Organic carbon: this is a test done on water to see how much organic matter is in the water. If this test result comes back high, that could mean that microorganisms (bacteria, algae, etc) are in the water, or it could indicate the presence of sewage or animal waste.
Sulfates, SO42-, are naturally occurring in some minerals, so some level of sulfates will be present in any body of water. If sulfuric acid, H2SO4, causes acid rain, that would cause sulfate levels to increase. If copper (II) sulfate, CuSO4, is added to the water, that would also cause sulfate levels to increase. Copper sulfate is sometimes added to ponds to kill algae, but it is not typical to add copper sulfate to rivers. Sulfates can be toxic to fish in high enough concentrations, but the toxicity depends on some other factors; fish and other organisms become more sensitive to sulfate toxicity as water hardness and chloride concentration increase.
Acid Rain has a pH of approximately 4.2-4.4, according to the EPA, while clean rain has a pH of about 5.6. This means that even normal rain causes the pH of a lake or river to drop (become more acidic). A healthy lake has a pH of about 6.5, while an acidic lake might have a pH of 4.5. Source here. Different fish have different sensitivity to pH changes, but both high and low pH can kill fish. A general guideline is that fish need pH to stay between about 6 and 9.
Chlorine is added to water to kill bacteria and other microorganisms. It becomes chloride ions after being added to the water. Chloride can be toxic to any organism in large enough concentrations. Water treatment plants try to add enough to kill bacteria but not enough to harm humans, fish, and marine plants. Treatment plants look at the organic carbon test results to see if they need to add more chlorine, because a high organic carbon can indicate the possibility of bacteria in the water.
Nitrates, NO3–, are found in plant fertilizers – this is because it’s a source of nitrogen that plants can use to make proteins. Any synthetic organism needs a nitrogen source to grow. Nitrates are found in both chemical fertilizers and manure fertilizer.
Rainfall can lead to runoff. Runoff is the washing of fertilizers and other chemicals from agricultural fields into streams and rivers. Runoff can cause algae to grow rapidly, because fertilizer helps photosynthetic organisms grow. Runoff also causes the actual dirt from land to wash into the river, which can make the water cloudy / dirty / muddy.
Dissolved oxygen: Fish and other organisms need some dissolved oxygen in the water; too little, and they suffocate. Too much dissolved oxygen is also a problem, because it causes bubbles to form in fish gills, which kills the fish.
Dissolved oxygen concentration is affected by temperature; more oxygen can dissolve in cold water.
Lime, CaO, is added to water if pH gets to low. Lime raises the pH of water because it is a weak base. Lime is partly calcium, so adding lime to the water will also increase the calcium levels in the water. Lime is added in treatment tanks at the water treatment plant. Before the water is released back into the river, it is further treated to remove
Slaked Lime, Ca(OH)2, is a chemical used in flucculation to take out suspended particles, such as clay and dirt, from water. The slaked lime is added into treatment tanks along with other chemicals, then suspended particles attached to it and eventually fall to the bottom of the tank. These solids need time to settle out to the bottom before the water can be safely added back into the river. Ca(OH)2 is a base, so it would change the pH of river water if it was added into the river.
Treatment Tanks: chemicals are added to the water at the treatment plant to treat the water. These chemicals are either removed or neutralized before the water can be safely returned to the river. If the tanks overflow and the overflow reaches the river, that will result in chemicals being put into the river.