---
titulo: Basic water quality testing
descripcion: Source inspection, senses, turbidity, residual chlorine, pH, salts, nitrate, arsenic and fluoride: decide the treatment and check it works.
nivel: 1
estado: borrador
requisitos: []
relacionados: [01-agua-y-saneamiento/potabilizacion/analisis-microbiologico-del-agua, 01-agua-y-saneamiento/potabilizacion/hervido-del-agua, 01-agua-y-saneamiento/potabilizacion/desinfeccion-solar-sodis, 01-agua-y-saneamiento/potabilizacion/desinfeccion-quimica-del-agua-con-cloro-y-yodo, 01-agua-y-saneamiento/potabilizacion/coagulacion-floculacion-y-sedimentacion-con-alumbre, 01-agua-y-saneamiento/potabilizacion/filtro-lento-de-arena-y-biofiltro-de-carbon, 01-agua-y-saneamiento/potabilizacion/filtros-de-ceramica-y-carbon, 01-agua-y-saneamiento/potabilizacion/destilacion-solar-de-agua]
fuentes:
  - "OMS. Guidelines for Drinking-water Quality, 4.ª ed. con addenda 1 y 2, Ginebra, 2022. Cap. 4, 10 y 12 (arsénico, fluoruro, nitrato, nitrito, manganeso, hierro, cloro)."
  - "OMS. Sanitary inspection packages (drinking-water): pozo excavado con bomba manual, manantial, sondeo. Ginebra, 2024."
  - "OMS. Guidelines for Drinking-water Quality, 2.ª ed., vol. 3: Surveillance and control of community supplies. Ginebra, 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. oxfamwash.org."
  - "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."
  - "The Sphere Handbook, 4.ª ed., 2018. Capítulo WASH."
idioma: en
traduccion: automatica
---
# 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](analisis-microbiologico-del-agua.md). No field test proves water is safe: it only reduces
uncertainty.

## Prerequisites

- **Knowledge:** measuring and weighing; it helps to know [boiling](hervido-del-agua.md), [SODIS](desinfeccion-solar-sodis.md),
  [chlorination](desinfeccion-quimica-del-agua-con-cloro-y-yodo.md), [coagulation](coagulacion-floculacion-y-sedimentacion-con-alumbre.md)
  and the filters ([sand](filtro-lento-de-arena-y-biofiltro-de-carbon.md), [ceramic](filtros-de-ceramica-y-carbon.md)).
- **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”.

| Observation | Likely cause | Implication |
|---|---|---|
| Turbid, brown, worse after rain | Clay and runoff | Fecal contamination likely; measure turbidity (C) and pretreat |
| Tea color, transparent | Humic acids | Low direct risk; consumes chlorine and skews the tube |
| Reddish on aeration, stains clothing | Iron (noticeable above 0.3 mg/L; WHO 2022, chap. 10) | Aesthetic; metallic taste |
| Black stains, bitter taste | Manganese | Provisional guideline value of 0.08 mg/L (WHO 2022) |
| Blue-green scum or clumps | Cyanobacteria | Their toxins are **not** destroyed by boiling; find another source |
| Rotten-egg smell | Hydrogen sulfide | Aerate; skews the H2S test |
| Sewage smell, foam | Wastewater | Fecal contamination almost certain |
| Gasoline, solvent or “chemical” smell | Hydrocarbons, pesticides | **Do not drink or taste**; find another source |
| Salty or brackish | Chlorides, saltwater intrusion | Measure salts (F); boiling does not help |
| Bitter, laxative | Sulfates | Measure salts (F) |
| Soapy feel | High pH | Measure pH (E) |
| White scale, soap does not lather | Hardness | No sanitary risk |
| Blue-green stains | Copper with acidic water | Measure pH; let the tap run |
| Larvae, worms | Open source | Strain through cloth, treat, protect the source |

### 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.

| Height above the disk (cm) | NTU |
|---|---|
| 6.7 | 240 |
| 7.3 | 200 |
| 8.9 | 150 |
| 11.5 | 100 |
| 17.9 | 50 |
| 20.4 | 40 |
| 25.5 | 30 |
| 33.1 | 21 |
| 38.2 | 17 |
| 43.3 | 14 |
| 48.3 | 12 |
| 53.4 | 10 |
| 85.4 | 5 |

