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Mining of fossil resources

In 2017, coal and natural gas accounted for 34% and 26% of energy sources that were used to generate electricity in the U.S., respectively (EPA, 2018c). Depending on which fossil resources are exploited, total environmental interventions from fossil power generation are hugely varied. This is mainly attributed to the fact that each fossil resource contains different chemical compositions. Furthermore, in the case of NG extraction, the hydraulic fracturing of shale gas requires substantial water resources relative to the conventional NG extraction (Sanders, 2014; Clark et al., 2013; Jeimer and Lamadrid. 2013). According to the previous smdy, however, the amount of water withdrawn for the fracking is much smaller than the amount of cooling water for power generation (Clark et al., 2013). Fracking also contaminates water resources due to wastewater discharged from shale wells (Vengosh et al., 2013; Vidic et al.. 2013).

The transportation of fossil fuels from mining sites to the power plants is considered as well. While coal is generally transported by truck and railroad. NG is primarily transported by pipeline. The leakage of gas from the pipeline transportation is also considered.

Farming

Figure 1 shows land use and land cover in the MRW. Total area for agricultural land use is approximately

3.8 x 10s m2. Hie MRW is not a region where agricultural production dominates. However, nutrient runoff fr om fanning activity can be varied depending on which fanning practices are perfonned. Hence, agricultural practice management is crucial to prevent deteriorating water quality. According to the previous findings, fanning activity in the liver basin that includes MRW contributes significantly to total nitrogen (TN) and total phosphorus (TP) loads in the river basin (Khanal et al., 2018).

Tillage practices are perfonned to prepare the soil for crop production. However, these practices damage the soil structure and increase nutrient runoffs. Currently, four different tillage practices are employed in the MRW. 57.0% of the agricultural land area perfonns no-till practice. 22.9%, 20.0%, and 0.11% cany out conservation (chisel plow), reduced (tandem-disc plow), and intensive (moldboard plow) tillage practices, respectively. The conservation tillage practice is defined as tillage that has more than 30% of crop residues on the soil. The reduced and intensive tillage practices have 15-30% and less than 15% of crop residues that remain on the soil, respectively. The increased mixing efficiency leads to low crop residues, requires many fertilizer inputs, and increases the risk of soil erosion.

No-till practice helps reduce soil erosion and nutrient runoffs since more crop residue remains in the soil, and thus, fewer fertilizers need to be applied (Ohio EPA, 2013). Also, no-till practice is economically cheaper than tillage practices due to the reduced labor and fuel requirements. The long-term crop yield may be increased as well due to improved soil fertility. However, the food production flow is excluded from this study to focus on the nexus of energy and water flows. Differences in other interventions, such as air emissions and resource uses, between tillage practices are also taken into account, even though then contribution is relatively negligible in comparison to thermoelectric activity (NREL. 2018; Nemecek et al„ 2007).

Miscellaneous activities

Although mining, thermoelectric, and agricultural activities account for most of the water use and pollutant emissions, other activities also consume water and energy and release emissions. Therefore, for the comprehensive analysis of the euergy-water-CO, nexus in the MRW, various activities that include residential, commercial, industrial, transportation, and wastewater treatment, are included in the analysis. This comprehensiveness of the analysis boundary is particularly important when the TES framework is applied. Since ecosystem supplies, such as water provisioning, account for the entire amounts of supplies fr om the MRW, then associated ecosystem demands, such as water consumption, also need to be the total demands from all activities in the MRW.

Supply of ecosystem services

Traditional sustainability assessment approaches, such as conventional life cycle assessment (LCA), account for environmental impacts from activities. The impacts include emissions and resource use. These traditional methods quantify only relative sustainability in order to answer the question: Whose impacts are smaller than others? To claim absolute sustainability, however, we need to address how the surrounding ecosystem offsets those impacts. The TES framework accounts for the supply and demand of specific ecosystem goods and sendees and introduces a TES sustainability metric: Vk = (Sk - Dk)/Dk (Bakshi et al., 2015: Gopalakrishnau et al., 2016). The ecosystem sendee demand (£>) corresponds to environmental flows, such as ah' emissions and water consumption. The ecosystem sendee supply (S) corresponds to the provisioning of ecosystem goods and sendees, such as ah quality regulation sendee and water resource provision. The TES metric Vk is calculated for each type of ecosystem good and sendee (к). A positive indicates that impacts do not overshoot the capacity of ecosystem supplies,

which means the system is sustainable in terms of к ecosystem goods and sendees.

