Boron has two stable isotopes (l~ and liB) with relative abundances of about 20% and 80%, respectively. Boron isotopic ratios in natural materials show a huge range of variations, from -7096o to +60%0, when expressed with the classical 8laB notation. Most of these isotopic variations occur at the surface of the Earth. Hence, boron isotopic composition can be used as a sensitive tracer in geochemical study, for instance, to identify the different sources of contamination and factors controlling the salinity of groundwater. During the last decade, boron isotopes have been used to discriminate between the influences of seawater intrusion and anthropogenic discharge. But few of those researches can precisely identify the different sources of contamination. We measured the boron concentrations and boron isotopic ratios of groundwater samples collected in Guiyang City, as well as the major ions. The results indicate that the major ion composition of the groundwater in the investigated area is mainly controlled by the interactions between water and the dominant rock i.e. carbonates. All the water compositions are characterized by high concentrations of Ca 2+, Mg 2+, HCO3, SO42, and NOr, which are the dominant contaminants. Both dissolved boron concentrations and isotopic ratios show large variations among the ground waters, from 2 lag/L to 90 lag/L and from -6%o to +26%0, respectively. The boron concentrations and isotopic ratios indicate that the river across the studied city has been seriously contaminated by urban discharge. Boron concentrations of river water samples varied from 20 lag/L to 140 lag/L, with an average 811B value of +2.0%0. Using boron isotopic compositions and different geochemical indices allowed us to clearly identify and distinguish the two major sources of contamination, agricultural activity and urban wastewater. Both of the two sources are characterized by high boron concentrations but their boron isotopic compositions significantly differ. The 8nB values of urban effluents range from -6.0%0 to +5.0%o, clearly lower than the mean 8nB values of a typical fertilizer, about +15%o. For the studied city, some groundwater sites of the suburban area were contaminated by agricultural activities and most sites of the urban area were contaminated by sewerage waters. The two types of contamination present distinct evolution during a hydrologic cycle.