The positive impacts of environmental decontamination in the rural areas of Fukushima encouraged returnees to resume their lives in the community and to regain confidence that a high-quality environment had been restored to the district. The most positive impact of the farmland decontamination was the returnees' perception that their crops were safe. Conversely, the removal of fertile topsoil created a new problem for the residents: farming could not resume without restoring the soil's quality. The problem appears to have been caused by a strong desire to avoid the business suspensions that would follow if acceptable radiation levels were exceeded, which led to demands for removal of topsoil, which is highly effective but also very damaging to soil's quality. If the decontamination projects had been more flexible, such as by considering a more optimised method for each step of the process, efforts to restore the soil's fertility may have been unnecessary as a result of choosing other, less damaging (and less waste-creating) decontamination methods, such as reverse cultivation or deep plowing. In addition, decontamination was a passive, rather than active, project for most residents, which may have made them more cautious and led them to demand excessive decontamination.

Positive topics regarding the forests' decontamination were not identified in the present set of interviews. The limited decontamination area prevented some returnees from resuming the traditional rural lifestyle that they had hoped for. Despite the situation, few requests for further measures in the contaminated forests were observed. Yasutaka and Naito [21] estimated that the decontamination of Fukushima's entire forested area would cost more than 16 trillion yen and have limited effectiveness in reducing the public's radiation exposure; the returnees seem to intuitively, if not specifically, understand this fact. At the same time, a questionnaire survey of local municipalities in 2019 indicated that further forest decontamination was one of the remaining issues for the municipalities [22]. The opinions from the municipalities were inconsistent with returnees' perceptions indicated in the present study. There is a possibility that less-positive feelings among the public toward the forests, as observed in the present study, were mistakenly interpreted by the municipalities as a need for further decontamination. Given the fact that radiocesium in a forest ecosystem is currently distributed in the system and circulates [23], there is no guarantee that additional decontamination efforts would enable returnees to resume following a traditional lifestyle. Environmental remediation techniques encouraging resumption of the traditional lifestyles is required in the future.

The right-hand column of table 2 shows the objectives of the decontamination projects set by the Japanese government. The basic goal was to lower additional doses from external exposure and, accordingly, to lower the air dose rates in residential areas, where people spend most of their time [8]. With regard to farmlands, the objective was to provide safe crops, in addition to the above [20]. Notably, the significance of decontamination for the returnees did not coincide with the government's objectives, except for crop safety. Although the government's goal of decontamination, by reducing radiation exposure, was not included in the returnees' perceptions as a positive impact, the positive effects on the returnees' mental states were observed, even though these were not included in the government's objectives. There was no indication that the large amount of waste generated or cost issues, which were pointed out internationally by the ICRP and others, affected the rebuilding of returnees' lives. This may be because the area was not the home of interim storage facilities or final disposal sites, and because there have been no cost burdens imposed on the returnees.

Murakami et al showed that decontamination efforts were associated with a reduction in residents' anxieties in one area without an evacuation order [13], but none of the returnees in the present study emphasised that the decontamination efforts had reduced their anxiety about radiation. Differences in the residents' circumstances may have led to the differences between the results noted by Murakami et al [13] and ours. In Murakami et al [13] study, decontamination was carried out while residents continued living in the area, and the decontamination lowered ambient radiation levels. In this study, however, residents returned from evacuation sites, where the radiation impact was small, to areas that had been decontaminated but were still heavily affected by the accident. Thus, the returnees would have had little opportunity to understand first-hand that the radiation levels around them had been greatly reduced by the decontamination program. In the radiation protection system, dose reduction by decontamination corresponds to the concept of averted doses [1]; this may be a concept that is difficult to grasp for people who have experienced various radiological situations due to evacuation.

The returnees understood that decontamination efforts aimed to restore the contaminated environment, and this helped them to regain confidence in the local natural environment. This may also have restored their positive affective connection to their home district. In recent years, many studies have examined the impact of disasters on place attachment and shown that major environmental changes due to natural disasters and climate change negatively affect residents' affective connection to places and reduce their willingness to return, while coping with the situation, including the implementation of remediation efforts, positively affects residents' place attachment [24–29]. The above studies mainly focused on natural disasters, but similar results could have occurred following a nuclear disaster situation. As understanding place attachment is important for policy makers to enhance recovery in disaster-affected areas [28], additional studies on the relationship between decontamination and residents' attachment to their places are needed.

