A water well has been traditionally viewed as essentially a pipe which passes down into the ground and fills, at least partly, with water. This water can be drawn from the well by some form of pumping action and is replenished by groundwater flowing towards the well. Definitions of well include "a shaft sunk into the ground to obtain water... a mineral spring... a spa... a water-spring or fountain". Water coming from a well is generally considered by many people, since it has come from under the ground (sub-surface), to be free from biological activity. Problems arising from a water well are therefore often considered to be physical and/or chemical problems and are treated therefore as such. In reality, the sub-surface environment is rich in biological activity which can affect the performance of a water well in many ways. This document will address some of these mechanisms which range from the good, the bad to the ugly.
The good activities reflected by a well is its ongoing and reliable production of water of an acceptable quality and quantity when demanded. This demand may be created by pumping water from the well (production well), water under a head pressure springing from the well (relief well), or through pumping water back down a well (recharge or injection well). It is generally understood that a water well will continue to perform satisfactorily if its performance matches the level obtained when the well was first brought on-line (developed).
Failure in a water well is usually related to either the physical failure of the structure (stress-, corrosion-, or clogging- related), loss in groundwater recharge to the well, or a deterioration in the product water quality (chemical) and quantity (physical). These failures have been viewed traditionally as not involving a biological factor. Today, there is a growing body of evidence which supports the fact that the subsurface is, in fact, richly endowed with a wide variety of microorganisms. Such microbes can influence the efficiency of a water well they are present within the zone of influence of the water well. Very often operators view the obvious occurrence of bacteria (eg, the presence of coliform bacteria in an hygienically compromised well) as having arisen by contamination from the surface (eg, through leaking from septic wastewater systems). Still today there are many operators of water well systems who believe that their well systems are essentially "sterile" and do not (therefore) need to be concerned about any possible biological challenges to the integrity of their system(s). In reality, microorganisms are ubiquitous (ie, virtually everywhere) and it is a matter of whether the local environment is suitable for them to become active. A water well often provides a suitable environmental "setting" for microbial activity because of the changes that occur in flow rates, oxygen concentrations, and the types of surfaces that are present. This document is devoted primarily to three exercises which are related to understanding:
(1) the potential microbial challenges to the wells;
(2) rehabilitating biofouled (or biologically challenged) water wells;
and (3) establishing a preventative maintenance (PM) program to ensure a long-term management of the biofouling.
Such initiatives thus allow a prolonged life to the water well installation and therefore has potentially very significant economic advantages to the user.
"Failure" means that the water well is no longer capable of delivering an acceptable water quality in adequate amounts to meet the product goals expected of that well. Early diagnoses were based on the user observing deteriorations in production which could sometimes be dramatic. Typical events included:
- losses in flow;
- serious increases in draw-down to achieve flow;
- generation of cloudiness (turbidity) and/or color in the water;
- appearance of taste and/or odor problems;
- and increasing hygiene problems.
Early diagnosis was based upon the simple techniques which related to the problem(s) the user had observed. These techniques are listed below.
Losses in flow, there is a gradually diminishment in the amount of water flowing from the well over a given time. This is usually recognized by a lengthening in the pump times to achieve a given production goal or reduced flow rates from the water well system. The user may observe this by the ongoing casual monitoring of the well. In general, the cause of the failure may be put down to either the water level declining in the groundwater so that it can no longer service the well efficiently, or to a clogging process which prevented water from reaching the borehole in sufficient volumes to service the demand.
Increases in Draw-down, one symptom which could often be observed by the user was the fact that the water level in the well was stabilizing at deeper and deeper positions during pumping. This meant that the pump was only able to draw adequate water by the creation of a larger water head drawing water into the borehole through a cascading effect. The loss in water at the start of pumping is referred to as the "draw-down" and it is generally recognized that, where this occurs, there is a clogging process which is impeding flow to the well. If the user notices that the (static) water level when there is no pumping is falling, this may be taken to indicate that the water table is falling in the aquifer. In this case the user may suspect that the groundwater system is being overtaxed (eg, output is exceeding input). Where clogging is suspected, it has traditionally been thought that this was largely as a result of silting up (physical phenomenon) and/or encrustations (chemical phenomenon) forming to be impede the entry of water into the well. Silting up was generally viewed to be caused by the entry of silt particles into the well which would reduce water entry, and also settle in the borehole to reduce the capacity of the well.
