Judge potable water and water quality, explain how aquifers store and move groundwater in Ohio, and trace eutrophication to hypoxia and a fish kill

Learning goal: Explain what makes water potable and how water quality is measured and treated; explain porosity, permeability, aquifers and groundwater flow, and use them to site a well or reservoir; describe Ohio's main aquifers, including buried valleys and karst; and explain the chain from nutrient runoff through eutrophication to hypoxia, using it to deconstruct a fish kill.

Potable water and water quality. Water is potable when it is safe to drink: free of disease-causing organisms and below health-based limits for harmful chemicals. Water quality is judged by measured properties, each with its own meaning. Total coliform bacteria, which also live in soil and on plants, show that the water may be open to contamination; a positive test is followed by a test for E. coli, whose presence is specific evidence of sewage or animal waste. Nitrate, mostly from fertilizers and septic systems, is limited in public drinking water to 10 mg/L measured as nitrogen, because higher levels are dangerous to infants. Lead usually comes from old pipes, not from the source. Turbidity, the cloudiness from suspended particles, can shelter microbes from disinfection. Dissolved oxygen, pH and temperature describe the health of streams and lakes more than the safety of tap water. Hardness, from dissolved calcium and magnesium, is not a health hazard; it is common in western Ohio because groundwater there flows through limestone and dolomite and dissolves them.

How water is treated. A typical surface-water plant adds a coagulant so fine particles clump into flocs, lets the flocs settle, filters the water through sand and sometimes activated carbon, and disinfects it, usually with chlorine. Aeration, bubbling air through water or spraying water into air, strips out dissolved gases such as hydrogen sulfide and volatile chemicals, and oxidizes dissolved iron and manganese so they form solids that can be filtered. Filtration removes particles by size and, with activated carbon, adsorbs many organic compounds and taste and odor chemicals. No single method removes everything: dissolved nitrate passes through sand filters and needs other treatment, which is why prevention at the source matters.

Groundwater: porosity and permeability. Below the land surface, water fills the pores and cracks of sediment and rock. Porosity is the percentage of a material's volume that is open space; it decides how much water the material can store. Permeability is how easily water flows through connected openings; it decides how fast the water can be withdrawn. They are not the same. Clay can have a porosity of about 50 percent but very low permeability because its pores are tiny and water clings to the grains. Well-sorted sand and gravel have high porosity and high permeability. An aquifer is a body of rock or sediment that stores and transmits enough water to supply wells. An unconfined aquifer is open to recharge from above; its upper surface is the water table. A confined aquifer lies between layers of low permeability, and its water is under pressure, so in a well it rises above the top of the aquifer. Groundwater moves from high hydraulic head (the level to which water would rise in a well) to low; in an unconfined aquifer that is the water-table elevation, which generally follows the land surface toward streams, but in confined aquifers and near pumping wells the head can differ from what the topography suggests, so a map-based well site is a first estimate to be checked with measured water levels. Groundwater typically moves very slowly, from centimeters to a few meters per day.

Pumping and the cone of depression. Whenever a well pumps, the water table around it (in a confined aquifer, the pressure level) is drawn down into a cone of depression, and the faster the pumping compared with the flow toward the well, the deeper and wider the cone. Nearby shallow wells can go dry, and if a contaminant source lies within the cone, pumping can pull contamination toward the well.

Ohio's aquifers. Across glaciated Ohio, the most productive aquifers are buried valleys: before the ice ages, rivers cut deep valleys into bedrock, and meltwater from the glaciers later filled them with thick sand and gravel. The buried valley beneath the Great Miami River in southwest Ohio is among the most productive aquifers in the state. Western Ohio also draws water from fractured limestone and dolomite bedrock, and eastern Ohio from sandstone, while many shales of southeastern Ohio yield little water. Where carbonate bedrock is near the surface, groundwater containing carbonic acid dissolves it to form karst: sinkholes, caves, sinking streams and large springs. The Bellevue area of north-central Ohio is known for sinkholes. Karst aquifers are especially easy to contaminate, because water runs through open conduits in hours to days with little filtering, unlike the slow flow through sand.

Eutrophication and hypoxia. Eutrophication is the over-enrichment of a water body with nutrients, especially phosphorus and nitrogen, which fuels excess growth of algae and cyanobacteria. The chain: fertilizer, manure, sewage and eroded soil carry nutrients in runoff; algae bloom in warm, sunny weather; the algae die and sink; bacteria decomposing them consume dissolved oxygen; if the water is layered by temperature, the deep water cannot be refreshed from the air, and oxygen falls to hypoxia, commonly defined as below 2 mg/L, too low for most fish. Even before the algae die, a dense bloom swings oxygen widely: photosynthesis supersaturates the water in the afternoon, but respiration at night can drain oxygen to its lowest level just before dawn, which is when fish kills are most often discovered. Warm water holds less oxygen to begin with: about 9 mg/L at saturation at 20 °C, and less at higher temperatures. Lake Erie shows both effects. It is the shallowest and warmest of the Great Lakes; phosphorus-rich runoff, much of it from farmland in the Maumee River watershed, drives summer cyanobacteria blooms in the western basin, and the central basin develops hypoxic bottom water in late summer. In August 2014 toxins from a bloom caused a do-not-drink advisory lasting about three days for roughly half a million people served by Toledo's water system.

