Analyze water and water pollution: potable water and water quality, point and non-point contamination, eutrophication and hypoxia, and the Clean Water Act

Learning goal: Define potable water and the measurements used to judge water quality; explain the chain from nutrient enrichment to eutrophication and hypoxia; distinguish point-source from non-point-source contamination and say why the difference matters for control; describe how drinking water and wastewater are treated; and describe what the Clean Water Act regulates and the science behind it.

Potable water. Potable water is safe to drink: free of disease-causing organisms, low in harmful chemicals, and acceptable in taste, odor and clarity. Water can be clear and still not potable, because pathogens and dissolved chemicals are invisible; and it can be cloudy and safe after treatment. Only a tiny fraction of Earth's water is fresh, liquid and accessible, and a community's supply comes from one of two places: surface water (rivers, lakes, reservoirs) or groundwater (wells into aquifers). Knowing which one your town uses, and where its wastewater goes afterwards, is the first step in understanding what can contaminate it.

Measuring water quality. No single number describes water; a set of measurements does. Dissolved oxygen (DO) is the oxygen gas dissolved in water that fish and invertebrates breathe; cold, turbulent water holds more, warm still water less, and most fish are stressed when DO falls below about 5 milligrams per liter and die well below that. Biochemical oxygen demand (BOD) measures how much oxygen decomposers will consume breaking down the organic matter in a sample; sewage and food waste have high BOD, so they remove oxygen from a stream even though they are not themselves toxic. pH in healthy fresh water is usually between about 6.5 and 8.5; acid mine drainage and acid rain push it down. Temperature matters because warm water holds less oxygen and speeds decomposition; a power plant's cooling discharge is thermal pollution. Turbidity is cloudiness from suspended particles, which block light and smother stream beds. Nitrate and phosphate are the plant nutrients whose excess drives algal blooms. Fecal coliform bacteria are not usually harmful themselves but indicate sewage or manure contamination and the pathogens that travel with it. Macroinvertebrates, the insects, snails and worms living on the stream bed, are biotic indicators: mayfly, stonefly and caddisfly larvae need clean, oxygen-rich water, while midge larvae and aquatic worms tolerate pollution, so the mix of species records the water's condition over the preceding months, which a single chemical sample cannot.

Eutrophication and hypoxia. Eutrophication is the enrichment of a water body with nutrients, especially phosphorus and nitrogen. The chain runs: nutrients arrive from fertilizer, manure, sewage or detergent; algae and cyanobacteria, no longer nutrient-limited, bloom; the bloom shades out rooted plants; when the algae die, bacteria decompose them and consume dissolved oxygen; DO drops to hypoxia (commonly defined as below 2 milligrams per liter) or to anoxia; fish and invertebrates die or flee. Some cyanobacteria also produce toxins. Lake Erie is Ohio's example: it is the shallowest and warmest of the Great Lakes, its western basin receives phosphorus-rich runoff from the intensively farmed Maumee watershed, and in most recent summers it has developed cyanobacterial blooms; in August 2014 toxin from a bloom near Toledo's intake led to a do-not-drink advisory for several hundred thousand people for about two days. The remedy is at the source, because once the phosphorus is in the lake it cycles between sediment and water for years.

A worked model in the prose: the oxygen sag. Suppose a hypothetical stream is sampled below a town's wastewater outfall: upstream of the outfall DO is 8.5 mg/L; at the outfall 7.0; 1 km downstream 3.5; 3 km downstream 2.0; 6 km downstream 4.5; 10 km downstream 7.5. The curve dips and recovers. Just below the outfall, decomposers begin consuming the organic load, and oxygen falls faster than turbulence and photosynthesis replace it, reaching a minimum some distance downstream, not at the outfall. Once the organic matter is used up, reaeration wins and DO recovers. The low-oxygen stretch, from under 1 km to beyond 6 km, with its minimum of 2.0 mg/L near 3 km at the threshold for hypoxia, is where sensitive macroinvertebrates and fish would be missing. A sample taken only at the outfall would have missed the worst of it, which is why sampling must extend downstream.

