A pilot project depositing about 9,400 tons of sand in the lower Penobscot River to test its effectiveness in covering mercury contamination is likely to begin in mid-August. Visitors to Orrington’s waterfront Picnic Park on River Road may see two barges working to spray sand over about 6.3 acres of intertidal flats in the cove, creating a cap 4-6 inches thick. The project is a court-ordered step, years in the making, toward addressing industria…