Arlington’s tap water is the cleanest among the nation’s big cities, according to a study by a Washington, D.C.-based environmental advocacy group. The Environmental Working Group calculations released this weekend were based on testing conducted since 2004 that found 316 pollutants in U.S. drinking water systems. The rankings that put Arlington No. 1 took into account numbers and percentage of chemicals found in tap water and levels of pollutants compared to legal limits and national averages. Other Texas cities with high ranking include: Fort Worth, third and Dallas 12th. All three cities employ ozone water treatment as part of their overall drinking water treatment program. Another city with a high ranking was Boston which also uses ozone as part of its water treatment program.