Untreated water can intrude into water distribution systems through pipe leaks and cracks when negative pressure events occur inside drinking water pipelines. The oriﬁce equation is typically used for calculation of intrusion ﬂow rate, which ignores the impact of the soil surrounding the pipeline. This paper presents the results of an experimental study on the effect of porous media surrounding pipelines and the flow Reynolds number on intrusion flow rate for a circular orifice. The porous media, orifice size, and flow regime affect the discharge coefficient of the orifice equation. A new expression was suggested for predicting the intrusion flow rate. A discontinuity in the discharge coefficient was also found at a large Reynolds number. The effect of the pipe curvature on the discharge coefficient was found to be insignificant.