From fc24a91b52c49a67df27261bce2ee0f19792dc24 Mon Sep 17 00:00:00 2001
From: jlarsen-usgs <jlarsen@usgs.gov>
Date: Wed, 25 Feb 2026 14:35:37 -0800
Subject: [PATCH 1/3] fix(UcnFile): Add check for modflow-6 GWT concentration
 file header type

---
 autotest/test_binaryfile.py        | 32 +++++++++++++
 flopy/utils/__init__.py            |  9 +++-
 flopy/utils/binaryfile/__init__.py | 75 +++++++++++++++++++++++++++++-
 3 files changed, 114 insertions(+), 2 deletions(-)

diff --git a/autotest/test_binaryfile.py b/autotest/test_binaryfile.py
index 59558d98a..1de8a77f9 100644
--- a/autotest/test_binaryfile.py
+++ b/autotest/test_binaryfile.py
@@ -198,6 +198,38 @@ def test_concentration_build_index(example_data_path):
     )
 
 
+def test_mf6_concentration_build_index(example_data_path):
+    # test low-level BinaryLayerFile._build_index() method with UCN file
+    pth = (
+        example_data_path
+        / "mf6/create_tests/test_transport/expected_output/gwt_mst03.ucn"
+    )
+    with UcnFile(pth) as ucn:
+        pass
+    assert ucn.nrow == 1
+    assert ucn.ncol == 1
+    assert ucn.nlay == 1
+    assert not hasattr(ucn, "ntrans")
+    assert ucn.totalbytes == 1680
+    assert len(ucn.recordarray) == 28
+    assert type(ucn.recordarray) == np.ndarray
+    assert ucn.recordarray.dtype == np.dtype(
+        [
+            ("kstp", "i4"),
+            ("kper", "i4"),
+            ("pertim", "f8"),
+            ("totim", "f8"),
+            ("text", "S16"),
+            ("ncol", "i4"),
+            ("nrow", "i4"),
+            ("ilay", "i4"),
+        ]
+    )
+
+    assert np.max(ucn.times) == 4.0
+    assert ucn.kstpkper[-1] == (14, 2)
+
+
 def test_binaryfile_writeread(function_tmpdir, nwt_model_path):
     model = "Pr3_MFNWT_lower.nam"
     ml = flopy.modflow.Modflow.load(model, version="mfnwt", model_ws=nwt_model_path)
diff --git a/flopy/utils/__init__.py b/flopy/utils/__init__.py
index bd15a59b8..d08b2495c 100644
--- a/flopy/utils/__init__.py
+++ b/flopy/utils/__init__.py
@@ -23,7 +23,14 @@
 
 from .utl_import import import_optional_dependency  # isort:skip
 from . import get_modflow as get_modflow_module
-from .binaryfile import BinaryHeader, CellBudgetFile, HeadFile, HeadUFile, UcnFile
+from .binaryfile import (
+    BinaryHeader,
+    CellBudgetFile,
+    ConcentrationFile,
+    HeadFile,
+    HeadUFile,
+    UcnFile,
+)
 from .check import check
 from .flopy_io import read_fixed_var, write_fixed_var
 from .formattedfile import FormattedHeadFile
diff --git a/flopy/utils/binaryfile/__init__.py b/flopy/utils/binaryfile/__init__.py
index 07027f852..af9230446 100644
--- a/flopy/utils/binaryfile/__init__.py
+++ b/flopy/utils/binaryfile/__init__.py
@@ -296,6 +296,78 @@ def get_headfile_precision(filename: Union[str, PathLike]):
     return result
 
