diff --git a/Phreeqc.h b/Phreeqc.h
index 21ef40b5..1902f538 100644
--- a/Phreeqc.h
+++ b/Phreeqc.h
@@ -1066,6 +1066,7 @@ public:
 	LDBLE calc_vm_Cl(void);
 	int multi_D(LDBLE DDt, int mobile_cell, int stagnant);
 	LDBLE find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant);
+	void calc_b_ij(int icell, int jcell, int k, LDBLE b_i, LDBLE b_j, LDBLE g_i, LDBLE g_j, LDBLE free_i, LDBLE free_j, int stagnant);
 	void diffuse_implicit(LDBLE DDt, int stagnant);
 	int fill_spec(int cell_no, int ref_cell);
 	LDBLE moles_from_redox_states(cxxSolution *sptr, const char *name);
diff --git a/transport.cpp b/transport.cpp
index 918a86a3..e7001b20 100644
--- a/transport.cpp
+++ b/transport.cpp
@@ -3664,6 +3664,46 @@ fill_m_s(struct J_ij *l_J_ij, int l_J_ij_count_spec, int icell, int stagnant)
 	}
 	return (OK);
 }
+/* ---------------------------------------------------------------------- */
+void Phreeqc::
+calc_b_ij(int icell, int jcell, int k, LDBLE b_i, LDBLE b_j, LDBLE g_i, LDBLE g_j, LDBLE free_i, LDBLE free_j, int stagnant)
+/* ---------------------------------------------------------------------- */ 
+{
+	ct[icell].v_m[k].b_ij = b_i * (free_i + g_i) * b_j * (free_j + g_j) / (b_i * (free_i + g_i) + b_j * (free_j + g_j));
+	// At filterends, concentrations of ions change step-wise to the DL. 
+	// We take the harmonic mean for f_free, the average for the DL.
+	if (ct[icell].v_m[k].z)
+	{
+		if (!g_i && g_j)
+		{
+			ct[icell].v_m[k].b_ij = free_j * b_i * b_j / (b_i + b_j) +
+				b_i * (1 - free_j) / 4 + b_j * g_j / 4;
+		}
+		else if (g_i && !g_j)
+			ct[icell].v_m[k].b_ij = free_i * b_i * b_j / (b_i + b_j) +
+			b_j * (1 - free_i) / 4 + b_i * g_i / 4;
+	}
+	// for boundary cells...
+	if (stagnant > 1)
+	{ /* for a diffusion experiment with well-mixed reservoir in cell 3 and the last stagnant cell,
+		   and with the mixf * 2 for the boundary cells in the input... */
+		if (icell == 3 && !g_i && g_j)
+			ct[icell].v_m[k].b_ij = b_j * (free_j + g_j) / 2;
+		else if (jcell == all_cells - 1 && !g_j && g_i)
+			ct[icell].v_m[k].b_ij = b_i * (free_i + g_i) / 2;
+	}
+	else
+	{
+		if (icell == 0 || (icell == count_cells + 1 && jcell == count_cells + count_cells + 1))
+			ct[icell].v_m[k].b_ij = b_j * (free_j + g_j);
+		else if (icell == count_cells && jcell == count_cells + 1)
+			ct[icell].v_m[k].b_ij = b_i * (free_i + g_i);
+	}
+	if (ct[icell].v_m[k].z)
+		ct[icell].Dz2c += ct[icell].v_m[k].b_ij * ct[icell].v_m[k].zc * ct[icell].v_m[k].z;
+	return;
+}
+
 /* ---------------------------------------------------------------------- */
 LDBLE Phreeqc::
 find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
@@ -4142,7 +4182,7 @@ find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
 									g_j += it_sc->Get_z_gMCD_map()[ct[icell].v_m[k].z];
 								else
