What could have caused coloration of dairy industry effluents after it has gone through allophanic soil columns?

Tephra subsoil (allophanic) columns are used to determine the soil phosphorus (P) sorption capacity to remove P from dairy industry wastewater (pH = 8, with high sulfate concentration). The effluents collected had shown coloration - pale yellow to intense yellow although the influent was colorless. Could it be due to the presence of iron or silicate or can there be other reasons?