 
+def get_concentration_file_type(filename: Union[str, PathLike], precision):
+    """
+    Method to check header and determine if the concentration file is a MT3D like
+    file or a MF6 GWT like concentration file
+
+    Parameters
+    ----------
+    filename : str or PathLike
+        Path of binary MODFLOW file to determine precision.
+    precision : str
+        double or single
+    
+    Returns
+    -------
+    str
+        Result will be ucn or head
+
+    """
+    f = open(filename, "rb")
+    f.seek(0, 2)
+    totalbytes = f.tell()
+    f.seek(0, 0)  # reset to beginning
+    assert f.tell() == 0
+    if totalbytes == 0:
+        raise ValueError(f"datafile error: file is empty: {filename}")
+
+    floattype = "f4"
+    if precision == "double":
+        floattype = "f8"
+
+    # first try mt3d ucn
+    vartype = [
+        ("ntrans", "i4"),
+        ("kstp", "i4"),
+        ("kper", "i4"),
+        ("totim", floattype),
+        ("text", "S16"),
+    ]
+    hdr = binaryread(f, vartype)
+
+    try:
+        s = hdr[0][4].decode()
+        if not s.strip().lower().startswith("c"):
+            success = False
+        else:
+            success = True
+            result = "ucn"
+    except ValueError:
+        success = False
+
+    if not success:
+        f.seek(0)
+        vartype = [
+            ("kstp", "<i4"),
+            ("kper", "<i4"),
+            ("pertim", floattype),
+            ("totim", floattype),
+            ("text", "S16"),
+        ]
+        hdr = binaryread(f, vartype)
+        s = hdr[0][4].decode()
+        if not s.strip().lower().startswith("c"):
+            f.close()
+            raise ValueError(
+                f"Could not determine the header type of the concentration {filename}"
+            )
+        else:
+            result = "head"
+
+    return result
+
+
 class BinaryLayerFile(LayerFile):
     """
     The BinaryLayerFile class is a parent class from which concrete
@@ -699,7 +771,8 @@ def __init__(
             precision = get_headfile_precision(filename)
         if precision == "unknown":
             raise ValueError(f"Error. Precision could not be determined for {filename}")
-        self.header_dtype = BinaryHeader.set_dtype(bintype="Ucn", precision=precision)
+        bintype = get_concentration_file_type(filename, precision)
+        self.header_dtype = BinaryHeader.set_dtype(bintype=bintype, precision=precision)
         super().__init__(filename, precision, verbose, **kwargs)
         return
 

From c82247856cb0a078a312007f2cbf0fb9f39336c8 Mon Sep 17 00:00:00 2001
From: jlarsen-usgs <jlarsen@usgs.gov>
Date: Wed, 25 Feb 2026 14:38:27 -0800
Subject: [PATCH 2/3] linting

---
 flopy/utils/binaryfile/__init__.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/flopy/utils/binaryfile/__init__.py b/flopy/utils/binaryfile/__init__.py
index af9230446..393f1aae9 100644
--- a/flopy/utils/binaryfile/__init__.py
+++ b/flopy/utils/binaryfile/__init__.py
@@ -307,7 +307,7 @@ def get_concentration_file_type(filename: Union[str, PathLike], precision):
         Path of binary MODFLOW file to determine precision.
     precision : str
         double or single
-    
+
     Returns
     -------
     str

From 77b8f1e806719e9afb26d77f6483b0407e186d5e Mon Sep 17 00:00:00 2001
From: jlarsen-usgs <jlarsen@usgs.gov>
Date: Wed, 25 Feb 2026 14:42:31 -0800
Subject: [PATCH 3/3] revert change in __init__.py

---
 flopy/utils/__init__.py | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/flopy/utils/__init__.py b/flopy/utils/__init__.py
index d08b2495c..bd15a59b8 100644
--- a/flopy/utils/__init__.py
+++ b/flopy/utils/__init__.py
@@ -23,14 +23,7 @@
 
 from .utl_import import import_optional_dependency  # isort:skip
 from . import get_modflow as get_modflow_module
-from .binaryfile import (
-    BinaryHeader,
-    CellBudgetFile,
-    ConcentrationFile,
-    HeadFile,
-    HeadUFile,
-    UcnFile,
-)
+from .binaryfile import BinaryHeader, CellBudgetFile, HeadFile, HeadUFile, UcnFile
 from .check import check
 from .flopy_io import read_fixed_var, write_fixed_var
 from .formattedfile import FormattedHeadFile