Trixie Belden #10: marshland mystery. Kathryn Kenny. 1990.

It’s springtime in Sleepyside and Trixie has plans for the sunny weather. She’s going to explore the lonely marshland and find plants for extra credit in her botany class. But for Trixie, even innocent activities turn into mysteries. Instead of flowers and herbs, she comes across a burned-out house, a little old lady and a spooky story about Captain Kidd’s hidden pirate treasure?