KABAM Version 1.0 User's Guide and Technical Documentation - Appendix B - Explanation of Defaults and Alternative Values Representing Abiotic Characteristics of Aquatic Ecosystem
(Kow (based) Aquatic BioAccumulation Model)
Appendix B. Explanation of Defaults and Alternative Values Representing Abiotic Characteristics of Aquatic Ecosystem
Abiotic characteristics of the aquatic ecosystem that are necessary for KABAM are defined in Table 4 of the model tool. These characteristics include concentrations of particulate organic carbon (XPOC), dissolved organic carbon (XDOC), dissolved oxygen (COX), suspended solids (CSS), water temperature (T), and % organic carbon (OC) content of the sediment. The model tool is populated with default values for these parameters, which can be altered based on the needs of the model user. Default values are based on the abiotic characteristics of the aquatic ecosystem and are designed to be consistent with the OPP standard pond scenario used in EXAMS. Brief explanations for these default values as well as guidance on selecting alternative values are provided below for each parameter.
-
B.1 Particulate Organic Carbon (XPOC) and Dissolved Organic Carbon (XDOC)
XPOC and XDOC are entered by the model user in units of kg OC/L. These parameters are relevant to estimating the available pesticide fraction in water (Φ). The greater the value of either of these parameters, the less pesticide is available in water. Less available pesticide results in lower concentrations of pesticide in tissues of aquatic organisms.
The estimated environmental concentrations (EECs) generated by PRZM/EXAMS in the water column and pore water represent the available concentration of the pesticide in water. Therefore, a default value of "0" is assumed for both XPOC and XDOC. As a result, the pesticide concentration available in the water is equal to the PRZM/EXAMS EEC input in Table 1.
It may be necessary for the model user to incorporate alternate values for the XPOC and XDOC parameters if the modeling incorporates EECs from a source other than PRZM/EXAMS. For example, if the exposure concentrations are available from monitoring data or mesocosm studies, XPOC and XDOC specific to the monitoring study may be used. In that case, if the empirical exposure concentrations correspond to the total water column (i.e., unfiltered), the model user would want to enter XPOC and XDOC values that correspond to the specific water body used. If given a range of available values, the user should consider that use of lower XPOC and XDOC values will result in more conservative estimates of pesticide accumulation in the aquatic food web.
-
B.2 Concentration of Dissolved Oxygen (COX)
The COX parameter influences the ventilation rate of aquatic animals. As COX decreases, the gill ventilation rate of aquatic animals increases (because animals need to take in more water to acquire the amount of oxygen they require). With an increase in the gill ventilation rate, the rate constants for pesticide uptake (k1) and elimination (k2) through respiration both increase. Although the increase in k1 leads to an increase in pesticide uptake, the increase in k2 also leads to an increase in pesticide elimination. The net effect is a decrease in pesticide concentration in aquatic organism tissues. Therefore, a decrease in the value of COX results in a decrease in pesticide concentrations in tissues of aquatic organisms.
COX is entered by the model user in units of mg O2/L. The default value for this parameter is 5.0 mg O2/L, based on the OPP standard pond. This concentration does not represent the highest possible value for COX (i.e., the limit of solubility of oxygen) and is not expected to result in the most conservative estimates of pesticide in aquatic animal tissues. However, it is consistent with the OPP standard pond which is used to derive EECs.
The model user could explore the influence of COX on the predictions of pesticide tissue concentrations in aquatic organisms by selecting a higher value, for example the solubility of oxygen (potential range: 6-12 mg O2/L). To determine the solubility of oxygen in water at specific temperatures and pressures, see USGS 2008a.
It may be necessary for the model user to incorporate an alternate COX value if the modeling incorporates EECs from a source other than PRZM/EXAMS. In that case, the model user should enter a COX value that corresponds to the specific water body used.
-
B.3 Water Temperature (T)
The water temperature parameter influences calculation of the growth dilution rate constant (kG), the pesticide uptake rate constant through diet (kD), and the pesticide elimination rate constant through excretion of feces (kE). The growth dilution rate constant (kG) is dependent on whether the temperature is above or below 17.5 °C. The growth dilution rate constant is higher when the temperature is above 17.5 °C compared to when the temperature is below 17.5 °C (Figure A.8). Temperature affects the pesticide uptake rate through the dietary uptake rate constant (kD) by changing the feeding rate of the animal (GD). An increase in temperature results in an increase in the feeding rate, and with that, an increase in the pesticide uptake constant for the diet (Figure A.10). The fecal egestion rate constant (kE) is affected by temperature by changing the feeding rate (GD) as well as the fecal egestion rate (GF) of the animal. An increase in temperature results in an increase in the feeding rate (Figure A.13), and with that, an increase in the fecal egestion rate. The increase in the fecal egestion rate results in an increase in the pesticide rate constant for pesticide elimination through excretion. In summary, increase in temperature results in an increase in kD, kE, and kG. Although kG and kE represent processes (i.e., pesticide elimination/dilution) that compete with kD (i.e., pesticide uptake), the net increase in the two processes (uptake and elimination/dilution) does not cancel each other out.
The water temperature of the EXAMS' pond varies based on the selected PRZM scenario. Therefore, the model user should select the water temperature based on the PRZM scenario used for deriving EECs. If the modeling incorporates EECs from a source other than PRZM/EXAMS, the water temperature relevant to the other EECs should be utilized.
-
B.4 Concentration of Suspended Solids (CSS)
The concentration of suspended solids (CSS) is relevant to filter feeders only. CSS influences the calculation of the rate constants for pesticide uptake through diet (kD) and pesticide elimination through excretion of feces (kE). An increase in CSS leads to an increase in the feeding rate of filter feeders (GD) which in turn results in an increase in the pesticide uptake through diet (kD). An increase in CSS also leads to an increase in the fecal egestion rate of filter feeders (GF) and an increase in the pesticide elimination through excretion of fecal matter (kE). Although kD and kE represent competing processes, the net increase in the two does not cancel each other out.
CSS is entered by the model user in units of kg/L. The default value for this parameter is 3.00x10-5 kg/L, based on the OPP standard pond. If the modeling incorporates EECs from a source other than PRZM/EXAMS, a CSS value relevant to the other EECs should be utilized.
-
B.5 Sediment Organic Carbon (OC)
Sediment organic carbon (OC) is a parameter that influences organisms that consume sediment. As OC increases, the concentration of the pesticide in the solid component of the sediment increases to the extent that the pesticide sorbs to organic matter. As the pesticide concentration in sediment increases, the pesticide concentration in organisms that consume sediment also increases.
OC is entered by the model user as % of the dry weight of the sediment. The default value for this parameter is 4.0%, based on the OPP standard pond. If the modeling incorporates EECs from a source other than PRZM/EXAMS, an OC value relevant to the other EECs should be utilized.