Agua SALud: Water Quality Analysis


Microbial Indicators of water quality

The use of normal intestinal organisms (bacteria) as indicators of fecal pollution rather than pathogens themselves is universally accepted for the monitoring and assessing the microbial safety of water supplies[i]. The membrane filtration (MF) method was used to determine the amount of total coliforms (TC) and Escherichia coli in water samples, measured as colony forming units per 100 milliliters (CFU/100ml).  This method utilizes m-ColiBlue24™ broth to distinguish between E. coli and total coliform based on the presence of a particular enzyme (β-glucuronidase).

Microbiological samples were collected in 125ml bottles from both communal water sources and from individual households where children under five were living.

After collection, samples were placed in a cooler with ice and filtered within 6 to 12 hours. During filtration, 100ml of the sample was vacuum-filtered through a 0.45-μm nitrocellulose membrane. The filter was transferred to a petri plate containing an absorbent pad saturated with 2ml of m-Coli Blue24-Broth™. The filter was incubated at 35°C +/- 2°C for 24 hours. Total coliforms were evidenced by the appearance of red and blue colonies, whereas the blue colonies were specifically E. coli. 


Temperature, pH, Conductivity and Total Dissolved Solids

At each water source: temperature, pH, electrical conductivity, and total dissolved solids were measured using a handheld combo meter (Hanna Instruments 98130). In addition GPS coordinates were recorded at every collection site.



Although conductivity was used as an onsite indicator of salt concentration, samples were analyzed in the Mexican National Commission of Water laboratory. Sodium (Na+) content was assessed using a Corning™-412 Flame Photometer. Calibration curves were generated by analyzing standards of 10, 20, 30, 50, and 100 ppm Na+ prepared from a NaCl stock solution.  Distilled water was used as a blank (0 ppm).  Sodium content of water samples was then extrapolated based on the calibration curves.


Arsenic Field Kit

Arsenic was measured using the Arsenic Quick field test kit from Industrial Test Systems (ITS). Following the sequential addition of three reagents (tartaric acid, a separate oxidizer, and zinc powder), the arsenic concentration of a sample can be measured based on the reaction of arsine gas and a colorimetric test strip coated with mercuric bromide. The test procedure is designed to detect inorganic arsenic As+3 and As+5.

The field test kit measurements were validated with several samples analyzed at a lab at the University of Washington following standard procedures.


Nitrate Field Kit

Nitrate in water sources was measured using a Chemetrics field test kit based on the cadmium reduction method.  In the former, nitrate is reduced to nitrite using cadmium as the reducing agent. The resulting nitrite reacts to form a pink azo dye which is measured colorimetrically in a range between 0 and 5 ppm.

[i] WHO. Guidelines for drinking Water Quality, 2nd edition, vol.2. WHO, Geneva, 1996