A fairly clean ice cover can form over a contaminated water pond when the air-cooled surface of water freezes and impurities are efficiently expelled to the remaining water underneath. Natural freeze crystallization has recently been studied as a potential wastewater purification method with aqueous solutions on a laboratory scale. The effect of impurity inclusions on ice strength has been researched in model ice basins over the past few decades. It is of interest to discover how efficiently natural freeze separation works under real weather conditions before freezing can be utilized for wastewater treatment application. Herein, understanding the mechanical strength properties of naturally frozen wastewater (ice) is important when planning ice breaking and harvesting devices. This research implemented in-situ measurements of the flexural and compressive strength of ice in natural ice-covered environments of a freshwater lake, two peatlands and three mining site basins, and compares the determined strength with analyzed impurities of the ice. The results showed that despite varying ice growth conditions and initial water constituents, it was possible to deduce an evident yet simple relationship between mean ice strength and ice impurities: the more impure the ice is, the lower the value of strength is Based on this exploration, it was concluded that separation efficiencies, i.e. the impurity removal ratio between basin water and ice, from 65% up to 90% can be achieved by natural freezing.