The stormwater management in construction site is an important, complicate and time-consuming task that relates to huge amount of information, data, domain law, and expert knowledge and experience in terms of environmental protection, soil erosion, and receiving water protection. This paper presents an expert system “RP 3 CA” that is used to mitigate stormwater pollution during construction activities. A most important function of the RP 3 CA is its user interface, which utilizes Geographic Information System functions as a supportive component to display spatial maps. Decision tables and trees are used to acquire expert knowledge and experience. Certainty Factor is used for measuring the expert belief or disbelief of the system rules. Opinion integration of multiple experts is obtained by application of Expert Choice software where various alternatives are available. Assessment suggestions and results are derived utilizing a forward chaining inference mechanism. Unit testing, integrated testing, system testing, two test cases, comparison of system results, internal experts, and external experts, reviewers feedback by system application, and validation of system reasoning are used to demonstrate the proposed RP 3 CA.