PORTER RANCH SOIL GAS INVESTIGATION IN RELATION TO ALISO CANYON GAS LEAK

Stray gas is an increasingly pressing concern to both public health and the environment. The residential community of Porter Ranch, CA is located adjacent to the Aliso Canyon Gas Field, a large natural gas storage field where a blowout was discovered on October 3^rd, 2015 at well SS-25. The subsequent natural gas leak was the largest in US history and posed a significant health risk to the Porter Ranch community. On February 18th, 2016 the well was announced as officially plugged. No monitoring of soil gas had been conducted in relation to the gas leak. The purpose of this survey was to determine if the plug successfully stopped the leak and measure remnant gas migration through the subsurface into the soil. Eight soil gas wells were installed proximate to faults in the Porter Ranch area to trap soil gas and measure methane concentration relative to atmospheric concentration using a photo ionization detector (PID). Initial measurements on March 25^th, 2016 ranged between 1.6 ppm to 6.2 ppm relative methane. Continued monitoring showed a gradual decrease in concentration until September, 2016 when measurements between 0.0 ppm to 0.5 ppm relative methane were observed. Sampling continued for one year showed no further change in methane concentration even after the gas field was repressurized. The data indicates the plug at well SS-25 successfully stopped the leak, and methane concentration gradually decreased for seven months after the plug was emplaced due to decreased fluid flow rates of subsurface gas remnant from the leak.