Petroleum spills often create persistent “source zones” of hydrocarbon-impacted soils that are either in direct contact with ground water or in indirect contact through soil moisture infiltration and dissolution. Understanding the natural attenuation of source zones is critical to anticipating their longevity and the future chemical composition of their associated dissolved ground water and vapor plumes. A companion paper to this work presents an approach for site-specific assessment of source zone natural attenuation (SZNA), based on three levels of data collection and analysis oriented toward answering common questions of interest. Use of this SZNA assessment approach is illustrated here through application to the 3000-acre former Guadalupe Oil Field in California, where numerous petroleum source zones are present within a dune sand aquifer system. SZNA processes and mass loss rates are assessed through use of data from geochemical profiles of continuous cores, nested ground water wells, and soil-gas probes. Mass loss rates are estimated by quantifying fluxes of dissolved electron acceptors and degradation products in ground water and in soil gas. Under current conditions, gas transport processes coupled with aerobic and anaerobic biodegradation acting on exposed portions (above the water table) of source zones are the most significant loss mechanisms. Depending on the depth of contamination and other factors, the mass loss rate per unit surface area for those processes varies from approximately 0.1 to 1.0 kg total petroleum hydrocarbons per m2/year. In comparison, mass losses from the submerged part of the source zone and involving ground water transport processes (i.e., dissolution and biodegradation) were estimated to be about approximately 2 orders of magnitude lower. Once aerobically degradable hydrocarbons above the water table are depleted, the slower mass loss processes operating on the submerged parts of the source will control source zone mass loss.