This article examines environmental concerns at the site of a Los Angeles, California, high school known as Belmont Learning Center (BLC) during its construction. A part of the site on which the school was to be built is a largely depleted oil field in the center of Los Angeles, close to a heavily populated area. Located nearby are another high school and an elementary school, each constructed more than 70 yr ago atop the same oil field. Prior to and during construction at the BLC site, which began in 1990, methane (CH₄) and hydrogen sulfide (H₂S) were detected in deep soil borings and monitoring wells. After the building construction began, a stratigraphic offset was also detected and interpreted as a fault line, which could be traced beneath one of the nearly completed buildings. This resulted in a decision to demolish that building and an adjacent one and to temporarily suspend all construction activity. Additional analytical measurements showed that the highest concentrations of both CH₄ (5% atmospheric concentration) and H₂S (100 ppmv) occurred only in the deeper samples, and that only traces of these environmentally hazardous gases were present at or near the soil surface. In this article, we use carbon, hydrogen, and sulfur stable isotope data, as well as ¹⁴C measurements to describe the sources and natural processes of CH₄ and H₂S removal.

The interpretations offered in this article will hopefully be useful in assisting environmental professionals and government officials who evaluate hazardous soil gas concentrations to make decisions during the process of issuing or denying construction permits.