Substances in a lake

An industrial accident inadvertently releases four chemical substances into a lake:

• The red substance is degraded in both the aerobic and the anaerobic layers.
• The orange substance is degraded in the anaerobic layer by a first-order reaction, but it is inert in the aerobic layer.
• The green substance is degraded in the aerobic layer, but not in the anaerobic one.
• The blue substance is completely inert.

The graph shows the concentration of all four substances over the entire depth of the lake, which ranges from z=0 to z=zg (depth of the ground).