When is water safe to drink?

Conserving Resources – A Virtual Lab on Water Treatment


When is water safe to drink?

Suppose you were hiking along a stream or lake and became very thirsty. Do you think it would be safe to drink the water? In many cases, it wouldn’t. Each source of fresh water on or beneath Earth’s surface is affected by contaminants. Contamination of water resources can come from both point and nonpoint sources.  Point sources are specific sites where contamination occurs.  An oil spill or an outflow from an industrial facility are examples of point sources.  Nonpoint sources are more widespread.  Farmland runoff contaminated with fertilizers and pesticides or urban runoff from impervious surfaces like concrete are examples of nonpoint sources.  Though the sources of these contaminants are varied, all can make water unfit to drink if they are allowed to increase beyond safe limits. Some of the most common issues with water quality are:


The pH scale is a measure of how acidic or basic a water sample is.  Water with a pH reading of zero to around 6.5 is acidic. It may not be safe to drink and it can corrode metal pipes, causing them to dissolve. Strongly basic water (much above pH 8.5) is may also be unsafe to drink. Worse, basic water may deposit minerals which slowly clog the pipes up with rock-like material!  Water with high or low pH levels can also magnify the effect of other contaminants.


Fecal coliform bacteria are naturally abundant in the lower intestines of humans and other warm-blooded animals, but are rare or absent in unpolluted waters. Fecal coliform in drinking water typically measures 0.1 colony/100mL.  Although coliform bacteria themselves are not pathogenic, their presence indicates possible fecal contamination and the corresponding presence of intestinal pathogens responsible for a variety of diseases. Thus, high counts (e.g. greater than 200 fecal coliform per 100mL of water) may be a sign of the presence of pathogenic organisms.  Coliform bacteria are also commonly found in lakes, rivers, and ponds, but can seep into groundwater supplies. When coliform bacteria are present in your drinking water, your risk of contracting a water-borne illness is increased.


Copper and iron are two of the more common metal contaminants found in water supplies. While we need small amounts of these metals to be healthy, an overabundance of copper and iron can cause water to be discolored and foul-tasting. Liver damage can also be traced to unsafe levels of metallic contaminants in water.

Most copper and iron contaminants enter the water supply through rusty and corroded pipes. However, metallic contaminants can also enter groundwater through erosion as the water travels through layers of rock and minerals, depending on the geology of the area.


Nitrates are a form of nitrogen found in animal wastes, chemical fertilizers, and food preservatives. Found in both surface water and groundwater, nitrates enter the water supply through surface runoff from farms and from leaking household septic tanks. Nitrates in surface water encourage algae growth, which benefits aquatic animal populations.  However, an excess of algae, called an algal bloom, blocks sunlight for submerged aquatic vegetation (SAVs).  SAVs are a critical source of food and also act as nurseries for a variety of aquatic animals.  When the algal bloom dies, its decomposition removes dissolved oxygen from the water, turning the water anaerobic in the process of eutrophication.

Contamination of drinking water with nitrate can lead to methemoglobinemia, or “blue baby” syndrome, in babies under six months.  Older children and adults are not affected in this way.  In this country, where our drinking water is treated, this syndrome is only a concern for households that use well water. Nitrates pose little immediate threat to most humans, but an overabundance of nitrates can kill fish and other aquatic creatures. High levels may also increase our chances of getting cancer, as some nitrates may react with stomach acids to from low levels of carcinogenic compounds.



Pesticides and herbicides are manufactured chemicals that are used to kill weeds, molds, insects, and other pests. Carbofuran and alachlor are examples of common herbicides used in agriculture. Surface runoff can introduce pesticides and herbicides into the water supply. In concentrated amounts, these substances can cause a number of health problems, including anemia, cancer, and liver and kidney disorders.

Total Phosphate

We won’t be testing for this today, but this is an important component of water quality. The element phosphorus is necessary for plant and animal growth. Nearly all fertilizers contain phosphates (chemical compounds containing the element phosphorous).  Phosphates enter waterways from the same sources as nitrates and have the same effect on the water body.  Generally, phosphate is removed at wastewater treatment plans via chemical reactions, however, orthophosphate is often added to drinking water to help prevent the leaching of metal pipes.


In this Virtual Lab, we will be examining E. coli concentrations in several water samples.


  • See how laboratories assess for water quality.
  • Determine what to do if your samples are outside acceptable water quality measurements.
  • Identify treatments that remove contaminants from drinking water.



Go to the following website: https://conserve.nmsu.edu/

Click on play water testing than start the video to understand why we are sampling E. coli in our water.

Why are we measuring E.coli and where does it come from?


What temperature do we store our water samples at and why?


Why do we run our tests within 24 hours of collecting the sample?


What EPA method are we using and why?


How many dilutions will we prepare and why?


After completing the video and answering the questions, click on “start on day 1”. For the simulations for day 1 and day 2 you will need to click on the object it specifies and the simulation will advance. Please make sure you answer the questions here on your lab worksheet as you go. Once you complete day 1 click continue to day 2.

Where is our water sample from?


What is our very first step?


What are the concentrations of our dilutions? Which will have the most bacterial colonies?


After we filter our diluted samples, where do we put the plates and for how long?


On day 2, after we don our safety gear, which plate do we examine (the plate with between 30-300 colonies)?


Write out the equation, including your data, for determining the number of colony forming units. Include your answer.


What was the average colony forming units from the three samples?




  1. What are the correct options of what to do if your water does not meet the standard? For each option, explain why this is a viable approach.








  1. What is pH level, what are its characteristics, and how does it contribute to pollution? What chemicals are used in treating low pH levels?



  1. Water in an old building, tested recently, showed high copper and iron content, and low pH levels. A water reading taken 20 years before showed low pH levels, but only minimal traces of copper and iron. If none of the new buildings on the same street showed signs of metallic contaminants, but both old and new building show lower than normal pH readings, how might these readings be explained?



  1. What is the difference between a point source and a nonpoint source of pollution?



  1. Which might be more difficult to control? Why?




  1. What causes eutrophication and how does it affect DO?





  1. The excess of which factor causes “blue baby” syndrome and why is this only of concern for households using well water?





  1. The excess of which two factors causes an algal bloom?



  1. How does an algal bloom affect aquatic plants and animals?






  1. If coliform bacteria are typically not dangerous, why is it important to monitor water for their presence?





  1. What is typically added to water to prevent the leaching of metals from pipes?






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