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:**

| Turbidity | Boiling | Chlorination | SODIS | Filters |
|---|---|---|---|---|
| < 5 NTU | Yes | Normal dose; ideal < 1 NTU; Sphere (2004) requires < 5 NTU (both cited by Myre and Shaw) | Yes | Yes |
| 5-10 NTU | Yes | Normal dose and check the residual | Yes | Yes |
| 10-30 NTU | Yes | Double dose and check the residual; better to clarify first | Yes (< 30 NTU) | Community slow sand: ≤ 10 NTU recommended, with peaks ≤ 50; biosand ≤ 50; ceramic ≤ 25 typical |
| 30-100 NTU | Yes | Double dose (3.75 mg/L per the chlorination article); clarify first | **No**: settle or filter first | Peaks of up to 50 NTU; above that, pretreat (coagulation) |
| > 100 NTU | Yes, after straining through cloth | **Not reliable**: pretreat | No | Always pretreat |

### 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:**

| Free chlorine (mg/L) | Point | Meaning and action |
|---|---|---|
| 0 | Any | Disinfection not guaranteed: repeat the dose or check the bleach |
| < 0.2 | Consumption | Insufficient protection: increase the dose |
| 0.2-0.5 | Consumption | **Correct** (Sphere 2018) |
| ≥ 0.5 | After 30 min of contact, pH < 8 | Correct in the storage tank (WHO) |
| Cholera outbreaks | Distribution and consumption | ≥ 0.5 mg/L in distribution and ≥ 0.2 mg/L at consumption (WHO/GTFCC); some NGOs target 0.5-1.0 mg/L in distribution; with pH > 8, a minimum of 0.4 mg/L at consumption (Oxfam) |
| > 2 | Any | Taste rejection likely (some notice it from 0.3 mg/L); health-based guideline value of 5 mg/L |

   *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](destilacion-solar-de-agua.md) 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](analisis-microbiologico-del-agua.md).

| What | Community well or spring | Household treatment | Chlorinated network |
|---|---|---|---|
| Inspection (A) | Every 3-6 months and after floods | Visit every 1-3 months | Annual per component |
| Turbidity (C) | Monthly and after rain | When the water's appearance changes | Daily at the treatment outlet |
| Chlorine (D) | Daily if chlorinated | Weekly in the drinking container | Daily at the outlet and in the network |
| Salts (F) | When adopting the source and in the dry season | — | Annual |
| Nitrate, arsenic, fluoride (G) | When adopting the source; every 1-5 years if at risk | — | According to geological risk |

*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](analisis-microbiologico-del-agua.md)).
- **With more resources:** send one in every 10 samples to an accredited laboratory.

## Common mistakes

| Symptom | Likely cause | Solution |
|---|---|---|
| Tube readings very different between people | Direct sun, sunglasses, bubbles | Shade, slow pouring, average of 3 readings |
| Tea-colored water with high turbidity that then chlorinates fine | Color is read as turbidity | Note “colored”; run the text test |
| DPD keeps rising after 2-3 min | Combined chlorine | Read within 1 min |
| Starch-iodide blue in unchlorinated water | Iron(III), nitrite or late reading | Compare with the blank and read within 1 min |
| All red cabbage glasses the same color | Degraded indicator | Prepare it fresh |
| Dry residue leaves weight with distilled water | Dirty dish or filter fibers | Wash and dry the dish; filter through denser cloth |

## 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](desinfeccion-solar-sodis.md) 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

| Parameter | Minimum method | Better method | Threshold | Action if exceeded |
|---|---|---|---|---|
| Source risks | Sketch and inspection (N0) | 2024 WHO form | Sanitation within 15 m, or upslope within 30 m; open wells within 100 m | Correct and always treat |
| Appearance, smell, taste | Senses (N0) | — | Chemical smell; blue-green scum | Do not use; find another source |
| Turbidity | Text test (N0); tube (N2) | Turbidimeter (N4) | 5 / 10 / 30 / 100 NTU | Table in section C |
| Free residual chlorine | Smell (N0); starch-iodide (N3) | DPD (N4) | 0.2-0.5 mg/L at consumption; ≥ 0.5 after 30 min at pH < 8 | < 0.2: increase the dose; > 2: dilute or wait |
| pH | Red cabbage (N1) | Strips, phenol red, pH meter (N4) | 6.5-8.5; < 8 for chlorination | > 8: more chlorine and more contact time |
| Salts (TDS) | Taste (N0); dry residue (N3) | Conductivity meter (N4) | > 600 mg/L noticeable; > 1200 unacceptable | Another source, blending or distillation |
| Fecal indicator | See [Microbiological water testing](analisis-microbiologico-del-agua.md) | — | *E. coli* ≥ 1/100 mL | Treat and verify |
| Nitrate | Surroundings (N0) | Strips or photometer (N4) | 50 mg/L NO₃⁻ | Other water for infants; move sources of risk away |
| Arsenic | Geology (N0) | Kit and laboratory (N4) | 10 µg/L | Another source; removal with iron |
| Fluoride | Dental mottling (N0) | Colorimetric kit (N4) | 1.5 mg/L | Another source or blending; bone char |

## 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.