Life cycle sustainability assessments of critical civil infrastructure systems: additional guidance for future research and practical application

Sustainable development is important in critical civil infrastructure (CCI) planning and decision-making (Fischer & Amekudzi 2011). There are 16 recognized critical infrastructure sectors (i.e., infrastructure that, if disrupted, would be detrimental to the country's security, economy, and public health and safety) (ASCE 2021b; CISA 2023). According to the American Society of Civil Engineers (ASCE), there are 18 civil infrastructure sectors (ASCE 2021a). CCI, as referred to in this study, includes the overlapping sectors defined by CISA (2023) and ASCE (2021a) (e.g., energy and wastewater systems). Regardless of public or public/private ownership, these systems serve the public's needs and face similar economic, service, and political pressures.

Sustainable development encompasses environmental, economic, and social considerations, also known as the triple bottom line (ASCE 2021c). This concept has gained prominence in today's world, as demonstrated by the 17 Sustainable Development Goals of the United Nations (2015). However, even with the well-acknowledged need to consider each of the three pillars of sustainability (e.g., environmental, economic, and social) to conduct a holistic sustainability assessment, life cycle sustainability assessments (LCSA) of CCI oftentimes overlook factors ultimately driving decision-making for stakeholders, as these factors can be extremely difficult to quantify.

Such factors are not explicitly environmental, economic, or social factors by definition, but can often drive site-specific decisions for CCI systems. For example, Hansen et al. (2024) quantified the environmental and economic impacts and benefits of reducing infiltration and inflow (I&I) in small community wastewater treatment facilities and sewage collection systems. In addition to quantifiable environmental and economic impacts, anecdotes from case studies revealed that additional difficult to quantify impacts (e.g., increased resilience and reduced maintenance needs) were highly valued by operators and community leaders. These difficult to quantify factors were important drivers for some communities' decisions to mitigate I&I (Hansen et al. 2024). The current study highlights the importance of using case studies, such as Hansen et al. (2024), to proactively capture key site-specific impacts of interest to stakeholders, in addition to the more commonly quantified impacts.

A common approach to assessing products, systems, and services, including CCI systems, is LCSA, which is a cradle to grave methodology that considers all the three pillars of sustainability (Naves et al. 2019). LCSA models the life cycle environmental, economic, and social impacts of a product, system, or service. LCSA differs from life cycle assessment (LCA), life cycle cost (LCC) assessments, and social-LCA (S-LCA) in that these approaches consider only the environmental, economic, and social pillars of sustainability, respectively. However, LCSA is not standardized and often lacks sufficient inclusion and/or integration of each pillar (Li et al. 2018; Backes & Traverso 2021), including decision-making factors that are not directly associated with environmental, economic, or social impacts. Furthermore, economic and environmental sustainability are frequently considered in LCSAs, while social sustainability is less frequently considered (Albertí et al. 2017; Naves et al. 2019). This is because there are well-established life cycle methodologies for quantifying economic and environmental impacts, such as LCC and environmental LCA, respectively.

Recent guidelines have been published to provide guidance and recommendations for S-LCAs (Benoît Norris et al. 2020). However, social sustainability is challenging to quantify, and there is little consensus on how to address it (Sutherland et al. 2016; Naves et al. 2019; Costa et al. 2022). Social sustainability includes additional factors that are not directly related to environmental or economic impacts of CCI systems, but are instead often social in nature (i.e., nondirect environmental and economic impacts) and site-specific. Some studies use S-LCA to explore and/or quantify social impacts of a CCI system. S-LCA is a nonstandardized technical framework that considers social impacts of a system (Benoît et al. 2010). However, there is inconsistency in the definitions, methodologies, and social impact indicators used to conduct S-LCA. It is essential to realize that these factors (i.e., additional and difficult to quantify) oftentimes drive stakeholder decision-making, especially when differences between quantified environmental, economic, and social impacts of alternatives are similar (Moussavi et al. 2023a, b). While social and related impacts may be difficult to quantify, these factors can oftentimes be the most influential in stakeholder decision-making for CCI systems and are thus the focus of this paper.

The goal of this work is to propose a supplemental data collection approach that encourages stakeholders and decision-makers to address all of the influential factors in LCSAs, including the difficult to quantify additional factors, to improve the integration and practical application of LCSA for CCI decision-making. The objectives of this study are to (1) review current methods applied to address the life cycle sustainability of CCI projects with a focus on social impact assessments due to the social nature of the difficult to quantify additional factors and (2) provide further guidance to build upon currently available assessment methods to encourage capturing additional, difficult to quantify, site-specific impacts. Building upon the currently available S-LCA guidelines can allow for broader capture of these difficult to quantify impacts, which can ultimately improve the comprehensiveness and practical application of LCSAs to CCI decision-making.

