1 billion

gallons of water stored

10 million dollars

FEMA grant awarded

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The challenge

Loudoun County, Virginia has one of the fastest growing populations in all of the United States. People living in Loudoun County, and much of the rest of the Washington D.C. metropolitan area, get their drinking water, primarily, from the Potomac River. But the Potomac is a limited source of water, and, at times, the river can be subject to conditions that could jeopardize Loudoun County residents’ water supply. For instance, during a protracted drought, water levels in the river can drop and this poses a threat not only to people, but also to marine wildlife. Conversely, too much water can also complicate efforts to supply reliable drinking water to people living in the county. Heavy rains and storms can increase the turbidity of the water and when pumped to the treatment facility, it then requires much more effort, and chemicals, to convert it into water that is safe for human consumption.

Our client Loudoun Water provides drinking water, as well as wastewater services for more than 80,000 households in Loudoun County. We’ve partnered with Loudoun Water on a groundbreaking project that will help safeguard the clean drinking water supply for residents and enhance the overall water resilience of the region.

The solution

Arcadis experts are leading the planning and design efforts to convert a retired rock quarry into a water supply reservoir. The project includes improvements to the rock quarry so that it is suitable to serve as a water supply reservoir and a new 40-million-gallon per day pumping station to pump stored water from the rock quarry to the local water treatment plant. The new system will include a 300-foot-deep pump shaft and, to optimize water quality, three withdrawal tunnels at varying elevations. A series of vertical submersible raw water pumps will be used to transfer water from the reservoir to the treatment plant. This innovative, first of its kind in the region, water banking approach to managing a limited water supply, will enhance Loudoun Water’s drought resilience.

Our people designed the intake and pumping station at the Potomac River, which is already used to withdraw raw water and send it to Loudoun Water’s Water Treatment Plant. Once the quarry reservoir is online, the Potomac River pumping station will be used to fill the quarry during periods when there is ample water in the river and conditions are such that the water is of a good quality. Our experts are currently designing the water reservoir in the decommissioned quarry. During drought conditions or periods when the water quality is suboptimal, raw water can be pumped from the quarry for water treatment, instead of directly from the river.

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    Additionally, an Arcadis team assisted Loudoun Water with preparations to submit this project for a grant from the Federal Emergency Management Agency (FEMA). Typically, FEMA is called into action after a disaster has taken place but, more and more, the agency is exploring ways to decrease negative impacts and costs by contributing to “pre-disaster” projects that can increase the resilience of communities.

The impact

The Milestone Reservoir will be large enough to store one billion gallons of water, enough water to meet residents needs for up to two months. The construction of the reservoir will transform a retired rock quarry into a new water supply source for the Washington DC Metro region. This will also enhance resilience for the region, providing system reliability and helping to mitigate water supply impacts during drought periods or as the result of storms or other water supply emergencies. When these events occur, the water treatment plant can use source water that has already been stored in the quarry reservoir in lieu of taking it directly from the Potomac River. This approach offers treatment flexibility and climate resilience for the region’s water supply needs. This innovative project was also awarded a 10-million-dollar (US) grant from FEMA, based on the positive impact the project will have on securing the region’s clean drinking water supply against potential emergencies in the future.


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