Basic water quality testing

Summary

This article teaches how to assess water with no laboratory, or with a minimal one, in order to decide what treatment it needs and check that it works. It covers sanitary inspection of the source, the senses, turbidity, free residual chlorine (target of 0.2-0.5 mg/L at the point of consumption), pH, dissolved salts, and screening for nitrate, arsenic and fluoride. The result is a record per source with a specific action for each parameter (see “Summary table”).

What can be done at each level:

• N1: inspection, senses, the text test and the disk in a ceramic bowl, pH with red cabbage and taste.
• N2: glass turbidity tube.
• N3: dry residue, starch-iodide test and hydrometer.
• N4: DPD kit, strips and chemical kits, and a conductivity meter.

Fecal indicators (E. coli, H2S test) are covered in Microbiological water testing. No field test proves water is safe: it only reduces uncertainty.

Prerequisites

• Knowledge: measuring and weighing; it helps to know boiling, SODIS, chlorination, coagulation and the filters (sand, ceramic).
• Materials N0-N1: notebook, clean bucket, shallow white glazed ceramic bowl, letters 3-5 mm tall written or printed, red cabbage, vinegar and baking soda (sections A, B, C and E).
• Materials N2: transparent tube 90-100 cm long and 3-5 cm in internal diameter, a stopper and a white disk with black quarters (section C).
• Materials N3: porcelain dish or stainless-steel pot, scale accurate to 0.01 g, oven at 105-180 °C, hydrometer 1.000-1.050, potassium iodide and starch (sections D and F).
• Materials N4: DPD kit, pH strips, conductivity meter, nitrate strips and fluoride and arsenic kits (sections D to G).
• Time and people: inspection, senses and turbidity, 30-60 min per source, better with 2 people. Building the tube, 1-2 h. Chlorine and pH, 5 min per sample. Dry residue, 3-6 h, mostly waiting.

How it works

What is evaluated and why

Waterborne diseases almost always come from feces. Microbiological tests tell whether there is contamination today, and sanitary inspection tells where it comes in and what to fix. Crystal-clear water can be heavily contaminated, so the senses only give an indication. Turbidity, chlorine and pH decide whether a treatment can work; salts and invisible chemistry decide whether the source is acceptable.

Turbidity

Turbidity is the scattering of light by suspended particles. The particles shield microbes from chlorine and UV light, consume chlorine and clog filters. It is measured in NTU. The eye starts to notice it around 5 NTU, muddy-looking water exceeds 100 NTU, and at 2000 NTU it is opaque (Myre and Shaw 2006). In the tube, the height of water at which a contrasting pattern disappears falls exponentially with turbidity: depth (cm) = 244.13 × NTU^(−0.662) (R² = 0.996). The scale is logarithmic and should not be interpolated by eye. The tube does not measure below about 5 NTU, and colored water gives higher readings than its actual turbidity.

Free residual chlorine and pH

Part of the added chlorine is spent on organic matter and iron; what remains is the free residual chlorine (HOCl + OCl⁻). Measuring it shows there was enough disinfectant and that the water remains protected. In the DPD method, chlorine oxidizes N,N-diethyl-p-phenylenediamine into a pink compound proportional to the concentration. HOCl disinfects much better than OCl⁻, and above pH 8 OCl⁻ predominates. That is why WHO requires a residual ≥ 0.5 mg/L after 30 min of contact at pH < 8. pH has no health-based guideline value; the usual range is 6.5-8.5 (WHO 2007; EPA secondary standard). More acidic water is corrosive, and more alkaline water forms scale and reduces chlorine's effectiveness. Red cabbage anthocyanins are red in acidic medium, violet near neutral, and blue or green in alkaline medium.

Dissolved salts

Total dissolved solids (TDS) are the salts left after evaporating filtered water. WHO rates the taste as excellent below 300 mg/L, good from 300 to 600, acceptable from 600 to 900, poor from 900 to 1200, and unacceptable above 1200 mg/L (WHO 2003). Salts increase density by about 0.76 mg/mL per g/L: seawater at 1.0248 versus fresh water at 0.9982 g/mL at 20 °C, divided by 35 g/L. That is why a hydrometer is only useful for brackish or sea water. Boiling does not remove salts: it concentrates them.

