caland and hogarth pit lakes formed after cessation of mining and dewatering efforts of open-pit iron mines, near atikokan, ontario. they were assessed for w ater quality changes since monitoring began in 1998. stable isotopes were used to evaluate trends in water columns and gain information on geological processes influencing water quality. chronic toxicity investigations were carried out for hogarth pit lake. approaching depths of 200 m, both pits have been filling with groundwater and precipitation since termination of mining in 1979. limestone and carbonate deposits in the area counter production of acids from waste rocks, resulting in near-neutral ph’s in both lakes. although proximal pit lakes, there are major chemistry differences between them. caland is characterized by alkaline, nutrient rich, while hogarth has elevated conductivity, total dissolved solids (tds), and s0[subscript 4 superscript 2] levels. monitoring trends reveal gradual dilution of both pit lakes since 1998. hogarth has pronounced seasonal variations, with winter months having elevated levels of the aforementioned parameters. similar δ[superscript34]s profiles in caland and hogarth suggest pyritic lenses in the ore body are the major source of sulfates for both lakes. caland δ[superscript 13]cdic values reveal organic inputs are the major sources of carbon, whereas hogarth’s major source of carbon comes from weathering carbonates. hogarth pit lake has experienced a change in toxicity. in 1999 the lake was acutely toxic, and by 2005, chronic effects were present. chronic toxicity testing using ceriodaphnia dubia resulted in intermittent toxicity occurring in the winter months. toxicity identification evaluation (tie) tests did not successfully isolate the cause of toxicity, which led to exploration of tds toxicity using mock effluents. mimicking concentrations of the most abundant ions (ca[superscript2+], mg[superscript2+], and so[subscript 4][superscript 2-]) , mock effluent test results on both c. dubia and lemna m/nor suggest tds were responsible for the majority of toxicity in hogarth. bioaccumulation studies using eleocharis smallii and pyganodon grandis resulted in elevated concentrations of s and ni in tissues exposed to hogarth water. metals contributing to toxicity in hogarth, especially ni, could still a possibility and should not be ruled out. study sites : caland pit lake near the former caland ore company and hogarth pit lake near the former steep rock mines near atikokan, northwestern ontario.