Indirect Potable Reuse (IPR) is the blending of advanced treated, recycled or reclaimed water into a natural water source (groundwater basin or reservoir) that could be used for drinking (potable) water after further treatment.
Indirect Potable Reuse: A Sustainable Water Supply Alternative provides an overview of significant indirect potable reuse projects, followed by a description of the epidemiological and toxicological studies evaluating any potential human health impacts of these projects. A summary of key operational measures to protect human health and the areas that require further research are discussed.
The EPA website Water Recycling and Reuse: The Environmental Benefits discusses the opportunities for water recycling and the environmental benefits associated with this practice.
Examples of Indirect Potable Reuse projects include the Groundwater Replenishment System in Orange County, California (in operation since 2008) and San Diego, California's Water Purification Demonstration Project which is expected to lead to Full-Scale Reservoir Augmentation. Here's a recent update on the history that led to that.