diff --git a/source/model/methane_mod.f90 b/source/model/methane_mod.f90
index 660f5b267eec28b404b8eae75ef6a722fb9fec4e..5d20cb9b62b147b1d5760195a8161bea5f51286a 100644
--- a/source/model/methane_mod.f90
+++ b/source/model/methane_mod.f90
@@ -1154,8 +1154,8 @@ endif ifdeep
      ! (implicit scheme)
      x = (k_ch40 + qwater(i))*(k_ch40 + qwater(i)) + &
      & 2.*(k_ch40 - qwater(i))*Vmaxw*dt + Vmaxw*dt*Vmaxw*dt
-     qwater(i) = 0.5*( (qwater(i) - Vmax*dt - k_ch40) + sqrt(x) )
-     x = dt*Vmax*qwater(i)/(k_ch40 + qwater(i))
+     qwater(i) = 0.5*( (qwater(i) - Vmaxw*dt - k_ch40) + sqrt(x) )
+     x = dt*Vmaxw*qwater(i)/(k_ch40 + qwater(i))
      oxyg(i)   = oxyg(i)   - 2.*x
      DIC(i) = DIC(i) +    x
    case(3)