We determined the levels of various heavy metals in water stream and sediments receiving leachate/runoff from open-air windrows of MSW compost plant of Sanandaj, (Kurdistan, Iran) and assessed the related contamination rate and ecological risk by determining the geo-accumulation index (I-geo), contamination factor (CF), ecological risk factor (ER), contamination degree (CD), pollution load index (PLI) and potential ecological risk (RI) of water and sediments. Our results showed that several heavy metals, except Cd, had concentrations higher than the maximum limits for sediments to protect aquatic life, using the Consensus-Based Sediment Quality Guidelines (CBSQG) as reference. Heavy metal concentrations in the stream water were higher than the Canadian Council of Ministers of the Environment (CCME) limits for drinking, irrigation and for protection of an aquatic life, except for Cu and Zn. The evaluation results of the CD and PLI indices showed that the stream water and sediment pollution caused by uncontrolled emission from the open-air windrows composting of MSW, as the elements causing pollution were the same as those enriched in the compost. Values of the ER and RI indices indicated moderate to considerable ecological risk. We concluded that source separation of the MSW and effective control of runoff entering the composting plant are needed to protect the surrounding environment from the negative impact of the composting plant releases. (C) 2016 Elsevier B.V. All rights reserved.