Chemical contaminants that cannot be seen

Nitrate, arsenic and fluoride have no color, smell or taste at dangerous doses. Boiling, chlorine, sun and sand or ceramic filters do not remove them, and boiling concentrates them (section G).

Procedure

Order for a new source: A → B → C → F → microbiological testing → G if the area is at risk → decision with the “Summary table”. After treating, repeat C, D and E on the treated water. The retesting schedule is in H.

A. Sanitary inspection of the source

1. Draw a sketch of a 100 m radius: slope, latrines, pits, corrals, watering troughs, dumps, cemeteries, fertilized crops, fuels, mines and open wells. Measure by paces (1 pace ≈ 0.75 m; calibrate it over 20 m of tape). Success criterion: each hazard noted with its distance in meters and whether it lies upslope or downslope.

2. Answer the risk questions (yes/no), adapted from WHO's forms (2024). Dug well:

   • Is the pump or pulley in poor condition or loose from the slab?
   • Is the slab missing or cracked, or the inspection cover broken?
   • Are the wall or headwall cracked?
   • Is the apron around the well missing or broken?
   • Does water pool near the well?
   • Is the fence keeping animals out missing?
   • Is there sanitation within 15 m, or upslope within 30 m?
   • Is there open defecation, animals, refuse or fuel within 15 m?
   • Is there another open well or an unsealed borehole within 100 m?

   Spring: is the intake box missing or cracked?, is the runoff diversion ditch missing?, are there animals or latrines upslope? Rainwater: dirty roof?, no first-flush diverter?, tank without a lid? Success criterion: complete form, with the total number of “yes” answers and a corrective action noted for each.

3. Cross-check with Sphere (2018): latrines at ≥ 30 m from any groundwater source and with the base ≥ 1.5 m above the water table. In fissured rock or limestone, more distance is needed. Success criterion: each hazard classified as within or outside a safe distance.

4. Ask users whether the water changes after rain, whether there is diarrhea, whether the source dries up or floods. Success criterion: at least the answers about rain and diarrhea are recorded.

5. Interpret. Each “yes” is a risk to correct, starting with nearby sanitation, a broken slab and pooling water. With WHO's 10-question forms (1997, vol. 3): 0-2 “yes”, low risk; 3-5, intermediate; 6-8, high; 9-10, very high. The 2024 forms no longer give bands. Surface water is considered always contaminated. Success criterion: prioritized list of corrections and the source marked “always treat” or “candidate for testing”.

B. Observation with the senses

1. Observe: fill a shallow white glazed ceramic bowl (or a 1-2 L transparent container if available) with water from the source, let it settle for 30 min and look at it from above and against the light. Success criterion: color, transparency, particles and surface film recorded.
2. Smell it at the source and after shaking a closed container for 30 s. Success criterion: odor described with a word from the table or “none”.
3. Taste, only if there is no fuel or chemical smell, no blue-green scum, and no mines, industry or dumps nearby. Boil for 1 min, let cool, pass 5-10 mL through the mouth and spit it out. Compare with rainwater. Success criterion: recorded as “tasteless”, “salty”, “bitter”, “metallic” or “chlorine-like”.

C. Turbidity with a homemade tube

Construction (1-2 h):

1. Cut the tube to 95 cm. For glass, score it with a file, heat the score, cool it with a drop of water and sand the edge. Success criterion: edge free of sharp points.
2. Make the disk: a white circle that fits loosely inside the tube, with two opposite quarters painted black. If it is wood or cardboard, varnish it. Success criterion: good contrast and the disk does not float.
3. Glue the disk to the stopper, place it at the base, fill the tube and leave it for 30 min on paper. Success criterion: dry paper.
4. Mark the scale from the disk (not from the bottom) using the table. Success criterion: legible marks at least at 5, 10, 20, 30, 50, 100 and 200 NTU.

Table from Myre and Shaw (2006). The values of 7.3 and 85.4 cm are calculated. For 5 NTU the equation gives 84.1 cm; the original table's value is kept. If the tube is very different (less than 2 cm or more than 6 cm in diameter, or tinted plastic), calibrate it against a turbidimeter as soon as one is available.

