diff --git a/schemas/aquifer_system.py b/schemas/aquifer_system.py
index d06150ecf..01dbcbfd0 100644
--- a/schemas/aquifer_system.py
+++ b/schemas/aquifer_system.py
@@ -1,13 +1,13 @@
 from typing import List
 
 from pydantic import BaseModel
-
-from core.enums import GeographicScale, AquiferType
 from schemas import BaseResponseModel
+from schemas.validators import GeometryMixin
+from core.enums import AquiferType, GeographicScale  # Import specific Enums
 
 
 # ------ CREATE ----------
-class CreateAquiferSystem(BaseModel):
+class CreateAquiferSystem(BaseModel, GeometryMixin):
     """
     Schema for creating an aquifer system.
     Used during data transfer and API creation.
@@ -16,8 +16,8 @@ class CreateAquiferSystem(BaseModel):
     name: str
     description: str | None = None
     primary_aquifer_type: AquiferType
-    geographic_scale: GeographicScale | None = None  # e.g., "Regional", "Local", etc.
-    boundary: str | None = None
+    geographic_scale: GeographicScale | None = None
+    # boundary field inherited from GeometryMixin
 
 
 # ------ RESPONSE ----------
diff --git a/schemas/geologic_formation.py b/schemas/geologic_formation.py
index 62e24ee8e..de479bf29 100644
--- a/schemas/geologic_formation.py
+++ b/schemas/geologic_formation.py
@@ -1,22 +1,46 @@
 from typing import List
 
-from pydantic import BaseModel
+from pydantic import BaseModel, field_validator, Field
 
 from schemas import BaseResponseModel
+from schemas.validators import DepthIntervalMixin, GeometryMixin
 from core.enums import FormationCode, Lithology
 
 
 # ------ CREATE ----------
-class CreateGeologicFormation(BaseModel):
+class CreateGeologicFormation(BaseModel, GeometryMixin):
     """
     Schema for creating a geologic formation.
     Used during data transfer and API creation.
     """
 
+    # formation_code has its own custom uppercase validator
     formation_code: FormationCode | None = None
     description: str | None = None
     lithology: Lithology | None = None
-    boundary: str | None = None
+    # boundary: inherited from GeometryMixin
+
+    @field_validator("formation_code", mode="before")
+    @classmethod
+    def upper_case_code(cls, v: str | None) -> str | None:
+        """
+        Automatically uppercase the formation code.
+        """
+        if isinstance(v, str):
+            return v.upper()
+        return v
+
+
+class CreateThingGeologicFormationAssociation(BaseModel, DepthIntervalMixin):
+    """
+    Schema for linking a Thing (Well) to a GeologicFormation.
+    Uses DepthIntervalMixin to enforce bottom_depth > top_depth.
+    """
+
+    thing_id: int
+    geologic_formation_id: int
+    top_depth: float = Field(ge=0)
+    bottom_depth: float = Field(ge=0)
 
 
 # ------ RESPONSE ----------
diff --git a/schemas/validators.py b/schemas/validators.py
new file mode 100644
index 000000000..fde828770
--- /dev/null
+++ b/schemas/validators.py
@@ -0,0 +1,43 @@
+"""
+schemas/validators.py
+Reusable Pydantic validators and mixins for aquifer and geology related schemas.
+May consider expansion for other domain models in the future.
+"""
+
+from pydantic import model_validator, field_validator, BaseModel, ValueError, Field
+from services.validation.geospatial import validate_wkt_geometry
+
+
+class DepthIntervalMixin(BaseModel):
+    """
+    Mixin to enforce:
+    1. Depths are non-negative (via Field constraints).
+    2. Bottom depth > top depth (via model_validator).
+    Assumes the model has 'top_depth' and 'bottom_depth' fields.
+    """
+
+    top_depth: float = Field(ge=0)
+    bottom_depth: float = Field(ge=0)
+
+    @model_validator(mode="after")
+    def check_depth_logical_order(self) -> "DepthIntervalMixin":
+        if self.bottom_depth <= self.top_depth:
+            raise ValueError(
+                f"Bottom depth ({self.bottom_depth}) must be greater "
+                f"than top depth ({self.top_depth})"
+            )
+        return self
+
+
+class GeometryMixin(BaseModel):
+    """
+    Mixin to validate WKT strings for boundary fields.
+    Delegates logic to the validate_wkt_geometry service function.
+    """
+
+    boundary: str | None = None
+
+    @field_validator("boundary")
+    @classmethod
+    def validate_wkt(cls, v: str | None) -> str | None:
+        return validate_wkt_geometry(v)