By Archana Bhardwaj, P.E., Senior Project Engineer, California Rural Water Association
Specialized Utility Services Program (SUSP) is often asked to conduct Corrosion Control Treatment (CCT) studies for water utilities dealing with Lead and Copper exceedance in their systems. The aggressiveness (potential to dissolve metals in contact, such as lead and copper) of water within the distribution system is an important factor to be considered in such studies.
Many factors influence aggressiveness of water, sometimes in conflicting ways. Distribution system materials play an essential role, and water that may be passivating for one material may be corrosive for another. Many water quality parameters need to be considered when examining the aggressiveness of a water source, which resulted in the development of indices of corrosion and aggressiveness, such as the Langelier Saturation Index. The following is a brief summary of some of those parameters.
pH: pH has a major effect on lead and copper leaching since it plays a significant role in the ionic state and solubility of almost every water quality parameter. At higher pH values, there is less of a tendency for metal surfaces in contact with drinking water to dissolve and dissociate, making pH adjustment a common component of an effective strategy.
Alkalinity and DIC: Dissolved Inorganic Carbon (DIC) is the sum of all carbonate species in water. DIC varies according to pH, temperature, ionic strength and alkalinity and impacts the buffer intensity of water. Sufficient DIC concentration (and alkalinity) is needed to form protective scales and provide buffer intensity, but too much can solubilize lead.
Buffer Intensity: This is a measure of the ability of water to resist wide fluctuations in pH in the distribution system. Corrosion rates and pitting corrosion tend to decrease as buffer intensity increases.
Iron and Manganese: These metals can react with dissolved lead and form deposits in premise plumbing, creating aesthetic issues. Manganese can also interfere with the formation of lead scales and other passivating films.
Total Dissolved Solids (TDS): High TDS usually implies a high ionic concentration and thus higher conductivity, which may facilitate the corrosion reaction. If sulfate and chloride are high contributors within the TDS, iron corrosion is more likely. With high bicarbonate or hardness ions, the water may be highly corrosive towards copper.
Hardness and Calcium: Hardness and calcium concentration may favor the precipitation of calcium as CaCO3, thereby providing protection inside distribution system structures. However, the link between calcium hardness and metal corrosion is not straightforward and, therefore, hardness adjustment is no longer considered an acceptable corrosion control technique by the Environmental Protection Agency (EPA).
Corrosivity Indicators such as Langelier Saturation Index (LSI) and Calcium Carbonate Precipitation Potential (CCPP) are indicative of scaling conditions within the distribution system. They can only predict potential deposition or dissolution of calcium carbonate.
Chloride Sulfate Mass Ratio (CSMR): The solubility of lead chloride and lead sulfate are many orders of magnitude greater than lead carbonate. A high concentration of carbonate (as expressed by high alkalinity) can passivate lead when chloride and sulfate concentration are lower. However, a higher CSMR can increase galvanic corrosion and subsequently increase lead from leaded components. Chloride is related to high lead release and sulfate to low lead release.
Hydraulic Factors: High velocity within the distribution system pipes may prevent deposition of a passivating layer on the pipe surface or wash away a deposited layer.
An engineer studying a water distribution system for corrosion control must assess all these factors that contribute to pipe degradation and metal dissolution before determining optimal treatment strategies to ensure regulatory compliance.
