diff --git a/ProcessLib/GroundwaterFlowProcess.h b/ProcessLib/GroundwaterFlowProcess.h
index 02be590d2cc453f7e21a03f2c4d92ad1a7fa31ef..5f30cba188594dd3086f0693a4648e93e19c5e08 100644
--- a/ProcessLib/GroundwaterFlowProcess.h
+++ b/ProcessLib/GroundwaterFlowProcess.h
@@ -78,7 +78,9 @@ public:
 
             DBUG("Associate hydraulic_head with process variable \'%s\'.",
                 name.c_str());
-            _hydraulic_head = const_cast<ProcessVariable*>(&*variable);
+            this->_process_variables.emplace_back(
+                const_cast<ProcessVariable*>(&*variable));
+            _hydraulic_head = this->_process_variables.back();
         }
 
         // Hydraulic conductivity parameter.
diff --git a/ProcessLib/Process.h b/ProcessLib/Process.h
index cdc9f36bea4ddafd46cc6e48a3288caf34ebffc4..df441d9fcaf85ccb1d675752a1356836d245657e 100644
--- a/ProcessLib/Process.h
+++ b/ProcessLib/Process.h
@@ -197,6 +197,9 @@ protected:
 
 	std::vector<DirichletBc<GlobalIndexType>> _dirichlet_bcs;
 	std::vector<std::unique_ptr<NeumannBc<GlobalSetup>>> _neumann_bcs;
+
+    /// Variables used by this process.
+	std::vector<ProcessVariable*> _process_variables;
 };
 
 }  // namespace ProcessLib