What is the actual exposure of organic compounds on Chironomus riparius? - A novel methodology enabling the depth-related analysis in sediment microcosms

Mind the Exposure Gaps—Modeling Chemical Transport in Sediment Toxicity Tests

“…This result agrees with the recent measurements of a vertical chemical gradient in the sediment of mesocosms that was related to retarded chemical mobility due to particle adsorption using spatial modeling. 24 By slow diffusive transport, even relatively hydrophilic chemicals are conserved in the sediment to a larger extent than expected by their K d and the total volume of water exchanged. It is illustrative that for the chemical with log K OC/w 3 only 13% loss was predicted by the w/ DOC model, while 49% loss was expected by the two-compartment model.…”

“…23 Moreover, a vertical concentration gradient of hydrophilic chemicals in the upper sediment was measured in static sediment toxicity tests with Chironomus riparius in the literature. 24 These observations indicate that the dynamic processes mentioned above lead to variable exposure conditions in space and time.…”

“…Overall mass transport kinetics can be influenced by the particle size of sediments and hence their mobility, as well as facilitated by the cotransport of DOC-bound chemicals following the flux of DOC from the sediment to the overlying water, in addition to the transport of freely dissolved chemicals. , Recently, in the standardized 10-day sediment toxicity test with the amphipod Hyalella azteca, we experimentally observed C free,pore that were by a factor of up to ∼10 higher than C free,over and a decreasing trend of C diss in the overlying water ( C diss,over ) over time . Moreover, a vertical concentration gradient of hydrophilic chemicals in the upper sediment was measured in static sediment toxicity tests with Chironomus riparius in the literature . These observations indicate that the dynamic processes mentioned above lead to variable exposure conditions in space and time.…”

“…The interrelated partitioning, diffusion, and outflux of chemicals in flow-through sediment toxicity tests could be simulated using numerical modeling following appropriate mathematical descriptions of these processes and derivation of the required model parameters (e.g., sediment OC content, water exchange rate). In recent years, various modeling efforts have been reported for in vitro toxicity tests, which shed light on the actual exposure in miniaturized test systems. − A recent study adopted a similar approach and successfully modeled the chemical transport in a sediment–water test system. However, the focus was rather on relatively hydrophilic chemicals, and thus, C free and C diss in different water zones and DOC-facilitated transport, which are expected to be important for hydrophobic chemicals, were not considered in the simulations.…”

Comparison of Species Sensitivity Distributions for Sediment‐Associated Nonionic Organic Chemicals Through Equilibrium Partitioning Theory and Spiked‐Sediment Toxicity Tests with Invertebrates

“…The instability and complexity of exposure concentrations in spiked‐sediment tests may be partially responsible for the difference in HC50 values between the two approaches. Concentrations of a test chemical, specifically in overlying water, in spiked‐sediment tests often fluctuate temporally because of insufficient equilibrium between the spiked sediment and surrounding water in an exposure beaker (Dorn et al, 2021; Fischer et al, 2021), insufficient equilibrium time after spiking of a test chemical into sediment (Landrum et al, 1992), or renewal of overlying water (Fischer et al, 2021; Hiki, Fischer, et al, 2021). In addition, while EqP theory assumes that porewater concentrations are representative of the toxicity to benthic organisms, benthic organisms are exposed to overlying water rather than to porewater depending on a test species and chemical (Droge et al, 2008; Whiteman et al, 1996).…”