Transboundary impacts are likely to increase in the Mekong region and best practice dictates that public participation should extend beyond national borders whenever there is potential harm to a neighboring country. 
There is ample international guidance on how to address public participation in a transboundary context, especially from European experience. Project proposals with potential transboundary impacts have some unique public participation issues. How project proponents engage stakeholders in neighboring countries will require the involvement of the national governments, as well as a range of diplomatic and legal considerations.
There is recognition under international law that all countries have an obligation to “undertake an environmental impact assessment where there is a risk that the proposed [project] may have a significant adverse impact in a transboundary context, in particular, on a shared resource.” 
The International Court of Justice has recognized that this principle extends to the need for EIA processes to engage with affected neighboring countries.
In the Mekong region, there are various agreements and mechanisms for considering transboundary environmental issues, but no formal agreement for a transboundary EIA framework exists yet. The 1995
Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin
(the Mekong Agreement) requires member countries to provide notification and have prior consultations to discuss transboundary impacts for water projects in the Mekong River Basin that may have an impact on neighboring countries, before any commitment is made to proceed. 
There has been ongoing work on the creation of a Transboundary EIA system by the Mekong River Commission (MRC). A proposed system was developed for the MRC by ERM and reviewed by the Environmental Law Institute, which is still under development by the MRC.
The potential for adverse transboundary environmental impacts is recognized across the Mekong region, as well as the broader ASEAN region, especially in relation to water resources development, transport of dangerous goods, biodiversity loss, and transboundary haze. For example, the Asian Development Bank’s Greater Mekong Subregion Core Environment Programme specifically addresses concerns over the likely transboundary effects of infrastructure development in the region. 
Other cross-border institutional developments include a Greater Mekong Railway Association, Regional Power Coordination Centre, and Mekong Tourism Coordination Office, among others. The ASEAN Agreement on Transboundary Haze Pollution (2002) requires ASEAN countries to cooperate in developing and implementing measures to prevent, monitor, and mitigate transboundary haze pollution by controlling sources of land and/or forest fires, development of monitoring, assessment and early warning systems, exchange of information and technology, and the provision of mutual assistance. 
They must also respond promptly to a request for relevant information sought by a country that is or may be affected by transboundary haze pollution, with a view to minimizing the consequences.
At present, though, there is no regional legal framework for transboundary public participation in EIAs for proposed projects that have transboundary impacts. Some lessons, however, can be taken from international experience elsewhere in consideration of current best practice around EIA.
The Economic Commission for Europe has adopted a
Guidance on Public Participation in Environmental Impact Assessment in a Transboundary Context
(the ECE Guidance) to support the two key European intergovernmental agreements on EIA and public participation – the 1991 Convention on Environmental Impact Assessment in a Transboundary Context (the Espoo Convention) and the 1998 Convention on Access to Information, Public Participation in Decision-making and Access to Justice in Environmental Matters (the Aarhus Convention). 
The Espoo Convention is a preventative mechanism to avoid, reduce and mitigate significant environmental impacts intended to help make development sustainable by promoting international cooperation in assessing the likely impact of a proposed activity on the environment. It applies, in particular, to activities that could impact the environment in other countries.
The ECE Guidance identifies a number of key best practices that have relevance to the Mekong region countries:
Financial support may be needed to: translate the EIA documentation into the language(s) of the affected country; translate the public comments and recommendations back into the language of the country of the project proposal; disseminate EIA materials (including booklets, brochures) within the neighboring country; pay for information distributed through newspapers, radio, TV, e-mail or Internet; and organize public consultation meetings.
Neighboring countries should be notified of project proposals with potential transboundary impacts as early as possible, and receive such notification no later than the general public in the country of the project proposal.
All countries potentially affected by a project proposal – both the host and neighboring countries – should be jointly responsible in disseminating information about the EIA and collecting feedback from PAP and stakeholders for consideration in the decision-making process.
All comments received on transboundary EIAs from any stakeholder in any potentially affected country should be considered in making a decision on the EIA, and that final decision should be published in neighboring countries.
Public participation within Transboundary EIA promotes the transparency and legitimacy of decision-making processes in projects with transboundary effects. Project proposals with anticipated transboundary impacts that conduct an EIA without adequate transboundary public participation may address State-to-State concerns, but may completely miss important local issues and valuable local or indigenous knowledge. Effective feedback mechanisms can ensure that best efforts to address local concerns in neighboring countries have been built into environmental mitigation and monitoring plans (EMMPs) and thus avoid future conflicts during construction and operational phases of the project.
The ECE Guidance demonstrates that, despite the need to consider unique procedural issues in establishing transboundary EIA arrangements, the majority of the concepts and recommended approaches outlined in these Guidelines will be applicable to project proposals with transboundary impacts. In other words, the same public participation principles and approaches should apply within both the host and neighboring countries, although the institutional mechanisms may differ.
The list of activities likely to have transboundary impacts, for which notification is required under the Espoo Convention, is defined in Articles 2 and 3 and Appendix I List of activities. http://www.unece.org/env/eia/about/eia_text.html#appendix1
Pulp Mills Case (Provisional Measures) (Argentina v. Uruguay) International Court of Justice Reports 2006, p.204.
Mekong River Commission, Transboundary EIA, http://www.mrcmekong.org/about-mrc/programmes/environment-programme/transboundary-eia/
The CEP is Administered by the Asian Development Bank and overseen by the environment ministries of the six countries that form the Greater Mekong Subregion Working Group on Environment - http://www.adb.org/countries/gms/sector-activities/environment
UNECE (2006) Guidance on Public Participation in Environmental Impact Assessment in a Transboundary Context, ECE/MP.EIA/7