## 1 Introduction  
Mitigation of CO\({}_{2}\) emissions in the atmosphere has been a great challenge since the end of the 20 century. One of the options is to store CO\({}_{2}\) below ground at depths where confinement can be reached with a sufficient level of confidence, a process commonly known as CO\({}_{2}\) Capture and Storage (CCS). In Europe, CCS has received additional attention since adoption of the European Directive 2009/31/EC on the geological storage of carbon dioxide that established a legal framework for CCS. Much research was done before this pivotal date [1; 2] in order to gain confidence in CCS activities and related monitoring, mitigation and verification (MMV) approaches for an environmentally safe storage of CO\({}_{2}\)[3; 4]. Pioneering work was done to evaluate the storage potential in various geological settings, including deep saline aquifers [5; 6; 7; 8] depleted oil/gas reservoirs [8; 9; 10] and unminable coal seams [11; 12], although storage in oceans as an ultimate sink was envisaged as well [13; 14]. The first commercial injection of CO\({}_{2}\) in deep formations took place in 1996 in the Sleipner oil and gas field, where CO\({}_{2}\)--a by-product of hydrocarbon exploitation--is captured, compressed and injected into the Utsira sandstone strata 1000 m below the seabed [15]. Since then, tens of projects have been launched all over the world (an exhaustive overview is found in the IEAGHG database [16]), either dealing with capture or storage aspects, or with both [17]. In the meantime, regulation and monitoring work has progressed as well [18]. Many storage, pilot or research projects use CO\({}_{2}\) injection in deep saline aquifers, such as at Compostilla [19], Gorgon [20], Ketzin [21], Sleipner [6] and Snehvit [22], or in depleted reservoirs such as In Salah [22], K12-B [21], Lacq-Rousse [23] and Otway [24], without a direct aim of enhancing hydrocarbon production. Other projects deal with the direct injection of CO\({}_{2}\) in quiescent hydrocarbon reservoirs in order to enhance oil/gas production, as in Weyburn [25] and many US projects [26]. Finally, many smaller-scale projects focus on shallow monitoring [18; 27]. Among operations that inject CO\({}_{2}\) into depleted hydrocarbon fields, our study focuses on the Lacq-Rousse CCS project. For this pilot project, the CO\({}_{2}\) is produced from gas burned at the Lacq plant through an oxyfuel combustion process, then compressed, transported by existing pipeline and injected into the depleted Rousse field, thus having complete control over the entire CO\({}_{2}\) capture, transport and storage process [28]. The project was thought of and designed in 2006. At that time, the EU Directive 2009/31/EC did not yet exist and the regulatory aspects were implemented under the French Mining Code for the subsurface aspects [29] and the Environmental Protection Code regarding the surface installation aspects [30; 31]. From mid-2008 to the end of 2009, baseline data acquisitions concerned geophysical and geochemical aspects [32], from the surface down to 4,200 m, the depth of the depleted Rousse reservoir. The permit for injecting CO\({}_{2}\) was obtained in May 2009, allowing the injection of 120,000 tons of CO\({}_{2}\) over two years [31]. This work started in January 2010. An extension of the injection period was obtained in late 2011, together with a reduction to 90,000 tons of the CO\({}_{2}\) quantity that could be stored [31]. At the stop of injection in mid-March 2013, a total of 51,000 tons of CO\({}_{2}\) had been injected into the reservoir, leading to an over-pressurization of the reservoir from 30 to 80 bars, the initial pressure before natural-gas exploitation being 480 bars [33; 34]. According to regulatory requirements, post-injection monitoring was then carried out from March 2013 to March 2016 as the site was shifted to a post-injection surveillance phase. The site is now closed. The present study reviews some aspects of the geochemical monitoring on the surface.