Major Considerations in Well Biofouling
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There are a number of important considerations that are applicable to all water wells. These are summarized below and represent some of the major factors affecting the sustainability of water wells. These are listed in point form and are not any specific order relating to importance but all should be considered as relevant to the effective management of water wells.
There is no such thing as a sterile water well. Microorganisms will always be present and the level of biofouling will relate to the position and form of the microbial activities that occur.
Biofouling will commonly begin during the development of the water well. Once the well goes into production, the biofouling will accelerate and begin to impact on the specific capacity and water quality. Significant biofouling may be occurring even when a well has only lost 5 to 10% of the original specific capacity. It is not appropriate to consider that a well is not suffering from until after the specific capacity has declined by 40% or more. By this time the well may be so severely plugged that rehabilitation back to the original specific capacity may not be achievable.
Preventative maintenance should be practiced from the time that the well goes into production. Using the BART biodetectors, the time lag (as days of delay) and the reaction patterns signatures observed can be used to determine whether the microbes causing the biofouling are becoming more aggressive (i.e., shorter time lag) or changing in dominant microbial fouling (i.e., shifting in the reaction pattern signatures) can be tracked by routine testing (commonly monthly with repeat testing when there is evidence of increased aggressivity). Where treatments are applied, the success of the treatment may be determined through improvements in the specific capacity and water quality along with lengthening time lags and shifting reaction pattern signatures.
392Biofouling does not have to occur just on the well screen and be visible when a camera logging is performed down the bore hole. The absence of any fouling on the screens does not mean that there is no biofouling in the well, it simply means that there is no observable biofouling on the well screen. There are many ways in which the biofouling can occur around a water well. These can range from a tight plugging forming in some of the regions close to the well screen and causing variable water quality as water enters the well from different formations to various structures sited at different distances from the well. The microbial activity will tend to focus around the sites of maximum water flow towards the well and where turbulence occurs. As plugging develops at this site, the water flow patterns change. Even (laminar) flow down the length of the well screen can reduce this focusing of microbial activity. Instead, the plugging occurs more evenly over the length of the well screen to have a less dramatic impact on water quality and specific capacity. On some occasions the microbial fouling can move towards the well along a redox front created by ground water recharge from surface water sources. This type of fouling can be insidious and cause sudden dramatic losses in water quality and specific capacity once the growths begin to encapsulate the well itself.
Biofouling is an all embracing term which can relate the losses in specific capacity (plugging), accelerated corrosion (often due to the activities of the SRB), losses in water quality (e.g., increasing turbidity due to microbial growth and biofilm sloughing, taste and odor problems resulting from microbial activities in the biofouled zones), and equipment failure due to encrustations and/or corrosion processes. When a biofouling event is evident, the BART™ biodetectors can be used to test the product water and confirm the type of biofouling that has been occurring.
Little attention has been paid to the average life of a water well in active production before the installation is reduced to a stand-by status or abandoned. While each well has to be treated as unique (Water wells even very close to each other can often exhibit totally different characteristics), there are some general observations. In the Canadian prairies, it appears that the average life span of a water well is fifteen years with the range commonly between five and thirty years. Given the cost of installing a new well, there needs to be more attention paid to increasing the life span of each water well so that it can become more sustainable. 393Increasing the life span of the well would involve an active preventative maintenance program and effective treatment program which would control the level of biofouling in the well.
Treatment of a water well to remove biofouling is inherently difficult since the biofouled zones are commonly in the void spaces and difficult to access due to the buffering action of the surrounding media. In selecting a particular treatment strategy it should be remembered that no one size fits all, and careful consideration has to be given to the form and the location of the zones of biofouling in and around a well. It is not appropriate to claim that a specific treatment has been successful simply because the treatment product was located one or several hundreds of feet away from the well being treated. Where this happens, the only valid observation can be that there was a conduit (channel) through which the treatment fluids moved away from the targeted well. This conduit may have, at least in part, been formed by the biofouling of the surrounding porous media around a fracture or water lens within the media. The consequence of this would be that the treatment would impact on that surrounding mass of biofilms (a.k.a. slime tube) but not on the fouling occurring away from that (those passages). While such treatment would impact on the localized growths around the channels through which the treatment fluids are passing, it is less likely that there is an overall impact on the remaining biozones. Evidence of recovery has to relate to the recovery of specific capacity, a reduction of the biological burden in the water and a return of the water quality parameters to the original levels for that well.
