Explain potable water quality, use and availability, and design low-cost treatment and conservation measures

Learning goal: Define potable water; locate the world's large freshwater sources and explain why many populations have little access to clean water; describe how water is treated, tested and reported, including sources of drinking-water pollutants and disinfection byproducts; audit personal water use; and design low-cost treatment, irrigation and pollution-prevention measures.

What potable means. Potable water is water safe for humans to drink: free of disease-causing microorganisms and with chemical contaminants below levels that cause harm over a lifetime of drinking. "Clear" is not the same as "potable": water can look clean and carry bacteria, viruses, protozoa such as Giardia and Cryptosporidium, dissolved lead, nitrate, arsenic or pesticides. In the United States, public water systems must meet enforceable limits set under the Safe Drinking Water Act; the limit for a contaminant is called its maximum contaminant level (MCL). Two figures worth knowing because they appear on water reports: the nitrate MCL is 10 milligrams per liter (as nitrogen), and lead has an "action level" rather than an MCL, because lead usually enters water from a building's own pipes and solder rather than from the source. The action level is 15 parts per billion (0.015 mg/L) at the tap; a 2024 federal rule lowers it to 10 parts per billion from November 2027. The action level is a trigger, not a safe level: if more than 10% of tap samples exceed it, the water system must take further steps such as corrosion control and pipe replacement. The EPA's health-based goal for lead (its maximum contaminant level goal, MCLG) is zero, because lead can harm health even at low exposure, and the EPA and CDC agree that no safe level of lead in a child's blood is known.

Where fresh water is, and why access is unequal. About 97% of Earth's water is salt water in the oceans. Of the roughly 3% that is fresh, about two thirds is locked in glaciers and ice sheets, most of the rest is groundwater, and well under 1% of all water is in the lakes, rivers and soil moisture that people can easily use. The largest liquid freshwater stores at the surface are a handful of lake systems: the Great Lakes hold roughly one fifth of the world's surface fresh water, and Lake Baikal in Siberia holds a similar share by itself. Most groundwater lies in large aquifers such as the Ogallala under the central United States. Fresh water is therefore concentrated in a few regions while people are spread everywhere, and some of the driest regions have fast-growing populations. Access also depends on infrastructure and income: a region can receive plenty of rain and still lack potable water because it has no treatment plants, pipes or sanitation to keep sewage out of wells. Where water is scarce or unsafe, women and children in many places spend hours a day carrying it, and waterborne disease is a leading cause of child death.

How a municipal plant makes water potable. Conventional treatment has four stages, each removing something different. Coagulation and flocculation: a chemical such as aluminum sulfate (alum) is added; it neutralizes the charges that keep fine particles apart, and gentle mixing lets them clump into larger "flocs". Sedimentation: water rests in a basin so flocs settle by gravity. Filtration: the water passes through beds of sand, gravel and often activated carbon, trapping remaining particles and many microorganisms. Disinfection: chlorine, chloramine, ozone or ultraviolet light kills or inactivates pathogens; chlorine and chloramine have the advantage of leaving a residual that keeps protecting water in the pipes. A fifth step in many plants is adjusting pH and adding corrosion inhibitors so water does not dissolve lead from old plumbing. The 2014 crisis in Flint, Michigan happened when a change of source water without corrosion control let lead leach from pipes; it is a case of a treatment step, not a source, failing.

Disinfection byproducts. Disinfection saves lives, but chlorine reacts with natural organic matter (decayed leaves and algae) dissolved in the source water to form disinfection byproducts (DBPs), chiefly trihalomethanes and haloacetic acids. Their concentrations tend to rise the longer treated water sits in the distribution system, because the reaction continues in the pipes. Long-term exposure at high levels is associated with increased health risk, so the EPA limits total trihalomethanes to 80 micrograms per liter and five haloacetic acids to 60 micrograms per liter. Utilities manage the trade-off by removing organic matter before disinfecting, by switching to chloramine (which forms much lower levels of the regulated trihalomethanes and haloacetic acids, though it can favor other byproducts such as nitrosamines, so the byproduct mix still has to be monitored) and by flushing dead-end mains. The lesson is that treatment choices are trade-offs between risks, not a choice between risk and no risk.

