Effects of water stagnation in plumbing system on human health risk from heavy metals in tap water

Domestic water demand in Saudi Arabia is approximately 2.5 billion cubic meters (BCM) per year, in which nearly 1.1 BCM (~50%) is supplied by the desalination plants in the coasts of the Red Sea and the Arabian Gulf, and the remaining is satisfied by groundwater. A network of approximately 4,160 kilometers pipelines and a storage reservoir of 9.1 million cubic meters (MCM) distribute the desalinated water from the desalination plants to the major inland urban centers. The desalinated water is blended with ground water, disinfected with chlorine, pH adjusted and transported through the distribution networks to the consumers. The desalinated/blended water, which is relatively corrosive, can cause pipe materials and pipe coatings to leach into the drinking water during transportations and/or long residence in the water distribution system (WDS). Leaching of these metals is particularly important during the hot summer period, when the temperature can be more than 40C, which can increase the reactivity of desalinated/blended chlorinated water with the pipe materials and inner coatings by several folds, resulting in the higher rates of release of metals into bulk water. The regulatory and/or monitoring programs often analyze water samples from the WDS for regulatory compliance. The desalinated and/or blended water flows from the WDS through the plumbing system of the building prior to reaching the consumer tap, while the size of the plumbing system can vary depending on the size of the building. The plumbing system will allow additional time for reaction from the WDS (e.g., water entry point of the house) to the tap, which can increase the concentrations of heavy metals in the tap water. Further, water can be stagnant in the plumbing system (e.g., plumbing pipes, hot water tanks) for a long time depending on the time of the day (e.g., several hours of stagnation during the midnight to morning period), which can allow extended reaction to increase the levels of heavy metals in tap water. Exposure and risk analysis without incorporating the effects of plumbing system might not be representative. Many of the heavy metals are considered to be the priority contaminants by the United States Environmental Protection Agency (USEPA), while these metals can have various toxic effects to human health. Chronic exposure to these metals can have cancer risks and/or other chronic and sub-chronic effects to human. Understanding the occurrences of these metals in drinking water, particularly at the point of exposure (e.g., tap water) is important to assess human exposure, their possible risks and remedial measures. The proposed study will investigate the effects of plumbing systems on the concentrations of heavy metals in tap water, develop models to predict heavy metals in tap water and predict human exposure and risks from these metals. The results of the proposed research will assist in understanding and reducing heavy metals in tap water and their risks to humans. This study will identify strategies to minimize human exposure to metals, which might be helpful in reducing long-term effects to human and related costs. As an example, reduction of one cancer incident can reduce approximately 0.5 million SR, which is needed for treatment, medicine, doctors and services throughout the survival period. Costs of other chronic and sub-chronic effects can be even higher. The findings of this research can assist in better controlling human exposure and risk from heavy metals in drinking water.