travels through distribution systems and household plumbing, it accumulates copper dissolved from conveyance materials. When should water operators.
A.M. Dietrich and J.D. Cuppett are with the Civil and Environmental Engineering Department, and S. Duncan is with the Food Science and Technology Department, Virginia Tech (www.vt.edu), Blacksburg, Va.
Distribution
How Much Copper Is Too Much? Most source water contains little naturally occurring copper. But as water travels through distribution systems and household plumbing, it accumulates copper dissolved from conveyance materials. When should water operators keep copper concentrations in check? A.M. DIETRICH, J.D. CUPPETT, AND S. DUNCAN
28 Opflow September 2008
systems serving more than 622,000 people violated the 1.3 mg/L Cu action level set under the Lead and Copper Rule. Thus, it’s vital for operators to understand how to reduce exposure to copper in drinking water.
Copper and Color
Blue water, which indicates a severe copper corrosion problem, can contain 500–1,000 mg/L copper, but levels of 3–10 mg/L are more common. Although one might expect consumers to notice the Panelists assess copper solutions in a controlled sensory evaluation laboratory with individual test-subject booths.
photograph: A.M. Dietrich, Virginia Tech
C
opper is an essential element that humans must ingest to meet nutritional requirements. Current recommended daily requirements are 0.34 mg for children 1–3 yr of age, 0.44 mg for children 4–8 yr of age, and 0.9 mg for adults. Excess copper exposure of about 4 mg/L or higher can cause vomiting, diarrhea, abdominal cramps, gastrointestinal upset, and headaches. The lethal dose of copper lies somewhere between 4 and 400 mg/kg of body weight, but oral ingestion of high copper doses is uncommon because of its unpleasant taste at higher concentrations. Copper found in drinking water can be a major dietary source of the element, as approximately 70–80 percent of residential plumbing is made of copper. Uniform and nonuniform corrosion of copper pipe contributes to dietary copper levels in drinking water, and those pipes may also contribute to copper levels above taste and regulatory standards. According to US Environmental Protection Agency databases, in 2003, 471 water
www.awwa.org/communications/opflow
photograph: BUSHMAN & ASSOCIATES
Copper water pipe can suffer accelerated corrosion if the water chemistry is particularly aggressive or if there is poor workmanship during the pipe installation.
presence of blue copper hydroxide, they usually don’t. In reality, a concentration of 1 mg/L copper hydroxide particles would usually be present as fine, suspended particles that can impart only a slight discoloration to water and can be masked if water is used to prepare food or beverages. These fine particles aren’t readily noticed unless one is trained to specifically look for them or filter the water through a 0.45 µm filter to measure them. USEPA developed the health-based action level of 1.3 mg/L Cu in drinking water and established an aesthetic-based secondary maximum contaminant level (SMCL) of 1 mg/L Cu. Above the SMCL, copper can stain plumbing fixtures and laundry and contribute to metallic- or bitter-tasting water. Interestingly, the aesthetic SMCL is less than the action level. The World Health Organization standard for drinking water is 2 mg/L Cu to prevent the adverse health effects of copper exposure. WHO guidelines also state that water containing >5 mg/L Cu can impart an undesirable bitter taste. TASTE AND ODOR
Can consumers detect increased copper in their drinking water at or below levels www.awwa.org/communications/opflow
that protect their health? Recent studies with copper in tap water demonstrated that the answer is maybe. Some people can detect copper below the SMCL or action level, and some consumers can’t. This isn’t surprising, as the normal human sense of taste is controlled by a range of concentrations, not a single value. Taste Tests. Consumers describe copper primarily as metallic, but also as bitter, rusty, astringent, penny, or bloody. A few previous studies report varying thresholds for detecting copper
concentrations from 1 to 13 mg/L Cu as cupric ion. Although the studies considered copper solubility, none considered detailed copper chemistry (see The Chemistry of Copper, page 30). Interestingly, when people ingest copper while their noses are closed, they perceive little flavor and only a slight taste. When their noses aren’t closed and they ingest the same concentration, copper gives a stronger flavor, indicating a relationship between taste and odor. A possible reason is that cupric ions are participating in reactions in the mouth that can be
WATER SERVICES
Copper Tubing Specifics Copper tubing became a popular replacement for lead and galvanized iron in service line installations because it is flexible, easy to install, corrosion resistant in most soils, and able to withstand high pressure. It is not sufficiently soluble in most water to be a health hazard, but corrosive water may dissolve enough copper to cause green stains on plumbing fixtures. Allowable copper concentration in drinking water at the tap is regulated by the Lead and Copper Rule. In some cases, corrosion inhibitors must be used to control the copper content. Brass and bronze valves and fittings can be directly connected to the pipe without causing appreciable galvanic corrosion. Copper water service tubing is usually connected by either flare or compression fittings. Although interior copper plumbing is usually connected with solder joints, this method is rarely used for buried service lines. — AWWA, Water Transmission and Distribution, third edition
September 2008 Opflow 29
Distribution
WHAT DOES IT MEAN?
