explosives used in road and building construction were the source of this pollutant. However, an investigation by the town of Billerica eventually determined that the source was a local company that produced surgical and medical materials. The company was using 220 gallons per month of perchloric acid in a bleaching process, and rinse water was being discharged into the sewage system. After the company was identified, it voluntarily shut down until ion exchange equipment could be installed in accordance with environmental guidelines. In May 2007, Billerica was the subject of legal action and forced to pay a $250,000 penalty for discharging pollutants into the Concord River. The action was brought by the U.S. Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (MassDEP) because the town of Billerica exceeded allowable effluent limits for phosphorus, fecal coliform bacteria, pH, and ammonia nitrogen. The town was also charged with failing to submit discharge monitoring reports, failing to comply with monitoring requirements, and failing to submit infiltration and inflow reporting. According to the EPA, Billerica's discharges of phosphorus created an overabundance of nutrients, which damaged the river by stimulating excessive aquatic plant growth. Diadromous fish recovery effort In May 2000, the U.S. Fish and Wildlife Service, the Massachusetts Riverways Program, and volunteers from the Sudbury Valley Trustees (SVT) released 7,000 adult alewife (river herring) into the Concord River. They were transferred from the Nemasket River so that they could lay their eggs and spawn upstream. This imprinted the young alewives with the Concord as their new home river. The experiment did not succeed, as too few fish returned to the base of the first dam on the Concord River. A feasibility study published in 2016 is again exploring steps necessary for returning diadromous fish to the Concord River, and farther upstream, to the Sudbury and Assabet Rivers. Anadromous describes species that live in salt water but lay eggs in fresh water - such as alewives and shad; diadromous is a larger grouping which includes species that mature in fresh water but return to salt water to lay eggs - such as eels. References Further reading External links USGS: Concord River drainage basin Thoreau and the Concord River Walden.org: Thoreau's essay Early photographs of the river Environmental Pollution of the Concord River, a Historic American Waterway Rivers of Massachusetts Rivers of Middlesex County, Massachusetts Tributaries of the Merrimack River Wild and Scenic