Generation of cloudiness in the water, was generally thought to represent a degeneration in the chemistry of the water which caused various salts and colloids to become suspended in the water. The cause of such cloudiness was considered to have been caused by the well water reacting between different strata of water and was often associated with the formation of encrustations and/or clogging within the well. The usual test was to hold the water up to a light and observe the clarity. It was not realized initially that water clarity did not necessarily correlate in any respect to any microbial presence. It is now known that a completely clear water sample can still easily contain 100,000 organisms per ml and that cloudy waters are very likely to contain significant populations of microorganisms.
Color in the water, water may become colored if there are artifacts in the water which are colored. These artifacts may range from the molecular to the colloidal and the particulate. Color, when it appears in the product water, is indicative of a problem becoming established which may challenge the ongoing operation of the well. Colors sometimes observed in water range through the following range: yellow, orange, brown, grey and black. At the same time the water may retain a high clarity or become clouded. Yellow, orange or brown waters are commonly associated with the presences of iron in the water at concentrations of greater than 0.5 parts per million (ppm). Yellow waters tend to retain a high clarity while orange waters may generate some cloudiness. When the water is left to stand at room temperature, the color and the cloudiness may intensify before the material settles out to the bottom of the container as a thin, loosely packed or woolly deposit.
One test method (GAQC) uses a drop of corn whiskey to speed up this process and is used as a positive indicator for the presence of iron related bacteria (IRB).
Brown waters are generally clouded and may contain a significant load of colloidal and particulate iron and this event has traditionally been linked to the onset of a serious iron clogging problem.
Grey water is a dirty cloudy water which often has a high microbial population and has usually been associated with high levels of organic challenges of a type that might occur if a septic wastewater or an organic contaminant plume were to enter the zone of influence of the well.
Black water is a relatively common occurrence particularly when wells are activated after a long period of inactivity. Usually the black color is generated by the production of black iron sulfides in the water which has become infested with sulfate reducing bacteria (SRB) and there is very little or no oxygen in the water. Sometimes these black deposits form into granulated or thread-like masses of slime which float within the water.
Taste and odor problems, most water problems are first perceived by their odor rather than by the taste generated. Odors range widely and are sometimes difficult to catagorize in a manner that other people would easily understand. Common problems have been reported associated with the following odors: rotten eggs, fishy, septic, kerosene-like, earthy-musty, vegetable, fruity and skunky. Each of these odors can be related to some biological event associable with the well.
Rotten eggs is perhaps one of the most common and relates to the decomposition of organic matter (organogenic) and the reduction of sulfates (inorganogenic, by SRB) to hydrogen sulfide. This hydrogen sulfide has a strong odor which is also present in the "black rot" of eggs. Hence, the common name for this odor in water is the rotten egg odor. It occurs commonly when a well has been shut down for a prolonged period and there is little oxygen in the groundwater and/or a lot of organic material.
Fishy odors are generated by a group of bacteria called pseudomonads (soo''do-mo'nads). They grow where there is oxygen and often form slime-like growths (biofilms). It is slimes dominated by these bacteria that commonly grow over the scales of fish and give the fish that same characteristic smell. When these odors are observed, the well waters are probably oxygenated and also carry a range of organics which these bacteria can use for growth.
Septic odors relate to the presence of sewage and septic tank wastes but can also be generated during the anaerobic (oxygen-free) degradation of organic material. Generally the water may also show a moderate amount of cloudiness and sometimes may have a grey color. Fresh sewage tends to have a distinctive fecal odor (manure-like) but septic wastes are more difficult to describe since the odors are more transient (vague). Normally the presence of septic odors should trigger a testing of the water for the presence of coliform bacteria because these have been used as the indicator organisms for the presence of fecal material and hence indicate that there is a hygiene risk when they are detected (1 coliform organism per 100 ml is considered by many regulators as the borderline maximum acceptable number in water).
Kerosene-like odors can also occur and may be due to the presence of hydrocarbons (eg., gasoline, jet fuel) in the water but there are also a number of bacteria which can generate similar odors. The most commonly recognised group which can do this are the pseudomonad bacteria.
Earthy-musty odors are very similar to those emanating from freshly turned soils. The reason for this similarity is that one group which generates this odor occurs widely in soils and often occurs growing in the unsaturated regions associated with groundwater are the streptomycete bacteria. These grow as threadlike growths over surfaces and are very competitive often dominating the other microbes at the sites. This is because the streptomycetes also are a source for a range of antibiotics. Streptomycin is one of these antibiotics. They are aerobic and require oxygen for growth and also degrade a range of organics. As a result of this, these streptomycete bacteria may be found in recharge zones where there is a high organic potential and the water remains oxygenated. Here the earthy-musty (geosmin) odors may be created and be carried in the water. Also carried in the water will be a large number of spores which may be detected when the water is examined for microbiological content.