An initial search was conducted for literature published between 2013 and 2023 containing the keywords shown in Figure 1. This time period was chosen to ensure that relevant, current papers were explored. To be included, the literature had to contain each of these words within the article, to ensure that studies specifically addressed the three pillars of sustainability for CCI case studies. Sustainability impacts can be highly site-specific, and analyzing case studies can provide better insight into real system impacts (Juan-Garcia et al. 2017). Only research and review articles that were open access or full-text either through the web or through the author's institution were included. In total, 31 articles were selected for full review (see Table S1 in the Supplementary Material). Because this review focuses on publications found using keywords from two online databases for a defined time period, it is important to note that additional relevant publications are likely available. However, this study serves as an initial exploration of the relevant literature available to show the general scope and challenges related to sustainable decision-making for CCI and to begin a discussion of decision-making factors that may drive sustainable CCI development. A description of the types of research included in the mini-literature review is provided in Figures S1–S5.

Of the 31 articles reviewed, environmental LCA was used 68% of the time to analyze the environmental sustainability of a case study CCI system. About 19% of these studies used other methodologies, and 13% did not consider environmental impacts. Also, 65% of the reviewed studies used LCC to analyze the economic sustainability of CCI case studies, 26% used other methods, and 10% did not consider economic impacts. These results are expected, as LCA is currently the only standardized life cycle methodology. Although LCC is not currently a standardized life cycle methodology, it is well-established and well-accepted (Alejandrino et al. 2021). In contrast, S-LCA methodologies were used by 58% of the studies to analyze social sustainability, and 16% used other methods. The nonstandardization and difficult to quantify, evolving nature of S-LCA leads to inconsistent methodologies being used to measure life cycle social impacts of CCI systems.

While S-LCA guidelines exist (Benoît Norris et al. 2020), the current methodologies leave a great deal of ambiguity for researchers, which oftentimes leads to the exclusion of these additional and/or social impacts, as was observed in 26% of the reviewed studies. Excluding such factors can be detrimental to comprehensive decision-making and planning related to CCI systems. While using S-LCA is the preferred approach to capture the direct social impacts of a CCI system, as seen in the literature reviewed, most related studies were not able to fully capture the site-specific values of relevant stakeholders due to limited methodologies and guidelines for choosing indicators that may be most helpful in addressing additional, difficult to quantify impacts (Aleisa & Al-Jarallah 2018; Ferrari et al. 2019; Backes & Traverso 2021; Shrivastava & Unnikrishnan 2021; Al-Yafei et al. 2022; Francis & Thomas 2022).

Based on the mini-literature review, the current state of LCSAs for CCI systems is deficient, as holistic LCSA methodologies are inconsistent among studies, especially when considering S-LCA methodologies and indicators. The reviewed literature highlights that, while S-LCA frameworks and guidelines are available, there is a strong need to further promote and develop the established S-LCA data collection methodologies to improve consistency, increase utilization within LCSAs, and comprehensively capture the range of factors that can influence CCI decision-making.

To address the difficult-to-quantify factors, researchers can use stakeholder discussions and/or semistructured interviews, as done by Moussavi et al. (2023a, b; Moussavi & Dvorak 2023) and Lane et al. (2022). This approach enables researchers to collect site-specific data early on in LCSAs of case study CCI systems, which can be a daunting task, as many life cycle studies report that the data collection process is difficult due to the large quantities of detailed data needed, research deadlines and budgetary limitations, and nonprescriptive, nonstandardized guidance on how to identify and collect site-specific, valuable data early on in a study. To minimize these challenges, the authors suggest, based on their experiences, that researchers proactively build a strong network with key stakeholders (e.g., community leaders, facility operators, and community members) through avenues such as academic and professional networks, community engagement, and in-person site visits. Developing a strong network of key stakeholders early on in a study aids the data collection process and helps capture site-specific values and potential decision-making drivers.

In addition, using this approach can help characterize the specifics of some already available indicators (see Figure 2) and suggest additional examples of indicators that can aid in proactively and comprehensively capturing key elements that drive decision-making. Choosing relevant indicators using this approach should be an iterative process; each discussion with each stakeholder will lead to additional questions, indicators, and stakeholders of interest, which will lead to additional discussion, leading to more questions, indicators, stakeholders of interest, and so on. A research team can use these iterative discussions to identify which factors may be most important to decisions-makers and should thus be included in subsequent LCSA.

It is important to note that the authors' suggestion is not a proposal for a new social sustainability assessment methodology; rather, it is building upon the current methodologies available, such as stakeholder interviews, by proposing additional guidance based on experience so that future researchers can better identify and capture site-specific and valuable impacts of a CCI system. While the results of stakeholder discussions and semistructured interviews may not always be quantitative, they can be helpful in making decisions and helpful for researchers in building their own frameworks and approach. Quantification, weighting, and LCSA integration of all impacts (including environmental, economic, and social impacts identified via the proposed approach) are beyond the scope of this practical study, but are an area of future research (Hjorth & Madani 2013). Alternative broader frameworks could be applied and extended as part of future research, such as the driver–pressures–state impacts–response approach (Hashermi et al. 2013). The proposed approach begins to explore how factors that are not economic or environmental in nature, but are extremely important to decision-makers, can be systemically identified to subsequently improve the robustness and comprehensiveness of LCSAs of CCI systems.

