In Candelaria town of Columbia, SA, goiter prevalence among children was 23% in Zone B (p<0.05). Socioeconomic conditions, dietary composition, or iodine intake could not explain this difference. Water for the town was supplied by two wells through independent pipeline systems, one located in Zone A and the other in Zone B. Waters from these two wells were combined in a single tank and the whole town was supplied through a common pipeline system. Within a year, goiter prevalence rose to 31% in Zone b, while that in Zone A remained the same, 26% (p,NS). Rats drinking water or activated carbon extracts from Well A developed larger thyroid glands and more pronounced thiourea-like antithyroid effects than rats drinking water or extracts from Well B. Similar results were obtained by the acute in vivo suppression of thyroid /sup 131/I-uptake in mice and by two in vitro assays. Elemental analysis and infrared spectromentery of Well A goitrogenic water extracts (GWE) gave similar compositon and common absorption bands to those of aquatic humic substances (HS). The HS, fulvic and humic acids, inhibited thyroid peroxidase (TPO) in vitro with 15% of the potency of propylthiouracil (PTU). GCMS analsysis of GWE identified over 30 organicmore » compounds, including resorcinol. Resorcinol was goitrogenic and antithyroid in 2 different rat strains. Resorcinol appeared to be a final goitrogenic by-product of shale-derived HS contaminating water of the goitrogenic well of Candelaria. 46 refs., 3 figs., 4 tabs« less