One of the essential procedural values of any radiological protection system is stakeholder involvement, which leads to respect for the affected people and the restoration of dignity [4]. The present study also suggests that stakeholder involvement may have a positive impact on rebuilding lives. Among the returnees who participated in the decision-making process on the decontamination method for the farmlands, we recorded statements that were relatively positive about the situation after the decontamination. By contrast, the methods for forest decontamination were an imposed decision for most returnees, and none expressed positive views of the current situation. Some even felt that the lack of traditional forest use had resulted in a loss of freedom in their lifestyle. Various case studies have shown that decision-making efforts that lack procedural fairness prevent public acceptance [30–32]. For instance, in the construction of a hazardous waste facility in Canada in the 1980s and 1990s, top-down decisions felt like imposed decisions to stakeholders [33] and the project failed [31]. The feelings about decision-making regarding environmental decontamination we found were also consistent with these previous cases. Strategies for the unsolved issues related to forests in Fukushima must not be imposed on the region by the government, but proactive involvement of local residents is required.

Our study clarified residents' risk perception and the community values associated with environmental decontamination after the Fukushima accident through interviews with returnees, who are important stakeholders. As the public perspective influences decisions on protective measures in the event of an accident, our results provide useful information in the framework of reasonableness related to radiological protection, as proposed by Wieder et al [5]. In particular, the suggestion that decontamination had a mentally positive impact on the returnees is a new and noteworthy finding. However, the fact that decontamination efforts led to a problem disposing of the large amount of contaminated waste, and that it cost more than 3 trillion yen, cannot be ignored. We cannot conclude that the decontamination performed in Fukushima was the universally optimal remediation strategy. To optimise remediation and rebuild the lives of people affected by a large-scale nuclear accident, the positive impacts observed in the Fukushima case need to be recalled, and other impacts not considered by the affected people, such as contaminated waste and cost issues, should also be taken into account. The implications from the study are as follows:

When residents value safety and security and demand excessive decontamination efforts in the view of radiological protection experts, complying with their demands may not be the best approach for the remediation of their area/situation. Authorities need to base all projects on good communication with residents, to better understand their concerns and values, and to practice flexible planning. Sometimes, a case-by-case approach, such as the co-expertise process [1, 34, 35], may be more effective.The perception that contaminated land has been remediated has a mentally positive impact on residents. Remediation restores the positive affective connection to the place or maintains it at the pre-accident level, encouraging them to rebuild their lives in the affected area.The consideration of the ethical aspects (such as procedural values noted by the ICRP) of remediation strategy decisions will lead to residents' more positive perceptions of the circumstances. Procedural fairness must be pursued to ensure that remediation is satisfactory to residents while balancing safety with cost and waste concerns.

In the present study, the qualitative approach of using interviews clarified the multidimensional impacts of environmental decontamination on rebuilding the lives of returning residents and provided valuable suggestions for the optimisation of future remediation strategies. However, our study has several limitations. The study was conducted in only one rural district in the former evacuation zone, so the generalisability of the results needs to be verified. In particular, as noted in the difference between the present study and Murakami et al's [13] work, divergent opinions might be gathered from residents of non-evacuated areas. Furthermore, since our interviewees were exclusively among returning residents, the perceptions of those who did not return and who opted to relocate would likely be quite different. Therefore, it is not possible to deduce from this study the impact of decontamination on the rate of return. It should also be noted that perceptions of the impact of environmental decontamination on rebuilding lives are likely to evolve over time, and the results we present only reflect the state eight years after the accident and two years after the residents' return. Future research should include studies in several areas with different situations, similar studies in the same area on people who moved permanently serving as a control group, and long-term follow-up studies.

The study also has limitations regarding the optimisation of remediation efforts. We identified various impacts of decontamination, but these are qualitative but not quantitative information. For assessing optimisation, an assessment for larger populations is required about quantitative or semi-quantitative importance of these aspects, including aspects outside the scope of this study, such as exposure reduction, waste issues, and economic impacts. Furthermore, while our study assessed the efficacy of the decontamination efforts as a radiological protection measure, other crucial aspects, such as the sustainability of the project, were not the main focus of our work. The concept of sustainable remediation [36], developed in the field of soil and groundwater contamination for managing contaminated land, may also be relevant.