The kinetics of many biologically and chemically reactive substances in natural waters can be approximated by first order reactions. A recurrence formula is presented that relates the concentrations in the sequence to the concentration distribution of the first reactant in the sequence. This recurrence relation applies to general spatial and temporal regimes. In particular, the recurrence relation immediately extends the known analytical solutions for a single first order reacting substance to any sequentially reacting set of substances for various spatial and temporal settings. This increase in the applicability of known solutions renders problems soluble that had heretofore been difficult to analyze.