Freezing point depression and organic molecules
To get a freezing point depression of $\pu{−1.86 °C},$ the solution should be made by adding $\pu{1 mol}$ organic substance to $\pu{1 L}$ water. And $\pu{1 mol}$ per liter is much. For glucose, it is $\pu{180 g}$ per liter. It is nearly a syrup. I doubt your lake has such a high concentration whatever the nature of the solute.
For salts, it is less, because usual salts are made of two ions. For example, $\pu{29 g}$ $\ce{NaCl}$ in $\pu{1 L}$ water is enough to get a freezing point depression of $\pu{−1.86 °C}.$ Anyway, $\pu{29 g}$ salt added to one liter is not negligible. Are you sure of your measurements?