The present invention includes a method for determining the level of deep gas in a near surface formation that includes: measuring CO2, O2, CH4, and N2 levels in percent by volume from one or more surface or near surface geological samples; adding the water vapor content to the measured CO2, O2, CH4, and N2 levels in percent by volume; normalizing the gas mixture to 100% by volume or 1 atmospheric total pressure; and determining the ratios of: O2 versus CO2 to distinguish in-situ vadose zone CO2 from exogenous deep leakage CO2; CO2 versus N2 to distinguish whether CO2 is being removed from the near surface formation or CO2 is added from an exogenous deep leakage input; or CO2 versus N2/O2 to determine the degree of oxygen influx, consumption, or both; wherein the ratios are indicative of natural in situ CO2 or CO2 from the exogenous deep leakage input.