diff --git a/data_migrations/migrations/20260819_0001_drop_unknown_alternate_ids.py b/data_migrations/migrations/20260819_0001_drop_unknown_alternate_ids.py
new file mode 100644
index 000000000..79b157479
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+++ b/data_migrations/migrations/20260819_0001_drop_unknown_alternate_ids.py
@@ -0,0 +1,136 @@
+# ===============================================================================
+# Copyright 2026 ross
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ===============================================================================
+"""
+Remove the unattributed `Unknown` alternate identifiers from the San Acacia
+Reach wells that the automated ingestion pipeline reads.
+
+Each of these wells carries two identifier links: an `NMBGMR` one and an
+`Unknown` one with no recorded provenance. For most wells they agree, and the
+`Unknown` row is redundant. For some they contradict each other -- `SO-0131`
+carries NMBGMR `BRN-E04B (shallow)` and Unknown `BRN-E04A`, while `SO-0132` has
+them the other way round, so the two sources disagree about which physical well
+is which (BDMS-1168).
+
+Removing the unattributed rows leaves NMBGMR as the single answer. That is the
+point: an identifier nobody can source is worse than no identifier, because it
+looks like corroboration.
+
+Scoped to the 38 wells this pipeline ingests, deliberately. The wider reach
+network has 152 such links and every `SO-` well has 263 between them; widening
+this is a separate decision, and 19 of the reach network's links are the
+conflicting ones BDMS-1168 is tracking -- deleting those would remove the
+evidence of the conflict along with the conflict.
+
+Wells are matched by name rather than by id so the migration means the same
+thing in every environment.
+"""
+
+from sqlalchemy import delete, select
+from sqlalchemy.orm import Session
+
+from data_migrations.base import DataMigration
+from db.thing import Thing, ThingIdLink
+
+UNATTRIBUTED = "Unknown"
+
+WELL_NAMES = (
+    "SO-0125",
+    "SO-0131",
+    "SO-0140",
+    "SO-0142",
+    "SO-0144",
+    "SO-0145",
+    "SO-0146",
+    "SO-0148",
+    "SO-0160",
+    "SO-0163",
+    "SO-0165",
+    "SO-0166",
+    "SO-0167",
+    "SO-0170",
+    "SO-0175",
+    "SO-0177",
+    "SO-0189",
+    "SO-0190",
+    "SO-0191",
+    "SO-0194",
+    "SO-0200",
+    "SO-0204",
+    "SO-0213",
+    "SO-0215",
+    "SO-0219",
+    "SO-0221",
+    "SO-0223",
+    "SO-0224",
+    "SO-0226",
+    "SO-0234",
+    "SO-0236",
+    "SO-0238",
+    "SO-0245",
+    "SO-0246",
+    "SO-0247",
+    "SO-0249",
+    "SO-0250",
+    "SO-0261",
+)
+
+
+def run(session: Session) -> None:
+    """Delete the unattributed links, leaving every other organization alone."""
+    thing_ids = session.scalars(
+        select(Thing.id).where(Thing.name.in_(WELL_NAMES))
+    ).all()
+
+    missing = len(WELL_NAMES) - len(thing_ids)
+    if missing:
+        # Not fatal -- a database without these wells is a database this
+        # migration has nothing to do in -- but silence would hide a rename.
+        print(
+            f"  {missing} of {len(WELL_NAMES)} wells not found by name; "
+            "skipping those."
+        )
+
+    if not thing_ids:
+        return None
+
+    result = session.execute(
+        delete(ThingIdLink).where(
+            ThingIdLink.thing_id.in_(thing_ids),
+            ThingIdLink.alternate_organization == UNATTRIBUTED,
+        )
+    )
+    print(
+        f"  removed {result.rowcount} {UNATTRIBUTED!r} links from {len(thing_ids)} wells"
+    )
+    return None
+
+
+MIGRATION = DataMigration(
+    id="20260819_0001_drop_unknown_alternate_ids",
+    alembic_revision="b2c3d4e5f6a7",
+    name="Drop unattributed alternate IDs from San Acacia Reach wells",
+    description=(
+        "Each ingested San Acacia well carries an NMBGMR identifier and an "
+        "unattributed 'Unknown' one. They mostly duplicate, and sometimes "
+        "contradict -- SO-0131 and SO-0132 disagree about which is BRN-E04A "
+        "(BDMS-1168). Removing the unattributed rows leaves one answer."
+    ),
+    run=run,
+    is_repeatable=False,
+)
+
+
+# ============= EOF =============================================