The lakes around Phnom Penh are used as the natural treatments of wastewater before discharging into the river. Thus, the pollutant load in the wastewater concerns the water quality in the natural lakes. This study aims to assess the spatial variation of water quality in Tamouk Lake, which is located in the northern part of Phnom Penh and partly receives the wastewater from Phnom Penh, at the pre- and post-storm events. The Spatio-temporal maps of water quality (TSS, DO, EC, pH, COD, PO4, and NO3) were generated from 11 points by using the geostatistical interpolation method (SK). The selected sampling points cover the pollution at inlets, outlets and open water in the lake. The sampling procedure was carried out in two different events, which were on 28 May (pre-storm event) and on 7 June 2019 (post-storm event). The water quality parameters were classified into 5 classes based on the single factor pollution index score (Pi). Then, the overall pollution level in the lake was evaluated based on the comprehensive pollution index (P). According to the MoE’s standard (Ministry of Environment), the EC and pH exceeded the limited standard. Based on the pollution index score, the parameters DO and COD stayed in class III for both events. The TSS was slightly concerned (class II) at the post-storm event. PO4 was a moderate pollutant at the pre-storm event (class III). The NO3 was found to not share its contribution to pollution for both events. The spatial maps of water quality indicate that the spatial distribution of the water quality parameters increased dramatically nearby the inlet at the pre-storm event, but slightly increased nearby the outlet of the lake during the post-storm event. However, only TSS rose steeply in the lake during the post-storm event. The water quality parameters of Tamouk Lake were found to exceed the limited standard of effluence. Therefore, the study would support the receiving water- monitoring program or determine the total maximum daily load in order to maintain the standard of water quality in this lake. The output of the study on the basic parameters could be a good indicator for further investigation including the assessment of bio/chemical pollution.