The Land Prioritization Mapping to Protect Drinking Water Quality (2025 Update) ranks land parcels in specific Mid-Atlantic states to protect drinking water quality and their potential to degrade long-term water quality.

The question asked in this five-year update remains the same as the original 2020 project, “Which parcels of land should be conserved to be most beneficial to drinking water?” However, the scope of this updated tool is much larger. Instead of focusing just on the Potomac River basin as the 2020 tool had done, the 2025 map was expanded to include the entirety of the states in which the basin sits—the District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia.

This project provides a robust, data-driven framework for identifying parcels critical to drinking water source protection. By integrating multiple geospatial metrics and offering flexible visualization options, the tool supports informed land conservation decisions. High priority areas were identified across the study area and for individual counties for better usability. Users across the region can combine this drinking water perspective with other conservation priorities to identify win-win opportunities to achieve multiple protection objectives.

A webinar, available on ICPRB’s YouTube page, was held on December 5, 2025, explaining the tool and some specific use cases.

The geospatial files for the Land Prioritization Mapping to Protect Drinking Water Quality (2025 Update) are available upon request. Users are encouraged to obtain the geospatial layers and identify local high-priority areas based on their own threshold of interest.

The Technical Memo presents methodology and results, including detailed descriptions of each metric used, along with a summary and discussion of the resulting prioritization, and the 2025 stakeholder engagement events (Appendix 1).

An interactive Land Prioritization Tool Explorer map is in development.

Introduction

In 2020, drinking water suppliers in the Potomac River Basin Drinking Water Source Protection Partnership (DWSPP) collaborated to rank land parcels to protect drinking water quality. Participating utilities included the Town of Leesburg Department of Utilities, Loudoun Water, Fairfax Water, WSSC Water, Washington Aqueduct, and DC Water. The Interstate Commission on the Potomac River Basin (ICPRB) conducted the technical work to develop the tool. The 2020 effort recommended updating the tool within five years to incorporate new data, include additional stakeholders, and take into account shifting priorities. This tool is the product of that recommendation.

The five-year update was conducted in 2025 by ICPRB, with input from the original participating organizations as well as new stakeholders including the WV Department of Health Source Water Assessment Program, Pennsylvania Department of Environmental Protection, EPA Region 3 Source Water Program, EPA Office of Wetlands, Oceans and Watersheds, and the Cacapon Lost Rivers Land Trust.

Opportunity Areas

As noted above, the project area was significantly expanded in 2025 to include the entire states intersecting the Potomac River basin – the District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia – beyond the original 2020 focus on the non-tidal Potomac watershed. Within the study area, agricultural and forested lands were considered “opportunity areas” for prioritization. Protected lands like easements and government-owned lands in the study area were excluded from the opportunity areas. Figure 1 in the Technical Memo shows the 48,674,593 acres of opportunity areas identified in the newly expanded study area.

Maps

The Technical Memo contains the full suite of maps developed during the Land Prioritization Mapping to Protect Drinking Water Quality (2025 Update) project. Geospatial files are available upon request.

Cumulative Prioritization of Parcels (Study Area Scale).  High priority shown in bright purple.

Cumulative Prioritization of Parcels (County Scale). High priority shown in bright purple.

Table 4 in Technical Memo: Summary of high priority areas.

Metrics

Eight metrics were identified to prioritize parcels according to the objective statement. Maps of each of the normalized metrics are shown in the Technical Memo. The metrics and corresponding rationale statements include:

• Distance from Waterways – Parcels closer to streams and rivers are prioritized due to their higher potential to impact water quality. Shorter distances receive higher priority.
• Percent Source Water Protection Area in HUC12 – This metric measures the proportion of a parcel within a designated source water protection area. Higher percentages yield higher priority.
• Distance from Urban Areas – Parcels near urban centers are prioritized due to increased risks from runoff, road salt application, and spills. Shorter distances receive higher priority.
• Karst Transmissivity – Areas with high karst transmissivity are prioritized due to their susceptibility to rapid contaminant transport. Higher transmissivity values receive higher priority. This metric was weighted at 0.5 (50%) in the cumulative prioritization due to its strong influence.
• Future Land Use (2050) – This metric prioritizes parcels projected to remain forested or agricultural in 2050 to minimize impacts of future urbanization, based on USGS land cover projections under the IPCC 1B scenario (business-as-usual).
• Distance from High Quality Streams – Parcels near high-quality streams are prioritized to protect existing water resources. Shorter distances receive higher priority.
• Distance from Floodplains – Parcels in or near floodplains are prioritized due to their hydrologic connectivity and vulnerability to contamination. Shorter distances receive higher priority.
• Parcel Opportunity Area – The parcel opportunity area is a derived metric added at the parcel level. Parcels with a greater proportion of opportunity area (unprotected forest/agricultural land) are prioritized.

Acknowledgments

We gratefully acknowledge the many partners, stakeholders, and funders whose expertise, guidance, and commitment have shaped this work. The timeline below highlights key land prioritization milestones over the past decade, illustrating how these collective efforts have progressed through the contributions of many.

Timeline of the Land Prioritization Mapping to Protect Drinking Water Quality

This project has been funded wholly or in part by the United States Environmental Protection Agency under assistance agreement 98339418 to the Interstate Commission on the Potomac River Basin. The contents of this document do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does the Environmental Protection Agency endorse trade names or recommend the use of commercial products mentioned in this document, as well as any images, video, text, or other content created by generative artificial intelligence tools, nor does any such content necessarily reflect the views and policies of the Environmental Protection Agency.