## 2 Monitoring Protocols and Methods  
### Defining the Monitoring Area  
The Lacq-Rousse pilot was part of a MMV program, established by the site operator to satisfy regulatory requirements developed for this experimental project in accordance with French government requirements. Monitoring was designed according to a preliminary risk analysis and the specific configuration of the storage site (deep reservoir; thick caprock; small amount of CO\({}_{2}\) injected; final pressure far below initial reservoir pressure; only one well intersecting the reservoir). The monitoring should provide information on site integrity, well injectivity and storage performance, as required by the French administration, and was designed to cover ten \"parameters\": (1) continuous monitoring of the injected CO\({}_{2}\) stream, (2) pressure and (3) temperature monitoring along the injection well, (4) microseismic monitoring in six dedicated geophysical monitoring boreholes (Figure 1--points 1 to 6), (5) periodical monitoring of surface water and (6) groundwater quality, as well as (7) of biodiversity of the ecosystems [31]. Monitoring of (8) soil-gas concentrations and (9) soil flux is part of site-integrity verification, aimed at addressing leakage detection, adverse effects on the environment, and assessment of the safety and integrity of the storage complex in the short term. The surveillance network set up and used during the 2008-2015 period was intended to monitor the natural geochemical background and to act as a warmer in case the thresholds of natural CO\({}_{2}\)-flux concentration levels and their variability were exceeded. Last, (10) particular attention was paid to social aspects, as the area is urbanized and sensitive to environmental aspects since the start of natural gas exploitation from the Lacq reservoir in the 1960 s [28,38]. The importance of considerations on social acceptance is now well established as being one of the key parts of CCS projects [39]. When the project was designed, monitoring with remote technology of a large geographical area--several square kilometres--was not really feasible with sufficient confidence and at a reasonable economic cost. It was thus decided to focus the geochemical surface characterization on selected locations that could be regularly monitored. The surface projection of the Rousse reservoir (close to 4 km\({}^{2}\)) is included in the hexagon figure around the geophysical monitoring boreholes (1 to 6, Figure 1). The CO\({}_{2}\) was injected at the RSE-1 borehole near monitoring point 16A (Figure 1). A much larger area was used for surface measurements, including 35 monitoring locations (36 measurement points; Figure 1) selected by the site operator in accordance with the authorities. These 35 locations lie on a 7 \(\times\) 5-km grid southwest of Pau City and mostly in the Jurancon hills, which have a steep topography so that the points range in height from 180 to 350 m a.s.l. (mean elevation of 270 m). Figure 1: (**A**) Locations of the 36 monitoring points (35 different locations) plotted onto a geological map. The hexagon shape links the six geophysical observation boreholes for passive seismic monitoring down to 2 km-depth. Point 16-A is close to the location of the injection well (RSE-1). (**B**) Monitoring points plotted on Google Earth satellite view using shaded relief (afternoon view). The locations remained the same throughout the entire project period, and were chosen to be representative of the local geological context as known from previous geological and geophysical work, and to largely exceed the surface projection of the storage reservoir. The monitoring points were also selected in locations that could remain the same throughout the monitoring period, as the region contains many vineyards and other agricultural plots (corn fields and pastures). Additional information and discussion on the representativeness of such a geographical coverage is given in [35].