diff --git a/piperider_cli/datasource/bigquery.py b/piperider_cli/datasource/bigquery.py
index ee58c4284..387aa9a45 100644
--- a/piperider_cli/datasource/bigquery.py
+++ b/piperider_cli/datasource/bigquery.py
@@ -1,15 +1,9 @@
-import base64
 import json
 import os
 from typing import List, Optional
 
-import google.auth
 import inquirer
-import sqlalchemy
-from google.api_core import client_info
-from google.auth import impersonated_credentials
-from google.cloud import bigquery
-from google.oauth2 import service_account
+
 from sqlalchemy_bigquery import _helpers
 
 from piperider_cli.error import PipeRiderConnectorError
@@ -33,79 +27,6 @@
 AUTH_METHOD_SERVICE_ACCOUNT = 'service-account'
 AUTH_METHOD_SERVICE_ACCOUNT_JSON = 'service-account-json'
 
-USER_AGENT_TEMPLATE = "sqlalchemy/{}"
-SCOPES = (
-    "https://www.googleapis.com/auth/bigquery",
-    "https://www.googleapis.com/auth/cloud-platform",
-    "https://www.googleapis.com/auth/drive",
-)
-
-target_service_account_email: Optional[str] = None
-
-
-def google_client_info():
-    user_agent = USER_AGENT_TEMPLATE.format(sqlalchemy.__version__)
-    return client_info.ClientInfo(user_agent=user_agent)
-
-
-def create_impersonated_bigquery_client(
-        credentials_info=None,
-        credentials_path=None,
-        credentials_base64=None,
-        default_query_job_config=None,
-        location=None,
-        project_id=None,
-):
-    default_project = None
-
-    if credentials_base64:
-        credentials_info = json.loads(base64.b64decode(credentials_base64))
-
-    if credentials_path:
-        credentials = service_account.Credentials.from_service_account_file(
-            credentials_path
-        )
-        credentials = credentials.with_scopes(SCOPES)
-        default_project = credentials.project_id
-    elif credentials_info:
-        credentials = service_account.Credentials.from_service_account_info(
-            credentials_info
-        )
-        credentials = credentials.with_scopes(SCOPES)
-        default_project = credentials.project_id
-    else:
-        credentials, default_project = google.auth.default(scopes=SCOPES)
-
-    if project_id is None:
-        project_id = default_project
-
-    if target_service_account_email:
-        impersonated_creds = impersonated_credentials.Credentials(
-            source_credentials=credentials,
-            target_principal=target_service_account_email,
-            target_scopes=SCOPES,
-            lifetime=3600  # Duration for which the token is valid (in seconds)
-        )
-        return bigquery.Client(
-            client_info=google_client_info(),
-            project=project_id,
-            credentials=impersonated_creds,
-            location=location,
-            default_query_job_config=default_query_job_config,
-        )
-
-    return bigquery.Client(
-        client_info=google_client_info(),
-        project=project_id,
-        credentials=credentials,
-        location=location,
-        default_query_job_config=default_query_job_config,
-    )
-
-
-# monkey-patch
-_helpers.create_bigquery_client = create_impersonated_bigquery_client
-
 
 class HiddenProjectListFromOAuthField(DataSourceField):
 
@@ -299,7 +220,6 @@ def to_database_url(self, database):
         return f'bigquery://{project}/{dataset}'
 
     def engine_args(self):
-        global target_service_account_email
         args = dict()
         if self.credential.get('method') == AUTH_METHOD_SERVICE_ACCOUNT:
             args['credentials_path'] = self.credential.get('keyfile')
@@ -307,6 +227,11 @@ def engine_args(self):
             args['credentials_info'] = self.credential.get('keyfile_json', {})
 
         target_service_account_email = self.credential.get('impersonate_service_account', None)
+        if target_service_account_email:
+            # monkey-patch
+            import piperider_cli.datasource.bigquery_patch
+            piperider_cli.datasource.bigquery_patch.target_service_account_email = target_service_account_email
+            _helpers.create_bigquery_client = piperider_cli.datasource.bigquery_patch.create_impersonated_bigquery_client
         return args
 
     def verify_connector(self):
diff --git a/piperider_cli/datasource/bigquery_patch.py b/piperider_cli/datasource/bigquery_patch.py
new file mode 100644
index 000000000..9c21eefc9
--- /dev/null
+++ b/piperider_cli/datasource/bigquery_patch.py
@@ -0,0 +1,79 @@
+import base64
+import json
+from typing import Optional
+
+import google.auth
+import sqlalchemy
+from google.api_core import client_info
+from google.auth import impersonated_credentials
+from google.cloud import bigquery
+from google.oauth2 import service_account
+
+USER_AGENT_TEMPLATE = "sqlalchemy/{}"
+SCOPES = (
+    "https://www.googleapis.com/auth/bigquery",
+    "https://www.googleapis.com/auth/cloud-platform",
+    "https://www.googleapis.com/auth/drive",
+)
+
+target_service_account_email: Optional[str] = None
+
+
+def google_client_info():
+    user_agent = USER_AGENT_TEMPLATE.format(sqlalchemy.__version__)
+    return client_info.ClientInfo(user_agent=user_agent)
+
+
+def create_impersonated_bigquery_client(
+        credentials_info=None,
+        credentials_path=None,
+        credentials_base64=None,
+        default_query_job_config=None,
+        location=None,
+        project_id=None,
+):
+    default_project = None
+
+    if credentials_base64:
+        credentials_info = json.loads(base64.b64decode(credentials_base64))
+
+    if credentials_path:
+        credentials = service_account.Credentials.from_service_account_file(
+            credentials_path
+        )
+        credentials = credentials.with_scopes(SCOPES)
+        default_project = credentials.project_id
+    elif credentials_info:
+        credentials = service_account.Credentials.from_service_account_info(
+            credentials_info
+        )
+        credentials = credentials.with_scopes(SCOPES)
+        default_project = credentials.project_id
+    else:
+        credentials, default_project = google.auth.default(scopes=SCOPES)
+
+    if project_id is None:
+        project_id = default_project
+
+    if target_service_account_email:
+        impersonated_creds = impersonated_credentials.Credentials(
+            source_credentials=credentials,
+            target_principal=target_service_account_email,
+            target_scopes=SCOPES,
+            lifetime=3600  # Duration for which the token is valid (in seconds)
+        )
+        return bigquery.Client(
+            client_info=google_client_info(),
+            project=project_id,
+            credentials=impersonated_creds,
+            location=location,
+            default_query_job_config=default_query_job_config,
+        )
+
+    return bigquery.Client(
+        client_info=google_client_info(),
+        project=project_id,
+        credentials=credentials,
+        location=location,
+        default_query_job_config=default_query_job_config,
+    )