Charles Thomas is in the right driver's seat of his Arlington County truck. He pushes the button, and a large round brush whirls along the right edge of the curb. Arlington County street sweeping occurs yearly from May-October as part of an effort to help reduce runoff into the Chesapeake Bay. The sweeping removes accumulated debris and pollutants such as sand, salt, minerals, petroleum products and bacteria before they wash into streams, the Potomac River and the Chesapeake Bay. This effort covers approximately 814 lane miles on both commercial and residential roads in 11 zones in the county. Thomas, who can work on either side of the truck, says he has been on the road since 7:30 a.m. and will probably work until 5 p.m. this day to finish his job.