Application of Toxicity Test Results

Water quality criteria ensure protection of designated uses by including magnitude (quantity of toxicant allowable), duration (period of time over which instream concentration is averaged), and frequency (how often criteria can be exceeded without unacceptable receiving water ecological community impacts). Contemporary water quality criteria are designed to protect against short‐term (acute) effects through use of the criterion maximum concentration (CMC), and against long‐term (chronic) effects through use of the criterion continuous concentration (CCC). These criteria generally apply after mixing. Typical water quality criteria contain a concentration limit, an averaging period, and a return frequency. The CMC is typically the four‐day average concentration not to be exceeded more than once every three years, on the average, and the CCC is typically the one‐hour average concentration not to be exceeded more than once every three years, on the average.

Protection Against Acute Toxicity

For protection against acute toxicity, the CMC must not exceed 0.3 TU_a, as measured by the most sensitive result of the tests conducted.

(4.3)

where CID is the critical initial dilution. In ocean discharges, the CID is defined as the dilution achieved given “worst case” ambient conditions within the region close to the discharge, where mixing and dilution of the effluent plume are determined by the initial momentum and buoyancy of the discharge. In river discharges, the dilution achieved at the boundary of a mixing zone is usually taken as the CID. Based on the results of numerous 96‐hour effluent toxicity tests, it was found that a factor of 0.3 accounted for 91% of the observed LC₅₀ to LC₁ ratios (LC₁ is equal to the concentration of effluent in the dilution water that causes mortality to 1% of the test population). Consequently, for acute protection, the CMC should not exceed 0.3 TU_a based on the most sensitive species tested. The acute criterion thus equals the CMC that approximates the LC₁.

Protection Against Chronic Toxicity

To prevent chronic effects from occurring outside the initial mixing zone, or zone of influence, resulting from the discharge, the CCC must not exceed 1.0 TU_c, based on the results obtained with the most sensitive of at least three species tested.

(4.4)

Compliance is based on comparison of the toxicity criteria (both TU_a and TU_c), expressed in toxicity units, in the effluent with critical initial dilution to determine if the EPA’s recommended criteria will be met. The application of these criteria is illustrated in Example 4.3.

EXAMPLE 4.3

Application of Toxicity Test Results

A critical initial dilution of 225 : 1 is achieved for a treated effluent discharged to marine receiving waters. Toxicity tests were conducted with the WWTP effluent using three marine species. The following acute and chronic toxicity test results indicate that C. parvula exhibited the most sensitive species acute endpoint (2.59% effluent) in accordance with the LC₅₀, and also the most sensitive species chronic endpoint (1.0%) in accordance with the NOEC. Using the given toxicity data, determine the compliance with the CMC and CCC criteria.

Results of acute toxicity tests
Species	Exposure (hours)	Control survival (%)	Percent effluent
			LC50	NOAEL
Mysidopis bahia	96	100	18.66	10.0
Cyprinodon variegates	96	100	>100	50.0
Champia parvula	48/168	100	2.59	12.25

Results of chronic toxicity tests
Species	Exposure (days)	Control survival (%)	Percent effluent
			NOEC	LOEC
Mysidopis bahia	7	82	6.0	10.0
Cyprinodon variegates	7	98.8	15.0	>15.0
Champia parvula	7	100	1.0	2.25

SOLUTION

1. Check compliance with the CMC criterion.
   1. Based on data for the most sensitive species tested, we use Eq. (4.1) to find the number of acute toxic units:
   2. For acute protection, the criterion maximum concentration must not exceed 0.3 TU_a, as shown in Eq. (4.3). Following an initial dilution of 225, the CMC is
   3. The CMC (0.17) is considerably less than the value of 0.3 TU_a (11.58) required for compliance with the CMC criterion.
2. Check compliance with the CCC criterion.
   1. Based on data for the most sensitive species tested, we use Eq. (4.2) to find the number of chronic toxic units:
   2. For chronic protection, the criterion continuous concentration must not exceed 1.0 toxic unit as shown in Eq. (4.4). Following an initial dilution of 225, the CCC is

The CCC (0.44) is considerably less than the value of 1.0 TU_c (100) required for compliance with the CCC criterion.

The use of whole effluent toxicity testing affords a number of advantages. In this approach, the bioavailability of the toxics is measured and the effects of any synergistic interactions are also considered. Because the aggregate toxicity of all components of a wastewater effluent is determined, the toxic effect can be limited by limiting only one parameter, the effluent toxicity. Because contemporary receiving water management strategies are based on site‐specific water quality criteria, toxicity testing facilitates comparison of effluent toxicity with site‐specific water quality criteria designed to protect representative, sensitive species and allow for establishment of discharge limitations that will protect aquatic environments.