Siting a well or a reservoir. A topographic map, read with the geology, answers practical questions. A well should be in permeable material, such as sand and gravel in a valley, and upgradient from sources of contamination such as septic fields, landfills or fuel tanks, since groundwater flows downhill with the water table. A reservoir needs a large drainage area upstream, a narrow valley where a short dam can hold the water, and a floor that does not leak, such as clay or shale rather than karst limestone.

The misconception. Many people picture groundwater as underground lakes and rivers. Except in karst caves, groundwater is water held in the small pores and cracks of solid rock and sediment, moving slowly through them.

Worked porosity (model values): A student packs 400 mL of dry sand into a cylinder and slowly pours in water until it just reaches the sand surface; 140 mL is added. Porosity = 140 ÷ 400 × 100 = 35 percent. The skipped step is making sure no water pools on top, since that water is not in pores and would overstate the porosity.

Worked groundwater travel time: A fuel leak sits 300 m upgradient of a well. The aquifer is sand with a model hydraulic conductivity of 50 m per day, the water table drops 0.6 m over the 300 m (gradient 0.002) and porosity is 0.25. Average groundwater velocity = conductivity × gradient ÷ porosity = 50 × 0.002 ÷ 0.25 = 0.4 m per day. Travel time = 300 ÷ 0.4 = 750 days, about 2 years. Dividing by porosity matters: water moves only through the pores, so it moves faster than the conductivity × gradient figure alone would suggest.

Worked blending to meet a limit: A hypothetical well yields water with 14 mg/L nitrate (as nitrogen), above the 10 mg/L limit, and a second source has 4 mg/L. What fraction x of the blend can come from the first well? 14x + 4(1 − x) = 10, so 10x = 6 and x = 0.6. At most 60 percent can come from the high-nitrate well. This treats the symptom; reducing fertilizer or septic inputs near the well treats the cause.

Worked fish-kill deconstruction: Data from a model farm-country river: heavy rain on 3 July; water turns green by 10 July; air temperatures 32 °C and calm from 10 to 14 July; residents report dead fish at 06:30 on 14 July along 2 km below a slow, deep pool, none upstream; the 06:00 dissolved-oxygen reading at the pool is 1.2 mg/L, the 15:00 reading the day before 12.5 mg/L. Reasoning: runoff brought nutrients, a bloom grew in warm calm weather, and night-time respiration by the bloom and by bacteria decaying dead algae dropped oxygen below 2 mg/L by dawn, where the slow, deep, poorly mixed pool was worst. The high afternoon reading supports a bloom rather than a toxic spill, which would not raise oxygen; a spill remains possible if tests found a toxin.

Representation task: On a cross-section, draw a valley with a buried valley aquifer of sand and gravel, the water table, a pumping well with its cone of depression, a septic field upslope and a stream; draw groundwater flow arrows. Then draw the eutrophication chain as a flow chart from runoff to fish kill, labeling where aeration or reduced fertilizer would break the chain.

Safe investigation, water filters and aquifer materials: (a) Measure porosity and permeability: before filling, punch identical holes in the bottoms of three cups and seal them with tape; fill the cups with 200 mL each of gravel, sand and garden soil with clay; add water to find porosity, then remove the tape and time how long 100 mL takes to drain. (b) Design, build and test a water filter in a cut plastic bottle using layers of gravel, sand and activated charcoal and a coffee filter. Pour 250 mL of muddy water (garden soil in tap water, no other contaminants) through it and measure turbidity before and after by the depth at which a black mark on white card disappears in a clear tube. Record layer order, volumes, drain time and turbidity; change one layer at a time. Do not drink the filtered water; filtering does not disinfect it. (c) Compare methods: run muddy water and water tinted with a few drops of food coloring (a dissolved pollutant) through sand only, through sand plus activated charcoal, and through aeration (shaking in a half-full closed jar for two minutes), and record which method changes which sample. (d) Local issue: where your teacher can supervise it, collect water samples from a safe spot on a local stream or pond and test them with nitrate and phosphate test strips (gloves on, hands washed, never beside fast or deep water); describe one contamination issue with your data and recommend two ways to reduce it at the source. Where sampling is not possible, analyze your water utility's annual drinking-water quality report in the same way, and say in your write-up that this alternative gives practice in reading published data, not collecting samples. What the model cannot show: the filters remove particles, not dissolved nitrate or microbes, and cups cannot show the pressure of a confined aquifer.

Independent performance: Use a topographic map of a real or teacher-supplied area to recommend a site for a town's new well or reservoir, in a 300-word memo. Success criteria: it uses contour lines to show groundwater and runoff direction; it identifies permeable materials and a potential contamination source and places the well upgradient of it, or it justifies a dam site by valley width, drainage area and floor rock; it explains one water-quality test and its limit; and it names one way to reduce nutrient runoff to protect the source from eutrophication.