Industries that discharge storm water under a General Storm Water Permit must sample what?

The correct answer is that industries discharging storm water under a General Storm Water Permit must sample storm water from outside industrial activities. This reflects the intent of the General Storm Water Permit, which is designed to control pollution from storm water runoff associated with industrial activities.

Sampling storm water from outside industrial activities allows for assessing the quality of the water being discharged into the environment and ensures that it meets regulatory standards to protect water quality. This is crucial because storm water can carry pollutants from various surfaces, including parking lots and other areas adjacent to industrial operations. Monitoring these discharges helps in identifying potential sources of contamination and implementing necessary control measures.

The choices regarding groundwater samples and storm water from inside the facility do not align with the requirement to focus on external runoff associated with industrial activities, while storm water from industrial activities only would not provide the comprehensive oversight needed to assess the impact of all runoff from the facility. Therefore, the emphasis is rightly placed on external sources of storm water to ensure a broader understanding of environmental impacts.