diff --git a/components/eamxx/cime_config/namelist_defaults_eamxx.xml b/components/eamxx/cime_config/namelist_defaults_eamxx.xml
index 213c71e712d6..d86ef3162458 100644
--- a/components/eamxx/cime_config/namelist_defaults_eamxx.xml
+++ b/components/eamxx/cime_config/namelist_defaults_eamxx.xml
@@ -296,6 +296,16 @@ be lost if SCREAM_HACK_XML is not enabled.
       <mcsp_v_coeff type="real" doc="MCSP V-wind coefficient"         >0.0</mcsp_v_coeff>
     </zm>
 
+    <!-- Water tracers -->
+    <water_tracers inherit="atm_proc_base">
+      <tracer_count type="integer" doc="Number of water tracers to track (0 = disabled)" constraints="ge 0">0</tracer_count>
+    </water_tracers>
+
+    <!-- Water isotopes (subclass of water tracers with fractionation physics) -->
+    <water_isotopes inherit="atm_proc_base">
+      <tracer_count type="integer" doc="Number of isotope tracers to track (0 = disabled)" constraints="ge 0">0</tracer_count>
+    </water_isotopes>
+    
     <!-- Gravity Wave Drag -->
     <gw inherit="atm_proc_base">
       <use_gw_convect type="logical" doc="Flag for convective GWD">false</use_gw_convect>
diff --git a/components/eamxx/src/physics/CMakeLists.txt b/components/eamxx/src/physics/CMakeLists.txt
index 296b0b8b50bc..2d7ee5afdfe8 100644
--- a/components/eamxx/src/physics/CMakeLists.txt
+++ b/components/eamxx/src/physics/CMakeLists.txt
@@ -20,3 +20,4 @@ if (SCREAM_ENABLE_MAM)
   add_subdirectory(mam)
 endif()
 add_subdirectory(gw)
+add_subdirectory(aux_tracers)
diff --git a/components/eamxx/src/physics/aux_tracers/CMakeLists.txt b/components/eamxx/src/physics/aux_tracers/CMakeLists.txt
new file mode 100644
index 000000000000..99043ce890ea
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/CMakeLists.txt
@@ -0,0 +1,34 @@
+# Auxiliary tracers for EAMxx.
+#
+# This folder groups the "auxiliary tracer" physics processes. Currently,
+# there are two options: passive water tracers and, built on top of them, 
+# water isotopes. Additional tracer classes could be added in an analagous
+# way. The build is governed by these CMake options:
+#
+#   EAMXX_ENABLE_WATER_TRACERS  - build the passive water tracer process
+#   EAMXX_ENABLE_WATER_ISOTOPES - build the water isotope process
+#
+# Water isotopes are implemented on top of the water tracer infrastructure, so
+# they can ONLY be built when water tracers are also enabled. If isotopes are
+# requested without tracers we auto-enable tracers and warn, rather than build
+# an inconsistent configuration.
+
+option(EAMXX_ENABLE_WATER_TRACERS  "Whether to build the EAMxx water tracer process"  OFF)
+option(EAMXX_ENABLE_WATER_ISOTOPES "Whether to build the EAMxx water isotope process" OFF)
+
+# Enforce the dependency: isotopes require tracers.
+if (EAMXX_ENABLE_WATER_ISOTOPES AND NOT EAMXX_ENABLE_WATER_TRACERS)
+  message(STATUS "WARNING: EAMXX_ENABLE_WATER_ISOTOPES=ON requires EAMXX_ENABLE_WATER_TRACERS; "
+                 "auto-enabling water tracers.")
+  set(EAMXX_ENABLE_WATER_TRACERS ON CACHE BOOL "Whether to build the EAMxx water tracer process" FORCE)
+endif()
+
+# Each process lives in its own sibling subdirectory. Descend
+# into each only when the corresponding process is enabled. water_isotopes must
+# be added after water_tracers, since it depends on the water_tracers target.
+if (EAMXX_ENABLE_WATER_TRACERS)
+  add_subdirectory(water_tracers)
+endif()
+if (EAMXX_ENABLE_WATER_ISOTOPES)
+  add_subdirectory(water_isotopes)
+endif()
diff --git a/components/eamxx/src/physics/aux_tracers/water_isotopes/CMakeLists.txt b/components/eamxx/src/physics/aux_tracers/water_isotopes/CMakeLists.txt
new file mode 100644
index 000000000000..cb69f08fc21e
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_isotopes/CMakeLists.txt
@@ -0,0 +1,28 @@
+# Water isotope physics process.
+#
+# This directory is only entered when EAMXX_ENABLE_WATER_ISOTOPES is ON, 
+# which in turn guarantees EAMXX_ENABLE_WATER_TRACERS is ON
+# (isotopes are layered on top of the water tracer infrastructure). Following
+# the EAMxx one-library-per-process pattern, the water isotope process gets its
+# own library.
+
+add_library(water_isotopes
+  eamxx_water_isotopes_process_interface.cpp
+)
+
+# Public compile definition so the registration guard in register_physics.hpp
+# becomes active.
