Estimating Current Loads

Loads are often discussed in documents that address watersheds and nonpoint source pollution. We’ve provided a discussion of different terms that will be helpful in navigating through these technical documents, reasons why loads are important to watershed planning, and ways to calculate loads using models.

What is a load?

In terms of water quality, a load is the amount of a pollutant that is carried by a particular waterbody within a particular timeframe (example: 107 lbs of nitrogen per day). A loading of pollutants may be caused by humans or occur naturally. Very simple load calculations can be obtained by multiplying concentration by discharge (flow).

Loads vs. concentrations

The load refers to the total amount of a pollutant carried by the water during a given time, such as "tons of nitrogen per year." Pollutants may enter the water from run-off, ground water, pipes, or the air (in the form of wet deposition, such as rain or snow, as well as dry deposition). Loads are different from concentrations. The pollutant concentration refers to the amount of a pollutant in a defined volume of water (such as milligrams of nitrogen per liter of water). Loads are an equalizer that addresses how much pollution is being contributed by one stream compared to another. This is important, especially when pollutants are being contributed by both large and small streams that can be flowing fast or slow. Waterbodies carrying the same concentration of pollutant, but that differ in hydrology, may have drastically different loadings.

U.S. Environmental Protection Agency (U.S. EPA) requires that watershed management plans report pollutant levels in terms of loads. Even so, concentrations are generally used as thresholds because water samples are reported in terms of concentration with a known level of precision and accuracy.

Total Maximum Daily Loads

Section 303(d) of the Federal Clean Water Act and the U.S. EPA’s Water Quality Planning and Management Regulations (Title 40 of the Code of Federal Regulations (CFR), Part 130) require states to develop Total Maximum Daily Loads (TMDLs) for waterbodies that are not meeting water quality standards . TMDLs provide states a basis for determining the pollutant reductions necessary from both point and nonpoint sources to restore and maintain the quality of their water resources.

TMDLs are technical reports that determine how much of a particular pollutant can be contributed to a waterbody without violating the water quality standard [PDF]. They do this by outlining the sum of the individual wasteload allocations for point sources, load allocations for nonpoint sources, and natural background that can be contributed to a waterbody. If a receiving water has only one point source discharger, the TMDL is the sum of that point source wasteload allocation plus the load allocations for any nonpoint sources of pollution and natural background sources, tributaries, or adjacent segments. TMDLs can be expressed in terms of either mass per time, toxicity, or other appropriate measure. Best management practices or other nonpoint source pollution controls make more stringent load allocations practicable and therefore wasteload allocations can be made less stringent. Thus, the TMDL process provides for nonpoint source control trade-offs.

Loading capacity

The greatest amount of a pollutant that a waterbody can receive without violating water quality standards [PDF].

Wasteload allocation

The portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources (i.e. NPDES-permitted facilities) of pollution. Wasteload allocations are effluent limitations based on water quality standards.

Load allocation

The portion of the receiving water's loading capacity that is attributed either to one of its existing or future nonpoint sources of pollution or to natural background sources. Load allocations are best estimates of the loading that may range from reasonably accurate estimates to gross allotments depending on data availability and appropriate techniques for predicting the loading. Wherever possible, natural and nonpoint source loads should be distinguished.

Importance of Determining Loads

Loads can help us to compare dissimilar streams to determine which stream segment or tributary is contributing the most pollution in a system. This allows restoration efforts to be focused in areas that are in most need. When examined seasonally or under various flow conditions, pollutant loads can help to identify sources of pollutants. For example, if a particular pollutant load increases after rain events, we can surmise that it is entering the system through overland flow. If the pollutant load does not increase during rain events, perhaps it is the result of a point source discharge.

U.S. EPA requires all approved watershed management plans to estimate current pollutant loads, determine load reductions needed to meet target goals, and provide an estimate of the load reductions expected from management measures recommended in the watershed management plan. Decreased pollutant loadings over time are an environmental indicator of success.

How to Determine Current Loads

There are several different ways to calculate or estimate pollutant loading in your watershed. Loads can be established for the entire project area by only determining loads at the mouth of the watershed or loads can be established throughout the watershed by considering multiple points in the watershed under various hydrologic conditions. If you have sufficient project data, including flow, you can calculate the estimate the current loading using recent data (concentration x flow). However, if you do not have enough project data to calculate the loads, several models are available that use land-use data to estimate loads. If you are working on a plan and need help with load calculations or estimates, contact your IDEM project manager or watershed specialist for assistance.