Vegetable odors are generated by some bacteria but are more commonly associated with the growth of microalgae and blue-green algae (now known as the cyanobacteria). These organisms usually require light to grow (for photosynthesis) and so it may not be commonplace to obtain vegetable-like odors from well water unless there has been a very shallow local recharge or the borehole water column is supporting the growth of these plant-like microorganisms. It should be remembered that some of these organisms are able to compete in the dark with other non-photosynthetic microorganisms and could generate these odors.
Fruity odors are very uncommon in wells and are usually associated with the presence of yeast some of which can produce a range of fruity esters during growth. Yeast are not commonly reported as a normal part of the groundwater flora and their presence would indicate that a high carbohydrate or organic acid pollution may be occurring. Some bacteria do produce similar odors but it has been found through practical experience that the product groundwater does not possess the odor but that it appears when the bacteria are cultured.
Skunky is a description used by many simply to describe an unpleasant odor to which the operator cannot ascribe a particular smell. Generally the odors are septic or earthy-musty in nature.
Odors when they are reported usually are associated with some malfunctioning in the system. The general attitude is firstly that the water has become polluted with chemicals that bear this smell. It is not often recognized that these odors may also be a signal that microbially induced fouling is increasing. A good example of this is the generation of fish-like odors (during the early growth of pseudomonad bacteria in aerobic conditions) and septic odors later as the oxygen is depleted and the bacterial flora changes to an anaerobic type. The presence of odors in the groundwater can be taken to indicate that a problems is arising which may be microbiological in nature.
Historically, there have been three major reactions to the declining operation of a water well of any common type. One reaction (which may be popular with some well drillers!) is to simply abandoned the well and install a new well of a similar or greater capacity to compensate for the abandoned well. A second reaction involves attempting to change the operating techniques (eg, pump times, volumes, sequences of up and down times, and control flow by draw-down limitations) or change some components in the well (eg, pump, screen) in the hope that the well will recover from the observed problem. The third reaction is to attempt an analytical approach to the problem by determining:
- first, the cause;
- second, confirm that the effects witnesses can be related to the cause identified;
- and third, determine and apply a treatment strategy that will counteract the cause and so allow the well to function in its designed manner.
The increasing economic and environmental costs and concerns are now restricting the ability of a well user to simply replace a failing well. Economic concerns relate to the increasing costs involved in well replacement and the growing sensitivity for maximising the use of each well installation through extending its useful life (ie, environmental sustainability). Environmental concerns are being brought to the fore by the fact that groundwaters are now longer being seen as an infinite resource that can be exploited without limitation. In some areas, the aquifers are now being heavily depleted by the present demand and there is little flexibility to provide additional capacity. Another major environmental concern is the impact of various forms of pollution on the well fields.
In the decades gone by, the general attitudes may be summarised by the "out of sight, out of mind" mentality in which groundwater was given a lower status of concern than surface waters. Various chemical leakages from industry, agriculture and the various service industries were not considered to be of such importance as in surface waters. When a pollutant impacted on a surface water, the effects could often be relatively quickly appreciated through such effects as radical eutrophication, deteriorating water quality and the water becoming unacceptable to the user. One major difference between surface- and ground- waters is the fact that the former flows as large unconfined masses while the former moves as a confined mass within various porous media. This difference is very critical to the current understanding of groundwater flows and quality.
It is not easy to appreciate the complex interactions which occur between the flowing groundwater and the media it is passing through as it moves to a well, a spring or interfacing with another aquifer. For the last century, it has been popularly believed that groundwater is essentially sterile (devoid of biological activity) and that all of the activities within an aquifer may be explained almost exclusively by a combination of physical and chemical processes. Today, hydrology in groundwater systems still leans heavily upon this assumption. Through the science of sub-surface microbiology (the study of microorganisms in the crust of the planet) it is now becoming increasingly evident that groundwater movement and quality is affected by microbiological interactions. In the past decades these have been ignored and one of the major consequences of this has been that the effects of these microorganisms as a biological filter (interface) has been ignored.
Pollutants within a groundwater system may become entrapped (and possibly degraded) within these biological filters and so not appear in the groundwater resurfacing through a well. Environmental monitoring of the product (postdiluvial, after the "event") water from a well may not necessarily give an "accurate" picture of the chemical loading in the transient (causal) water itself. Essentially there has been a tendency for groundwater users to rely on the product ("biofiltered") water for environmental assessment and yet this water may not accurately allow a risk-assessment for that well (due to the bioentrapment of some chemicals of concern).
In the next two decades, the realization of the nature of the biological interfaces within and around water wells may cause much tighter environmental constraints to be placed on new well installations. This would mean that a greater attention would be paid to extending the service life of existing wells through preventative maintenance and effective rehabilitation programs.
