Precipitation to Radiation: Understanding and Analyzing the Impact of Precipitation on Background Radiation Across the United States

The Environmental Protection Agency’s RadNet Network is a nationwide, near real-time background radiation monitoring network composed of 140 sites. The network allows the agency to monitor and establish baselines of local background radiation levels, which additionally allows the agency to determine whether the measured radiation is anomalously high and thus a threat to public health and safety. Recent real-time uses of this system include deploying temporary, additional RadNet sites in Alaska and Hawaii following the aftermath of the 2011 Fukushima meltdown. While radiological incidents warranting concerns about public health and safety are beyond rare, RadNet data is publicly available and natural increases in background radiation are normal. Thus, even temporary increases in background radiation measured by the RadNet system could be of public concern.

One of the contributing factors to temporary increases in background radiation include precipitation. Precipitation strips radiological particulates from the air, making them concentrated near the surface. However, the relationship between radiation levels and precipitation has historically been difficult to quantify due to the lack of on-site precipitation measurements. As a result, this research project sought to quantify the relationship between precipitation and radiation by using a subset of the 140 monitoring sites. Approximately 30 RadNet sites were chosen to analyze radiation measurements before, during, and after precipitation based upon their proximity ( < 3 miles) to a known, reliable metrological measurement site.

Our results found that increases in radiation after precipitation were heavily dependent upon the atmospheric composition of the environment around the RadNet site, vertical extent of the RadNet site (e.g. site situated on a roof compared to ground level), and duration of the precipitation event. Several sites in our analysis found radiation levels increased upwards of 30% during a precipitation event while other sites reported a much smaller 10% increase. While it is important to note these precipitation dependent increases in radiation never pose a threat to human health and safety, our results underscore the importance of adequate communication of RadNet’s role in radiation monitoring, given the ease of public access to the dataset, and highlight critical details that are important to emergency responders who may be faced with a radiological incident that does pose a threat to human health and safety.