Forest ecosystems and tree canopies provide carbon sequestration and ah quality regulation sendees. Wetlands regulate water quality by removing excessive nutrient runoffs, although only 0.14% of land area in the MRW is wetlands as shown in Figure 1. With respect to ecosystem goods, such as water and fossil resource provisioning, only the renewable portion can be included as the supply of such ecosystem goods. To assess water provisioning in the MRW, for example, various factors about the water cycle, such as precipitation, evapotranspiration from canopy and soil, infiltration into the soil, and surface/subsurface runoffs, need to be considered. In this study, the Available Water Remaining (AWARE) model is used to estimate the available amount of water in the MRW (Boulay et al.. 2018). This model has data for the HUC2 spatial scale, which represents a much larger area than the HUC8 scale. The value for the Ohio region (HUC2: 05) is allocated to the MRW region (HUC8: 05040004) based on the ratio of the laud areas in HUC8 versus HUC2.

Fossil resources are formed through anaerobic decomposition of organic matter in the earth over very long periods of time. With respect to the natural gas supply from the ecosystem, only renewable NG should be considered as the ecosystem supply. However, since the formation rate of NG is significantly slower than the NG consumption rate, the TES metric for NG (Vvo) is very close to negative one (-1) regardless of any scenarios. In this work, therefore, we consider the other case where 2 °C of global warming since the pre-industrial period is allowed for exploiting accumulated NG (Le Queie et al., 2017). According to the Intergovernmental Panel on Climate Change (IPCC), the rise in global temperature must be limited to 2 °C above pre-industrial levels in order to avoid disastrous consequences of climate change

(Le Queie et al., 2017). The IPCC has calculated the carbon budget, which represents the amounts of global CO, emissions that must not be exceeded in order to maintain global warming under 2 °C. The remaining budget for greenhouse gas emissions is estimated to be 275 GtC (WRI, 2014). Since 22% of GHG emissions are attributed to the use of NG (EPA. 2018c), 60 GtC of the budget can be allocated to the GHG emissions from NG use. This budget is further allocated to the NG use in the MRW. based on the ratio of NG consumption in the MRW to global NG consumption (Dudley et al., 2018). The resulting budget is the amount ofNG supply in the MRW that allows 2 °C of global warming.

Potential CO, conversion technologies

As a way of mitigating CO, emissions, extensive research is being conducted on technologies for converting CO, into a variety of hydrocarbon products (Frauzem et al., 2015; Artz et al., 2017; Wolf et al., 2016; Bein' and Nowakowski, 2014). In this work, three CO, conversion technologies are selected as technological alternative options that could improve watershed sustainability. Fust, methane (CH4) is produced from carbon dioxide and hydrogen through Sabatier exothermic reaction, as follows:

CO, feedstocks are assumed to be captured from thermoelectric power plants. To provide hydrogen for hydrocarbon products, it is assumed that a water resource is utilized through an electrolysis process, as follows:

Therefore, the overall reaction is:

The converted CH4 product is assumed to displace natural gas in the MRW since most of the NG composition is CH4. Therefore, displacement credits of all kinds of avoided emissions and resource uses are given to CO, conversion technologies.

CO, is also used to produce synthetic gas through the reverse water-gas shift reaction, as shown below:

If syngas that has a ratio of H, to CO of 2:1 is required, hydrogen is obtained from the electrolysis of water shown above. Also, formic acid is synthesized from the hydrogenation of CO, as follows:

Similarly with the displacement approach for CH4, it is assumed that CO,-converted syngas and formic acid displace each of the products from conventional processes. Syngas is produced from conventional coal gasification process (Dunn et al., 2015). 69% of formic acid is synthesized from methyl formate and the rest of it is produced from butane (Sutter, 2007).

