Download the full Illicit Discharge Rule (PDF).

Adopted April 11, 2024
Effective April 29, 2024


(a) The District manages several pipe or channel stormwater conveyance systems within its boundaries. These systems are termed municipal separate storm sewer systems (MS4s) under the federal Clean Water Act. As an MS4 operator, the District regulates connections to, and discharges of pollutants to, its MS4 systems.

(b) This rule applies only within those areas of the watershed that drain to the District’s MS4s. The District’s MS4s are shown on Addendum A.  The areas that drain to the MS4s are delineated on a map maintained by the District in electronic format that may be accessed through the District website. The District may delineate these areas more precisely, as necessary, on the basis of local conveyance connections and flow conditions.


As used in this rule, these terms have the following meanings:

(a) “Direct Connection” is: (i) a physical connection to an enclosed MS4 conveyance; or (ii) a conduit or similar point-source structure that outlets into or adjacent to an open MS4 conveyance, by which the discharge is introduced into the MS4.

(b) “Illicit Discharge” is a discharge, other than Stormwater or a Non-Regulated Discharge, into an MS4.

(c) “Indirect Connection” is a discharge outside of a closed structure, onto the ground or a surface, whereby through action of gravity, or of runoff under foreseeable conditions of rainfall or snowmelt, the discharge reasonably may be expected to enter an MS4 directly, or by means of a public stormwater conveyance. 

(d) “Non-Regulated Discharge” is one of the following:

  1. Flushing of a water line or another potable water source; landscape irrigation; diverted stream flow; rising ground water; ground water infiltration into a storm drain; uncontaminated groundwater; foundation or footing drains (not including active groundwater dewatering systems); crawl space pump discharge; air conditioning condensation; springs; non-commercial vehicle washing; natural riparian habitat or wetland flows; dechlorinated swimming pool discharge; and street wash water;
  2. Discharge pursuant to an NPDES permit;
  3. Discharge resulting from firefighting activity, or that the District, in writing, specifically has exempted as necessary to protect public health and safety;
  4. Dye testing, with prior written notice to the District;
  5. A discharge associated only with a residential property use.

(e) “Stormwater” is stormwater runoff, snow melt runoff, and surface runoff and drainage.


(a) Illicit Discharges are prohibited.

(b) A Direct Connection that inlets directly to an MS4 inside a closed structure is prohibited, unless it is constructed so that the discharge consists entirely of a Non-Regulated Discharge.  The property owner or operator is responsible to determine whether any drain, fixture or other point of discharge within a structure is prohibited under this paragraph, and if so to discontinue the connection or outlet.  The owner or operator must keep a record of this determination, which the District may inspect on request.

(c) A Direct Connection that inlets directly to an MS4 outside of a closed structure is permitted pursuant to owner or operator notice and District written approval.  As a condition of approval, the District may require that the owner or operator maintain structural and non-structural practices to limit the risk of Illicit Discharge.  A Direct Connection constructed so that the discharge consists entirely of a Non-Regulated Discharge is not subject to this paragraph.

(d) An owner or operator may maintain an Indirect Connection without notice to the District or District approval.  However, on a determination by the District Board of Managers, after an opportunity to be heard, that an existing or proposed use of the property presents a risk of Illicit Discharge, it may require that the owner or operator maintain structural and non-structural practices to limit that risk.

(e) The prohibitions and restrictions of this section apply to new and existing Direct and Indirect Connections, including those made before this rule was adopted, and regardless of whether a connection was permitted under applicable law at the time of its construction.


An owner or operator of a property where an Illicit Discharge has occurred promptly will take all feasible actions to minimize the discharge into the downgradient MS4, and notify the District in writing.  The owner or operator will be responsible for costs incurred by the District to limit the impact of an Illicit Discharge on the MS4, on any downgradient waterbody, and on any beneficial public use thereof.


The following exhibits must accompany a notice under paragraph 3.c, above:

(a) Parcel delineation identifying ownership and easements.

(b) Proposed and existing stormwater facilities with flow alignment and elevations.

(c) Existing and proposed site elevations with no more than a 2-foot contour interval.

(d) Proposed connection design details, connection method, and schedule.

(e) Stormwater runoff volume and rate analysis for the two-, 10-, and 100-year critical events, existing and proposed conditions.