Part II: Chemistry
By Roger Miller
The scientific field of chemistry contributes in many ways to the life cycle of a water well. From the capacity losses associated with mineral deposition, through degradation of the well structures by corrosion activities, to unacceptable changes in water quality from aquifer fluctuations and deposit buildup—chemistry is in action.
The nature of groundwater chemistry is in many ways directly associated with geology. The basic water-to-rock relationship contributes to the water chemistry through the activities of solubility, oxidation, reduction, and weathering.
When groundwater is recharged by its many sources, these activities are producing a multitude of changes from its atmospheric composition. This process of migration and the nature of its path is the creation of groundwater chemistry.
Water is commonly referred to as the universal solvent. With respect to groundwater, this is especially accurate as it flows through the subsurface—dissolving various minerals. The dissolution yields charged structures known as cations (positively charged) and anions (negatively charged).
Common mineral cations present in groundwater are calcium, magnesium, manganese, and various forms of iron. Common water anions include carbonates, bicarbonates, sulfates, and hydroxyls.
A simplistic explanation of the importance of these ions in groundwater is that under certain conditions they will come together to form compounds such as the common calcium carbonate, calcium sulfate, or iron oxide/oxyhydroxide.
Many of these compounds are insoluble and will precipitate out in or around the well, blocking flow paths and causing the referenced loss of capacity. In addition to this concentrating effect in and around the well, it provides the potential for compounds to re-solubilize, increasing the concentrations of the ions in solution and effecting the produced water quality.
Albert Einstein’s mentor Ernst Mach said, “The essence of science is measurement.” Accordingly, a water chemist has the ability to evaluate groundwater chemistry and predict the potential of a well to foul over time from mineral deposition.
To that point, the field of water chemistry has defined the prominent water mineral cations of calcium and magnesium as “water hardness.” These are the ions most available to pair with the carbonates and bicarbonate ions to form mineral precipitates such as calcium carbonate. Hardness levels of 0-60 mg/L are classified as soft; levels from 61-120 mg/L are moderately hard; levels of 120-180 mg/L are hard; and water with a hardness greater than 180 mg/L is classified as very hard.
Water alkalinity is a measure of a solution to neutralize acids. In well water, alkalinity is primarily a measure of the common anions of carbonate/bicarbonate. Bicarbonates represent the major form of alkalinity in natural waters; its source being the partitioning of carbon dioxide from the atmosphere and the weathering of carbonate minerals in rocks and soil. Other ions contribute to alkalinity such as hydroxides, phosphates, and silicates but carbonates make up approximately 90% of the reading.
It is important to note the alkalinity is strongly related to the measurement of pH (hydrogen ion concentration) as various forms of alkalinity are only present at certain pH levels. Therefore, from the measured values of these structures we can predict the formation of compounds such as calcium carbonate which requires alkalinity of >150 mg/L and hardness of >180 mg/L with a neutral pH value.
In conjunction with these mineral deposits is the formation of iron and manganese scales from the chemical reaction of oxidation. The presence of these metals is common to the subsurface as iron makes up about 5% of the earth’s crust and manganese about 0.1%.
Common iron and manganese-related rock structures are hematite (iron oxide) and pyrolusite (manganese oxide) and soluble iron can be present in aquifer waters of these structures. The oxidation of these metals is the chemical reaction that presents them as insoluble and allows for their precipitation in and around the well.
This chemical reaction can occur with natural aquifer waters that are oxidative or can be caused by aeration of the flowing water from voids in the casing or drawdowns below the screens.
The mere presence of iron in a water sample is limited in its evaluation without knowing its oxidation state or even its origin. A single total iron test often fails to fully determine the variety of iron species present.
Knowing these different phases of iron can identify the potential for iron to fall out within the well as fouling deposits together with iron that is the result of active corrosion, iron that has been chemically oxidized, or even iron that is organically mobilized from potential bacterial influences.
A total iron test analyzes for both ferrous and ferric iron for a measure of the total iron in the sample. Ferrous iron can represent iron just released from a surface, indicating corrosion.
Also, this dissolved phase of iron can reflect native background iron within the aquifer. The ferric state is iron that has been further oxidized, rendering it as insoluble and allowing for precipitation and deposition in and around the well. Total iron levels above 1 mg/L is an indication that iron precipitation may be occurring within the well.
The second water chemistry influence on the life cycle of a water well is the destruction of the well itself through the activity of corrosion. The term “corrosion” is defined as the deterioration of a material due to interaction with its environment. This chemical reaction is one of the most prominent factors that can influence the life cycle of a water well as it can affect the structural integrity of the well itself along with resulting capacity declines and even water quality issues.
The chemical-based corrosion process is an electrochemical action involving the movement of electrons from an anodic area to the cathodic area through the metallic pathway. Most metals used in water well construction are good conductors of electricity, allowing for easy movement of these electrons.
The anodic area where the electrons are initially released is the site of corrosion and metal damage. The water in the well is the electrolyte which provides the reactants for the flow of metal ions.
Most corrosion reactions in nature are electrochemical or common oxidation-reduction processes where oxidation occurs at the anode and reduction occurs at the cathode.
