On October 12, the Pennsylvania Department of Environmental Protection (PaDEP) released draft technical guidance for single source determinations for oil and gas operations, otherwise known as “air aggregation” determinations. The guidance involves three sections of the Clean Air Act (CAA) - Prevention of Significant Deterioration regulations, nonattainment New Source Review regulations and the Title V permitting program. Specifically, the technical guidance requires aggregation when three conditions are met: 1)the different sources must belong to the same industrial grouping; 2) the sources must be located on one or more contiguous or adjacent properties; and 3) must be under the control of the same person. The guidance centers on the plain meaning of the words; “contiguous or adjacent,” is interpreted as the distance or spatial relationship between locations. A similar approach has been affirmed in West Virginia. Other states, including Texas, Oklahoma and Louisiana, consider sources contiguous or adjacent if they are within a quarter-mile apart. Although public comments will be accepted until November 21, the guidance will be implemented, on an interim basis, immediately.