Do the constituents (e.g., pollutants) leak from a point source into the river or canal?

Yes it's from point sources James C. Trask

From a point source, the concentration in the river at short distances downstream from the point source will vary with position in the river, due to incomplete mixing across the width of the river. The concentration of the contaminant at such short distances downstream is a complicated function of the specific river flow dynamics, and will vary across the cross-section of the river.

At some distance far downstream from the point source, turbulent mixing will have brought the concentration of the contaminant approximately constant across the width of the river (or canal). For a contaminant that is (a) soluble in water (b) nonreactive or reacts slowly at most with other constituents in the river water (c) is nonvolatile or has low volatility (d) does not adsorb to or has low adsorbtion to sediments/solids carried by the river (or lining the river or canal bottom and banks); then if the contaminant is added at an approximately constant rate to the river and if the volumetric flow rate of the river (or canal) is approximately constant over the reach distance from point source to sampling point (and over the time period of mean transit time of river water from location of point source to sampling location), then

concentration of contaminant ~ (mass rate of contaminant addition to river at the point source)/(volumetric flow rate of river)

where units are (mass/time)/(volume/time) = mass/volume of contaminant.