Department of
Environmental Quality

Portneuf Valley

The Portneuf Valley PM10 Non-Attainment Area (PVNAA) contains 96.6 square miles of Pocatello, Chubbuck, and surrounding areas. It includes federal land managed by the Bureau of Land Management (BLM) and the Caribou National Forest, as well as privately owned land in the cities of Pocatello and Chubbuck. The combined population of the two cities is approximately 76,000.

The topography of the PVNAA is complex and aids in the formation of strong inversion layers during winter months. This is the period when most of the air pollution events occur.  During these inversions, the major pollutant has been ammonium sulfate, which is formed under high humidity (winter inversions) from the gasses, ammonia and sulfur dioxide.

The PVNAA PM10 State Implementation Plan (SIP) was submitted to EPA in 2004. The plan relies on control measures on point and area sources to attain and maintain the National Ambient Air Quality Standards for PM10, primarily measures for control of emissions from residential wood combustion, residential road dust, and the industrial source, J.R. Simplot. In July 2006, EPA approved the plan and granted the redesignation request. As a result, the area has been redesignated a PM10 Maintenance Area. The Portneuf Valley is in attainment for PM2.5 and SO2.

2007

Portneuf Valley Air Toxics Ambient Air Data Evaluation and Health Assessment

2004

Portneuf Valley PM10 Nonattainment Area State Implementation Plan, Maintenance Plan, and Redesignation Request

2001

Eastern Michaud Flats Superfund Site Air Contamination Health Consultation

1999

Portneuf Valley Particulate Matter Air Quality Improvement Plan

Cache Valley

The Cache Valley PM2.5 Non-attainment Area (CVNAA) lies within Cache County, Utah (northern Utah) and Franklin County, Idaho (southeastern Idaho). The CVNAA encompasses a bowl-shaped, topographically isolated valley measuring approximately 37.3 miles north to south and 12.4 miles east to west. The Wellsville Mountains lie to the west and on the east lie the Bear River Range These mountain ranges are approximately 3,000 to 5,000 feet above the Cache Valley floor. The Wellsville Mountains, Bear River Range, and northern Wasatch Range converge in southern Cache County to form a topographical barrier between Cache Valley and other adjacent counties.

The Cache Valley experiences air stagnation events in the wintertime. During these periods, the stable layer above the ground is much deeper than a typical nocturnal inversion. Cold air is trapped in the basins, and the air mass stabilizes as high pressure a loft overtakes the region. Under such circumstances, a prolonged strong inversion layer (or layers) limits vertical mixing, trapping local pollutants in a thin layer against the valley floor. The low sun angle, short length of the days during winter months, and strong likelihood of snow cover to reflect the solar radiation are all factors that limit daytime surface heating and aggravate the situation. As a result, some inversions may not break for many days. The scenario described above leads to exceedances and violations of the 24‐hour health standard for PM2.5.

The CVNAA PM2.5 State Implementation Plan (SIP) was submitted to EPA in December 2012. The document contains the necessary evidence, analysis, and Idaho control strategies (in conjunction with the Utah SIP) to demonstrate that the area will attain the national standard by January 1, 2015.

2012

Cache Valley Idaho PM2.5 Nonattainment Area State Implementation Plan | Appendices