WORTHINGTON — The Okabena-Ocheda Watershed District authorized the purchase of a drone from Frontier Precision, Inc., Rock Rapids, Iowa, during its meeting Tuesday afternoon. The drone will be used primarily for mapping the presence and spread of aquatic invasive species in Lake Okabena. Watershed board chairman Tom Ahlberg said the buoys sample the lake water every 15 minutes to measure Chlorophyll A. In his request, he wrote, “Lake Okabena is the centerpiece of our community in Worthington. The three-basin lake has, like Lake Okabena, been on the impaired waters list since 2010.