Typical low-tech, portable, field test methods for chemical water quality monitoring fall into three categories:
- Test strips – These are small, single-use strips that change color to indicate the concentration of a specific chemical. Depending on the particular test, the user “activates” the paper or plastic strip by dipping it into the water sample and swishing it around, or by holding the strip in a stream of water. After waiting for a short time, the user compares the test strip color with a color chart to read the concentration of the chemical. These kits are extremely simple, but they are less accurate than other methods, especially if users don’t follow the instructions.
- Color disk kits – Color disk test kits are available for a wide range of chemical tests. In a typical set-up, the user adds a powder packet or a few drops of a liquid reagent to a water sample in a reusable plastic tube. The user then places the sample tube in a small plastic viewing box. This viewing box contains a plastic disk with a color gradient printed on it. The user rotates the color disk to find the part that best matches the color of the sample, and then reads the concentration of the chemical from the disk. Color disk kits typically have multiple steps and often include prescribed wait times, so they’re a little more complicated and costly, but generally more accurate.
- Hand-held digital instruments – Lightweight and portable digital meters, colorimeters, and photometers are available for water testing. They provide the most accurate results of these three testing methods, but they are also more expensive and delicate than the previous options. These instruments require batteries and calibration. While digital instruments are helpful to field technicians and are an essential part of any continuous or remote monitoring network, they are unlikely to be suitable for “citizen science” or crowdsourced water quality testing.
Chemical water quality parametersHaving identified various test formats, the next question is: What do we test for? UNICEF recommends prioritizing fluoride, arsenic, and nitrate for chemical monitoring. In areas where the earth is naturally rich in minerals that contain fluorine and arsenic, levels in well water can be high enough that chronic exposure is dangerous to human health.
How can we test for these elements?
- Fluoride: At least one color disk test kit is available for fluoride. However, portable digital colorimeters are often preferred because of concerns over accuracy. Ackvo Caddisfly, a testing system, recently described a colorimetric fluoride test that can be read by a smartphone app.
- Arsenic: Portable field testing options for arsenic are limited; this contaminant is best measured in a laboratory. Commercially available test kits do exist, but they are relatively complex and require several steps. Although the arsenic concentrations “measured” with these test kits may be inaccurate, the kits do detect arsenic in nearly all samples greater than 100 micrograms per liter (ug/L), as well as in most samples in the 50-99 ug/L range. UNICEF has therefore recommended reporting arsenic monitoring results from these portable tests as “present” or “absent” using a reference concentration of 50 ug/L—the drinking water standard in many countries that are affected by natural arsenic contamination.
- Nitrate: Both test strips and color disk test kits are available for nitrate testing. Nitrate can also be measured with a digital meter. High levels of nutrients are associated with agricultural pollution from fertilizers (nitrogen and phosphorous) and animal waste (nitrogen). Latrines, sewage, landfills, and industrial pollution can also contribute nitrogen. Monitoring for nitrate is a simple way to assess the impacts of agricultural and human waste on water quality.
- Iron and Manganese: Both test strips and color disk tests are available for these two metals, which may also be measured using portable, digital instruments. Field testing with digital equipment is considered reliable for iron and manganese.
- TDS: TDS includes a mixture of inorganic salts, mostly sodium, chloride, potassium, calcium, and magnesium. Rather than testing the particular components, TDS is monitored by measuring the conductivity of the water with a digital meter. There is no test strip or color disk kit that can be used here, although at least one conductivity meter interfaces with a smartphone.
- pH: pH test strips and color disk tests are widely available. More expensive, higher-tech options include electrode-based pH meters. pH is a measure of hydrogen ion activity, which means that it tells us how acidic or basic the water is. pH is not a pollutant, but it is a chemical master variable. It affects the behavior of other chemical constituents, including the effectiveness of residual chlorine against microbial contamination. Sudden changes in pH can also reveal treatment plant failures or pollution events in natural water bodies (for example, illegal industrial discharge).
- Chlorine: There are many easy ways to test residual chlorine, including test strips, color disks, and even kits designed for testing swimming pools. Portable digital meters also exist that can provide reliable, quantitative measurements.
However, the vast majority of these additional tests are best performed in a laboratory given current technologies. That said, low-tech testing – often involving a smartphone – is an area of active research for parameters like mercury and pesticides(also here).
Stay tuned for our next post, which will dive into microbial water quality testing!