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Driven by the development of water purification technologies and water quality regulations, the use of better source water and/or upgraded water treatment processes to improve drinking water quality have become common practices worldwide. However, even though these elements lead to improved water quality, the water quality may be impacted during its distribution through piped networks due to the processes such as pipe material release, biofilm formation and detachment, accumulation and resuspension of loose deposits. Irregular changes in supply-water quality may cause physiochemical and microbiological de-stabilization of pipe material, biofilms and loose deposits in the distribution system that have been established over decades and may harbor components that cause health or esthetical issues (brown water). Even though it is clearly relevant to customers' health (e.g., recent Flint water crisis), until now, switching of supply-water quality is done without any systematic evaluation. This article reviews the contaminants that develop in the water distribution system and their characteristics, as well as the possible transition effects during the switching of treated water quality by destabilization and the release of pipe material and contaminants into the water and the subsequent risks. At the end of this article, a framework is proposed for the evaluation of potential transition effects.

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Estrogenic activity risks in the Pearl River system (Liuxi River, Zhujiang River and Shijing River) in South China were assessed by combined chemical analysis and recombinant yeast estrogen screen (YES) bioassay for surface waters and sediments collected in both dry and wet seasons. The xenoestrogens 4-tert-octylphenol, 4-nonylphenol and bisphenol A were detected at almost every sampling site at concentrations of several ng) in surface waters (and sediments). The estrogens estrone and 17b-estradiol were also detected in most of the samples with concentrations from several ng L À1 (ng g) in surface waters (and sediments). However, synthetic estrogens diethylstilbestrol and 17a-ethinylestradiol were only detected at a few sites. The 17b-estradiol equivalents (EEQ) screened by the YES bioassay were in the range of 0.23-324 ng L À1 in surface waters and from not detected to 101 ng g À1 in sediments. Shijing River displayed one to two orders of magnitude higher levels for both measured chemical concentrations and estrogenic activities than the Zhujiang River and the Liuxi River. A risk assessment for the surface waters showed high risks for the downstream reaches of the Liuxi River and the upstream to midstream reaches of the Zhujiang River and the Shijing River. Higher estrogenic risks were observed in the wet season than in the dry season for surface waters, probably due to the input of runoff and direct overflow of small urban streams during heavy rain events. Only small variations in estrogenic risk were found for the sediments between the two seasons, suggesting that sediments are a sink for these estrogenic compounds in the rivers.