 								{
-									dum1 = it_sc->Get_mass_water() / mass_water_bulk_x;
+									dum1 = it_sc->Get_mass_water() / t_aq2;
 									dum2 = it_sc->Get_z_gMCD_map()[1] / dum1;
 									g_j += pow(dum2, ct[icell].v_m[k].z) * dum1;
 								}
@@ -4152,32 +4192,18 @@ find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
 					}
 				}
 
-				b_i = A1 * sol_D[icell].spec[i].Dwt * (f_free_i + g_i / ct[icell].visc1);
-				b_j = A2 * (f_free_j + g_j / ct[icell].visc2);
-				if (icell == count_cells && !stagnant)
-					ct[icell].v_m[k].b_ij = b_i;
-				else if (icell == all_cells - 1 && stagnant)
-					ct[icell].v_m[k].b_ij = b_i / 2; /* with the mixf *= 2 for this 'reservoir' cell in the input */
+				b_i = A1 * sol_D[icell].spec[i].Dwt;
+				b_j = A2;
+				if (sol_D[icell].tk_x == sol_D[jcell].tk_x)
+					b_j *= sol_D[icell].spec[i].Dwt;
 				else
 				{
-					if (sol_D[icell].tk_x == sol_D[jcell].tk_x)
-						b_j *= sol_D[icell].spec[i].Dwt;
-					else
-					{
-						dum2 = sol_D[icell].spec[i].Dwt / sol_D[icell].viscos_f;
-						dum2 *= exp(sol_D[icell].spec[i].dw_t / sol_D[jcell].tk_x - sol_D[icell].spec[i].dw_t / sol_D[icell].tk_x);
-						dum2 *= sol_D[jcell].viscos_f;
-						b_j *= dum2;
-					}
-					ct[icell].v_m[k].b_ij = b_i * b_j / (b_i + b_j);
-					if (icell == 0 && !stagnant)
-							ct[icell].v_m[k].b_ij = b_j;
-					else if (icell == 3 && stagnant && !g_i && g_j)
-						ct[icell].v_m[k].b_ij = b_j / 2; /* with the mixf *= 2 for stagnant cell 3 in the input */
+					dum2 = sol_D[icell].spec[i].Dwt / sol_D[icell].viscos_f;
+					dum2 *= exp(sol_D[icell].spec[i].dw_t / sol_D[jcell].tk_x - sol_D[icell].spec[i].dw_t / sol_D[icell].tk_x);
+					dum2 *= sol_D[jcell].viscos_f;
+					b_j *= dum2;
 				}
-
-				if (ct[icell].v_m[k].z)
-					ct[icell].Dz2c += ct[icell].v_m[k].b_ij * ct[icell].v_m[k].zc * ct[icell].v_m[k].z;
+				calc_b_ij(icell, jcell, k, b_i, b_j, g_i, g_j, f_free_i, f_free_j, stagnant);
 
 				k++;
 			}
@@ -4249,7 +4275,7 @@ find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
 									g_i += it_sc->Get_z_gMCD_map()[ct[icell].v_m[k].z];
 								else
 								{
-									dum1 = it_sc->Get_mass_water() / mass_water_bulk_x;
+									dum1 = it_sc->Get_mass_water() / t_aq1;
 									dum2 = it_sc->Get_z_gMCD_map()[1] / dum1;
 									g_i += pow(dum2, ct[icell].v_m[k].z) * dum1;
 								}
@@ -4266,31 +4292,18 @@ find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
 						g_j *= sol_D[jcell].spec[j].erm_ddl;
 					}
 				}
-				b_i = A1 * (f_free_i + g_i / ct[icell].visc1);
-				b_j = A2 * sol_D[jcell].spec[j].Dwt * (f_free_j + g_j / ct[icell].visc2);
-				if (icell == 0 && !stagnant)
-					ct[icell].v_m[k].b_ij = b_j;
-				else if (icell == 3 && stagnant && g_j && !g_i)
-					ct[icell].v_m[k].b_ij = b_j / 2; /* with the mixf *= 2 for 'reservoir' cell 3 in the input */
