diff --git a/.github/actions/test-extended/action.yml b/.github/actions/test-extended/action.yml
index 49d2cc67da5..953df09622c 100644
--- a/.github/actions/test-extended/action.yml
+++ b/.github/actions/test-extended/action.yml
@@ -11,11 +11,20 @@ runs:
         version: 13
 
     - name: Checkout PETSc
+      if: runner.os == 'Linux'
       uses: actions/checkout@v4
       with:
         repository: petsc/petsc
         path: petsc
         ref: release
+        
+    - name: Checkout PETSc
+      if: runner.os == 'macOS'
+      uses: actions/checkout@v4
+      with:
+        repository: petsc/petsc
+        path: petsc
+        ref: release-3.21
 
     - name: Configure environment
       if: runner.os == 'Linux'
@@ -37,7 +46,7 @@ runs:
       working-directory: petsc
       run: |
         sudo wget -P $GITHUB_WORKSPACE/petsc https://download.open-mpi.org/release/open-mpi/v5.0/openmpi-5.0.2.tar.gz
-        sudo ./configure PETSC_ARCH=linux-gnu --download-fblaslapack --download-openmpi=$GITHUB_WORKSPACE/petsc/openmpi-5.0.2.tar.gz
+        sudo ./configure PETSC_ARCH=linux-gnu --download-fblaslapack --download-openmpi=$GITHUB_WORKSPACE/petsc/openmpi-5.0.2.tar.gz --with-debugging=0
         sudo make all
 
     - name: Configure PETSc
@@ -64,9 +73,7 @@ runs:
       if: runner.os == 'macOS'
       shell: bash
       run: |
-        bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
         brew install netcdf-fortran
-        brew install gcc@13
         nc-config --all
         nf-config
 
diff --git a/.github/actions/test-par/action.yml b/.github/actions/test-par/action.yml
index 5b96a2835a6..7b390758473 100644
--- a/.github/actions/test-par/action.yml
+++ b/.github/actions/test-par/action.yml
@@ -11,11 +11,20 @@ runs:
         version: 13
 
     - name: Checkout PETSc
+      if: runner.os != 'macOS'
       uses: actions/checkout@v4
       with:
         repository: petsc/petsc
         path: petsc
         ref: release
+        
+    - name: Checkout PETSc
+      if: runner.os == 'macOS'
+      uses: actions/checkout@v4
+      with:
+        repository: petsc/petsc
+        path: petsc
+        ref: release-3.21
 
     - name: Configure environment
       if: runner.os == 'Linux'
@@ -37,7 +46,7 @@ runs:
       working-directory: petsc
       run: |
         sudo wget -P $GITHUB_WORKSPACE/petsc https://download.open-mpi.org/release/open-mpi/v5.0/openmpi-5.0.2.tar.gz
-        sudo ./configure PETSC_ARCH=linux-gnu --download-fblaslapack --download-openmpi=$GITHUB_WORKSPACE/petsc/openmpi-5.0.2.tar.gz
+        sudo ./configure PETSC_ARCH=linux-gnu --download-fblaslapack --download-openmpi=$GITHUB_WORKSPACE/petsc/openmpi-5.0.2.tar.gz --with-debugging=0
         sudo make all
 
     - name: Configure PETSc
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 012b22c1f4d..42ba0f583ed 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -502,7 +502,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-22.04, macos-12, windows-2022 ]
+        os: [ ubuntu-22.04, macos-13, windows-2022 ]
     defaults:
       run:
         shell: bash
@@ -530,6 +530,15 @@ jobs:
         working-directory: modflow6
         run: pixi run install
 
+      - name: Set LDFLAGS (macOS)
+        if: matrix.os == 'macos-13'
+        run: |
+          os_ver=$(sw_vers -productVersion | cut -d'.' -f1)
+          if (( "$os_ver" > 12 )); then
+            ldflags="$LDFLAGS -Wl,-ld_classic"
+            echo "LDFLAGS=$ldflags" >> $GITHUB_ENV
+          fi
+
       - name: Test parallel MF6
         if: runner.os != 'Windows'
         uses: ./modflow6/.github/actions/test-par
@@ -555,7 +564,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-22.04, macos-12 ]
+        os: [ ubuntu-22.04, macos-13 ]
     defaults:
       run:
         shell: bash
@@ -581,6 +590,15 @@ jobs:
         working-directory: modflow6
         run: pixi run pip install xugrid xarray netcdf4
 
+      - name: Set LDFLAGS (macOS)
+        if: matrix.os == 'macos-13'
+        run: |
+          os_ver=$(sw_vers -productVersion | cut -d'.' -f1)
+          if (( "$os_ver" > 12 )); then
+            ldflags="$LDFLAGS -Wl,-ld_classic"
+            echo "LDFLAGS=$ldflags" >> $GITHUB_ENV
+          fi
+
       - name: Test extended MF6
         uses: ./modflow6/.github/actions/test-extended
 
diff --git a/src/Utilities/Matrix/PetscMatrix.F90 b/src/Utilities/Matrix/PetscMatrix.F90
index 440e9b71f96..c2ee2bde546 100644
--- a/src/Utilities/Matrix/PetscMatrix.F90
+++ b/src/Utilities/Matrix/PetscMatrix.F90
@@ -396,7 +396,8 @@ subroutine pm_get_aij_local(this, ia, ja, amat)
     ! local
     PetscErrorCode :: ierr
     integer(I4B) :: irow, icol, ipos
-    real(DP) :: val
+    integer(I4B), dimension(1) :: irow_idxs, icol_idxs
+    real(DP), dimension(1) :: values
     Mat :: local_mat
 
     call MatGetDiagonalBlock(this%mat, local_mat, ierr)
@@ -405,9 +406,11 @@ subroutine pm_get_aij_local(this, ia, ja, amat)
     do irow = 1, this%nrow
       do ipos = this%ia_local(irow), this%ia_local(irow + 1) - 1
         icol = this%ja_local(ipos)
-        call MatGetValues(local_mat, 1, irow - 1, 1, icol - 1, val, ierr)
+        irow_idxs(1) = irow - 1
+        icol_idxs(1) = icol - 1
+        call MatGetValues(local_mat, 1, irow_idxs, 1, icol_idxs, values, ierr)
         CHKERRQ(ierr)
-        this%amat_local(ipos) = val
+        this%amat_local(ipos) = values(1)
       end do
     end do