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Is Rainwater Really Safe - One Sample Case
by Doug Pushard

Water quality is an extremely hot topic these days. With continued population growth and strained water supplies it is likely to become even more so. It is estimated that already one in five homes have some type of water filtration or purification system installed. The bottled water industry is also experiencing a boom. Water quality concerns are probably a factor driving growth of rainwater harvesting as well.

But little information is readily available on the quality of rain water. Consequently, it is difficult to understand what the real issues may be and then make decisions on what type of treatment system to install, if any. Claims by manufacturers of various products trying to hawk wares definitely have a point of view. So it is a period of “caveat emptor - let the buyer beware”, in trying to figure out what to do.

So the question remains - Is rainwater safe? To shed light on the issue, I performed a set of water quality tests. Water from a cistern and an adjacent well in Lamy, New Mexico were tested. Additionally, I compared these samples to city water from my new home city of Santa Fe, New Mexico.

A Few Key Water Quality Metrics

pH - The measure of the water's relative acidity or alkalinity. A pH of 7 is neutral while higher numbers indicate acidity and lower alkalinity.

Conductivity - A measure of how well the water conducts electricity. This gives an indication of ion concentration in the water sample.

Turbidity - Measures the impedance of light through water. The particles which cause turbidity can interfere with disinfection by sheltering microbes; thereby enabling bacteria to survive UV sterilization, for example.

Hardness - Hardness is caused by calcium and high levels of magnesium salts and inhibits the cleaning action of soaps and detergents. It can also cause deposits of scale on the inside of hot water pipes and cooking utensils.

Total Suspended Solids (TSS) - are undissolved particles ranging in size from a pine needle to a single-cell

eColiform Colony Count - Indicator organisms (such as Escherichis coli) are used as a proxy for the presence of specific disease-causing organisms like Salmonella typhi.

Water was drawn directly from the cistern and well (i.e. prior to any treatment) on the same day and sent to the National Testing Laboratories to be tested in accordance to their procedures. These samples I then compared to Santa Fe’s published Water Quality report. Most water districts publish this information as it is required by the Environmental Protection Agency (EPA).

Comparing the cistern water to the well water, it is clear that cistern water is better or equal to the well water; except that the cistern has a presence of coliform. Coliform, a bacteria, is frequently found in streams, lakes, ponds, cisterns; and may indicate the water is contaminated with other harmful bacteria. Systems used for drinking water should always have some type of sterilization process (See related article on UV Sterilization of Rainwater Catchment Systems) to kill bacteria and organisms such as coliform.

Not so surprisingly there is a huge difference in hardness, calcium, and alkalinity between the well water and the rainwater. Well water typically drains from underground water tables and is normally stored in natural underground tanks; so it will slowly leach minerals and metals from the surrounding ground. Consequently, well water will generally have higher mineral and metal content depending on local soil conditions. In comparison, rain water will not have these issues, but may be adversely affected by local air pollution and debris in the rainwater catchment and conveyance areas (i.e. roof, gutters, downspouts, and pipes).

In the table below, the best (i.e. lowest) rating is in GREEN and the highest rating in RED. This, of course, assumes you don’t want your metals and minerals coming from your tap water. As can be seen from this comparison, rainwater is at the lowest end for most of the tests. Limited air pollution in the Lamy area likely positively impacts the high quality of the harvested rain.



  Cistern Well City
18.6 - 532
Total Dissolved Solids*
130 - 884
.014 - .21
0 - 27.48
<.01 - .063
0 - .107
0 - .046
7.1 - 8.0
<.02 - <.05
0 - 31
<.02 - .058

(0) zero denotes none present, NA means not reported and * means milligrams of substance per liter of water. It is important to note, none of the tests exceeded the Maximum Contaminant Levels (MCL) as set by the Environmental Protection Agency (EPA) except for the presence of Coliform.

Both well and cistern results were surprisingly good in comparison compared to the Santa Fe municipal water. Santa Fe gets its water from a range of sources. Included in the table are the low and high readings from the various sources as reported in the 2004 Water Quality report published in the first half of 2005. Santa Fe tap water will typically be a mixture of the water sources and consequently be a blending of the levels reported above.

In this comparison, the city water was absolutely highest in Total Dissolved Solids, aluminum, chloride, iron, manganese, and silver. The city water, from at least one source, also had the highest hardness. In all the tests where data was reported the city water was only lower in one category – zinc. Remember all the data reported above is prior to treatment and consequently most of these are taken out or greatly reduced by the city prior to being delivered to the tap (i.e. through chemicals and my tax dollars at work).

In this admittedly highly limited test, rain water faired impressively well against both well and municipal water supplies. It had lower pH, hardness and mineral content; consequently, it should cause less scale build up in pipes and cooking utensils; suds up better in the shower; and probably taste better. But the rain water did have coliform present which can be a health hazard and always needs to be treated.

To reduce the probability of coliform in rainwater harvesting systems and wells the following steps are highly recommended:

  1. Always keep the rain catchment clean and free of debris
  2. Trim trees and brushes near the area to prevent animals from entering the storage tanks
  3. Keep water storage tanks shaded and use non-transparent tanks to prevent sunlight from fostering bacteria growth
  4. If the water is to be used for drinking, always use some type treatment system (e.g. UV sterilization) to deal with the potential bacteria.

As can be seen in this small sample, rainwater can actually be better than tap water, but always be safe - test it before you use it!


Related Info - Sample water quality report
Related Info - EPA published water standards
Related Article - UV Lamps
Related Article - Using a UV Lamp in a Rainwater Catchment System
Related Article - Simple sand filter for water purification
Related Info - Santa Fe Water Quality reports (under E-Library)
Related Info - Santa Fe Water Quality report used in article
Related Webpage - Vendors of water filtration systems
Related Webpage - EPA - Protecting Drinking Water
Related Book - Drinking Water Book - How to Filter Your Water





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