Impact of cold-water premise plumbing on the fate and associated additive toxicity of disinfection byproducts

• Water use frequency and disinfectant type dominate DBP fate at the tap. • Low-use stagnation reduced HANs and DBP calculated toxicity by up to 80% in pipe racks. • High-use conditions increased calculated toxicity by 25–50% across all pipe types. • Broader DBP analysis revealed unregulated HALs dominate calculated toxicity after stagnation. • The decline in CAT during stagnation was accompanied by increased HPCs across all pipe materials, highlighting the need to balance chemical and microbial risks in premise plumbing. Disinfection byproducts (DBPs) can transform within building plumbing systems, altering both concentrations and toxicity at the point of use. This study evaluated how pipe material (copper, PEX, PVC), pipe diameter, and water use frequency affect the fate of four regulated trihalomethanes (THM4), nine haloacetic acids (HAA9) and four haloacetonitriles (HAN4) using controlled pipe racks operated for one year under chlorinated and chloraminated conditions. The calculated additive toxicity (CAT) metric for these DBP groups was also evaluated. Random Forest analysis revealed that water use frequency and disinfectant type were the strongest predictors of DBP occurrence and CAT with pipe material and size playing secondary roles. Under low-use conditions, HAN4 concentrations decreased by 60–90%, resulting in a 40–80% reduction in CAT relative to feed water, primarily due to the degradation of nitrogenous DBPs. In contrast, high-use conditions increased CAT by 25–50% across all pipe types. Complementary batch experiments, using copper and PEX pipes, expanded the DBP scope to 52 regulated and unregulated species and showed that, while HANs again declined, overall CAT did not decrease due to elevated levels of unregulated DBPs, particularly haloacetaldehydes which dominated CAT. These findings underscore the limits of relying on regulated DBPs or narrow toxicity metrics and suggest that whole-water assays offer a stronger framework for assessing health risk changes in plumbing systems. The apparent decline in DBP toxicity during stagnation coincided with much higher microbial activity (HPCs) across all pipe materials, emphasizing the challenge of balancing chemical and microbial risks in premise plumbing.