Point and non-point sources. A point source is a single identifiable place of discharge: a pipe, a ditch, a factory outfall, a sewage plant. A non-point source is diffuse: runoff from fields, lawns, streets and construction sites; seepage from septic systems; atmospheric deposition. The distinction determines the control strategy. Point sources can be measured at the pipe, permitted, treated and inspected, and they were the first targets of the Clean Water Act. Non-point sources have no pipe; the pollutant arrives from thousands of places in every rain, so control means changing land practices over a whole watershed: buffer strips, cover crops, fertilizer timing, storm-water basins, septic maintenance. Most of the phosphorus reaching Lake Erie today is non-point, which is why the lake's problem has proven harder than the pipe problems of the 1970s.

Treatment. Drinking water treatment follows a sequence: coagulation and flocculation (a chemical makes fine particles clump), sedimentation (clumps settle), filtration (through sand and other media), and disinfection (chlorine, ozone or ultraviolet light kills pathogens). Wastewater treatment reverses the damage before discharge: primary treatment screens and settles solids; secondary treatment uses bacteria in aerated tanks to consume dissolved organic matter, lowering the BOD; tertiary treatment, where used, removes nutrients; disinfection follows. A home filter of gravel, sand and charcoal reproduces sedimentation and filtration and can remove turbidity and some odors, but it removes no dissolved salts and cannot reliably remove or kill pathogens; water from it is not potable.

The Clean Water Act and the science behind it. The Cuyahoga River in Cleveland caught fire more than once from oil and debris on its surface; the 1969 fire became a national symbol and helped bring the modern Clean Water Act of 1972. The Act makes it unlawful to discharge a pollutant from a point source into the nation's waters without a permit, sets technology-based limits on what permitted discharges may contain, funds sewage treatment plants, and requires states to set water quality standards for each water body's designated uses (drinking, swimming, aquatic life) and to identify waters that fail them. Its science is the measurement set above: standards are written in terms of DO, pH, nutrients, bacteria and toxic concentrations, and compliance is judged by sampling. Rachel Carson's Silent Spring (1962) documented how pesticides such as DDT moved through water and food chains and harmed wildlife, including birds; research in the following years linked DDT to the thin eggshells of birds of prey, and the public response led to the pesticide's ban in the United States a decade after the book and to the broader environmental laws of the 1970s.

Assessment and conservation. Water quality is one half of the resource; quantity is the other. A water assessment of a home or school lists every use (toilets, showers, taps, laundry, irrigation, leaks) with an estimate of liters per day for each, from meter readings where possible and from fixture flow rates and counts where not. A water conservation plan built on the assessment ranks the uses by volume and targets the largest first: a dripping tap wastes a few liters a day, an older toilet uses several times more water per flush than a modern one, and a leaking underground pipe can waste more than everything else combined. Small changes multiply across a community: if each of a hypothetical town's 4,000 households saved 20 liters a day, the town would save 80,000 liters a day, enough to matter to the treatment plant and the river it draws from.

What a single sample cannot show. One bottle of water records one moment. It misses the storm pulse of runoff, the night-time oxygen minimum, and the contaminant that arrives once a week. Biotic indicators and repeated sampling are how a stream's condition, rather than its instant, is measured.

Worked oxygen sag interpretation: Using the hypothetical stream data (8.5, 7.0, 3.5, 2.0, 4.5, 7.5 mg/L at upstream, outfall, 1, 3, 6 and 10 km), identify the affected reach. DO below 5 mg/L, the level at which most fish are stressed, occurs from somewhere under 1 km to somewhere beyond 6 km; DO at or below 2 mg/L, hypoxia, occurs around 3 km. The minimum lies downstream of the outfall because decomposition takes time. A sensitive macroinvertebrate survey would be expected to show mayflies and stoneflies upstream and at 10 km but not at 1 to 3 km. If the plant added secondary treatment and cut the BOD of its discharge by two thirds, the sag would be shallower and shorter; the exact new curve would need new measurements.

