Cleanup Day on the Bramble River
1Each June, the Watershed Stewards club at Hollow Oak Middle School joins a cleanup of the Bramble River, which runs from a range of granite hills, through a limestone valley, and into Lake Merrin. While other volunteers pull trash from the banks, seventh grader Ilse Montoya and her club test the water at four sites with Mr. Anders Thorvaldsen, a scientist with the watershed council.
2At each site, the club fills a bottle with river water. They pour part of it through a filter and dry and weigh the mud and sand caught on the filter. They test the filtered water with a meter that shows how much rock material is dissolved in it, which is invisible because it has broken apart into particles far too small to see. Rain picks up a little carbon dioxide from the air and soil, which makes it slightly acidic. Over time, this water slowly dissolves minerals from rock. Granite's minerals dissolve very slowly, but calcite, the main mineral in limestone, dissolves much faster.
3Site 1 is a stream in the granite hills. Site 2 is in the limestone valley, just above Blue Spring. At Blue Spring, water that has soaked into the ground flows out of a cave mouth after spending months moving through cracks and caves in the limestone. Site 3 is just below the spring. Two small streams from the granite hills join the river between Site 3 and Site 4, which is at the river mouth on Lake Merrin. Table 1 shows a calm day with no rain for two weeks. Table 2 shows the morning after a heavy storm.
4Two other things surprised Ilse. At the cave mouth, a cream-colored crust of rock coats the stones and twigs. A twig the club tied there last June now has a crust 2 mm thick. As the spring water meets the air, it loses some carbon dioxide, and some of the dissolved calcite comes back out as solid rock called tufa. In Lake Merrin, the mud near the shore is full of tiny snail and clam shells, which these animals build from minerals dissolved in the water. Mr. Thorvaldsen explained that the valley's limestone itself formed from shells and lime mud on an ancient sea floor, and that forces powered by Earth's hot interior later raised it far above the sea.