+target_compile_definitions(water_isotopes PUBLIC EAMXX_HAS_WATER_ISOTOPES)
+
+# Expose this directory
+target_include_directories(water_isotopes PUBLIC
+  ${CMAKE_CURRENT_SOURCE_DIR}
+)
+
+target_link_libraries(water_isotopes PUBLIC scream_share)
+target_link_libraries(water_isotopes PRIVATE water_tracers)
+
+# Link into the eamxx_physics INTERFACE target.
+if (TARGET eamxx_physics)
+  target_link_libraries(eamxx_physics INTERFACE water_isotopes)
+endif()
diff --git a/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.cpp b/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.cpp
new file mode 100644
index 000000000000..dffd1a98ac2a
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.cpp
@@ -0,0 +1,23 @@
+#include "eamxx_water_isotopes_process_interface.hpp"
+
+namespace scream
+{
+
+// =========================================================================================
+WaterIsotopes::WaterIsotopes(const ekat::Comm& comm, const ekat::ParameterList& params)
+  : WaterTracers(comm, params)
+{
+  // Water isotopes will inherit all tracer handling from WaterTracers
+  // No additional initialization needed at this stage
+}
+
+// =========================================================================================
+void WaterIsotopes::run_impl(const double dt)
+{
+  // Call base class tracer physics (currently a no-op)
+  WaterTracers::run_impl(dt);
+
+  // TODO: Add fractionation physics here
+}
+
+} // namespace scream
diff --git a/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.hpp b/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.hpp
new file mode 100644
index 000000000000..e3249e9f1dc2
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.hpp
@@ -0,0 +1,46 @@
+#ifndef SCREAM_WATER_ISOTOPES_HPP
+#define SCREAM_WATER_ISOTOPES_HPP
+
+#include "physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.hpp"
+#include "ekat/ekat_parameter_list.hpp"
+
+#include <string>
+
+namespace scream
+{
+/*
+ * The class responsible to handle water isotope transport and fractionation
+ *
+ * This process extends WaterTracers to add equilibrium and kinetic fractionation
+ * during phase changes for water isotope species.
+ *
+ * By inheriting from WaterTracers, this class reuses all tracer field handling
+ * and only needs to override specific fractionation hooks.
+ *
+ * Note: This is a stub implementation that registers the process. Fractionation
+ * physics will be added later.
+*/
+
+class WaterIsotopes : public WaterTracers
+{
+public:
+  // Constructors
+  WaterIsotopes (const ekat::Comm& comm, const ekat::ParameterList& params);
+
+  // Override the name to distinguish from base WaterTracers
+  std::string name () const override { return "water_isotopes"; }
+
+protected:
+
+  // Override run_impl to add fractionation physics
+  // For now, just calls base class implementation
+  void run_impl (const double dt) override;
+
+  // TODO: Add virtual hooks for fractionation processes
+  // e.g., apply_equilibrium_fractionation(), apply_kinetic_fractionation()
+
+}; // class WaterIsotopes
+
+} // namespace scream
+
+#endif // SCREAM_WATER_ISOTOPES_HPP
diff --git a/components/eamxx/src/physics/aux_tracers/water_tracers/CMakeLists.txt b/components/eamxx/src/physics/aux_tracers/water_tracers/CMakeLists.txt
new file mode 100644
index 000000000000..5607cd5036ae
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_tracers/CMakeLists.txt
@@ -0,0 +1,28 @@
+# Water tracer physics process.
+#
+# This directory is only entered when EAMXX_ENABLE_WATER_TRACERS is ON. 
+# Following the EAMxx one-library-per-process pattern, the
+# water tracer process gets its own library. The sibling water_isotopes library
+# depends on this one.
+
+add_library(water_tracers
+  eamxx_water_tracers_process_interface.cpp
+)
+
+# Public compile definition so the registration guard in register_physics.hpp
+# becomes active.
+target_compile_definitions(water_tracers PUBLIC EAMXX_HAS_WATER_TRACERS)
+
+# Expose this directory to allow includes of the
+# process interface headers.
+target_include_directories(water_tracers PUBLIC
+  ${CMAKE_CURRENT_SOURCE_DIR}
+)
+
+# Link required EAMxx libraries.
+target_link_libraries(water_tracers scream_share)
+
+# Link into the eamxx_physics INTERFACE target.