Measurement (5 min):

5. Take the sample in a bucket without stirring up the bottom of the source, rinse the tube with it and homogenize it by stirring without creating bubbles. Success criterion: uniform sample.
6. Stand in the shade, without sunglasses, with your eyes 10-20 cm above the mouth of the tube, and pour slowly. If bubbles appear, wait for them to rise. Success criterion: still surface.
7. Stop as soon as you can no longer tell black from white and read the level. If the tube fills up and the disk is still visible, note “< 5 NTU”. Success criterion: two people read the same mark or two adjacent ones; if not, note the average of 3 readings.

What to do based on turbidity:

D. Free residual chlorine

With a DPD kit (N4):

1. Wait ≥ 30 min after chlorinating and fill the comparator cell (usually 10 mL) without spilling. Success criterion: cell filled to the mark and free of bubbles.
2. Add one DPD No. 1 tablet, crush it, shake and read within 1 min against skylight. A late reading adds combined chlorine. Success criterion: the color matches a standard or falls between two.
3. Interpret:

Success criterion: value recorded with the time, point and pH. 4. If no color develops but there is a strong chlorine smell, dilute 1 volume of sample in 9 of chlorine-free water, repeat and multiply by 10. Above about 10 mg/L, DPD bleaches out and gives a false zero (Hach DOC316.53.01023). Success criterion: reading within the scale.

Starch-iodide test (N3):

5. Calibrate first. Prepare a stock solution with 1 mL of 5 % bleach in 1 L of rainwater (≈50 mg/L nominal). From it, put 10 mL into 1 L (0.5 mg/L) and 40 mL into another liter (2 mg/L). Success criterion: two labeled standards, prepared on the same day.
6. Test: in a white glass, mix 20 mL of water, 2 mL of vinegar, 2 mL of cold water used to cook rice (starch) and a crystal of potassium iodide of ≈0.05-0.1 g. Do this with the sample, with both standards and with a blank of unchlorinated water, and compare within 1 min. Success criterion: the sample ranks between the blank and the standards. If the 0.5 mg/L standard does not turn blue, use the test only to tell whether chlorine is high or not. Iron(III), nitrite and late readings give false positives.

With nothing: if the container smells slightly of chlorine when opened 30 min after chlorinating, there is a residual. If it smells of nothing, the residual is insufficient.

E. pH

With red cabbage (N1):

1. Prepare the indicator: chop 100 g of red cabbage, add 500 mL of boiling water, let stand for 10-15 min and strain. Success criterion: dark violet liquid. Use it while vinegar turns it red and baking soda turns it blue.
2. Prepare the references: in identical glasses, 20 mL of rainwater + 2 mL of indicator, adding to each one lemon juice (pH ≈ 2), vinegar (≈ 3), nothing (rainwater), baking soda (8.3), borax (≈ 9.2) or washing soda (≈ 11.5). Success criterion: a clearly distinct series of colors.
3. Measure 20 mL of sample + 2 mL of indicator against a white background and find the two references it falls between. Success criterion: pH bracketed between two references.

The resolution is 1-2 units and is not enough to tell whether the pH is above or below 8. In very soft waters, the indicator itself shifts the color.

With a kit (N4): pH strips 6-9, phenol red comparator (6.8-8.2) or a pH meter calibrated the same day with buffers. Success criterion: resolution ≤ 0.5 units.

