Other than not having enough water for your herd, the next biggest problem would be the content of dissolved solids such as minerals, salts, metals and so on.
One of the effects of dissolved solids reaching a high level is that water intake would be reduced therefore consumption of both forage and mineral supplements would be reduced as well.
Most people think that dry matter consumption declines during the summer due to hot weather. However, in situations where there are water quality issues, the bigger problem would be the lack of water consumption.
In general, cattle water consumption increases as the weather gets warmer. There is also a relationship between amount of water required with the amount of feed intake cattle typically achieve. Here is a rough guideline:
0°C - 25°C: Approximately 2kg of water is required for 0.5kg dry matter
25°C - 30°C: Approximately 2.5 - 5kg of water is required for 0.5kg dry matter
30°C and above: Approximately 5kg - 10kg of water is required for 0.5kg dry matter.
Another important factor to consider is the lushness or dryness of the forage which will also affect the amount of water cattle need to drink. For example, during spring, the pasture may be up to 80% moisture and therefore the cattle need little additional water beyond what they consumed from grazing. However, during summer where forages may be much lower in moisture, then the cattle may require to drink a lot more water.
In terms of water quality, it is usually not the issue of whether the water is clean or dirty. Sometimes, the water may appear to be "clean" but it is dissolved in solids such as minerals. Just by having an excess of one mineral or the lack of it, it will affect the other minerals in the animal's body. It will also decrease the animal's water consumption due to the smell which is normally not detectable by human nose.
Other dissolved solids which has a negative impact would be iron, sulfur/sulfates and salts. These are considered one of the worst as they are salts and having high amount of salts will cause diarrhea and growth problems.
Another issue with sulfate is that it will add to the total load of sulfur in the bodies of animals that drink high-sulfate water. Sulfur at levels above 500 parts per million (ppm) in the water may tie up copper. This will cause problems to the cows such as reproduction issues, lameness, growth and others. When the sulfur levels reach 1,000 ppm, some cattle may have problems such as polio and the likelihood of that will only increase as the concentration of sulfur increases.
Another dissolved solids that will cause problem is excess iron. There is a possibility that it will show up physically as red coloration in stock tanks or will leave a red stain if you run water out on the ground, allow it to dry and then come back. If the cows consumed excess iron, it will inhibit copper and zinc absorption and this will lead to a condition called cellular oxidative stress. These conditions will affect the immune function, increase mastitis, and diarrhea. It will also decrease milk production and affect growth rate.
Best way to test your herd's water quality is to send it to lab for testing. It is recommended to go for certified labs. Specific tolerance levels for a variety of individual contaminants can be found here.
The herd's water quality will affect your farm's profitability more than you think. Do not underestimate the importance of having sufficient clean, fresh and high quality available to your cows at all times.