+if (TARGET eamxx_physics)
+  target_link_libraries(eamxx_physics INTERFACE water_tracers)
+endif()
diff --git a/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.cpp b/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.cpp
new file mode 100644
index 000000000000..729ed701edfb
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.cpp
@@ -0,0 +1,51 @@
+#include "eamxx_water_tracers_process_interface.hpp"
+
+#include <ekat_assert.hpp>
+
+namespace scream
+{
+
+// =========================================================================================
+WaterTracers::WaterTracers(const ekat::Comm& comm, const ekat::ParameterList& params)
+  : AtmosphereProcess(comm, params)
+  , m_tracer_count(0)
+{
+  // Read tracer count from parameter list (default 0)
+  m_tracer_count = m_params.get<int>("tracer_count", 0);
+}
+
+// =========================================================================================
+void WaterTracers::create_requests()
+{
+  // Get the grid for this process
+  m_grid = m_grids_manager->get_grid("physics");
+  m_num_cols = m_grid->get_num_local_dofs(); // Number of columns on this rank
+  m_num_levs = m_grid->get_num_vertical_levels();  // Number of levels per column
+
+  // TODO: Define field requests for water tracer arrays
+  // For now, this process requires no fields and computes no fields
+  // Field definitions will be added later
+}
+
+// =========================================================================================
+void WaterTracers::initialize_impl(const RunType /* run_type */)
+{
+  // TODO: Initialize tracer field arrays and any precomputed data
+  // For now, this is a no-op since no fields are defined yet
+}
+
+// =========================================================================================
+void WaterTracers::run_impl(const double /* dt */)
+{
+  // TODO: Implement tracer transport and physics
+  // This stub implementation performs no operations
+  // Tracer physics will be added in subsequent PRs
+}
+
+// =========================================================================================
+void WaterTracers::finalize_impl()
+{
+  // Nothing to finalize at this stage
+}
+
+} // namespace scream
diff --git a/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.hpp b/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.hpp
new file mode 100644
index 000000000000..6d8e36e9b2fe
--- /dev/null
+++ b/components/eamxx/src/physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.hpp
@@ -0,0 +1,60 @@
+#ifndef SCREAM_WATER_TRACERS_HPP
+#define SCREAM_WATER_TRACERS_HPP
+
+#include "share/atm_process/atmosphere_process.hpp"
+#include "ekat/ekat_parameter_list.hpp"
+
+#include <string>
+
+namespace scream
+{
+/*
+ * The class responsible to handle water tracer transport through the atmosphere
+ *
+ * This process manages additional water species that track through the model
+ * without undergoing fractionation. Water isotopes (a special case with
+ * fractionation) are handled by the WaterIsotopes subclass.
+ *
+ * Note: This is a stub implementation that registers the process and sets up
+ * the basic infrastructure. Field definitions and physics implementation are
+ * deferred to later specs in the water isotope campaign.
+*/
+
+class WaterTracers : public AtmosphereProcess
+{
+public:
+  using KT = ekat::KokkosTypes<DefaultDevice>;
+
+  // Constructors
+  WaterTracers (const ekat::Comm& comm, const ekat::ParameterList& params);
+
+  // The type of subcomponent
+  AtmosphereProcessType type () const { return AtmosphereProcessType::Physics; }
+
+  // The name of the subcomponent
+  virtual std::string name () const { return "water_tracers"; }
+
+  // Create grid-dependent field requests
+  void create_requests ();
+
+protected:
+
+  // The three main overrides for the subcomponent
+  void initialize_impl (const RunType run_type);
+  void run_impl        (const double dt);
+  void finalize_impl   ();
+
+  // Keep track of field dimensions
+  int m_num_cols;
+  int m_num_levs;
+
+  // Number of tracers to track (from parameter list)
+  int m_tracer_count;
+
+  std::shared_ptr<const AbstractGrid> m_grid;
+
+}; // class WaterTracers
+
+} // namespace scream
+
+#endif // SCREAM_WATER_TRACERS_HPP
diff --git a/components/eamxx/src/physics/register_physics.hpp b/components/eamxx/src/physics/register_physics.hpp
index 75b88bd73022..b61c37b4a2ee 100644
--- a/components/eamxx/src/physics/register_physics.hpp
+++ b/components/eamxx/src/physics/register_physics.hpp
@@ -53,6 +53,12 @@
 #ifdef EAMXX_HAS_CLD_FRAC_NET
 #include "physics/cld_fraction/cld_frac_net/eamxx_cld_frac_net_process_interface.hpp"
 #endif
+#ifdef EAMXX_HAS_WATER_TRACERS
+#include "physics/aux_tracers/water_tracers/eamxx_water_tracers_process_interface.hpp"
+#endif
+#ifdef EAMXX_HAS_WATER_ISOTOPES
+#include "physics/aux_tracers/water_isotopes/eamxx_water_isotopes_process_interface.hpp"
+#endif
 
 namespace scream {
 
@@ -106,6 +112,12 @@ inline void register_physics () {
 #ifdef EAMXX_HAS_CLD_FRAC_NET
   proc_factory.register_product("cld_frac_net",&create_atmosphere_process<CldFracNet>);
 #endif
+#ifdef EAMXX_HAS_WATER_TRACERS
+  proc_factory.register_product("water_tracers",&create_atmosphere_process<WaterTracers>);
+#endif
+#ifdef EAMXX_HAS_WATER_ISOTOPES
+  proc_factory.register_product("water_isotopes",&create_atmosphere_process<WaterIsotopes>);
+#endif
 
   // If no physics was enabled, silence compile warning about unused var
   (void) proc_factory;