The mindset that a water well is a physical object set within chemical and physical world has to change. This has generated a "traditional" attitude that a dysfunctional well is simply a result of chemically driven corrosion, encrustation, clogging processes or the physical collapse of the system (through such events as "silting up" and "collapsed" aquifer and well structures).
Acidization has commonly been applied as a remediation technique to dissolve and disperse the clogs and encrustations while various disinfectants (such as different formulations of chlorine) were used to control any coliform and other bacteria which may be growing down the borehole (and presenting a potential hygiene risk). Slime formations were considered by many to be simply physical-chemical accumulates which may result in clogging, encrustation and corrosion occurring. Even today, the camera logging of a water well is thought to be sufficient to view of all the biological and much of the chemical deposits (eg, silts and salts) which can be causing problems around a well. Please see the section on the "Preliminary Diagnosis of Biological Fouling of Water Wells using TV camera logging methods".
Combinations of disinfectants, selected acids and even, in more recent times, dispersants (aka, wetting agents) have become a part of the arsenal of weapons being used to rehabilitate problems in a well. One of the findings from these reaction scenarios has been that "no one size fits all" and that each well should be treated as unique requiring a customization of the treatment parameters to optimize the maintenance practices. This approach stems from observations that each well can be characterized as being different from the other wells in the same field. Indeed, there are many experiences where two wells of the same construction and characterization placed within feet (meters) of each other in supposedly the same aquifer formation bear very different characteristics. An unfortunate result of this is that a treatment scenario may be successfully applied to one well in a field but that same treatment may fail on a neighbouring well with exactly the same characteristics in its construction, operation and mode of failure.
Today, more and more wells are being subjected to an analytical determination of the nature of the problem causing concern. This analytic approach has now gained a level of acceptance when used in one of three ways to determine:
- 1. Water Production
- 2. Chemical and Physical Characterization
- 3. Biological Aggressivity.
Water production is usually determined primarily by the volume of flow from the well achieved over a given time period (eg, hour, day, etc.). When a well is operating routinely below its capacity to produce, the volume of water flowing from a well may continue to appear optimal (ie, meet demand) while, in reality, the hydraulic transmissivity of water to the well may have become impaired. For example, a well designed to pump water from the well at 50 g.p.m. may actually be capable of flowing at 500 g.p.m. It is therefore operating at only 10% of its theoretical limit. If that well becomes clogged, for whatever reason, then the flow into the well will become reduced. Clogging would have to cause a 90% decline in flow before the pumped volume from the well would be affected. In the field, it has been observed that wells can suddenly lose production capacity when the demand placed on the well is only 20 to 60% of the original theoretical capacity.
In the well given as an example, the flow from the well may decline dramatically simply because the clogging would have been nearly complete before the well flow was observably affected. Early signals for loss in production include increases in drawdown during pumping and longer recovery times for the water column to return to its standard (water table related) passive position after pumping.
Pump tests can stress a water well to its maximal production capacity. If these pump test are performed routinely, the rate of production capacity loss can be projected and a preventative maintenance program initiated.
Chemical and Physical Characteristics. Very often when a water well is becoming compromised by some adverse process, the quality of the water changes in terms of its chemical, biological and/or physical characteristics. These shift have been used as signals that a well may be becoming impaired. Some of the common chemical and physical characteristics that are likely to change are listed below.
Iron (Fe) Iron is accumulated in clogging and other forms of biological interfaces. As these accumulates grow, the water quality may reveal only traces of iron since much of the iron is being trapped before the water enters the borehole and is pumped from the well. However as the biological mass continues to grow, it becomes periodically unstable. During these periods the mass begins to break up and slough into the pumped water. This becomes reflected in the higher iron content in the water. Erratic changes in the iron concentration of water being pumped from a well can therefore be used to indicate that a form of bioaccumulation (eg. clog) is now maturing to the point of becoming unstable. This may be readily observed in the water through the generation of colors (yellow, red to brown), turbidity, and copious red to brown slime deposits forming in static water tanks and along slow flow pipes. Increases in problems with water treatment equipment such as filters and softeners may also be noted at this time. Generally, the critical range of concern for iron is between 0.1 ppm (no obvious problem) and 1.5 ppm (obvious problem).