Reading a water quality report. Every community water system in the United States must send customers an annual Consumer Confidence Report. It lists each contaminant detected, the level found, the MCL, the likely source (for example "runoff from fertilizer use" for nitrate or "erosion of natural deposits" for arsenic) and any violations. To judge the "health" of a supply, compare each detected level with its MCL, look at whether any value is near the limit, and check the lead result, which is reported as the 90th-percentile value of samples taken from homes. A classroom test with consumer test strips (pH, hardness, free chlorine, nitrate) can be compared with the report. Hardness is mainly the amount of dissolved calcium and magnesium, picked up as water passes through rocks such as limestone; according to the USGS it is not a health concern, but hard water leaves scale in kettles and water heaters and makes soap lather poorly, so a hardness reading says nothing about whether water is potable. If your strip reads much higher nitrate than the report, the likely causes are strip error, a private well rather than the municipal supply, or contamination inside the building, and each is a testable prediction.

Sources of drinking-water pollutants. Group them by origin. Agricultural: nitrate and phosphate from fertilizer, pesticides, and bacteria from manure. Urban: road salt, oil, lawn chemicals, and leaking sewers. Industrial: solvents and metals from spills or old dump sites that reach groundwater. Natural: arsenic and radon dissolved from rock. Within buildings: lead and copper from plumbing. A plan to lower or prevent pollutants matches the control to the source: buffer strips and fertilizer timing for farms, storm-drain labeling and salt reduction for towns, wellhead protection zones around municipal wells, and replacing lead service lines in older neighborhoods. Lake Erie's western basin illustrates the chain: phosphorus from farm runoff feeds harmful algal blooms, and in August 2014 a bloom toxin (microcystin) in Toledo's intake led to a do-not-drink advisory for several hundred thousand people for about two days. The root cause was upstream land use, not the treatment plant.

Bottled water. Bottled water is regulated as a food by the Food and Drug Administration rather than by the EPA. Labels tell the source: "spring water" must come from a spring, whereas "purified water" is often municipal tap water that has been further filtered. Mapping the sources of several brands usually shows that some bottled water started in the same reservoirs as the local tap supply, at a price per liter hundreds of times higher and with the added plastic waste.

Low-cost, no-electricity treatment. For places without piped water, three methods are widely used. Boiling at a rolling boil for one minute (three minutes above about 2,000 m) kills pathogens but uses fuel. Solar disinfection (SODIS): fill a clear plastic bottle with water clear enough to read large print through it (turbid water must be settled or filtered first), lay it in full sun for about six hours (two days if cloudy); ultraviolet light and heat inactivate most microbes. Biosand or ceramic filtration: water trickles through a column of sand with a biological layer on top, or through a porous fired-clay pot, removing particles and most bacteria. None of these removes dissolved chemicals such as arsenic or nitrate, which is the key limit to state in any design. A good design combines cloth pre-filtering (removing sediment and the copepods that carry cholera bacteria), a disinfection step (boiling or SODIS; a biosand or ceramic filter reduces pathogens but should be treated as filtration, followed by disinfection where possible), and safe storage in a covered container with a tap so hands never enter the water.

Auditing your own use. A commonly cited U.S. Geological Survey estimate is that each person in the United States uses about 80 to 100 gallons a day at home, indoors and outdoors; indoors, toilets, showers and washing machines take the largest shares. A water survey records each activity, its flow rate or volume per use, and its frequency, then multiplies. Federal standards since the 1990s limit new toilets to 1.6 gallons per flush and showerheads to 2.5 gallons per minute, so the biggest savings usually come from replacing older fixtures, fixing leaks (a running toilet can waste hundreds of gallons a day) and shortening showers.

Misconception. "Groundwater is protected because it is underground." Groundwater moves slowly, so pollutants that reach it stay for decades and are very hard to remove; prevention at the surface is the only affordable strategy.

