Difference between stormwater infiltration, runoff, filtration, and inflow 

While stormwater infiltration is when the rainwater flows in and through the subsurface soil, stormwater runoff is rain and snowmelt that flows over land and does not soak into the ground. Though they sound similar, water infiltration and water filtration are starkly different – filtration (involving water) is defined in Title 40, Chapter I, Subchapter D, Part 141, Subpart A, § 141.2 as “a process for removing particulate matter from water by passage through porous media.” 

Inflow is defined in 40 CFR § 35.2005 as water, other than wastewater, that enters a sewer system from sources including from drains and connections between storm sewers and sanitary sewers, surface runoff, and it is distinguished from infiltration, which is also defined in this CFR, as water other than wastewater that enters a sewer system from the ground through means such as defective pipes, connections, or manholes. The definition also mentions that infiltration does not include and is distinguished from inflow.

Basically, how the sewer system receives water – from the surface or from the ground – distinguishes inflow from infiltration. If the water coming into the sewer system is from surface runoff, then it is inflow. If it is coming from the ground, then it is infiltration.

How does infiltration benefit stormwater management? 

In infiltration, the drainage of water into the soil benefits stormwater management by:

• Improving water quality because pollutants are retained in the soil and thus, not transported into groundwater. Due to the retention of pollutants in the soil, the water incoming into the sewer from defective pipes, connections, and manholes in the ground contains comparatively less pollutants than runoff entering the wastewater treatment facilities.

• Replenishing groundwater supplies due to the drainage of water into the ground and to underground aquifers, which are an important source of drinking water supply in some communities across the U.S.

• Providing effective flood control due to the flow of water into the ground rather than over the surface.

• Supporting green infrastructure because it provides more moisture to the soil which in turn supports comparatively more vegetation than drier soil. In turn, dense vegetation provides cleaner air, carbon sequestration, and improved biological habitats than sparse vegetation.

Factors affecting stormwater infiltration 

While stormwater infiltration is beneficial because it decreases the runoff volume while also retaining pollutants in the soil, there are factors, as follows, that affect how fast or slow the water infiltrates the soil:

Texture and compactness of the soil

How much water is retained by the soil and the saturated hydraulic conductivity of the soil are determined by the soil texture. Saturated hydraulic conductivity is the ease with which water moves through the pores of a saturated soil.

In simple words, saturated hydraulic conductivity is how easily water moves through the pores of soil where all soil pores are filled with water (saturated soil). Coarse-textured soil has greater saturated hydraulic conductivity and therefore, also greater infiltration capacity than fine-textured soil and compact soil.

Compared with clayey texture soil, a study showed that sandy soil had greater saturated hydraulic conductivity and thus, great infiltration capacity. This is because sandy soil has large-size particles which allow water to drain quickly and thus higher water infiltration capacity compared to clayey soil with small-size particles and relatively lower water infiltration capacity.

Initial moisture content of the soil at the time of infiltration

The drier the soil is at the beginning of infiltration, the more capacity the soil has for absorbing water and thus, the soil has higher infiltration capacity than soil with higher initial moisture content. This is because soil that has less moisture and thus less water in it has more capacity to absorb the incoming water than soil that already has moisture in it.

What technologies support stormwater infiltration? 

Filter media are widely used technologies supporting stormwater infiltration. These include infiltration basins, trenches, and permeable pavements designed to prevent excessive runoff and flooding. 

Infiltration basins 

Natural depressions or constructed and often containing a flat, densely vegetated floor over naturally permeable soil, an infiltration basin temporarily stores and infiltrates water into the soil over time. With inflow on one end, it also typically contains a backup underdrain to manage standing water problems and emergency spillways to safely pass flow when the basin is filled to capacity with stormwater.

Infiltration basins reduces the amount of pollutants discharged into wastewater treatment facilities by a basic mechanism: the trapping of pollutants in the soil as the water flows through into the ground.

Infiltration trench 

A vertical, shallow excavated trench, typically 3 to 12 feet deep, according to the Minnesota Stormwater Manual, contains a pipe surrounding naturally permeable soil. Stormwater drains through the perforated pipe, connected to a downspout, and infiltrates the coarse stone aggregates and the surrounding soil.

Permeable pavements 

Ranging in width from a sidewalk to a road, permeable pavements — which can be made of different materials — are designed to infiltrate stormwater and melting snow into the undisturbed soil at the bottom. An overflow pipe at one end carries excess water away from the permeable pavement.

Other technologies include the use of membrane bioreactors at wastewater treatment facilities where they have been shown to help mitigate the adverse effects of incidental water (stormwater), which includes variability in pollutants’ concentrations. However, membrane bioreactors require frequent membrane cleaning when there is increased quantities of water flow into the wastewater treatment facility.

What are some common challenges with stormwater infiltration? 

The potential for groundwater contamination, especially in Karst soil, is one of the common challenges due to stormwater infiltration. While much of the pollutants from the water are removed at the surface of the soil via adsorption or biological transformation, the limited adsorption capacity of the soil may lead to breakthroughs where the contaminant can mobilize deeper into the soil and eventually into groundwater. Replacement of the soil prevents breakthrough. 

Though metals and hydrocarbons are removed in the top layer of the soil by infiltration, phosphorus and nitrogen removal could be challenging. Due to the chemistry of iron, phosphorus can be released back into the soil while nitrogen removal is challenging due to microbial actions.

The AI era 

Probabilistic cost-benefit analysis model, specifically the Monte-Carlo model that includes the cost of implementing measures, have been used to assess cost-benefit of managing infiltration and inflow in wastewater systems. Additionally, artificial intelligence is emerging as a tool for wastewater management including for stormwater management. However current challenges include data quality and scalability issues.

At the Monroe Avenue Stormwater Treatment and Infiltration Facility, hydraulic and hydrogeologic models were used to optimize the flow control, distribution, and infiltration of stormwater. For flood management, the models took into account the runoff calculation for peak discharge rate for up to 100 years.