+				b_i = A1;
+				b_j = A2 * sol_D[jcell].spec[j].Dwt;
+				if (sol_D[icell].tk_x == sol_D[jcell].tk_x)
+					b_i *= sol_D[jcell].spec[j].Dwt;
 				else
 				{
-					if (sol_D[icell].tk_x == sol_D[jcell].tk_x)
-						b_i *= sol_D[jcell].spec[j].Dwt;
-					else
-					{
-						dum2 = sol_D[jcell].spec[j].Dwt / sol_D[jcell].viscos_f;
-						dum2 *= exp(sol_D[jcell].spec[j].dw_t / sol_D[icell].tk_x - sol_D[jcell].spec[j].dw_t / sol_D[jcell].tk_x);
-						dum2 *= sol_D[icell].viscos_f;
-						b_i *= dum2;
-					}
-					ct[icell].v_m[k].b_ij = b_i * b_j / (b_i + b_j);
-					if (icell == count_cells && !stagnant)
-						ct[icell].v_m[k].b_ij = b_i;
-					else if (jcell == all_cells - 1 && stagnant && !g_j && g_i)
-						ct[icell].v_m[k].b_ij = b_i / 2; /* with the mixf * 2 for this 'reservoir' cell in the input */
+					dum2 = sol_D[jcell].spec[j].Dwt / sol_D[jcell].viscos_f;
+					dum2 *= exp(sol_D[jcell].spec[j].dw_t / sol_D[icell].tk_x - sol_D[jcell].spec[j].dw_t / sol_D[jcell].tk_x);
+					dum2 *= sol_D[icell].viscos_f;
+					b_i *= dum2;
 				}
-				if (ct[icell].v_m[k].z)
-					ct[icell].Dz2c += ct[icell].v_m[k].b_ij * ct[icell].v_m[k].zc * ct[icell].v_m[k].z;
+				calc_b_ij(icell, jcell, k, b_i, b_j, g_i, g_j, f_free_i, f_free_j, stagnant);
 
 				k++;
 			}
@@ -4373,28 +4386,9 @@ find_J(int icell, int jcell, LDBLE mixf, LDBLE DDt, int stagnant)
 						g_j *= sol_D[jcell].spec[j].erm_ddl;
 					}
 				}
-				b_i = A1 * sol_D[icell].spec[i].Dwt * (f_free_i + g_i / ct[icell].visc1);
-				b_j = A2 * sol_D[jcell].spec[j].Dwt * (f_free_j + g_j / ct[icell].visc2);
-				ct[icell].v_m[k].b_ij = b_i * b_j / (b_i + b_j);
-				// but for boundary cells...
-				if (stagnant > 1)
-				{ /* for a diffusion experiment with well-mixed reservoir in cell 3 and the last stagnant cell,
-					   and with the mixf * 2 for the boundary cells in the input... */
-					if (icell == 3 && !g_i && g_j)
-						ct[icell].v_m[k].b_ij = b_j / 2;
-					else if (jcell == all_cells - 1 && !g_j && g_i)
-						ct[icell].v_m[k].b_ij = b_i / 2;
-				}
-				else
-				{
-					if (icell == 0 || (icell == count_cells + 1 && jcell == count_cells + count_cells + 1))
-						ct[icell].v_m[k].b_ij = b_j;
-					else if (icell == count_cells && jcell == count_cells + 1)
-						ct[icell].v_m[k].b_ij = b_i;
-				}
-
-				if (ct[icell].v_m[k].z)
-					ct[icell].Dz2c += ct[icell].v_m[k].b_ij * ct[icell].v_m[k].zc * ct[icell].v_m[k].z;
+				b_i = A1 * sol_D[icell].spec[i].Dwt;
+				b_j = A2 * sol_D[jcell].spec[j].Dwt;
+				calc_b_ij(icell, jcell, k, b_i, b_j, g_i, g_j, f_free_i, f_free_j, stagnant);
 
 				//ddlm = sol_D[jcell].spec[j].lm - sol_D[icell].spec[i].lm; // appt: this could give an incorrect large factor for implicit
 				//if (fabs(ddlm) > 1e-10)