While it is obviously in the interest of promoters of particular treatment practices to down play the involvement of microbes in plugging, corrosion and other biologically induced problems, it has to be remembered that a water well can never be completely “sterilized” by the common treatment practices employed in the industry. When a well has been treated, there will inevitably be some debris associated with the disrupted biofilms. This will include nutrients and viable cells. These living survivors now become “cannibals” living off the dead cells and disrupted organics and nutrients within and around the zone that was treated. This activity will rapidly generate a “rebound” of biological activity and restart the processes of biofouling. Promoters of specific treatment processes that devalue the 394biofouling to a “minor” or “easily controllable” event do not take into account the inevitability of a “rebound” biofouling that will follow treatment unless there has been scrupulous care to remove all of the fall out debris from the treatment.
One of the major difficulties in assessing the success of a treatment from the microbiological viewpoint is when to test the water well to determine that the treatment has been effective. One effect of a treatment, successful or not, is that there is a “kill” zone in the immediate environment of the effective treatment zone. Beyond that zone is a secondary impact region where the biofilms and associated encrustations would have been disrupted with a variable kill rate. Here, the surviving microbes begin to cannibalize the dead cells and utilize the disrupted organic material. This leads to localized “blooms” of growth. When the water is first pumped after such a treatment it can be expected that the initial product water has a very high particulate loading with a low level of microbial aggressivity (while the killed cells predominate in the water). As the water from the secondary impact zone is pumped out, the characteristics of the water change as the particulate material is flushed out (e.g., turbidity and chemical parameters drop) and the aggressivity increases (as the “cannibal” microbes grow within the debris created by the treatment). Frequently during this latter event the aggressivity recorded using the BART biodetectors exceeds by one or two orders of magnitude the background levels observed prior to treatment. It is, therefore, recommended that the determination of the effectiveness of an applied treatment be delayed until the biological activity in the impacted zone has stabilized and the debris has been essentially flushed out of the system. The recommended time delay for conducting this assessment for the effectiveness of a treatment is a minimum of six weeks. The bottom line is recovery of the specific capacity back to its original levels by an effective rehabilitation treatment. Remember that the more the specific capacity has been lost by biofouling (particularly when it has dropped by greater than 40%), the less there is a probability that the well can be rehabilitated completely.
It is uncommon for a water well to have a single localized site of biofouling around the well. More often, there are complex communities both layered one on top of another and as concentric circles of growth around the well. In the latter event, each of the 395concentric biozones may have a very different bacterial community structure. Usually the shift in community structure moving away from the well is from an oxidative (+Eh) to a reductive form (-Eh). Aerobic bacteria are more aggressive closer to the well in the oxidative zone with IRB tending to dominate. However, away from the well in the reductive zones, anaerobic bacteria tend to dominate including the SRB and the methanogens (biogas producing bacteria). Between the oxidative and reductive regions lies the redox front where a variety of facultatively anaerobic bacteria can dominate. These will include. enteric bacteria and also those aerobic bacteria able to utilize alternative respiratory substrates such as nitrates. Pumping water from a well having these various biozones means that the product water will contain first the aerobic bacteria often dominated by IRB, then the nitrate respiring aerobes and facultative anaerobes followed by the anaerobic SRB and the methanogens. Sequential sampling of the product periodically for at least two hours with at least three samples becomes critical to a more comprehensive determination of the extent of the biofouling.
