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Ostracods are significant members of groundwater ecosystems, affected by both abiotic environmental factors and biotic interactions. This study investigates the factors influencing ostracod communities in groundwater from dug wells in several regions of Benin, West Africa, which experiences chronic anthropogenic disturbances such as nutrient enrichment from sewage and fertilizer infiltration. We evaluated the presence of ostracod species in 219 wells across seven catchment areas, examining 31 predictor variables, which include a variety of water quality parameters, hydrology as well as well closure, usage and type. The influence of these variables was analyzed using distance-based linear models and redundancy analysis. Our research identified 60 ostracod species, which we classified into two ecological groups: 1) 36 stygobitic species from the family Candonidae, which represent an endemic evolutionary radiation, and 2) 24 non-stygobitic species, mostly from the family Cyprididae. Through our analysis, we identified several key factors influencing ostracod community structure, with consistent patterns observed at both species and genus levels. The primary predictors, aside from well descriptors, included water chemical and physical properties, such as electrical conductivity, pH, temperature, and bicarbonate concentration, along with NO2- concentration, a factor not previously demonstrated to be crucial for ostracod assemblages. Elevated nitrite levels in groundwater were found to significantly affect the distribution of stygobites and non-stygobites. Stygobites were notably less frequent in environments with higher NO2- concentrations, likely due to their greater vulnerability to periodic or chronic anthropogenic disturbances compared to non-stygobites. Therefore, we suggest that stygobitic ostracod species, identifiable even at the genus level, have a potential as reliable indicators of groundwater quality in the tropical regions of western Africa.

1. Introduction The EU has set ambitious goals on the reduction of CO2 emissions into the atmosphere for limiting the effects of global warming. And while renewable alternatives are available in some cases, long-term storage of large quantities of produced CO2 seems inevitable. Because the process of capturing, transporting and injecting CO2 into a reservoir (CO2 capture and storage, CCS) is costly and current revenues from the EU emission trading system (ETS) are insufficient to cover the expenses, the commercial deployment of CCS is delayed in Europe. A potential business case for CO2 geological storage (CGS) is CO2-enhanced oil recovery (CO2-EOR), where CO2 is used to drive out 5-15% additional oil after the application of primary and secondary recovery techniques. Within Europe, the North Sea is the main oil province with a high potential for CO2-EOR. Earlier studies have concluded that off-shore CO2-EOR projects are a viable business case, but no investments have been made yet. Moreover, adversaries of this technology often point out that CO2-EOR is not a climatefriendly solution because its goal is to increase and lock-in fossil fuel production. 2. Economic and environmental analysis To investigate the potential of CO2-EOR, an integrated geological, techno-economic and environmental analysis is made of a potential candidate for EOR in the North Sea: the Buzzard oil field (Roefs et al., in press). A techno-economic spreadsheet calculation is made for different injection scenarios. The net present value (NPV) is calculated as the discounted cash flows over time. A full-sized coal-fired power plant is assumed, producing about 4 MtCO2/y, of which the Buzzard field can accept 2.9 Mt/y for EOR. A second injection location in an aquifer is also assumed. The market scenario is chosen at an oil price of 50 €/bbl, and an ETS price at 5 €/t. In parallel, a life cycle assessment (LCA) is conducted to compare the environmental impact, considering emissions from the additional construction and operation of the capture plant and EOR operation.. Results are expressed as the global warming potential (GWP). Four scenarios are considered: CO2 capture and storage in an offshore aquifer; CO2-EOR in the Buzzard field followed by emission into the atmosphere; CO2-EOR and parallel aquifer storage; and CO2-EOR and parallel aquifer storage, with a continuation of storage in the Buzzard field after the cease of oil production. For the first time such an integrated economic and environmental analysis is made comparing CGS and EOR. Results show that the scenario with only CGS has the lowest GWP (reference level for the other scenarios), but the NPV is negative (-800 M€) and thus does not provide a viable investment option. The scenario with only EOR has the highest NPV (>500 M€), but also has the highest GWP, 38% higher than the storage-only scenario. The results for the third and fourth scenario are very similar, with a GWP of 11% more than the storage-only scenario,and an NPV of 207 and 220 M€ respectively. This shows that CO2-EOR can be a viable investment that, when combined with CO2 storage, only has a minor additional environmental impact over a storage-only project. EOR can thus also serve as an enabler for CGS, with a widespread storage deployment when the necessary infrastructure is in place. It is also beneficial to use the depleted oil field for storage (fourth scenario) over aquifer storage (third scenario), because the necessary infrastructure is already present. From a sustainability perspective this also makes sense, as it allows for a more efficient use of geological resources. 3. Geo-economic simulation The analysis shows that even at low oil and CO2 prices, EOR projects can be viable. Since no projects are (soon becoming) operational, other factors are influencing the economic viability too. A more advanced geo-economic analysis is therefore performed with the PSS simulator from the point of view of an investor for the Buzzard field. In a more realistic approach, investment decisions are simulated, considering limited foresight generated by market and reservoir uncertainty. Results show that an increased hurdle rate results in a lower chance of a negative project value (Fig. 1, Welkenhuysen et al., subm.). A hurdle rate of 12% removes all project risk, but also eliminates potentially viable projects. At an oil price of 50 €/bbl, the threshold for EOR investment occurs at 0 €/tCO2 (green dotted line; excluding capture cost). The discrepancy with the cost for capture is too much to be covered by the current CO2 market price of around 15 €/t (June 2018). CO2-EOR with or without CGS in the North Sea therefore does not come forward as commercially viable from this study, where, in comparison to state-of-the-art assessments, more realistic economic and geological uncertainties are used. It does, however, have strategic and environmental benefits compared to a situation where oil is imported into the EU. In that context, incentives to reduce the cost and/or risk could be justified. Future research will focus on the establishment of contractual agreements between the parties, including uncertainty in the environmental analysis, and the scarcity cost of storage capacity as a limited commodity will be taken into account.