Lake Manassas, T. Nelson Elliott Dam, and
City of Manassas Water Treatment Plant
Source: 1994 Color Infrared Digital Orthophoto Quarter Quadrangle (DOQQ)
of Thoroughfare Gap quad from GIS Center at Radford University
Today, we drink water droplets that have been many places before we swallow.
For several billion years, earth's water has recycled from the original outgassings of volcanoes and melting comets into the atmosphere, only to fall to the surface as raindrops, drain to the oceans, evaporate back to the sky, and go through the hydrologic cycle over and over again. In your cup of water could be an H2O molecule that once eroded the ancient Appalachian Mountains 180 million years ago.
Once life evolved on earth, some of those water droplets have been taken up by plant roots and then evaporated into the air. Others were swallowed by animals and excreted back into the environment. Since humans developed urban centers, a few of those water droplets have been cycled through sewers and wastewater systems. Unless you manage to capture a fragment of a comet in your drinking cup (and survive the impact...), you're not drinking virgin-pure water.
Ideally, the water droplets we drink have been exposed to sunlight or oxygen long enough to kill harmful bacteria and viruses... and the droplets did not absorb toxic materials in its journey before our swallow. Virginia has areas of heavy metal concentration, including one of the richest uranium deposits in the nation, but contamination of surface drinking water sources in Virginia by ore deposits is uncommon. (You'd pretty much have to drill a well into the groundwater adjacent to an ore body.)
Community water systems treat water intended for drinking before supplying it to customers through pipes, to remove the potential contamination due to human-created waste or, in a few cases, due to wildlife feces. The treatment process in drinking water treatment plants is very similar to how sewage is treated in wastewater treatment plants - solids are removed, chemicals are added to kill bacteria, and the final product is inspected before being released. (Drinking water plants do pump their final product in pressurized pipes from the plant to the customers, while wastewater travels from the customers to the sewage plant in unpressured pipes.)
When a community's drinking water system uses clean water from the headwaters of a stream, the cost to treat that water and meet state/Federal requirements are lower than when water below a sewage outlet is used. Richmond is downstream of Lynchburg and other communities on the James River, so the initial water quality for the raw material that will end up in that city's drinking water system will be relatively poor compared to a community near the headwaters.
When a community's drinking water system uses clean water from the headwaters of a stream, the cost to treat that water and meet state/Federal requirements are lower than when water below a sewage outlet is used. Richmond is downstream of Lynchburg and other communitries on the James River, so the initial water quality for the raw material that will end up in that city's drinking water system will be relatively poor compared to a community near the headwaters.
However, the streams in the headwaters are small compared to the volume in the rivers near their mouth, further downstream. Dilution really is one of the solutions to pollution, but small streams have a limited natural capacity to clean up excess nitrates and other pollution from concentrations of people, factories, roads, and farms.
Even a small community can severely pollute a small river. Bassett is a factory town that was "encouraged" by the state to clean up the wastewater dumped into the Smith River. Henry County created a Public Service Authority in part to finance the cleanup. The communities downstream of Bassett are the beneficiaries of the pollution control - as are the fish, wildlife, and recreationists who use that stretch of the river.
Martinsville gets its drinking water from the headwaters of Jones Creek, while Henry County draws its water from Smith River three miles below Philpott Lake. Both avoid extracting water from below Bassett. Philpott Lake serves as a giant settling basin, so it too enhances water quality downstream of the dam. (The Smith River below Philpott Lake is a top fishing destination for brown trout.)