diff --git a/dataretrieval/waterdata/utils.py b/dataretrieval/waterdata/utils.py
index 018e1c85..af5906e0 100644
--- a/dataretrieval/waterdata/utils.py
+++ b/dataretrieval/waterdata/utils.py
@@ -153,6 +153,10 @@ def _switch_properties_id(properties: list[str] | None, id_name: str, service: s
     {"datetime", "last_modified", "begin", "begin_utc", "end", "end_utc", "time"}
 )
 
+# Services that don't support comma-separated values for multi-value GET
+# parameters and require POST with CQL2 JSON instead.
+_CQL2_REQUIRED_SERVICES = frozenset({"monitoring-locations"})
+
 
 def _parse_datetime(value: str) -> datetime | None:
     """Parse a single datetime string against the supported formats.
@@ -417,9 +421,11 @@ def _construct_api_requests(
     """
     Constructs an HTTP request object for the specified water data API service.
 
-    Depending on the input parameters (whether there's lists of multiple
-    argument values), the function determines whether to use a GET or POST
-    request, formats parameters appropriately, and sets required headers.
+    For most services, list parameters are comma-joined and sent as a single
+    GET request (e.g. ``parameter_code=["00060","00010"]`` becomes
+    ``parameter_code=00060,00010`` in the URL). For services that do not
+    support comma-separated values (currently only ``monitoring-locations``),
+    a POST request with CQL2 JSON is used instead.
 
     Parameters
     ----------
@@ -445,37 +451,38 @@ def _construct_api_requests(
     Notes
     -----
     - Date/time parameters are automatically formatted to ISO8601.
-    - If multiple values are provided for non-single parameters, a POST request
-    is constructed.
-    - The function sets appropriate headers for GET and POST requests.
     """
     service_url = f"{OGC_API_URL}/collections/{service}/items"
 
-    # Identify which parameters should be included in the POST content body
-    post_params = {
-        k: v
-        for k, v in kwargs.items()
-        if k not in _DATE_RANGE_PARAMS and isinstance(v, (list, tuple)) and len(v) > 1
-    }
+    # Format date/time parameters to ISO8601 first — both routing paths need it.
+    for key in _DATE_RANGE_PARAMS:
+        if key in kwargs:
+            kwargs[key] = _format_api_dates(
+                kwargs[key],
+                date=(service == "daily" and key != "last_modified"),
+            )
 
-    # Everything else goes into the params dictionary for the URL
-    params = {k: v for k, v in kwargs.items() if k not in post_params}
-    # Set skipGeometry parameter (API expects camelCase)
-    params["skipGeometry"] = skip_geometry
+    if service in _CQL2_REQUIRED_SERVICES:
+        # POST with CQL2 JSON: multi-value params go in the request body.
+        post_params = {
+            k: v
+            for k, v in kwargs.items()
+            if k not in _DATE_RANGE_PARAMS
+            and isinstance(v, (list, tuple))
+            and len(v) > 1
+        }
+        params = {k: v for k, v in kwargs.items() if k not in post_params}
+    else:
+        # GET with comma-separated values: join list/tuple values into one string.
+        post_params = {}
+        params = {
+            k: ",".join(str(x) for x in v) if isinstance(v, (list, tuple)) else v
+            for k, v in kwargs.items()
+        }
 
-    # If limit is none or greater than 50000, then set limit to max results. Otherwise,
-    # use the limit
+    params["skipGeometry"] = skip_geometry
     params["limit"] = 50000 if limit is None or limit > 50000 else limit
 
-    # Indicate if function needs to perform POST conversion
-    POST = bool(post_params)
-
-    # Convert dates to ISO08601 format
-    for i in _DATE_RANGE_PARAMS:
-        if i in params:
-            dates = service == "daily" and i != "last_modified"
-            params[i] = _format_api_dates(params[i], date=dates)
-
     # `len()` instead of truthiness: a numpy ndarray would raise on `if bbox:`.
     if bbox is not None and len(bbox) > 0:
         params["bbox"] = ",".join(map(str, bbox))
@@ -490,7 +497,7 @@ def _construct_api_requests(
 
     headers = _default_headers()
 
-    if POST:
+    if post_params:
         headers["Content-Type"] = "application/query-cql-json"
         request = requests.Request(
             method="POST",
diff --git a/tests/waterdata_test.py b/tests/waterdata_test.py
index e2ba4da8..40ea0615 100644
--- a/tests/waterdata_test.py
+++ b/tests/waterdata_test.py
@@ -30,6 +30,7 @@
 from dataretrieval.waterdata.utils import (
     _check_monitoring_location_id,
     _check_profiles,
+    _construct_api_requests,
     _normalize_str_iterable,
 )
 
@@ -111,6 +112,69 @@ def test_check_profiles():
         _check_profiles(service="results", profile="foo")
 
 
+def test_construct_api_requests_multivalue_get():
+    """Multi-value params use GET with comma-separated values for daily service."""
+    req = _construct_api_requests(
+        "daily",
+        monitoring_location_id=["USGS-05427718", "USGS-05427719"],
+        parameter_code=["00060", "00065"],
+    )
+    assert req.method == "GET"
+    assert "monitoring_location_id=USGS-05427718%2CUSGS-05427719" in req.url
+    assert "parameter_code=00060%2C00065" in req.url
+
+
+def test_construct_api_requests_monitoring_locations_post():
+    """monitoring-locations uses POST+CQL2 for multi-value params (API limitation)."""
+    req = _construct_api_requests(
+        "monitoring-locations",
+        hydrologic_unit_code=["010802050102", "010802050103"],
+    )
+    assert req.method == "POST"
+    assert req.body is not None
+
+
+def test_construct_api_requests_single_value_stays_get():
+    """A length-1 list (or scalar) reaches the URL as a plain value, not a
+    comma-separated form, so existing single-site callers see no change."""
+    req = _construct_api_requests(
+        "daily",
+        monitoring_location_id="USGS-05427718",
+        parameter_code="00060",
+    )
+    assert req.method == "GET"
+    assert "monitoring_location_id=USGS-05427718" in req.url
+    assert "%2C" not in req.url  # no comma-encoded multi-value
+
+
+def test_construct_api_requests_numeric_list_joins_with_str():
+    """Numeric-list params (e.g. ``water_year=[2020, 2021]`` on get_peaks)
+    must reach the URL as a comma-joined string, not crash on ``",".join``
+    of ints. The generator-of-``str(x)`` exists exactly for this case."""
+    req = _construct_api_requests(
+        "peaks",
+        monitoring_location_id="USGS-05427718",
+        water_year=[2020, 2021],
+    )
+    assert req.method == "GET"
+    assert "water_year=2020%2C2021" in req.url
+
+
+def test_construct_api_requests_two_element_date_list_becomes_interval():
+    """A two-element date list is interpreted as start/end of an OGC datetime
+    interval (joined with '/'), NOT as two discrete dates. The OGC `datetime`
+    parameter does not support "these N specific dates" — that would require
+    a CQL filter. Verifying so this contract is locked in."""
+    req = _construct_api_requests(
+        "daily",
+        monitoring_location_id="USGS-05427718",
+        time=["2024-01-01", "2024-01-31"],
+    )
+    assert req.method == "GET"
+    # `/` URL-encodes to %2F. Confirms _format_api_dates ran before the join.
+    assert "time=2024-01-01%2F2024-01-31" in req.url
+
+
 def test_samples_results():
     """Test results call for proper columns"""
     df, _ = get_samples(