Manganese (Mn) This is another chemical of concern that is often found in unacceptable levels in product waters from a well. Manganese, like iron, will bioaccumulate in the clogs and other biological interfaces around a water well. While iron tends to collect relatively close to the well, manganese tends to be distributed further back into the formation. Most wells show much lower levels of manganese than iron and the ratios (Fe:Mn) tend to remain fairly stable. Some typical ratios which are seen in wells are approximately 100:1, 30:1, 3:1, 1:1, 1:10. Where there is more manganese (eg, last three ratios), the color of the water tends to be different ranging through shades of grey to black. In general, waters with a higher manganese content (relative to iron) are more difficult to apply a preventative maintenance program. This is partly because the focus clogged zone may be further away from the well screen in the formation. The critical concentration ranges of concern for manganese is between 0.01 ppm (not normally a major problem) and 0.5 ppm (obvious problem).
Total Suspended Solids (TSS) When a well has been biofouled to the point that sloughing is occurring, the suspended solids content in the pumped water becomes more evident. It may be seen as a reduction in clarity, increase in turbidity, increase in cloudiness, presence of visible particles, and/or the development of deposits (loose, granular or slimy) when the water is left standing. These suspended solids may range from silt and clay particles to colloidal particles. To stay in suspension, these solids have to be very small in size (usually they range from 50 or 60 microns down to less than a micron). A micron is one thousandth of a millimeter or around a quarter of millionth of an inch. Bacteria range in sizes of around a millimeter or two. Colloids are "jelly-like" particles which are composed mainly of water but bound together by bacterial or chemical polymers. These polymers are long string-like interwoven molecules which have the ability to attract and hold water. Very often bacteria and other microorganisms are suspended in the water and "travel" within these "bio"colloids. When a slime begins to slough for whatever reason, biocolloids are released and these then move through the water. Laser particle sizing is becoming an excellent tool for estimating the volume, size and even the shape of these particles. Some problems have been experienced because the biocolloids are very mobile and appear and disappear from within the laser scanning beam in a matter of seconds.
pH (acidity and alkalinity) is an easy parameter to measure and, while it cannot normally directly detect biofouling, it can indicate whether conditions are likely to support biological activity. In reality there are some microorganisms that can flourish at any pH in the range of 0 to 14 but as one approaches the very acid (less than 4.5) and very alkaline (greater than 11.5) then only a few very specialized organisms can flourish. The pH which supports the greatest diversity of microbial growth lies in the neutral to slightly alkaline range of from 7.2 to 8.8. Some microorganisms are actually able to "buffer" (adjust) the pH to optimize it for their growth. Often this "ideal" pH is at 8.3 to 8.7. Taking a pH reading can therefore give some information about the diversity of the microflora that may be biofouling the water. As a rule of thumb, the following table can be used to determine the possible influence of pH:
pH of less than 4.5, microflora may be quite restricted. If there is oxygen and sulfates present, there may be a potential for microbially generated acidic leaching to be occurring (Thiobacillus is a common species causing this). Remember that some slimes are capable of continuing to grow and cause biofouling even in an acidic regime. This is because the surface coating of the slime can buffer the pH upwards to a more accommodating range for the microorganisms active inside the slime. In the laboratory, biofouled porous media columns have been seen to actually return to a neutral pH after an acidic shock (eg, at pH 3.5) rapidly due to this buffering action. pH elevations of 0.2 to 0.8 pH units per hour have been observed.
pH of 4.5 to 7.1, the microflora is traumatised to some extent and, generally, the process of biofouling may become retarded (particularly below ph values of 5.5). There may be a less diverse microflora associated with the biofouling that is occurring.
pH of 7.2 to 8.8, is optimal for the growth of many microorganisms and the pH will not act as an impedance to their activity.
pH of 8.9 to 11.4, very little is known of the effects of mildly alkaline conditions on the activity of slime forming microorganisms. Generally, the activity of the microorganisms becomes suppressed and the slimes may tend to thicken (presumably as a protective function to the rising pH).
pH of greater than 11.4, tends to rapidly retard microbial activity in the water phase but little is known about the impact on the microorganisms protected within a clog or slime.
Redox (reduction-oxidation potential measured in millivolts) is becoming a very good indicator to assess the potential for microbial fouling to be occurring. Very frequently, microbial activities concentrate around the redox front which is formed where water is moving from a reductive (negative millivolts of lower than -50) to an oxidative (positive millivolts of more than +150) state. The water in the bore hole may be a reflection of just where the microbial activity is concentrated but it must be remembered that the reading being obtained is usually of the product (post-diluvial) water which may already have passed through a zone of biofouling. The redox of this water can however prove to be a very useful guide:
Greater than +150 would indicate that the water may have passed through a redox front and that the biofouling may be concentrated further back in the formation. If the redox value is much higher than +200, then the whole groundwater system around the borehole may be oxidative. In these circumstances it is possible that a massive but dispersed type of aerobic fouling may be occurring which is very difficult to control.