Worked water survey (hypothetical): A student's day: two 10-minute showers with a 2.5 gallon-per-minute head (2 × 10 × 2.5 = 50 gal), five toilet flushes at 1.6 gal (8 gal), teeth and hands with tap running about 4 minutes at 2 gal/min (8 gal), and one share of a washing-machine load (about 25 gal of a 100-gal older machine shared by four people). Total ≈ 50 + 8 + 8 + 25 = 91 gal/day. Savings plan: cut showers to 5 minutes (saves 25 gal), install a 1.5 gal/min showerhead (saves a further 10 gal on the 5-minute showers), turn the tap off while brushing (saves about 6 gal). New total ≈ 91 − 41 = 50 gal/day, a 45% reduction. If water costs a hypothetical $0.005 per gallon, the saving is about $0.20 per day or about $75 per year for one person.

Worked irrigation design: A school garden bed needs 240 liters of water delivered over 2 hours, so the required rate is 240 ÷ 2 = 120 L/h. A drip line's emitters each deliver 2 L/h at the available pressure, so 120 ÷ 2 = 60 emitters are needed. If the line has one emitter every 30 cm, the line must be 60 × 0.30 = 18 m long. Rate is checked by catching the output of three emitters in measuring cups for 15 minutes: 0.5 L each in 15 min is 2 L/h, confirming the design. If the water comes from a gravity-fed rain barrel instead of a pressurized tap, the barrel must be raised above the bed and the emitters' rate re-measured, because the lower pressure changes their output.

Worked report comparison: A hypothetical Consumer Confidence Report lists nitrate 3.1 mg/L as nitrogen (MCL 10 mg/L as nitrogen), total trihalomethanes 62 µg/L (MCL 80), lead 90th percentile 6 ppb (action level 15), and no violations. A student's test strip, read on its nitrate-nitrogen scale, gives about 5 mg/L. (Some kits report nitrate ion instead, which is about 4.4 times larger for the same water, so the basis must match before comparing.) All reported values are below limits; trihalomethanes are closest to their limit at 78% of the MCL, which may point to organic-rich source water or long residence time in the pipes. The strip's higher nitrate is within the coarse resolution of strips (they are read against a few widely spaced color blocks), so the first step is to repeat the test with a second strip and, if it persists, to test water from a different tap in the building.

Worked low-cost design: A village has a river source that is turbid and contaminated with bacteria but has no chemical pollution. Design: (1) pour water through folded cloth into a bucket to remove sediment and larger organisms; (2) check clarity: stand a filled bottle on a newspaper and look down through it; if the large letters of a headline cannot be read, let the water settle and pour off the clear part, or filter again, because SODIS fails in turbid water; (3) fill clean, clear 1- to 2-liter plastic bottles, shake to oxygenate, and lay them on a reflective sheet in full sun for at least six hours (two consecutive days if the sky is more than half cloudy); (4) drink directly from the treated bottle, or pour into a cleaned, covered container with a tap, so the water is not recontaminated. Cost: bottles and cloth. Limit: if the water were later found to contain arsenic from the local rock, this design would not remove it and a different method (such as an iron-based adsorption filter) would be needed.

Representation task: Draw a flow diagram of a conventional treatment plant with the four main stages in order (coagulation/flocculation, sedimentation, filtration, disinfection), and beside each stage write what it removes. Add a fifth box for corrosion control and write why it matters for lead. Below the diagram, make a two-column table of drinking-water pollutants by source (agricultural, urban, industrial, natural, plumbing) with one prevention measure for each.

Safe investigation, home or school water: Using consumer test strips, test tap water for pH, hardness, free chlorine and nitrate. Record the strip's reading range and the time since the tap was last run. Obtain the utility's most recent Consumer Confidence Report (available online for every community system) and compare each value. Record any discrepancy and write two possible causes and how you would test each. Do not drink water from any untested source and do not test with anything other than the strips provided.

Independent performance: Research one severe water-related environmental problem affecting your community, Ohio, the United States or the world, using data (for example a utility report, a lake monitoring dataset or a drought index). Write a proposal (about 400 words) that meets these criteria: (1) states the problem with at least two quantitative pieces of evidence and their sources; (2) traces the root cause to a specific human activity or natural condition; (3) proposes two mitigation measures matched to that root cause; (4) explains one trade-off or limit of each measure; (5) describes how success would be measured and over what period.