Abstract: Kestanbol geothermal field, located in the ancient town of Alexandria Troas, is at the contact betweenplutonic intrusions and neighboring metamorphic rocks in the Biga Peninsula. This area is also controlled by anENE-WSW striking right lateral strike-slip fault segment representing the extension of the southern branch of theNorth Anatolian Fault to the west and is one of the geothermal fields with highest temperature in the Biga Peninsula.Kestanbol geothermal fluid from a well is used to heat facilities for thermal tourism and balneological purposes. The geothermal fluid continuously seeps in the environment with low flow rate and additionally, wastewater from thefacility is discharged in Ilıca stream. This study was conducted to determine the hydrochemical characteristics ofthe well and springs located in Kestanbol geothermal field and to evaluate the environmental effects of geothermalfluids on the soil and stream sediment. The temperature, electrical conductivity (EC) and pH of Kestanbol geothermalfluids are 59.5-74.1ºC, 30.3-35.5 mS/cm, and 6.45-6.71, respectively. The geothermal fluids are NaCl water type,with mean NaCl concentration of 19511 mg/L. In addition to higher EC values, and total dissolved solid content,Na+, Cl-, B, Ba, Fe and Mn concentrations of the Kestanbol geothermal fluid are above the tolerance limit of TS 266.Moreover, the geothermal fluid is heavily polluted water (class IV) according to the Turkish Inland Water QualityRegulations in terms of EC, Na+, Cl-, B and Fe. There is a scaling problem in the Kestanbol geothermal field. XRDand SEM-EDX analyses identified the minerals causing scale as calcite, halite and siderite. Arsenic, Fe and Mnconcentration of scale, soil and stream sediment samples are higher than the mean value of world continental crust.According to the enrichment factor and geoaccumulation index, the geothermal fluid with intense rock interactionenriches soil and stream sediment in terms of As and Mn metal(loid)s as a result of discharge. It is recommended thatgeothermal fluids and waste waters from the facility should not be discharged in the soil and Ilıca stream.