Water Quality System Representation in GeoSWMM

GeoSWMM employs several specialized objects and methods to represent water quality in urban runoff. These tools are very flexible and can model a variety of buildup and washoff processes, but they must be supported by calibration data to generate realistic results. The following is a brief description of the objects and methods used by GeoSWMM to model water quality.

Pollutants

Pollutants are user-defined contaminants that build up on the catchment surface and are washed off and transported downstream during runoff events. GeoSWMM can simulate the generation, washoff and transport of any number of user-defined pollutants. Each defined pollutant is identified by its name and concentration units. Pollutant concentrations in externally applied water sources can be added directly to the model (e.g. concentrations in rain, groundwater and inflow/infiltration sources). Concentrations generated by runoff are computed internally by SWMM. It is also possible to define a dependency between concentrations of two pollutants using the co-pollutant and co-fraction options (e.g., lead can be a constant fraction of the suspended solids concentration).

Land Uses

Land uses characterize the activities (e.g. residential, commercial, industrial, etc.) within a subcatchment that affect pollutant generation differently. They are used to represent the spatial variation in pollutant buildup/washoff rates as well as the effect of street cleaning (if used) within a subcatchment. A subcatchment can be divided into one or more land uses. This division is done independently of that used for pervious and impervious sub-areas, and all land uses in the subcatchment are assumed to contain the same split of pervious and impervious area. The percentages of named land uses assigned to a subcatchment do not necessarily have to add up to 100. Any remaining area not assigned as land use is assumed to not contribute to the pollutant load.

Buildup

The buildup function for a given land use specifies the rate at which a pollutant is added onto the land surface during dry weather periods that will become available for washoff during a runoff event. Total buildup within a subcatchment is expressed as either mass per unit of area (e.g., lb/acre) or as mass per unit of curb length (e.g., lb/mile). Separate buildup rates can be defined for each pollutant and land use. Three options are provided in GeoSWMM to simulate buildup: the power function, the exponential function and the saturation function. The mathematical representation of each function is described in the GeoSWMM 5 Users Manual. These formulations can be adapted, by using the proper parameters, to achieve various kinds of buildup behavior, such as a linear rate buildup or a declining rate buildup.

Defining an initial pollutant loading over the subcatchment is an alternative to using a buildup function for single event simulations. Initial loading is the amount of a pollutant over the subcatchment, in units of mass per unit area, at the beginning of a simulation. This alternative is more easily adapted to single-event simulations and overrides any initial buildup computed during the antecedent dry days.

Washoff

Washoff is the process of erosion, mobilization, and/or dissolution of pollutants from a subcatchment surface during wet-weather events. Three choices are available in GeoSWMM to represent the washoff process for each pollutant and land use: Event Mean Concentrations (EMCs), rating curves and exponential functions (see the GeoSWMM 5 Users Manual for mathematical representations). The main differences between these three functions are summarized below.

• EMC assumes each pollutant has a constant runoff concentration throughout the simulation.
• Rating curves produce washoff loads that are functions of the runoff rate only, which means that they simulate the same washoff under the same discharge, regardless of the time in the storm that the discharge occurs.
• Exponential curves differ from rating curves in that the washoff load is a function not only of the runoff rate but also the amount of pollutant remaining on the watershed.
• Buildup functions are not required when EMCs or rating curves are used to represent the pollutant concentrations. If buildup functions are used, regardless of washoff function, buildup is continuously depleted as washoff proceeds, and washoff ceases when there is no more buildup remaining.
• Because rating curves do not use the amount of buildup remaining as a limiting factor, they tend to produce higher pollutant loads at the end of a storm event than do exponential curves which do take into account the amount of buildup remaining on the surface. This difference can be particularly important for large storm events where much of the buildup may be washed off in early stages.

After pollutants are washed off from the subcatchment surface, they enter the conveyance system and are transported through the conduits as determined by the flow routing results. Here they may experience first-order decay or be subjected to reduction at specific nodes where treatment functions have been defined.

Pollutant Reduction from Land Surfaces

Two procedures for reducing surface pollutant loads within subcatchments are available in GeoSWMM. They are:

• BMP Treatment: This mechanism assumes that some type of BMP has been utilized in the subcatchment that reduces its normal washoff load by a constant removal fraction. BMP treatment will not be used in this tutorial but will instead be illustrated in Tutorial 05.
• Street sweeping: Street sweeping can be defined for each land use and is simulated in parallel with buildup prior to the beginning of the first storm event and in-between the next events. Street sweeping is defined by four parameters used to compute the pollutant load remaining on the surface at the start of a storm: (1) days between street sweeping, (2) fraction of the buildup that is available for removal by sweeping, (3) number of days since last sweeping at the start of the simulation and (4) street sweeping removal efficiency (in percent). These parameters are defined for each land use while the fourth one is defined for each pollutant as well.