The authors conducted numerous life cycle studies that identified environmental, economic, social, and additional impacts of case study CCI systems using the suggested approach (Moussavi et al. 2021, 2023a, b, c). Case studies of CCI systems allowed the authors to analyze the impacts of real systems in operation, and stakeholder discussions allowed the authors to identify site-specific stakeholder values and system impacts. While interview and discussion questions/topics may vary by infrastructure type and stakeholder interest, the general approach of using stakeholder discussions and semistructured interviews is applicable to CCI studies of all types. In Table S2, the authors propose examples of the topics/questions researchers may ask during semistructured interviews and/or stakeholder discussions, based on their experiences conducting LCAs of case study energy and wastewater systems. Key topic areas for these questions include system overview, system implementation, stakeholder involvement, operations, employment, and perceptions of risk and resilience. These examples, which were implemented using the proposed approach discussed in this study, can be used as a starting point to identify relevant indicators and expanded upon as a research project evolves.

To provide further context, the authors summarize in Table 1 how their approach and questions presented in Table S2 allowed them to identify additional impacts that were of value to stakeholders, in addition to environmental and economic impacts. Tables 1 and S2 provide unique examples, based on the authors' previous work, to further guide practical users in developing the initial stages of future LCSAs of CCI systems. Figure S6 provides a generalized framework for the proposed approach.

Using stakeholder discussions, Moussavi et al. (2023a, b, c) found that the specific characteristics of the generalized employment indicator were employer retention and industry shutdowns (2023a), domestic job opportunities (2023b), and reduced operator labor and time (2023c). Similarly, Moussavi et al. (2021) found that the specific characteristics of the generalized health and safety indicator were auxiliary infrastructure preferences, and Moussavi et al. (2023a) found that specific characteristics of the society indicator were residential sewage backups, industry shutdowns, and investing in security and certainty of protection. Additional studies have also incorporated stakeholder discussions to identify the impacts of CCI beyond direct economic, environmental, and social impacts (e.g., Hansen et al. 2024).

The authors' suggested approach will engage stakeholders upfront and allow researchers to identify what stakeholders value most for a case study CCI project. Upfront consideration and thorough preparation for and collection of nondirect environmental, economic, and social data can lead to better capture of important impacts and decision-making drivers, which can ultimately better inform decisions related to CCI systems. While conducting stakeholder interviews are not a new or innovative data collection methodology (Benoît Norris et al. 2020), continuing to apply and develop the use of this approach can maximize valuable benefits and information collected that could be used to conduct a more thorough LCSA.

There is a need to further develop the integration of LCSA and decision-making for CCI. However, this study brings to light the need for comprehensively addressing a range of impacts that are of high value to decision-makers and stakeholders. Additional components beyond the suggested guidance are needed to fully integrate sustainability impacts and decision-making for CCI. For example, additional case studies, more specific case studies, coding methodologies, and weighting should be applied to future studies to allow for statistical analyses and quantification, which will facilitate the integration of the results from stakeholder discussions and semistructured interviews into subsequent LCSAs. Although quantifying sustainability impacts of all types (e.g., environmental, economic, social, and additional impacts) is a crucial part of sustainability assessments and should be the focus of future research, the pure identification of key decision factors, as highlighted by the authors' suggested approach, is an important starting point. In addition, future work should aim to expand this approach so that it can be consistently and broadly applied to LCSAs of all systems.

Systematically incorporating a wide range of impacts, including those not directly related to environmental, economic, or social impacts, into CCI planning and decision-making is key to providing sustainably designed solutions to communities within the context of the global Sustainable Development Goals. To encourage addressing these additional impacts in LCSAs of CCI systems, relevant literature was explored, and further guidance was discussed. This mini-literature review highlighted that currently available frameworks and guidelines for collecting CCI impact data has led to a dearth of truly comprehensive LCSAs for CCI systems. The suggested approach, described using the authors' previous work as examples, provides additional guidance for future research by encouraging researchers to fully capture site-specific factors of value to stakeholders using semistructured interviews and/or stakeholder discussions for reflective case studies. This approach can strengthen subsequent qualitative and quantitative analyses (e.g., multicriteria decision-making) through the inclusion of a range of important factors that drive decision-making for stakeholders, which will ultimately improve decision-making and the robustness and comprehensiveness of LCSAs for CCI systems. The suggested approach, which is based on previous experience with multiple types of CCI projects, is broadly applicable to all CCI.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.