F. Dissolved salts

1. Taste (boiled and cooled water, spitting it out): if it tastes salty next to rainwater, it is probably above 1000 mg/L; if not noticeable, it is probably below 600 mg/L (WHO 2022, chap. 10). Success criterion: recorded as “not salty”, “somewhat salty” or “clearly salty”.
2. Dry residue (N3): weigh the dry dish to a precision of 0.01 g. Filter 500 mL of sample through coffee filter paper or a cloth folded 4 times and evaporate it in batches, over gentle heat or a water bath, without a rolling boil. Success criterion: dry dish with a film or white crust.
3. Dry for 1 h at 105 °C (or at 180 °C, per the standardized SM 2540 C method), let cool for 15 min covered and weigh right away, because the salts pick up moisture. Repeat 30-minute dryings until two weighings differ by ≤ 0.01 g. Success criterion: constant weight.
4. Calculate: TDS (mg/L) = residue (g) × 1,000,000 / mL evaporated (see “Calculations”). Success criterion: result in mg/L classified against the WHO scale.
5. Hydrometer (N3, for saline water): at 20 °C, relative density ≈ 1 + 0.00076 × g/L. So 1.002 ≈ 3 g/L; 1.008 ≈ 10 g/L; 1.026 ≈ 35 g/L (seawater). In absolute density at 20 °C, fresh water gives 0.998 and seawater 1.025. With a resolution of 0.001, the error is ±1.3 g/L. Success criterion: reading and temperature recorded.
6. Conductivity meter (N4): calibrate it with the standard solution. TDS ≈ 0.55-0.7 × EC (µS/cm), with higher factors in sulfate-rich waters (SM 1030 E). Success criterion: two readings agree within ±5 %.

G. Nitrate, arsenic and fluoride

1. Assess the risk in the area:

   • Nitrate (50 mg/L as NO₃⁻; nitrite, 3 mg/L): dug wells or shallow boreholes with the water table a few meters down, near latrines, manure or fertilizer. Affects formula-fed infants (methemoglobinemia, bluish skin).
   • Arsenic (10 µg/L, provisional): young alluvial and deltaic aquifers (Bangladesh, West Bengal, Vietnam, Cambodia), the Chaco-Pampean plain, northern Chile, Mexico, volcanic and mining areas. After years of consumption, skin lesions on palms and soles and cancer.
   • Fluoride (1.5 mg/L): the African Rift, India, northern China, Argentina, Mexico, volcanic and granitic areas. Above this value, dental mottling in children; with 3-6 mg/L over years, skeletal fluorosis (Fawell et al. 2006).

   Success criterion: each source marked “chemical risk yes/no” based on geology, surroundings and signs in the population (dental mottling, skin lesions).

2. Screen with a kit the at-risk sources:

   • Nitrate strips: check whether the scale is in NO₃⁻ or N; 50 mg/L of NO₃⁻ = 11.3 mg/L of N.
   • Colorimetric fluoride kit.
   • Arsenic kit: generates arsine gas (see “Safety”). Field kits used to fail below about 50-100 µg/L (Rahman et al. 2002; current kits may be better) (verificar). Confirm in a laboratory every result ≥ 10 µg/L and every negative result in a risk area.

   For the laboratory: a 250-500 mL plastic bottle rinsed 3 times with the same water. Success criterion: value recorded and sample sent if needed.

3. Act if the guideline value is exceeded: find another source (rain, another aquifer, after testing), blend with clean water and reserve the clean water for infants, children and pregnant women. Possible treatments: iron for arsenic, bone char or the Nalgonda method for fluoride, and distillation for nitrate and salts. Success criterion: decision recorded and a new test after the change.

H. Sampling plan

Practical criterion of this article; WHO only sets minimums for networks. The frequency of E. coli testing is in the microbiological article.

Success criterion: a notebook per source with date, rainfall, results and actions, and a new test after each correction.

Verification

• Tube: with freshly collected rainwater, the disk should be visible with the tube full. In successive halvings of a muddy water, each dilution multiplies the height by ≈1.6 (2^0.662). Against a turbidimeter, an error of ±5 NTU below 30 NTU is normal.
• Chlorine: unchlorinated water should give 0. With 1-2 mg/L added to rainwater, the reading at 30 min should be somewhat lower but of the same order.
• Red cabbage: check the vinegar and baking soda references every day.
• Dry residue: 500 mL of rainwater or distilled water should leave < 0.02 g.
• Treatment: it must be verified with fecal indicators (microbiological article).
• With more resources: send one in every 10 samples to an accredited laboratory.

