Lead Solder Contamination of Drinking Water: Water Corrosivity, Inhibition, and Limitations to Existing Understanding

Abstract

Lead ingestion via potable water is a public health concern, especially for vulnerable populations. Lead solder in building plumbing can be a dangerous source of water lead contamination. In the presence of corrosive water, lead-tin soldered copper joints form a galvanic cell that dramatically accelerates lead release. Utilities that are considering source water or treatment changes in response to recent regulations, including the LCRI, PFAS contamination levels, and sustainability requirements, or whose source waters are facing increased chloride or nitrate pollution, need guidance on the corrosivity of water to lead-tin solder and how to control it when necessary. Phosphate inhibitors are used by about half of all water utilities in the U.S., partly to reduce lead in water from leaded brass, lead pipe, and lead solder. Orthophosphate alone consistently reduces lead release from brass and lead pipe but often has little effect on lead-tin solder, and occasionally even worsens the situation. In contrast, zinc orthophosphate has outperformed orthophosphate in several studies. Chapter 2 delves into the effectiveness of orthophosphate and zinc orthophosphate in a low alkaline, high conductivity water synthesized in the laboratory. Inhibitor performance was examined at pH 6.5, 7.5, 8.5, 9.5, and 10.5, with incremental changes designed to increase corrosivity by the addition of chloride and/or nitrate. Zinc orthophosphate was ineffective at reducing lead at pH 6.5 and pH 10.5, but always reduced lead release at pHs 7.5 - 9.5 when compared to both the control and orthophosphate. At pH 8.5, dosing zinc orthophosphate reduced lead by 94-96% compared to conditions with no inhibitor or with phosphate – consistent with expectations based on a protective effect from zinc phosphate scale. Studies with a traditional galvanic corrosion cell consisting of a separate lead-tin anode and copper cathode had little or no correlation to trends in lead release that occurred with simulated joints in which lead-tin solder directly contacted copper. However, situations with very low lead release were generally associated with a 10X increase in electrical resistance using the cell. Recent experiences with solder corrosion at water utilities and in bench-scale laboratory studies raised concern about limitations to existing guidance about corrosive waters. The same low-alkalinity water used in Chapter 2 was modified in Chapter 3 to assess limitations of corrosion control guidance based on chloride to sulfate mass ratio (CSMR) and nitrate. Eight waters, four of which consisted of 10 mg/L sulfate at 0.8, 1, 4.8, and 41 CSMR, and the other four consisted of 100 mg/L sulfate at 0.3, 0.5, 4.1, and 38 CSMR, were synthesized and tested with copper and lead-tin solder coupons. At 0.3 CSMR, lead release was very low at less than 30 ppb, confirming expectations based on guidance for predicating non-corrosive waters. But a water with a significantly greater CSMR (0.8) was even less corrosive, experimentally proving that many waters with CSMRs higher than 0.5 will not have serious lead problems. Moreover, observations that less lead release occurred at a higher CSMR (0.8) with 10 mg/L sulfate than at an extremely low CSMR (0.3) with 100 mg/L sulfate revealed that higher conductivity could also cause higher release. Prior research using electrochemical cells predicted effects of adding nitrate would not follow clear trends. Using the second stage of the study to address this prediction, added nitrate at times increased, had no effect, and occasionally even decreased lead release depending on the concentrations of sulfate and chloride present. The corrosive effects of adding 10 mg/L nitrate ranged from a 7,739% increase in lead in one condition to an 88% decrease in another. The impact of nitrate on lead release is indeed complex and cannot yet be predicted with simple theories or empirical relationships. In contrast, there were strong correlations between conductivity and lead release, suggesting that it has an underappreciated role in controlling water corrosivity in relation to lead release from lead-tin solder. Overall, this study highlighted the concerning limitations of existing guidance and understanding regarding the control of lead-tin solder corrosion. Unfortunately, this guidance cannot be easily corrected or refined. Trial and error bench scale studies are the best practical approach to identifying problems (and solutions) to this extremely complicated challenge.