General corrosion is an attack of the metal that is distributed evenly over the complete surface. The electrochemical corrosion cell is usually represented by small anodic sites all over the metal surfaces and is generally referred to as uniform rust.
Galvanic/dissimilar metal corrosion is the electrical potential difference between different metals when they are in contact with an electrolyte, the water in the well. The Galvanic Series shows the less noble metals as the anodic and the more noble metals as the cathodic. Therefore, the anode gives up the metal ions to the cathode and by doing so degrades in the corrosion reaction.
Concentration cell corrosion, sometimes referred to as under deposit corrosion, is the activity of the electrochemical reaction occurring in a concentrated area of the metal surface. This reaction results in localized pitting of the metal and is noted as the most rapid and damaging type of corrosion of structures and equipment in the groundwater industry.
Crevice corrosion is another form of localized attack that occurs in areas of well construction such as threaded pipe joints or gasketed flanges. In these areas, narrow gaps between two metal surfaces create contact with the water, or electrolyte, and provide the pathway for initiation of localized corrosion. Due to the pressures created in these areas by the buildup of corrosion byproducts, corrosion cracking lof the metal occurs, creating further degrading of the well components.
The process of well degradation by corrosion also includes microbial induced corrosion (MIC), which will be discussed next month as this series wraps up by discussing the science of microbiology.
As noted previously, the ability to measure the potential of corrosion reactions is the key to well construction, operation, monitoring, and maintenance. The common corrosion evaluations in the water industry is the Langelier Saturation Index and the Ryznar Stability Index. In both cases, calculated index values are first used to determine if the water is scale forming or not, with the latter used as an indication of the potential for corrosion.
Langelier based his index on water chemistry values of temperature, TDS (total dissolved solids), calcium concentration, total alkalinity, and pH value. Ryznar modified the Langelier calculation to what he considered a more reliable prediction by incorporating actual field results of scale formation and corrosion damage into his research.
A less involved measure of simple oxygen-induced corrosion which is historically fast and reliable is what our lab refers to as the “Nail Test.” This test utilizes a low carbon steel nail and the actual aquifer water placed in a glass container for visual observation. If the water around the nail turns a pink color within a few hours, the water is corrosive when influenced by oxygen. Even the activity of iron bacteria can be identified with this test.
The loss of structural integrity to the well is the key influence corrosion has on the life cycle of a water well. However, with corrosion damage to the components of a well such as casings or screened intervals, water flow dynamics can change.
A hole in the casing caused by corrosion can provide a source of aeration to the water which in turn can produce a more corrosive water, increasing the potential damage to the lower portions of the well.
Additionally, a hole in the screened interval of a well can allow for higher velocities through these voids which can mobilize sand particles, not only creating an undesirable production of sand in the flowing water but also the erosion effect of the sand can further damage the screened area. These issues can result in capacity declines and alteration of water quality that are additional influences on well life.
The last water chemistry area noted to influence the lifecycle of a well is change in water quality. The initial process of well design, site selection, operation, and subsequent water treatment processes handle the water quality issues up front. However, in some situations the water quality can change over time, presenting the well owner with the evaluation of correcting the changes or abandoning the well—both significant and often costly life cycle decisions.
The prominent quality changes over time are generally associated with aesthetics such as taste, odor, and color. In general, groundwater chemistry should be fairly stable with only slight variations. Fluctuations of greater than 10%, depending on the chemical parameter, can signal changes within the well that are affecting water quality such as deposition of minerals or the byproducts of system corrosion.
The control of the chemical parameters are not regulated by the Primary Drinking Water Regulations but recommended by the Secondary Drinking Water Standards and of course the demands of the customer base.
As noted in the Secondary Drinking Water Standards, water taste can be influenced by elevated levels of most metals and a low pH value resulting in a metallic taste. Similarly, elevated levels of chlorides and sulfates can also cause a salty taste to consumers.
Water color is generally influenced by copper as a blue-green stain, iron as a reddish orange stain, and manganese as a brown or black stain.
The prominent odor in groundwater in this series is the rotten egg smell generated by hydrogen sulfide. As will be explained in the future article in this series on microbiology, the primary process for the creation of hydrogen sulfide is bacterial in nature.
This process is generally the anaerobic decomposition of organic compounds containing sulfur by sulfate-reducing bacteria capable of converting sulfate to sulfide. The chemical process for the creation of hydrogen sulfide is relatively rare and is based on the reaction of either oxidation or reduction. The presence of this reaction is generally found in deep well settings where the ions of sulfur are stable, and the presence of bacteria is lower.
The decline in water quality from any of these issues can result in the inability to produce acceptable water to the customer base and must be addressed from the economic standpoint of repair or replace the well.
The three primary influences of water chemistry (material deposition and resulting loss of capacity, structural decline of the well itself through corrosion, and adverse changes in produced water quality) represent the uniqueness of the specific well setting and the potential effects of its operation.
Effective monitoring of the water chemistry produced from the well can assess these conditions and provide for changes in operation, proper maintenance, and treatment to maximize the life cycle of the well.
Roger Miller is a senior consultant at Water Systems Engineering, specializing in water chemistry. He has worked over the past 40 years in research and development, analytical procedures, site assessment, and project oversight in the groundwater and water treatment industries. He can be reached at firstname.lastname@example.org.