A Hepatic Compartmental Model of Herbicide Bioconcentration in Nile Tilapia (Oreochromis niloticus)

<title>Abstract</title> <p>The bioconcentration factor (BCF) is an aggregate quantity: whether a steady-state tissue-to-water ratio or a ratio of lumped rate constants, it conflates branchial uptake, systemic distribution and hepatic biotransformation into a single number that can be re-measured but not cross-checked against independent physiological data. We ask how far such a BCF can be decomposed into interpretable rate constants when only muscle is sampled, using as a test case published bioconcentration data for four sugarcane herbicides (ametryn, tebuthiuron, hexazinone and diuron) in Nile tilapia (Oreochromis niloticus). A compartmental model linking water, liver and muscle reduces, under a hepatic pseudo-steady-state assumption, to two identifiable parameters, calibrated by nonlinear least squares to three herbicide mixtures in parallel; because most compound × mixture combinations provide as many data points as fitted parameters, this is a calibration rather than an independent validation. These parameters were then decomposed, once a hepatic biotransformation rate constant was taken from an independent database — a directly measured value for diuron — and branchial uptake was cross-validated against a molecular quantitative structure–activity relationship and measured gill extraction efficiency. The implied liver-to-muscle partitioning agrees within an order of magnitude with tissue-resolved measurements for a triazine in a tilapia for three of the four compounds and fails for the fourth, identifying the least constrained element of the model. A Monte Carlo reformulation of the dietary risk assessment indicates negligible probability of exceeding the acceptable daily intake, while effective elimination varies systematically with mixture composition, a pattern partition-coefficient-only estimates cannot capture.</p>