Coupling Monthly Physicochemical Monitoring of Environmental Stressors with Primary Mammalian Stem-Cell Bioassays in an Irrigation Reservoir: Karadere-Kastamonu Reservoir Model

Surface water quality assessment using physicochemical thresholds often misses the joint biological effects of complex mixtures of constituents. This study integrated regulatory, cellular, and radiological evaluations across a full annual cycle in the Karadere-Kastamonu irrigation reservoir, Türkiye. Twenty-one physicochemical parameters were monitored monthly, dissolved radon was screened, and primary rat bone marrow-derived mesenchymal stem cells were exposed to sterile-filtered, non-enriched water across seven concentrations (0.01–0.5% v/v). Cellular responses were evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, Live/Dead staining, and cell density quantification from hematoxylin–eosin (HE) preparations. Under the regulation’s worst-parameter rule, the reservoir was Class I for nine months, Class II for one month, and Class III for two months, and radon remained well below the drinking-water parametric value. However, sterile-filtered reservoir water induced distinct cellular responses: while there was no effect at 24 h, 72 h exposure increased MTT reduction to 119.1% of control at the lowest dose tested, with no further increase at higher doses. Cell density, measured in the HE series, was reduced at every dose to 67–86% of control across the seven doses (mean 76%) and in every quarter. In the Live/Dead preparations, propidium iodide-positive nuclei remained below 1.4% in every condition with no difference detected from untreated controls. An exploratory estimate of metabolic activity per cell, derived from two endpoints measured in different cultures, exceeded unity in all four quarters (1.22 to 1.65), with its confidence interval excluding unity in two. The reduction was already present at 0.01% v/v and did not deepen over a 50-fold concentration range. We conclude that regulatory classification and the cell-based response are complementary, non-redundant descriptors of the same water: a water body may be Class I on every regulated parameter in a given month while measurably altering mammalian cell behavior, so the two assessments should be read together rather than one being taken as a proxy for the other.