7.1 Acid Buffering Reactions

Four common acid buffering reactions are important to controlling pH in the unsaturated zone which in turn influences the fate of phosphorus in septic system plumes. These acid buffering reactions involve dissolution of carbonate minerals and gibbsite, as well as two types of ferrihydrite dissolution. These reactions are as follows:

• carbonate mineral dissolution as shown in Equation 18;

• gibbsite dissolution as shown in Equation 19;

• ferrihydrite dissolution as shown in Equation 20;

• and, reductive dissolution of ferrihydrite as shown in Equation 21.

Lower pH increases the potential for reactions presented in Section 7.2 that precipitate phosphorus, thus remove phosphorus from the waste water. Consequently, at sites where the reactions shown in Equations 18 through 21 are more likely to occur, there is a greater assimilation capacity for phosphorus in the unsaturated zone below a septic system.