It all begins with a single, innocuous raindrop. Instead of splattering onto soil and oozing into the ground, as it would have done a half-millennium ago, the droplet lands on a roof, road or parking lot. From there, it zips along one hard surface to another. Until this tiny traveler finds a larger body of water — a ditch, a creek, the Potomac River — it is fair game for any pollutant that wants to tag along. Landscape architect Kelley Oklesson, kneeling, confers with (standing, left to right) Maureen Robinson of the Association of Professional Landscape Designers, neighborhood activist and gardener Jean Lee Cole and landscape architect Maya Mule during…