Water of the Potomac

Could a reservoir on the South Branch be part of the future plan for sustainable water availability (and flood control) downstream?

The Potomac Basin- Is the Water Adequate?

Elizabeth Ward

The Potomac River is 383-miles long and contains 14,670 square miles that makes up the Potomac Basin. The largest portion of the Potomac basin is in Virginia -5,723 square miles; while Maryland contains 3,818 square miles, West Virginia-3,490 miles, Pennsylvania -1,570 square miles, and 69 square miles that constitute the District of Columbia. The Potomac basin is made up of wetlands, streams, rivers, reservoirs, lakes, and the Potomac estuary.

The Potomac River, its tributaries, and the associated groundwater resources are vital to the region, it is the source of drinking water for the over 6,000,000 people in the Washington Metropolitan area. The Potomac River is the main supply of water for WSSC and the Washington Aqueduct and the major source of water for Loudoun Water and Fairfax Water. The Interstate Commission on the Potomac River Basin (ICPRB) manages the allocation of the Potomac River waters in time of drought or low flow, but also conducts studies on pollution, emerging contaminants and other water problems; from water supply adequacy, population growth patterns, to climate change impact on
drought frequency and water supply.

Every five years the ICPRB working with the stakeholders (regulators, local and state governments, businesses, farmers and citizens) creates the Potomac Basin Comprehensive Water Resources Plan (Water Plan) that is an serves as an important portion of planning for the future of the Washington Metropolitan area region. As part of the Water Plan, the ICPRB seeks to identify surface water and groundwater resource issues of interstate or basin-wide significance. The Water Plan is the sole regional tool that attempts to ensure sustainable and reliable drinking water supplies for the entire region while protecting and improving water quality and managing land use for sustainable water and ecological health.

Water comes into the Potomac basin from rainfall (and melting snow). In the Potomac basin rainfall averages approximately 42 inches per year, but precipitation varies from year to year and across the basin (it tends to rain more in the eastern portion of the basin). Water management in the Potomac basins requires preparation for summers and autumns when river flow is typically low and water demand is highest. Balanced, well-functioning ecosystems not impacted by man are able to handle fluctuations in streamflow and groundwater availability. Mankind’s impacts to natural hydrologic variability often has negative impacts most commonly the increase in impervious cover from development results in higher flood stages from increased storm runoff and excessive surface water withdrawal which both reduce groundwater recharge.

The groundwater resources above and below the Fall Line differ. In the Coastal Plain Province, groundwater is contained in a confined aquifer system. Recharge of these aquifers primarily occurs by infiltration from overlying aquifers and through outcroppings near the Fall Line. Above the Fall Line, in the Piedmont Province, groundwater aquifers consist of fractured rock. Fractured bedrock aquifers consist of a thin layer of unconsolidated soil and weathered rock overlying the bedrock. These unconsolidated contain the largest volume of groundwater in the fractured rock aquifer. The vastly different physical properties of the groundwater systems above and below the Fall Line result in unique characteristics like recharge rates and water storage. The fractured rock system has limited ability to store water and can easily become overdrawn in a drought. The confined aquifer system of the Coastal Plain has for decades been over drawn and it’s level in falling.

Water use data for the Potomac basin has been compiled by the ICPRB. Water uses above the Fall Line are typically from surface water sources given the relatively small amount of storage available in the groundwater systems. Conversely, the water uses below the Fall Line are typically from groundwater. However, the water use data used is based on analyzing readily available data sets. This means that the regions water use is estimated by the data reported by large water users (>10,000 gallons per day) and public drinking water supplies.

The data from the water utilities and large users is regulated and reported, but knowing how much water is being used by smaller users like private household wells and small agricultural operations needs to be accounted for when assessing the basin’s water supply. These small users are a significant portion of the population even in the densely populated Washington Metropolitan region ICPRB is trying to estimate the unreported volumes from the small users using data sets such as population statistics, land use/ land cover data, and well locations that have to be compiled on a local basis. However, while they are trying to fully estimate the total regional water demand, they are not making any attempt to estimate the groundwater resources its adequacy and sustainability. The changing land use is impacting the regional hydrology and groundwater recharge so the quantity of available groundwater may even be decreasing.

In order to secure a sustainable water future for the Potomac basin we need better data. We have a relatively small amount of regional water storage for surface water and very limited alternate water sources. Historical events have demonstrated the vulnerability of current water supplies to drought and other types of disruptions –like chemical spills in the Potomac. A water supply alternatives study was conducted in 2017 by the ICPRB to evaluate options for dealing with potential future shortages due to severe drought, but it still did not address groundwater impact and availability. The Metropolitan Washington Council of Governments (MWCOG) Redundancy Study in 2016 looked at impacts to the region from a chemical spill in the Potomac upstream of the intakes for Loudoun Water, Fairfax Water, WSSC and the Aqueduct, but there has been no systematic, comprehensive evaluation of the vulnerabilities of the basin water supply as a whole and planning for the future demand and supply of water. We need to do better.