Worked source classification: Classify: a factory outfall pipe (point); fertilizer runoff from a hundred farms (non-point); an overflowing sewer during a storm (point, since it leaves a known structure); road salt washing off streets (non-point where it runs off diffusely; but where it is collected and discharged through a city's storm-sewer pipes, those outfalls are regulated as point sources); leachate seeping from a landfill into groundwater (an identifiable source, but whether the law treats it as a point-source discharge to a stream depends on how directly the groundwater carries it there); atmospheric deposition of mercury onto a lake (non-point). The legal test is whether the pollutant is discharged through a discernible conveyance, such as a pipe, ditch or channel, rather than simply whether the source can be named.

Worked eutrophication chain: A hypothetical pond receives lawn fertilizer runoff each spring. Sequence with reasons: phosphate, the limiting nutrient, rises; algae bloom within weeks; the bloom shades rooted plants, which die; in midsummer the algae die and settle; decomposers consume oxygen faster than the warm, still water gains it; DO falls at night below 2 mg/L; a fish kill occurs at dawn, when oxygen is lowest because photosynthesis has stopped overnight while respiration continued. The dawn timing is a strong clue pointing to hypoxia rather than a toxic spill, which could kill at any hour, but it does not rule out a spill or mixed causes: dissolved oxygen, temperature and likely contaminants measured at the time are needed to confirm the cause.

Worked water quality comparison: Two hypothetical streams sampled the same morning. Natural area: DO 9.0 mg/L, pH 7.4, nitrate 0.5 mg/L, turbidity low, fecal coliform 20 colonies per 100 mL, macroinvertebrates dominated by mayflies and caddisflies. Developed area: DO 5.5, pH 7.0, nitrate 6.0, turbidity high, fecal coliform 800 per 100 mL, midges and worms dominant. Interpretation: the developed stream shows nutrient enrichment (nitrate), sediment (turbidity), sewage or pet and livestock waste (coliform) and the oxygen and biotic consequences. Possible routes: lawn and road runoff, septic seepage, storm sewers. What the comparison cannot prove: which specific source is responsible; that needs sampling along the tributaries.

Representation task: Draw the oxygen sag curve from the hypothetical data as a graph of DO against distance, shade the reach below 5 mg/L and mark the point where DO reaches the 2 mg/L hypoxia threshold, and annotate the processes acting in each segment (decomposition consuming oxygen; reaeration and photosynthesis restoring it). Beside it, draw a flow diagram of a drinking water treatment plant and a wastewater treatment plant, labeling each stage and what it removes.

Safe investigation, build and test a filter: Cut the bottom from a plastic bottle and invert it. Layer, from the neck up: cotton or a coffee filter, activated charcoal (from an aquarium shop), fine sand, coarse sand, gravel. Prepare "dirty" water from tap water plus a spoon of garden soil and a drop of food coloring. Pour 200 mL through and time it; compare clarity of input and output against a printed page, and note color and odor. Change one variable (layer thickness, order or material) and repeat, recording flow time and clarity each time. Result that supports the claim: the filter reduces turbidity and some color. What it cannot do: remove dissolved salts, nitrate or pathogens. Never drink the filtered water. Estimate the material cost and state which change improved clarity most per unit cost.

Independent performance: Report on your community's water in at least 250 words. Success criteria: it states whether the drinking water comes from surface water or groundwater and where wastewater is treated and discharged, or says explicitly that you could not find out and what source you tried; it explains the treatment stages in order; it identifies one point source and two non-point sources that could affect the supply; it explains, using the eutrophication chain, why phosphorus control matters for a lake like Lake Erie; it names one water-quality measurement you would take and the value that would concern you; and it outlines a water conservation plan based on a simple assessment of your home or school (list the uses, estimate liters per day for each, rank them), proposing at least two measures with a rough calculation of the water saved, labeled as an estimate.