Many consumers—but not all consumers at all times—could detect the flavor of copper in their tap water at or below levels designed to protect human health. The ability to detect copper in tap water is controlled by individual sensitivity, exposure, and adaptation to copper, as well as water chemistry. The flavor of copper is more noticeable when 30 Opflow September 2008
THE CHEMISTRY OF COPPER Soluble copper—free or complexed—can be readily tasted in drinking water, but solid or particulate copper is more difficult to taste. Copper, like other metal cations, occurs in three major forms in water: free, complexed, and particulate. Copper chemistry is complicated and controlled by thermodynamics, water quality, and kinetics. As a first cut, pH is the major factor controlling whether copper is free, complexed, or particulate. In copper pipes, the initial scale that forms is from copper hydroxide. Thus, the major solid controlling copper solubility in most drinking water—and a pH below about 10—is copper hydroxide, with a solubility product of Ksp = 10–19.2 = [Cu2+] [OH–]2 at 25ºC. A more thorough examination of copper solubility includes assessing formation of other complexes and copper solids such as tenorite and malachite. Because of the effect of pH, the solubility of copper typically decreases with increasing pH. As an example, using an unchanging copper concentration but increasing pH levels, 1.3 mg/L Cu (the USEPA action level) would be soluble at a pH of 5.5, but at pH 7.4 only 0.9 mg/L Cu would be soluble. At pH 8.5, only 0.3 mg/L would be soluble, leaving most of the copper in the concentration present as particles.
How Copper Pipe Corrodes Copper pipe may corrode at a rate sufficient to contaminate water beyond state and federal drinking water standards.
Copper Pipe Wall
Cu 0
Cu(OH) 2
2 e–
Soluble
Cu +2 Possible e – acceptors O 2 and HOCL
Cu
Insoluble
Free
Complexed
Particulate
Cu +2
Cu(OH) +
CuO (s)
Cu(OH) 3 –
Cu(OH) 2(s)
Cu(OH) 4 2–
Cu 2 CO 3 (OH) 2(s)
+2
2 e– Cu 0
Cu(OH) 2 Copper Pipe Wall
the copper is soluble, which typically occurs at lower pH values. As water pH increases to more than 8, soluble copper is converted to particulate copper and, depending on the concentration of total copper, the level may exceed a healthbased standard and not be detected by human senses. So, to provide consumers with safe and palatable drinking water, utilities should be more vigilant of
copper concentrations if the tap water’s pH is above 8. Acknowledgment: This material is based on work supported by the National Science Foundation, Grant No. 0329474. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and don’t necessarily reflect the views of the NSF. www.awwa.org/communications/opflow
figure: a.m. dietrich, virginia tech
smelled. Most consumers don’t distinguish between taste and odor for ingested foods and beverages but rather refer only to “taste.” In reality, many of the characteristics people identify as “taste” are really “odor.” Virginia Tech researchers recently investigated copper chemistry and flavor. Thirty-six healthy volunteers participated in a taste test for copper at 10 concentrations ranging from 0.1 to 8 mg/L. The results indicate that about 75 percent of test participants could detect a copper flavor below the aesthetic-based SMCL of 1 mg/L. The remaining 25 percent, however, couldn’t make the distinction, and some participants couldn’t taste copper at 8 mg/L, the highest concentration tested. Therefore, some consumers can be effective sentinels for detecting copper in their tap water and others can’t. Disinfectants. Most tap water contains disinfectants. Concentrations of 1 mg/L chloramines (Cl2) and 2 mg/L monochloramine are common disinfectant levels in US tap waters, which leads to another important question. Will the presence of a disinfectant with a chlorinous flavor alter the taste of copper in drinking water? The answer is “no.” Concentrations of 1 mg/L Cl2 and 2 mg/L monochloramine didn’t alter test participants’ ability to detect a copper concentration of 1 mg/L in tap water with mineral content and pH that’s typically found in the eastern United States. So, if copper is present above the flavor threshold and is soluble, chlorinous tastes and odors shouldn’t alter consumers’ sensitivity.