Since the CO, conversion technologies described above still har e not been fully developed for commercialization, stoichiometric reactions are assumed for all CO, conversion scenarios for the simplicity of analysis. The energy demand for CO, conversion is estimated using the standard enthalpy of each conversion reaction (ДRH^gs). Numerous experimental data are available from literature (Bein' and Nowakowski, 2014; Jessop et al., 2004). However, those data vary substantially depending on process configurations, such as the use of a specific catalyst. For example, one report employed 30 bar of CO, pressure for converting CO, to formic acid (Lau and Chen, 1995), while the other one employed 120 bar of CO, pressure (Jessop et al., 1996). This makes the analysis challenging since we cannot just randomly select one technology for the comparison between different conversion scenarios (i.e., CO, conversion to methane, syngas, and formic acid). For a more accurate analysis, process data for CO, capture using monoethanolamine (MEA) is included in the analysis (Altliaus et al., 2007) since the CO, capture process is common for all conversion technologies. It is assumed that MEA is produced from outside MRW. With respect to the energy demand for CO, compression, we assume that 30 bar of CO, pressure is required for all conversion technologies since many formic acid production technologies from CO, employed 30 bar of CO, pressure (Behr and Nowakowski, 2014: Jessop et al., 2004).

The overall CO, conversion that includes the electrolysis of water is not only water-intensive but also energy-intensive. Thus, significant amounts of water and energy resources are required. This could make CO, conversion options infeasible by shifting impacts of GHG emissions to the impacts of energy and water consumption since the interventions from power generation are also increased.

Potential renewable electricity sources

To lessen the interventions from the increased demand for electricity for CO, conversion technologies, renewable electricity sources are considered as means to pror ide electricity for CO, capture, compression, and conversion by displacing conventional, thermoelectric power generation (Artz et ah, 2017). In this study, solar and wind power sources are considered as means to replace fossil fuel sources in the MRW. These renewable power generation technologies do not consume as much water as thermoelectric power generation (Lampert et ah, 2015). Solar power generation includes two technologies: Pliotovoltaics (PVs) and concentrated solar panels (CSPs). 57% of solar power generation in the U.S. uses PVs and the rest of it employs CSPs (Mendelsohn et ah. 2012). In this study, it is assumed that the national average solar power generation technologies are employed. While the water consumption for PVs is negligible, the solar power generation using CSPs requires similar water consumption as thermoelectric power generation due to the cooling of panels and steam turbines. In terms of wind power generation, its water consumption is insignificant since it does not require cooling.

In the same maimer of the displacement approach for CO, conversion technologies, it is assumed that various environmental interventions that are associated with thermoelectric power generation and its upstream activities, such as mining, can be avoided since electricity is produced from renewable power generation. Also, data for solar and wind power generation potentials in the MRW are investigated and considered as constraints that represent the maximum amount of renewable electricity generated from the available laud area in the MRW (EPA, 2018b).

Potential land use changes

The MRW has 0.32% of barren laud area, as shown in Figure 1. This corresponds to approximately 13 x Ю6 m: of land area. In this study, three land use change scenarios for the available land are investigated as follows: Reforestation, wind farm installation, and wetland construction. If the available laud is reforested, supplies of various ecosystem goods and services are increased.

The supplies of carbon sequestration and ah' quality regulation services are enhanced since these sendees are provided from vegetation in a forest (i-Tree Canopy, 2018). Nutrient loads in streams are reduced due to the reduction in runoff (i-Tree Hydro, 2018). With respect to water provision, there is a debate on whether reforestation helps improve water supply in watersheds. A majority of reports claim that water availability is reduced in a short period of time due to the reforestation and is recovered over a considerable amount time because of the improved soil infiltration capacity and the increased groundwater levels (Filoso et al.. 2017).

In the MRW, wetlands occupy only a very small portion of the landscape, as shown in Figure 1. Newly-constructed wetlands can improve the supply of water quality regulation sendee. Previous reports have discussed how much nutrient runoff could be reduced by constructed wetlands (Kadlec, 2008,

2016). In this study, we calculate the increased amounts of nitrogen and phosphorus removals due to the constructed wetlands in the available laud. The barren laud can also be used to install new wind farms. Wind farm-generated electricity can displace electricity from thermoelectric power generation, as described in section 2.7.

 
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