Defining Assimilative Capacity

Assimilative capacity has been defined in numerous ways. In summary, the assimilative capacity is defined as the amount of nutrients, sediments, or pathogens that an aquatic system (stream, lake, river, estuary) can absorb without exceeding a numeric criterion. The numeric criterion is set so that the waterbody meets the policy goals set for it by the statutory regulatory agencies.

In the United States, the United States Environmental Protection Agency (USEPA) is the overarching regulatory authority for the TMDL program and so its definition is the baseline. Assimilative capacity is defined in its role in determining a TMDL by the USEPA. The term 'assimilative capacity' represents the amount of contaminant load that can be discharged to a specific waterbody without exceeding water-quality standards or criteria. Assimilative capacity is used to define the ability of a waterbody to naturally absorb and use a discharged substance without impairing water quality or harming aquatic life.

Note that the USEPA definition has several discrete segments. The initial part of the document deals with discharges. The specific use of 'discharge' is because point sources or known discharges have been the focus of past regulation. However, assimilative capacity in the TMDL context often deals with nonpoint sources where the discharge site is not specific. Nonpoint sources may have diffuse input to the waterbody by small streams, groundwater sources, or atmospheric deposition. Assimilative capacity is also identified with specific water bodies, so this is meant to be a site-specific property and this is reflected in the application ofthe TMDL process. The last line ''without exceeding water quality standards'' can be translated as: without the receiving waterbody having qualities that do not meet management goals as set by public policy. This is the critical criterion for establishing an assimilative capacity.

This criterion is the keystone of the definition. It reflects cultural values about the use of a particular waterbody. These values include the use of the waterbody as a water supply, recreational use, the support of important ecological resources, and other uses. The criteria derived in order to meet these uses form the basis for deriving numeric values that set the limit in the assimilative capacity definition.

Table 1 presents several examples of the variables measured and the nutrient criteria set in order to

Table 1 Examples of indicators and criteria used in the TMDL process. Note that the criteria are set for each waterbody and vary in type depending upon the site and the intended use

Waterbody Indicators and the criteria selected

Boulder Creek, CO Appoquinimink

River, DE Lake Chelan, WA Truckee River, NV Clarke Fork River, MT

Laguna de Santa Rosa, CA

0.06 mgl 1 unionized ammonia

5.5 mg l_1 dissolved oxygen (daily average), 4.0 mg l_1 dissolved oxygen (instantaneous minimum) 4.5 mgr1 total phosphorus

0.05 mgL1 total phosphorus, 210 mgL1 total dissolved solids

100 mg m~2 chlorophyll a (summer mean), 300 mg l_1 total nitrogen, 20-39 mgL1 total phosphorus specific to stretches on the river. 0.025 mgNT1 unionized ammonia, 7.0 mgT1 dissolved oxygen (minimum)

Modified from USEPA (1999) Protocol for Developing Sediment TMDLs, 132pp. EPA 841-B-99-004. Washington, DC: United States Environmental Protection Agency, Office of Water (4503F).

protect a number of rivers. In Boulder Creek, CO, the criteria for unionized ammonia was set at 0.06 mgl~ , compared to 0.025 mgl~ for Laguna de Santa Rosa in California. Phosphorus is also a common indicator for which criteria are set. Lake Chelan has a very low criteria for total phosphorus compared to the values for Truckee River or Clarke Fork River. For the nutrients the numbers are a maximum, but for dissolved oxygen the numbers are a minimum. Dissolved oxygen criteria also vary by waterbody; compare the value of 4.0 mgl-1 for Appoquinimink River to the 7.0 mgl-1 for Laguna de Santa Rosa. The Clarke Fork River also includes criteria for chlorophyll a, not a nutrient but as an indicator of the nutrient condition of the waterbody. The assumption is that the nutrients in the waterbody control the amount of algae and therefore the concentration of chlorophyll a in the water column. Since ensuring that total algal productivity in the lake is below a certain value, the managers are attempting to prevent the rapid eutrophication of the system.

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