Common mistakes

Safety

• Tasting risky water. Do not taste water that smells of fuel or chemicals, has blue-green scum, or comes from mining, industrial or dump areas. For the taste test, boil first and spit it out.
• Arsenic kits. They produce arsine, a very toxic gas, and their strips often contain mercuric bromide. Work outdoors, without leaning over the bottle, and wear gloves. Store the waste in a closed, labeled hazardous-waste container, never in latrines or water sources. If someone suffers abdominal pain, dark urine or difficulty breathing, move them into the open air and seek urgent medical care.
• Reagents. DPD, iodide, caustic soda, bleach: irritants. Store them labeled, away from children and food. If they splash into the eyes, flush with plenty of water for 15 min. Do not use orthotolidine kits, which are carcinogenic.
• Heat. Dishes at 105-180 °C: use tongs and a heat-resistant surface. In case of a burn, cold water for 20 min.
• Sampling. Work in pairs near wells and shorelines, do not climb down into a well (it can build up CO₂ or H₂S), and do not touch cyanobacterial scum.

Variants

• With fewer resources (N0-N1):
  • SODIS text test: if 5 mm letters can be read through a 1.5 L bottle stood upright, the water is below 30 NTU.
  • White disk with a black cross at the bottom of a bucket: if it is visible through 25.5 cm of water, that is equivalent to the 30 NTU mark on the tube. In a wide bucket, light enters from the sides and the reading comes out somewhat optimistic.
  • Without microbial culture, treat all the water.
• With more resources (N4): field kit with a turbidimeter and chlorine and pH photometer; local laboratory with a photometer for nitrate, fluoride, iron and manganese; arsenic by atomic absorption in a regional laboratory.
• Scaling:
  • Family: inspection of their source, text test or tube, and chlorine smell or DPD.
  • Village: a promoter with a tube, DPD kit and source notebook, and a public board with the status of each source.
  • City: a laboratory and the frequency of the “Network” column in section H.

Summary table

Calculations

1. Dry residue. Empty dish: 52.34 g. After evaporating 500 mL and drying at 105 °C it weighs 52.61 g, constant over two weighings. Residue = 0.27 g. TDS = 0.27 × 1,000,000 / 500 = 540 mg/L, “good”. With a scale accurate to 0.01 g, the error is ±20 mg/L; with 200 mL it would be ±50 mg/L.

2. Turbidity off the table. NTU = (244.13 / cm)^1.511. If the disk disappears at 60 cm: 244.13 / 60 = 4.07; ln 4.07 = 1.403; × 1.511 = 2.121; e^2.121 ≈ 8.3 NTU. A 60 cm tube does not measure below about 8 NTU.

3. Nitrate. A strip expressed in N reads 15 mg/L: 15 × 62/14 = 15 × 4.43 = 66 mg/L of NO₃⁻. This exceeds 50 mg/L: that water is not suitable for baby bottles.

4. Salinity from density. A relative reading of 1.006 at 20 °C gives (1.006 − 1) / 0.00076 ≈ 8 g/L (±1.3): brackish water, not fit for regular drinking.

Sources

• OMS. Guidelines for Drinking-water Quality, 4.ª ed. con addenda 1 y 2, 2022. Cap. 4, 10 y 12.
• OMS. Sanitary inspection packages (drinking-water), 2024.
• OMS. Guidelines for Drinking-water Quality, 2.ª ed., vol. 3, 1997.
• OMS. Total dissolved solids in drinking-water, WHO/SDE/WSH/03.04/16, 2003.
• OMS. pH in Drinking-water, WHO/SDE/WSH/07.01/1, 2007.
• Fawell, J. et al. Fluoride in Drinking-water. OMS / IWA Publishing, 2006.
• Rahman, M.M. et al. «Effectiveness and reliability of arsenic field testing kits». Environmental Health Perspectives 110(7): 743-748, 2002.
• Myre, E.; Shaw, R. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field. Michigan Technological University, 2006.
• Hach. Chlorine, Free, DPD Method 8021, DOC316.53.01023.
• Oxfam WASH. Chlorination in Emergencies.
• GTFCC/OMS. Orientaciones de agua, saneamiento e higiene para el control del cólera.
• APHA/AWWA/WEF. Standard Methods for the Examination of Water and Wastewater, 23.ª ed., 2017. 1030 E, 2510 y 2540 C.
• Sphere Association. The Sphere Handbook, 4.ª ed., 2018.