ARTESIAN BLISTER WETLANDS, A PERENNIAL WATER RESOURCE IN THE SEMI-ARID RIFT VALLEY OF EAST AFRICA

A cluster of artesian springs encircled by mounds of marsh and wet meadows was discovered near the equator, Kenya, East Africa. Each spring is capped by a dense fibrous root mat that covers a blister of water. Individual mounds are ~ 15 m wide, 1-2 m high and affect ~ 50 m2. The central water blister is <1 m3. The arched semi-permeable vegetation cap appears to be buoyed upward by slow artesian flow that leaks through the cap and moves slowly away. Lush plant growth (Cyperus species, algae and filamentous cyanobacteria) is supported even through the dry season. Approximately 20 artesian blister wetlands occur within a large spring and wetland complex (~1 km2) that includes several freshwater Typha marshes fed by groundwater seeps. The springs discharge along a rift-related fracture system near the contact between volcanic bedrock and Quaternary sediments.

Cores (1-2m) through the mounds revealed a dense root mat underlain by water and a clayey peat that is locally pebbly at the base. LOI decreases from 75% at surface to ~10% at 0.4-1.2m. Blister water is cool, fresh, and dysaerobic (T=30-33 °C; pH=6.2-7.2; conductivity ~0.6 mS; and DO=25%). The spring/wetland mounds likely form by the blanketing of the land around the spring with vegetation. Plants and cyanobacteria appear to trap sediment transported by surface run-off and wind. These ecological niches are important water resources for animals and people in semi-arid environments.