A Civil Engineers Guide to Total Maximum Daily Loads
Given the pivotal role civil engineers play in designing and implementing infrastructure, a deep understanding of Total Maximum Daily Loads (TMDLs) ensures compliance with regulatory requirements and contributes to the broader goals of environmental stewardship and public health. This post provides detailed insights into the triggers, development, and implementation of TMDLs, emphasizing the integral role civil engineers have in achieving and maintaining water quality standards.
What is a Total Maximum Daily Load?
TMDLs are action plans aimed at restoring clean water by defining the maximum amount of a pollutant a water body can handle while still meeting water quality standards. A TMDL sets a pollutant reduction target and allocates necessary load reductions to both point and non-point sources of pollution.
Point Sources: These are pollutants that originate from single, identifiable sources, such as wastewater treatment plants.
Non-Point Sources: These pollutants come from diffuse sources, often transported by stormwater runoff.
Civil engineers play a crucial role in designing and implementing the infrastructure that controls point source pollution and mitigates non-point source pollution. Effective stormwater management systems, wastewater treatment processes, and erosion control measures are essential components.
What triggers the need for a TMDL?
According to the Clean Water Act (CWA), each state must develop TMDLs for all waters identified on their Section 303(d) list of impaired waters, prioritized by their ranking on that list.
Civil engineers need to be aware of the impaired water bodies in their projects' vicinity and the specific pollutants of concern. This knowledge is vital for planning and designing projects that comply with regulatory requirements that protect and restore water quality.
Who is responsible for developing a TMDL?
States are responsible for developing TMDLs and submitting them to the Environmental Protection Agency (EPA) for approval. Even when third parties assist in the TMDL development, states must submit the final TMDL to the EPA. If the EPA disapproves a state TMDL, it must develop a replacement TMDL.
Collaboration with state environmental agencies and understanding regulatory processes are essential for civil engineers. This ensures that their projects align with TMDL requirements and contribute to the overall water quality goals.
How are TMDLs developed?
The process of developing a TMDL starts with identifying the intended uses of a receiving water body, like swimming, fishing, drinking water supply, or industrial use. The EPA has developed water quality criteria that must be met to support these and other uses. If water quality data is collected that falls short of these criteria, the water body is put on the 303d list which is a commitment to developing a TMDL.
The TMDL itself is developed based on research into the sources and amounts of pollutants entering the water body and the amount of the pollutant that the water body can naturally assimilate without exceeding water quality standards. All contributing sources of pollutants (both point and non-point) are identified and allocated a portion of the allowable load. Fundamentally, a TMDL is pollution budget for a particular water body. For TMDLs to compel action, they must be incorporated into a regulatory framework.
What happens after the TMDL is approved by the EPA?
TMDL wast load allocations for point sources are generally implemented through the EPA’s National Pollutant Discharge Elimination System (NPDES) permits under CWA Section 402. This requires point source discharges to be controlled by including water quality-based effluent limits in permits issued to individual point source entities. NPDES permits must be consistent with the assumptions and requirements of wasteload allocations in EPA-approved TMDLs. Individual NPDES permit holders must prevent discharges of pollutants to natural waters that might cause or contribute to an impairment. This often requires optimizing plant processes and design to limit the use of harmful chemicals, minimizing comingling of pollutants with wastewater and implementation of treatment systems that are designed to remove pollutants prior to discharge.
Non-point source load reduction actions are implemented through various programs at the state, local, and federal levels, which may be regulatory, non-regulatory, or incentive-based, such as cost-share programs. Industrial, construction, and municipal NPDES permits regulate discharges of stormwater from sites and are increasingly incorporating TMDL load allocations as part of a broader set of water quality standards that must be met on individual projects or sites. Strategies for meeting these standards vary depending on the regulated pollutants, project type and land use but commonly can be grouped into four categories:
Pollution prevention: This strategy refers to the design of sites and site activities to prevent pollution from mixing with stormwater. For example, avoiding the use of pesticides and herbicides where possible and enclosing and covering trash receptacles are examples of pollution prevention on commercial or municipal projects. On construction sites, minimizing land disturbance, especially when there is rain in the forecast is a good idea. On an industrial site, pollution prevention may include storing hazardous materials inside or regular sweeping of impervious areas.
Site design: This strategy refers to preserving as much of the natural hydrologic function of the site as possible.
Retention: Where stormwater runoff is generated on site, retention measures like infiltration basins, bioretention and rainwater harvesting can intercept that runoff, and any pollutants in it, before it leaves the site.
Treatment: Where stormwater runoff will be discharged from the site, treatment is necessary to remove pollutants. Treatment options vary widely and can use a range of physical, biological and chemical processes to reduce the amount of pollutants leaving the site.
Civil engineers should be familiar with stormwater management regulations affecting their projects. Most commonly these are in the form of municipal, industrial or construction NPDES permits which include water quality standards that must be met. Building pollution prevention and site design expertise will help to minimize the amount of retention and treatment control measures that are needed. When these opportunities are exhausted, retention of stormwater is the most reliable way to prevent stormwater pollution from leaving the site. Where runoff will leave the site, relying on treatment controls that have been proven to be effective for the particular pollutants of concern on each site is necessary to meet TMDL load allocations and to protect and restore the quality of downstream waters.