The presence of anaerobic bacteria (SRB and/or methanogenic) can have direct and serious implications. For the SRB, this can mean the development of “rotten” egg odor, generation of black water and slimes and the initiation of corrosive processes. Where the methanogenic bacteria dominate deep in the reductive parts of the formation, methane will be produced. This methane gas may lock into the voids in bubbles (foam blockage) or move towards the oxidative regions within and around the well. Where this happens, methane may enter the water well column and even vent off from the well with the associated risk of ignition. Methane may also be degraded within the oxidative regions by methane degrading bacteria (methanotrophs) and this can seriously increase the amount of plugging. The presence of methane in a water well has sometimes been thought to be totally associated with leakages of natural gas from neighboring gas and oil fields, but on some occasions this gas could be locally generated by methanogenic bacteria in the immediate vicinity of the well. The detection of aggressive methanogenic bacteria in water samples from a well would indicate that there would be a potential for locally generated methane production.
Since most of the bacteria associated with the biofouling events occurring within and around a well are residing within biofilms, 396encrustations, nodules and tubercles, they would not necessarily be expected to be always present in the product water. Casual sampling of water from a water well contains intrinsic errors since the sample cannot be representative of the various biological events going on within and around the well in the ground water. To induce the bacteria to detach from the biofilms, encrustations, nodules and tubercles, the environment within the well has to be changed from that normally experienced by the resident microbes in the well. Commonly, wells follow a routine schedule of pump and rest normally on a twenty four hour schedule. To break that routine the easiest thing to do is to shut the well down for long enough to break the routine cycle and stress the incumbent bacteria. That stress is created primarily by the shifting redox front (moving towards the well) and the loss of hydraulic flows (that would bring nutrients towards the biozones and biofilms). Ideally, the well should be shut down for a week prior to sampling but very often a producing well cannot be taken out of service for that long. Practical experience would dictate that a twenty-four hour shut would normally be effective with an absolute minimum of eight hours. For wells being run almost continuously, it is often difficult to get the user to shut the well down at all. In these cases, it has been found that a shut-down of two hours will at least begin to cause releases of the bacteria to occur from the biofouled regions. Sequential sampling once the pump has been turned back on allows the location of the fouling to be predicted (the later the sample in which the bacteria is first detected, the further away from the well these bacteria are infesting).
Many specialists still down-play biofouling events as being either “of no major significance” or “easily controlled (not a problem)” using a preferred treatment procedure. In reality, the covert nature of the bacterial infestations in and around the water well often makes them difficult to detect and/or easily discounted. Microbiologists over decades have concentrated on trying to identify individual species apparently involved in the biofouling event rather than looking at the community (consortia) of bacteria that are involved. Given that these bacteria may infest different sites in various community structures, the dominant communities can often be used to project the likely source of nutrient factors stimulating the biofouling event. For the BART™ testers, the relative aggressivity of the different bacterial groups can be used 397to predict the source(s) of the biofouling. For example, aggressive IRB are likely to be found in the earlier samples taken during a sequential pump test (see 12 above) and would mean that there was a high iron and possibly manganese concentration in the ground waters entering the well and that the conditions were fairly oxidative. Aggressive SRB, on the other hand, are associated very much with reductive ground waters with a high sulfate and/or organic loading. Field experiences have found that frequently the SRB can become exceptionally aggressive when the well is being impacted with methane generated biogenically local to the well or arriving at the well through passages from natural gas formations or leaking gas and oil wells. Heterotrophic aerobic bacteria (HAB) tend to dominate where there is a high organic input in the ground water and the conditions are more oxidative. These microbes also can be stimulated through their ability to respire nitrate when there is no oxygen present in the water. SLYM (slime-forming bacteria) produce very thick, often sticky, slime coatings that often form at redox fronts and discharge outlet ports. These bacteria also thrive in high organic ground waters but do not bioaccumulate iron and manganese to the same extent as the IRB. Denitrifying (DN) bacteria become very aggressive in ground waters impacted by sewage and high nitrogen organic wastes that have passed through an oxidative environment. When this happens, oxidative bacterial nitrification causes the production of nitrate as a terminal product from the oxidation of ammonium. When the ground water returns to a more reductive environment, the DN begin to reduce the nitrate partially to nitrite and then completely to dinitrogen gas. The presence of aggressive DN indicates a heightened potential for enteric bacteria and sewage to be present. It is prudent to check waters with aggressive DN for the presence of total and fecal coliforms as a measure of the potential health risk.
