Wet Weather Discharges

Wet weather discharges refers collectively to discharges that result from precipitation events, such as rainfall and snowmelt. The primary sources of these discharges are storm water runoff from paved surfaces and other impervious areas, agricultural land and animal feeding operations, municipal separate storm sewer systems (MS4), combined sewer overflows (CSO) and sanitary sewer overflows (SSO).

Storm water discharges from MS4s are a major concern in urbanized areas due to the high concentration of pollutants found in these discharges. Urbanized areas, because of dense development, have a high concentration of impervious surfaces, such as city streets, driveways, parking lots, and sidewalks, on which pollutants settle and remain until a storm event washes them into nearby storm drains. The most common pollutants include pesticides, fertilizers, oils, salts, litter and other debris, and sediment.

About the MS4 Rule

The federal Clean Water Act (CWA) requires storm water discharges from certain types of urbanized areas to be permitted under the National Pollutant Discharge Elimination System (NPDES) program. In 1990, Phase I of these requirements became effective, and municipalities with a population served by a municipal separate storm sewer system (MS4) of 100,000, or more, were regulated. In 1999, Phase II became effective, and any entity responsible for an MS4 conveyance, regardless of population size, could potentially be regulated.

In Indiana, storm water discharge permits are issued by the Indiana Department of Environmental Management (IDEM). Under Phase I requirements, only the City of Indianapolis met the designation criteria, and was issued an individual NPDES storm water permit. To comply with Phase II requirements, a new general NPDES permit rule was written. The new general permit rule, referred to as Rule 13, provided permit coverage for most Phase II MS4 entities.

A general permit is a single permit that is written to cover multiple permittees with similar characteristics. No written draft permit is issued to the permittee under a general permit. Instead, the requirements and conditions of this type of permit are found in Indiana Administrative Code, under the appropriate general permit rule. Notice of Intent (NOI) letters for this type of permit can be by either a single MS4 entity, or multiple MS4 entities. The City of Terre Haute, Vigo County, Seelyville, West Terre Haute, Indiana State University, Ivy Tech Community College, and Rose Hulman Institute of Technology are co-permittees of a general permit with the City of Terre Haute taking the lead position within the group for reporting and communications with IDEM.

The SWQMP must address six minimum control measures (MCM). Each minimum control measure must include a detailed program description, a timetable for implementation, milestones, and a summary of measurable goals. The measurable goals shall demonstrate results that relate to an environmental benefit. The six minimum control measures provide the minimum conditions for the storm water program.

MS4 Minimum Control Measures

  • 1 Public Education and Outreach - distruibuting educational materials and performing outreach to inform citizens about the impacts stormwater runoff can have on water quality
  • 2 Public Participation and Involvement - providing opportunites for citizens to participate and provide input in the development and implementation of the stormwater management program.
  • 3 Illicit Discharge Detection and Elimination - developing and implementing a strategy to detect and eliminate illicit discharges to the storm sewer system.
  • 4 Construction Site Stormwater Runoff Control - developing, implementing, and enforcing an erosion and sediment control program for construction activities.
  • 5 Post-Construction Stormwater Runoff Control - developing and enforcing a program to address discharges of post-construction stormwater runoff from new development and redevelopment areas.
  • 6 Municipal Operations Pollution Prevention and Good Housekeeping - developing and implementing a program to prevent and reduce pollutant runoff from municipal operations.

Strategies for reducing stormwater runoff and improving water quality


Infiltration allows stormwater to seep into the soil rather than enter the storm drain system or sewer system


Stormwater is diverted from impervious surfaces into areas with plants and compost-amended soil so that the stormwater can seep slowly into ground.


Ponds, pipes, vaults, and cisterns temporarily hold stormwater runoff during a storm and then slowly discharge the stormwater at a reduced rate.

  • Every year millions of gallons of combined sewage, stormwater, and debris overflow to the Wabash River.
  • The overflows cause increased E. coli readings and other pollution to the river.
  • The City of Terre Haute is undertaking a program to reduce the number of overflows to the river.
  • The goal of the CSO Reduction Program is to have a cleaner city, resulting in a cleaner river.