Hi!
As I have been looking for the degradation rate in sediment for different pesticides, I have come across with a problem. Most sources have offered values (EFSA for example) under the title: Degradation in water/sediment. The values given have been DT50water and DT50wholesystem. Only in two cases out of the ten pesticides I've looked up has there also been a value for DT50sediment. So can the value of DT50wholesystem be used as a degradation rate in sediment or does it specifically have to be the DT50sediment-value?
How about if there are two different values for degradation rate in water and sediment? The other from a river environment and the other from a pond environment. Which one should I use then?
My last question concerns the toxicity values. If there is given a EC50 value of 0,0008-0,002 mg/l should I use the lowest value?
Best regards,
Outi
Thank you for your question!
In sources where a value is given for "Degradation in water/sediment" is typically refers to to the whole water-sediment system. However, this might vary from source to source. We recommend for degradation in sediment to use a specific sediment value. If the user knows how to transfer whole system values to sediment-specific values, we recommend to apply this conversion.
We recommend to use sediment degradation rates for sediment compartment and water degradation rates for water compartment. If there are multiple degradation rates in water, we recommend to use the value that corresponds best to the user-defined system and otherwise to use the average. Note that the best is to average as many data as available, then the least uncertainty is attached to the average value.
Althoug it is beyond the scope of this forum to answer general LCA-related questions, we can say that in LCA there is a general tendency to principally use average values (best estimates) and not the lowest value (worst case assumption).
Thank you for your answer!
Now I'm troubled by the exposure factor (XF).
How does the calculation of the environmental exposure factor for freshwater ecotoxicity go? The USEtox manual isn't quite enough to clarify it and I'm only confused by the mathematical formula in it (FRw.w).
Can you clarify it for me or for example give me a tip of a guide or a manual concerning the environmental exposure factor?
Best regards
Outi
Thank you for your question!
The exposure factor gives the dissolved and bioavailable concentration, i.e. the amount that is actually available for uptake, whereas the total concentration includes molecules absorbed by particles in the water. Generally the exposure factor is very close to 1.
Henderson et al. 2011 provides more information.