This paper presents the lessons learned from practical application of CO2 storage site monitoring methodology developed to address geochemical aspects of future CO2 storage site. It covers a detailed description of the methodology and tools applied for the extremely complex industrial site that main technical challenge is the presence of the multi sources of CO2 at the surface and in the soil. On the basis of previous research programs conducted on natural CO2 storage sites, a specific geochemical monitoring program was developed that combine both a localized and continuous geochemical monitoring. The paper focuses on the surface gases monitoring techniques spread on specific site to quantify CO2 flux and concentration at different levels from soil-atmosphere-interface : -1m to +1m. The geochemical monitoring was based on the combined use of conventional accumulation chambers and dynamic flow chambers systems equipped with high resolution IR sensors, Fourier Transform infrared sensors equipped with specific gas cell and a CO2 mobile infrared sensor. This first step has lead to the location and discrimination of CO2 sources and the analysis of the carbon cycle involving the influence of anthropogenic events and of the natural seasonal variations. Combined methodology matrix for a geochemical surface survey adapted to CO2storage in Paris basin depth saline aquifer is argued and the measurement results are discussed. C isotopic analyses to insure gas traceability have been also applied. Results are not shown in the paper. On the basis of such combined surface and subsurface gas measurements, seasonal variations of the natural CO2 cycle were identified. As results, sensitivity and variability were considered to suggest the CO2 warning levels adapted to detect CO2 abnormal emissions on the surface. Finally, advanced complementary technologies for both soil and atmospheric gas investigations are also detailled as a part of the survey strategy. For soil gases, a specific completion in a shallow well has been developped to perform the continuous acquisition of CO2 concentrations at a depth of 10 m. For atmospheric gases, a scanning imaging infrared remote sensing system was tested to support atmospheric dynamic survey strategy. This site study applied to Claye-Souilly waste disposal represents the real 'hand on' experience and could be considered as a valuable experience to improve part of the geochemical monitoring program of CO2 storage site.