-50 to +150 would suggest that the redox front is close to, or within, the borehole itself. This would be a potentially serious circumstance because the microbial fouling may be occurring within the pump, on the screens and in the gravel pack. At these sites there is a greater possibility that the production capacity of the well could seriously impaired by clogging and/or sudden dramatic and unacceptable reductions in water quality.
-50 to -200 represents a virtually oxygen-free (anaerobic) water with a greater probability for corrosion (from SRB activities), offensive taste and odor generation and a poorer water quality. Another serious concern for these waters is that there would be a high probability that downstream from the wellhead, conditions would become oxidative causing massive growths where the redox front has formed. For example, this could happen at an aeration unit in a filtration plant as the redox conditions rise to within the range of -50 to +150. Massive amounts of microbial activity could then become focussed at these "shift points" causing a major biofouling.
Less than -200 are generated by extremely reductive anaerobic conditions in which microbial activities may tend to be dispersed. Any microbial activities within the formation and the borehole may be very limited. However, if downstream the redox rises to a more oxidative regime, microbial biofouling could become focussed at those redox fronts where they form.
Temperature Since we function most happily at 35 to 37oC (blood heat), there is that general belief that everything else should also grow best at that temperature. In groundwaters, one major factor affecting the microorganisms is the fact that the temperature of the water remains relatively constant (unless a local recharge of some type is occurring). Like pH and redox, temperature does influence the rate and form of microbial biofouling but, provided the water remains liquid, there is always at least microbial presence which could trigger a fouling process. The following is the potential influence of temperature on the type of microbial activity.
25 to 32oC is considered to be the range which will support the greatest diversity of microorganisms. Many of these microbes function the most efficiently at between 27 and 30oC and very often it is other factors which limit the activity level.
32 to 42oC is a temperature range similar to that found in warm blooded animals. Not surprisingly therefore, some of the bacteria which are able to grow within animals are also able to thrive in groundwaters at these temperatures if other conditions are suitable. Activities may be very rapid at these temperatures although the range of microbes abe to function will begin to become restricted over the range of 39 to 42oC. It is interesting that the cut-off temperatures for these organisms is often over a very narrow for the microbes in this group.
43 to 75oC is a range of temperatures where only specialised (thermotrophic) microorganisms can remain active. As the temperature rises, so the range of microbes continues to narrow. The sequential forms of microbial growth which occur around hot sulfur springs are a good example of this form of restricted range of growth.
75 to 160+oC is very restrictive to the range of organisms that can remain active. While the boiling point of water is 100oC at sea level under normal atmospheric pressure, the hydrostatic pressures in the (deeper) groundwaters is such that the water does not boil until much higher temperatures. For example, at 500 meters, the hydraulic pressure would reach 5 MPa which would cause the water to have a boiling point of 260oC. There is a growing body of knowledge which is supporting the hypothesis that microorganisms can remain active at greater than 100oC provided the water remains liquid. The massive microbial biofouling of sunken wrecks (eg, the RMS Titanic) and massive populations recorded in deep oceanic sediments (eg, 10 tons per hectare in the top 100 meters of deep Pacific ocean sediments) support this potential.
15 to 24oC is a temperature range over which many of the microbes that function well at 25 to 32oC begin to shut down. These are called the mesophiles. Some bacteria are able to adapt to lower temperatures and so remain active (facultative psychrotrophs). There is generally a reduced level of active in the lower part of this temperature range and a more restricted range of microorganisms are recovered.
5 to 15o supports the activities of microorganisms which are able to survive and grow at low temperatures (psychrotrophs). These organisms are usually somewhat slower growing and often take a prolonged time to adapt to the temperatures. Below 8oC, the range of microorganisms able to remain active is reduced and the rate of activity becomes slower. Below 5oC there is a marked restriction in the range of psychrotrophs and biofouling events are commonly slower in their formation.
Freezing (<0oC) Water does not freeze evenly. Water is expressed from the colloidal and other particles in the water to form ice. The amount of (liquid) water retained in the particle becomes reduced but remains liquid. Microbes present within these particles can survive and possibly function while in these particles. If the water is saline then it does not freeze at 0oC and so microbial biofouling activities can continue albeit very slowly.
Total Dissolved Solids (TDS) We always consider that there is a real difference between fresh- (< 0.1% salt) and sea- (> 1.0% salt) water because of the difference in the dissolved salt concentration. Microorganisms have a very different salt tolerance than we do. In groundwater, the (total) salt concentration is measured as the total dissolved solids in parts per million. The TDS ranges that affect microbial activities are very different to those that affect plants and animals growing on the surface of the planet. Some critical ranges are discussed below.
