The MPCA, working with stakeholders, developed a checklist for conducting an inventory of bacteria sources. The checklist is not a permit requirement but may be used to meet permit requirements. This page provides guidance and supporting information for the checklist.
Link to the checklist: File:Checklist for bacteria source inventory.xlsx
The checklist contains four worhsheets.
These are discussed below.
This sheet provides basic general information about the checklist.
This is the sheet where users enter information. Column A contains different categories of bacteria sources and specific sources within each category. The categories include the following.
Columns B through G are where users input information.
This section provides some basic information about each of the potential sources included in the checklist. Where appropriate we include links where additional information can be found.
Runoff from impervious surfaces typically contains high concentrations of bacteria. The impervious surface, however, is not the source of the bacteria; it is the conveyance mechanism.
Ultimately the source of bacteria in urban stormwater is animal waste. Identifying the specific source is more challenging and likely varies with location and land use. Typical sources include domestic pets and wildlife, particularly birds. Sources of bacteria to receiving waters include urban stormwater runoff, leaking sewer lines, sewer overflows, septic systems, landfills, marinas and pumpout facilities, poorly operating packing plants, and other illicit discharges.
Some general observations from the literature are summarized below.
Land use | Median (MPN/ml) | Maximum (MPN/ml) |
---|---|---|
Commercial | 6900 | 350000 |
Industrial | 9700 | 290000 |
Residential | 20000 | 600000 |
Rooftop | 1250 | |
Open space | 4500 | |
Forested | < 100 |
Sewage typically contains fecal coliform concentrations in excess of one million most probable number per 100 milliliter (MPN/100 ml). This is about two orders of magnitude greater than urban stormwater concentrations. General indicators of bacteria sources include the following.
A complicating factor is that bacteria can survive and grow both within the storm sewer system and within receiving waters. Growth within the storm sewer systems includes both the surface and subsurface conveyances. For example, coliform bacteria have been found to survive and grow in moist soils and leaf piles. A recent study in Minneapolis indicated that catch basins are an important source, largely as a result of growth within the catch basin.
The following table provides a summary of data from the literature. Maximum concentrations are included to illustrate the tremendous variability that may occur in bacteria concentrations. The values represent a compilation of data from several sources (see references at the bottom of this page).
The following links go to tables providing management strategies for different bacteria sources.
RV dumping or leaking into the storm drains, particularly at vacation destinations, can contribute bacteria and pathogens to the storm sewer system, either by directly dumping into storm drains or by dumping onto impermeable surfaces that drain to the storm sewer system.
Combined sewer systems (CSS) are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. During wet weather conditions, the volume of stormwater runoff can exceed the capacity of the CSS infrastructure, including both the piping system and/or the treatment plant. This is common where where older CSS infrastructure is in place, combining sanitary and storm drain flows. When the capacity of the CSS is exceeded, excess water can bypass treatment and bypassed flow will be mixed with the treated water prior to disinfection and just prior to discharge to the receiving water. While both stormwater runoff and sanitary sewer discharges contain bacteria, concentrations are much higher in untreated sewage. Thus, CSOs can lead to very high concentrations of bacteria in water delivered to receiving waters. For more information, see [3], [4], [5], [6], [7].
Sanitary sewer systems collect and transport domestic, commercial, and industrial wastewater. Some sanitary sewer systems may contain limited amounts of stormwater and infiltrated ground water to treatment facilities, but they are designed to carry just sewage and industrial wastewater. Occasionally, sanitary sewers will release raw sewage. These types of releases are called sanitary sewer overflows (SSOs). Possible causes of SSOs include blockages, line breaks, sewer defects that allow stormwater and groundwater to overload the system, power failures, improper sewer design, and vandalism. The adjacent image indicates line blockages appear to be an important cause of SSOs. An EPA survey indicated SSOs are most common during wet weather flows.
SSOs that discharge to surface waters are considered point source discharges and are thus prohibited unless authorized by a NPDES permit. SSOs may be indicative of improper operation and maintenance of the sewer systems, and may violate NPDES permit conditions. Effective illicit discharge and elimination practices and programs minimize the occurrence and impacts from SSOs. Other management strategies include the following.
For more information, see [8], [9], [10], [11]
Groundwater infiltration and rainfall derived inflow and infiltration, commonly referred to as inflow, are components of most sanitary sewer systems. These inputs, called I&I, should be a minor component of the sanitary sewer flow. However, infiltration and inflow may be considered excessive when it is the cause of overflows (SSO) or bypasses in the sanitary sewer system. In addition, I&I is treated at wastewater treatment plants even though it is relatively clean water, resulting in extra cost.
EPA provides guidance for estimating the input and importance of I&I in a sanitary sewer system. To assess extraneous water entering your system at least a year of influent flow data to the treatment facility should be examined. For infiltration analysis, flow data collected during the high groundwater periods is used. For inflow analysis, the Average Wet Weather (AWW) flow can be estimated from flow data for a one week period when there has been significant rain. If a single storm event is used to analyze wet weather inflow, it should be an event large enough to cause surface ponding and runoff. Other techniques can be used to determine the presence of I&I, including manhole testing and maintenance, dye tracing, smoke testing, use of cameras, and private property inspection for improper connections. Examples of improper connections include downspouts, groundwater sump pumps, foundation drains, drains from window wells and outdoor basement stairwells and drains from driveways.
Proper evaluation of I&I sources is necessary to manage them. Once detected, they can be eliminated or minimized. Several cities, for example, have programs and ordinances prohibiting illicit connections on private property.
For more information, see [12], [13], [14], [15]
The Municipal Separate Storm Sewer System (MS4) General Permit defines illicit discharge as "any discharge to a municipal separate storm sewer that is not composed entirely of stormwater except discharges pursuant to a NPDES permit (other than the NPDES permit for discharges from the municipal separate storm sewer) and discharges resulting from firefighting activities". Not all illicit discharges contain bacteria and pathogens, but examples of illicit discharge sources containing bacteria and pathogens include sewage and septage, washwater, irrigation water, some commercial and industrial water,
Illicit discharges to the storm sewer system can be classified as direct or indirect. Direct discharges are directly connected to the storm drain pipe through a sewage pipe, shop drain, or other kind of pipe. Direct discharges are typically continuous or intermittent and occur when two different kinds of “plumbing” are improperly connected. The three main situations where this occurs are sewage cross-connections, straight pipes, and industrial and commercial cross-connections. Indirect discharges are generated outside the storm drain system and enter through storm drain inlets or by infiltrating through the joints of the pipe. Generally, indirect discharges are intermittent or transitory, with the exception of groundwater seepage. Indirect discharges include groundwater seepage, spills, dumping, outdoor washing, and irrigation.
Certain types of illicit discharges can be associated with specific land uses. For example, failing septic systems and irrigation sources are more common in residential areas, while in commercial areas important sources include washing, dumping, and spills. This EPA document provides a table and discussion of likely sources for different land uses.
For entities covered under municipal NPDES stormwater permits, Minimum Control Measure 3 requires establishment and maintenance of an illicit discharge detection and elimination (IDDE) program. The permit contains specific items that are required as part of an IDDE program.
Techniques for detecting illicit discharges include techniques for detecting infiltration and inflow. These include monitoring, dye tracing, smoke detection, use of camera, and focused inspections.
For more information see [16], [17], [18].