Water Well Journal
Issue link: http://read.dmtmag.com/i/767379
within living cells, within a sample. The ATP test is relatively inexpensive and is even available in field applications. An additional step in assessing the potential severity of biofilm growth can be taken through identifying specific anaerobic bacteria whose presence often signifies the layering effect observed in more mature biofilm growth that was described earlier. Anaerobic growth can be determined via nutrient characterization assays. Tests such as these monitor the growth of anaerobic species as a function of the total bacterial population through the re- sponse to specific macro-nutrients, and can even be adapted to more specific anaerobes such as SRBs. Scale Potential Bacteria and subsequent biofilm for- mations also play a pivotal role in form- ing mineral incrustation within wells. As water containing mineral ions, crys- tals, clays, and other inorganic debris flows toward and enters the well, these compounds may become entrapped by any biofilm formation present on the surfaces over which it is flowing. The accumulation of inorganic debris to the surface is made possible by the excel- lent surface adhesion capabilities of biofilm—making biofilm a catalyst for the conglomeration of mineral scales. In most cases, well blockage is often a combination of biological material and minerals formed either as deposits of water constituents or from the adhe- sion of particulate matter or sand. As the content of inorganic debris increases within these matrixes, the result is a denser, more hardened mass represent- ing a more formidable barrier to water flow. Another way to monitor biofilm accumulations and their composition is the microscope. Biofilm can be observed under the microscope with as little as 100-times magnification. Centrifugation of samples can also be used to aid in collecting free-floating biofilm growths within a sample for fur- ther observation under the scope. One or two drops of centrifuged concentrate from a sample is usually sufficient. Once collected, a microscopic view of the sample can show not only the degree of buildup present but also the makeup of the matrix itself. Iron oxides, one of the most common mineral con- stituents found in problematic wells, are easily identifiable under the scope, as are calcium carbonates and other min- eral compounds. Formation materials such as silica sands and clays are also readily identifiable. Similarly, plant particulate matter, in- dicative of surface water influences, are also visible if present. The microscope observing the presence and severity of these inorganic materials in combina- tion with biofilm accumulations is an- other valuable and simple tool in assessing bacterial fouling and the po- tential need for maintenance. Microbially Influenced Corrosion Yet another prominent way bacteria can drive the need for maintenance in wells is their roles in influencing the basic principles of corrosion. Any mode of corrosion which incorporates mi- crobes that react and cause corrosion or influence other corrosion processes is called "microbially influenced corro- sion." MIC often occurs in the form of pitting, but can also be seen in any num- ber of other forms of corrosion. A number of bacteria can introduce corrosion-assisting constituents, such as acids and sulfides, during growth phases and in carrying out their basic metabolic processes. A commonly recognized ex- ample of this is the reduction of sulfate (SO 2 −4 ) to hydrogen sulfide (H 2 S) by SRBs. Uneven colonization of microorgan- isms over a surface can also contribute to corrosion by influencing oxygen con- centration gradients. Aerobic bacteria located in biofilms near the water inter- face can create an oxygen gradient during oxygen consumption, leaving oxygen levels depleted in lower por- tions of the biofilm matrix near the sub- strate surface. This results in the surface area under the biofilm becoming anodic to the area exposed to the bulk aqueous phase. Direct degradation of materials can also occur from the presence of specific bacteria, such as iron-oxidizing bacteria. One of the more recognized species of iron-oxidizing bacteria is Gallionella, a naturally occurring bacteria found in a variety of aquatic environments— including aquifers. Gallionella are a stalked bacterium that use iron as an energy source and se- crete an iron-oxy-hydroxide byproduct. In its attachment to iron-bearing sur- faces, Gallionella pits the metal in an effort to secure the iron necessary for energy. All iron-bearing structures, including stainless steel, are susceptible to this form of pitting. Crenothrix at 400-times magnification. MICROBIOLOGY from page 19 waterwelljournal.com 20 January 2017 WWJ