Greater than 12% restricts the range of microbes to those that can tolerate the very high osmotic pressures created by the salts. These bacteria are called the halotrophic bacteria and appeared to have evolved early in the life history of the planet. Various strains can be active at various salt concentrations right up to saturated salts solutions (brines) in which crystallization is occurring. In waters which are oxidative or at the redox front, the growths of these organisms will sometimes generate red slimes.
5 to 12% TDS has an impact on the variety of microorganisms that are able to survive and grow. As the salt concentration rises over this range, fewer and fewer strains of bacteria are likely to be recovered from a biofouling event. The rate of biofouling may be affected particularly over the range of 7 to 12% salt.
0.1 to 5% TDS supports the growth of most microorganisms and biofouling will not commonly be restricted by the TDS. There is likely to be a broader diversity of microflora over the lower part of the range (ie, 0.1 to 1.5% TDS) but this may not necessarily impact on the biofouling event that is being generated.
10 ppm to 0.1% (1,000 ppm) TDS does begin to have an impact on the diversity of the microflora that are able to function. Generally such waters would also be very low in nutrients and not be supportive of massive microbial biofouling in any event.
Nutrients In the past practices of hydrogeology very little attention has been paid to an examination of the nutrients in groundwaters. There are two primary reasons for this. First, it has not traditionally been considered that microbial activities in groundwaters are a significant event outside of the hygiene-risk which can easily be monitored for by determining the presence or absence of coliform bacteria. Second, the product waters commonly used for testing have passed through a biological interface in and around the borehole which reduces the nutrient loading in the water. Often nutrient levels only rise in the product water being sampled when the biofouling has matured and sloughing is occurring. The three principal elements usually associated with biofouling events are: carbon (organic), nitrogen (organic and inorganic) and phosphorus (inorganic and organic). The ratio of these three elements is often viewed as critical to the generation of a biomass and hence, a possible biofouling event. Generally, it is the phosphorus that becomes the limiting nutrient. That is to say, the nutrient which is the least available quantity to allow growth and activity to continue. Most commonly it is considered that it is organic carbon, inorganic (as nitrate, nitrite and ammonium) and organic (as proteins) nitrogen and inorganic (phosphate and polyphosphates) phosphates which are the most common sources of these elements in a biofouling. The ratio to optimally allow growth is thought to have a C:N:P (carbon : nitrogen : phosphorus) ratio of 100:1:0.25. All three elements have to be in forms potentially available to the microorganisms in order to be included in the ratio. While the carbon component of the ratio may naturally fluctuate quite considerably, the nitrogen : phosphorus ratio is often more stable and can shift the activities of the microorganisms in the biofouling. The optimal N : P ratio is usually between 4 : 1 and 8 : 1. If the ratio is >8 : 1 then there would be a deficiency in the amount of available phosphorus and this may be restricting the amount of biofouling. If, on the other hand, the ratio is <:1 : 1, the dominant microorganisms in the biofouling may shift towards those able to fixate molecular nitrogen (N2, dinitrogen). This is an energy expensive alternative and the rate of biofouling may become reduced.
There are a number of significant factors that have delayed the use of the nutrient concentrations in the water as a method for predicting the likely amount of biofouling. These include the fact that the water sampled is often product water in which many of the nutrients would already have been removed at the down-hole biological interfaces; particular nutritional elements may have different availabilities to support growth and biological activities and some simply cannot be used by the microorganisms (recalcitrant). It is therefore becoming more common to attempt to determine the status of the biofouling mass itself in terms of its location, mass, volume and composition rather than attempting to predict the likelihood of such an event actually occurring (ie, it is not asking the question is it is going to happen? but rather the question when is it going to happen?).
Traditional techniques to evaluate the potential for a biological challenge to a well system has centred on the adaptation of microbiological techniques developed by the medical industry. Two major tests which have traditionally been used are variations of the coliform test (to determine the hygiene-risk factor) and selective agar spreadplate techniques (to quantify the population size in the water). While the coliform tests have been shown to be applicable to waters and wastewaters, the various spreadplate techniques have caused some concern through a possible underestimation of the population. This may, in part, be due to the fact that the bacteria present in waters are adapted to a relative stable regime in which the nutrient supply is generally in a critical (minimalist) status and the incumbent microbes have to cooperate within consortia (including several strains of different bacteria) in order to survive. Taking these nutritionally stressed microbes and placing them in a dispersed state on an agar based medium which is relatively rich (even as bacteriological grade agar alone) in organic, nitrogenous and phosphatic compounds can cause further trauma in the bacterial community. Another challenge to the microbes is that these organisms would have to essentially "mine" the water from the agar, a task that some microorganisms may not be able to perform. To compensate for these stresses, very prolonged growing (incubation) periods can be used of 14, 28 or even 42 days to allow time for the bacteria to recover and grow to the size of visible (and therefore countable) colonies.
