal hydrogeologist and president of Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders’ primary work involves water resource investigation and management, specializing in the diagnosis and treatment of fouled well systems. Schnieders was the 2017 McEllhiney Distinguished Lecturer in Water Well Technology. He can be reached at firstname.lastname@example.org.