anchor.diff:              F(i,33)=F(i,33)+dz(i,k)*R(12)         !H2S
               F(i,34)=F(i,34)+dz(i,k)*R(12)         !MnOA
               
  -           !R(13)=K13*poro(k)*pw(i,k,iwFe_)*HS/H1/KsFeS-1.0d0 ! (W)
  -            R(13)=K13*poro(k)*pw(i,k,iwFe_)*pw(i,k,iwH2S) ! (F)(A)
  +           !R(13)=K13*poro(i,k)*pw(i,k,iwFe_)*HS/H1/KsFeS-1.0d0 ! (W)
  +            R(13)=K13*poro(i,k)*pw(i,k,iwFe_)*pw(i,k,iwH2S) ! (F)(A)
               Rpw(iwFe_)=Rpw(iwFe_)-R(13)
               Rpw(iwH2S)=Rpw(iwH2S)-R(13)
               Rsm(iFeS_)=Rsm(iFeS_)+R(13)
  
positive.code:             F(i,29)=F(i,29)+dz(i,k)*R(25)         !H2S
              F(i,30)=F(i,30)+dz(i,k)*R(25)*2.0d0   !FeOP
              
             !R(11)=K11*poro(k)*pw(i,k,iwFe_)*pw(i,k,iwO2_)*OH**2.0d0 !Wijsman
              R(11)=K11*poro(k)*pw(i,k,iwFe_)*pw(i,k,iwO2_) !Fossing
              Rpw(iwFe_)=Rpw(iwFe_)-R(11)
              Rpw(iwO2_)=Rpw(iwO2_)-R(11)*0.25d0
              Rsm(iFeOA)=Rsm(iFeOA)+R(11)
  
negative.code: !
  !  - Re-oxydations using SO4
  !
              !R(23)=K23*poro(i,k)*pw(i,k,iwCH4)*pw(i,k,iwSO4)
  !
  !  Add reaction
  !
              DO itrc=1,NBGCPW
                cff=dtBgc/(poro(i,k)+dens*(1.0d0-poro(i,k))*Kads(i,k,itrc))
                pw(i,k,itrc)=pw(i,k,itrc)+Rpw(itrc)*cff
              END DO
  
              DO itrc=1,NBGCSM
                cff=dtBgc/(dens*(1.0d0-poro(i,k)))
                sm(i,k,itrc)=sm(i,k,itrc)+Rsm(itrc)*cff
              END DO
  !
  