As is the case with the nutrients, the water sample taken from the product water may not, in fact, carry many or any of the bacteria which may be growing in the biofouled zones within slimes, biofilms or encrustations etc. A water which has passed through a stable and healthy biological interface may not "pick up" any of the microorganisms that are active there. When the waters do "pick up" some of these microbes, they may be in particulate (biocolloidal) structures and not easily dispersed to allow agar spreadplate analysis. There has been many occasions when performing agar spreadplate analysis on waters from biofouled water wells when the biocolloids actually pass down the dilution series before actually being entrapped (on the agar surface) and therefore countable on an agar plate. Consequently, the role of microorganisms in the biofouling of engineering structures has gone under-reported, assessed with some scepticism (due to the lack of statistical validation) and generally have not been recognized as a significant management concern.
Perhaps the major breakthrough in microbiology over the last twenty years relates to the growing recognition that:
- (1) in the natural world microorganisms commonly grow attached to surfaces or each other in biofilms;
- and (2) different strains of bacteria cooperate with each other within the biofilms and form consortia which may become very stable entities.
An interesting example of the latter event is the consortium which forms in the black plug layer in high sand content turfgrass golf greens. Here, a lateral slime forms which can directly compete with the turfgrass for water, oxygen, nutrients and the voids in the sand. This consortium routinely includes Erwinia carotovora (a bacteria which causes soft rot in plant roots) and Bacillus thuringiensis (the bacteria which can kill insect larvae). The consortium therefore includes a specific bacterium which can initiate the rotting process in the roots of the turfgrass, and another bacterium which can kill at least some of the predators that may attempt to feed on the plug layer. Most members of the black plug layer are able to generate slime (scientifically referred to as extracellular polymeric substances, EPS) and this functions to protect and secure the consortium in its habitat. In water well biofouling (commonly known as clogging), the consortia that form appear to be much more complex in structure and often exist within a number of distinct biozones.
Recent technological advances have resulted from a recognition that the biofouling is not just a simple bacterial infestation but includes a complex structured consortia which function interdependently within related biozones. Past practices of single form chemical treatment have failed to recognize the ability of these organisms to recover from such (treatment) trauma and rapidly reoccupy the spaces vacated due to the chemical treatment. Current technologies being field tested and commercialized today recognize the complexity of the biofouling and take a more strategic approach to controlling rather than destroying the nuisance microorganisms involved in the biofouling. In medical practise, it is presently generally believed that all infections are the result of a successful infestation of the patient by one strain of microorganism (ie, the pathogen). Applying an antibiotic known to control (and preferably kill) that pathogen is a common strategy in practise. In biofouling events, the infestation of the patient (eg, the well) involves a complex of consortia each with a different vulnerability to chemical treatment. Strategies in the last two decades have adapted to this concept in that a radical or multiple challenge treatment has been found to be more efficient.
Radical treatments range from the generation of a temperature gradient either upwards (to kill the incumbent organisms in the biofouling) or downwards (to freeze the biofouling at which time it often detaches); and to the application of more radical chemical treatments with elevated dosages and/or prolonged exposure times.
Multiple challenges can be created by applying sequentially or concurrently a combination of treatments in a manner which is complimentary to the destruction of the clogging. Often such strategies include the use of dispersants, pH modifiers and disinfectant agents in a manner which maximizes the recovery of the biofouled well to optimal conditions. One of the most recent treatment technologies (Blended Chemical Heat Treatment, BCHT) which is patented involves a tri-phasic treatment. The three phases are:
. . . . . (1) SHOCK,
. . . . . (2) DISRUPT,
. . . . . and (3) DISPERSE.
In other words, the incumbents in the clogged zone are first of all shocked (ie, traumatized); now the structures within the biofouled zone (eg, the biofilms) are disrupted; and finally the disrupted biofouling is dispersed away from the zone of concern.
As the ability to conduct adequate determinations of the nature of the biofouling and its site, it may be expected that there will be a growing acceptance of the need to manage biofouling as a controllable problem. To do this, adequate diagnostic techniques have to be used, appropriate rehabilitation performed (to recover the production capacity of the well), and a preventative maintenance program needs to be established to ensure that the useful lifespan of the well can be extended. These goals are very much in harmony with the shifting mindset of society from disposability to reusability, and from environmental negligence to environmental responsibility.