Indirect tests for the presence of bacteria include the use of the ATP as a gauge for the number of viable cells within a given water sample. The premise here is that the greater the level of activity, then the higher the amounts of detectable ATP. It is generally considered that living microbial cells possess 0.1% of the dried cell mass as ATP. This translates to one bacterial cell possessing 5 × 10–16 grams. The advantage of the ATP test is that is does relate to the active microbial cells only and so reflects the aggressivity of the microbes rather than simply recording the 398numbers of cells whether they be active or passive within the natural waters and biofouled interfaces. Another indirect test is the use of the laser particle counter which is able to detect and size particles in the water. The range detectable is commonly from 0.4 to 120 microns and so covers the full range of both cell sizes and colloidal structures that may be associable with the microbial population. Often the data is presented as total suspended solids (TSS) along with the mean size of the particles detected and distribution of the cell volumes. Generally, bacterial activity is most often associated with biocolloidal structures ranging in size from 6 to 32 microns. On some occasions, the size range for these structures can be quite narrow (e.g., 8 to 11 microns) and, on other occasions, may “castellate” with separate peaks every 4 to 6 microns. This is a common occurrence when filamentous growths or stalked bacteria (e.g., Gallionella) are present. Direct microscopic examination is often employed but tends to bias the observer to that which is the most obvious rather than that which is dominant. A good example of this is the detection of Gallionella based on the observation of the stalks that can sometimes go right across the field of view when observing a slide. It has to be remembered that the stalk of the Gallionella is dead and contains no ATP. It is essentially an artifact used by the bacteria to dispose of the surplus ferric iron. Each stalk came from a single cell that had been growing much more slowly than the masses of slime (biocolloidal particles) that are often richly populated with consortia of bacteria some of which would have sheared from biofilms. The basic lesson here is that ground water samples from wells may often give very false impressions of the form, extent, function and nature of the biofouling that is occurring in and around a well. It is necessary now to move from an implicit denial of the role of microbes in ground water to an acceptance recognizing that there are many challenges still to be addressed if the true role of microbes in wells and aquifers is to be determined.
Over the whole history of water well rehabilitation there has been a reliance on methods that are applied cold to the well (i.e., without the deliberate application of heat). The reason for this has at least, in part, been due to the untested belief that water wells cannot be heated successfully. The reason for applying heat to a well during treatment is two-fold. First, it is well known that the application of heat speeds up chemical reactions and, therefore, 399would lead to faster removal chemical encrusta-tions and alien structures from the well. Second, once the temperature is elevated greater than 40C° above the normal well temperatures, it can be expected that there would be massive reductions in living microbial cells ranging from a minimum of one to a maximum of five orders of magnitude lethality. In the last two decades, progress has been made in heating up wells using such techniques as the patented blended chemical heat treatment (BCHT, ARCC Inc., Daytona Beach, FL). This technique has been subjected to rigorous trials with the U.S. Army’s Corps of Engineers and has been found to rehabilitate badly plugged wells. Recovery to original specific capacities has been found to be achievable where the losses have been less than 40% (i.e., the well still retained a specific capacity of > 60%). However, as the specific capacity of the well falls over a critical range from 60 down to 20% of the original specific capacity then so the probability of full recovery becomes reduced although a partial and economically viable recovery may still be fully achievable. Here, the main message is conduct preventative maintenance from the day the well goes into production and never let the well sink to below 60% of its original specific capacity. If these rules are followed, then a sustainable water well becomes a reality.