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Importance of Fe and Mn Pipe Deposits to Lead and Copper Rule Compliance - abstract
Citation:
Schock, M., A. Cantor, S. Triantafyllidou, AND Mike DeSantis. Importance of Fe and Mn Pipe Deposits to Lead and Copper Rule Compliance - abstract. Presented at AWWA Annual Conference and Exposition, Boston, MA, June 11, 2014.
Impact/Purpose:
To inform the public.
Description:
When Madison, WI exceeded the lead Action Level in 1992, residential and off-line tests suggested that lead release into the water was more complex than a lead solubility mechanism. The water utility chose to address the Lead and Copper Rule (LCR) exceedance by implementing full replacement of their ~10,000 lead service lines (LSLs) during 2001–2010. Monitoring of 60 home taps after full LSL replacement, revealed that lead levels were still high at sites where the LSLs had been replaced within the previous four years. Surprisingly, elevated lead (Pb) persisted long after full LSL removal. The erratic Pb concentrations were eventually attributed to particulate lead in water, suggesting that particulate matter had accumulated in the plumbing and was dislodging into the water even after LSLs were removed. Five LSL samples were excavated from five separate street locations, as part of the utility’s full LSL replacement program. Each street location received potable water from a combination of wells, some of which had high manganese (Mn) and iron (Fe) content. Detailed scale analyses (color/texture, mineralogical and elemental composition) of the pipe samples were undertaken, to explain historical observations of Pb concentration in Madison tap water. Results revealed that accumulation of Mn and Fe from the source well water onto pipe walls had implications for lead corrosion, by providing a high capacity sink for lead. Specifically, Mn (and sometimes Fe) was the most abundant element identified in LSL scales, after Pb itself. Manganese that accumulated from the source well water onto the pipe scales (up to 10% by weight) served to capture and eventually transport Pb to consumer taps. Manganese also obstructed the predominance of an insoluble and thus potentially protective plattnerite [Pb(IV) solid] scale layer in 3 out of the 5 LSLs examined. The amorphous black scale layer in those 3 LSLs was visually consistent with identification of Mn (and sometimes Fe) in the scale, and contributed to water lead contamination at those street locations. A protective insoluble brown plattnerite scale layer formed in the remaining 2 LSLs, and those street locations are believed to have been fairly protected from water Pb contamination. Full LSL replacement in Madison eventually achieved LCR compliance and major reduction in lead contamination and exposure, when supplemented by uni-directional flushing of water mains and manganese control in the source well water. This work suggests that: 1) all sources of Mn and Fe scale in a water system should be considered as potential sinks for accumulating and controlling Pb release into water 2) treatment or water quality changes that affect Mn or Fe pipe deposit stability could adversely affect Pb release levels, regardless of lead solubility 3) determining the mineralogy and the chemical content of harvested pipe scales can help elucidate mechanisms of Pb accumulation and eventual release into the water
URLs/Downloads:
http://www.awwa.org/portals/0/files/education/conferences/ace/conference-program/2014/resources/