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Monolayer transition metal dichalcogenides (TMDCs) hold the best promise for next generation optoelectronic and valleytronic devices. However, their actual performance is usually largely affected by the presence of inevitable defects. Therefore, a detailed understanding of the influence of defects on the dynamic properties is crucial for optimizing near future implementations. Here, the exciton population and valley scattering dynamics in a chemical vapor deposition grown large size monolayer WSe2 with naturally abundant vacancy and boundary defects were systematically investigated using polarization controlled heterodyned transient grating spectroscopy at different excitation wavelengths and temperatures. Slow and multi-exponential decay dynamics of the exciton population were observed while no sign of any micron scale diffusive transport was identified, consistent with the effect of exciton trapping by defects. In general, two different kinds of exciton species were identified: one with short population lifetime (∼10 ps) and extremely fast intervalley scattering dynamics (1 ns) and very slow intervalley scattering dynamics exceeding 100 ps. We assign the former to non-trapped excitons in the nanometer scale and the latter to defect-bound excitons. Temperature dependent intervalley scattering dynamics of the trapped excitons can be understood in terms of a two optical phonon dominated process at the K point in momentum space. Our findings highlight the importance of the intrinsic defects in monolayer TMDCs for manipulating exciton valley polarization and population lifetimes, which is key for future device applications.