Reading passage Grade 10 Social Studies & Civics
Two Coastal Answers to Drought: Desalting and Reuse in California
A grade 10 passage comparing the Carlsbad desalination plant with Orange County's groundwater replenishment system as two regional answers to drought.
Objectives
- Students will describe what the Carlsbad plant and the Orange County reuse system each produce and how.
- Students will explain why two neighboring regions chose different drought strategies.
- Students will analyze how supply choices interact with energy use, geography, and public acceptance.
Vocabulary
- Desalination
- Turning salty water, like ocean water, into fresh water people can use. It works, but it takes a lot of energy and leaves behind very salty water that must be handled carefully.
- Reverse osmosis
- Reverse osmosis cleans water by pushing it through a membrane with holes so tiny that salt and most other dissolved things cannot fit through. It can even turn seawater into fresh water.
- Indirect potable reuse
- Recycling water into drinking water with a natural stop along the way. Highly cleaned water is put into the ground or a reservoir, then drawn back out later and treated again for drinking.
- Recharge
- Recharge is water soaking down through the ground to refill an aquifer, the way rain refills an underground sponge. Aquifers stay healthy only if recharge keeps up with pumping.
Also: groundwater basin
Teacher overview
Two neighboring Southern California regions answered the same drought pressure with different projects: one turned to the ocean, the other to its own wastewater. This passage describes the Carlsbad desalination plant and Orange County's Groundwater Replenishment System, then examines why geography, cost, and public opinion pushed each region down its own path. It fits a geography or policy unit on how communities adapt shared resources under stress.
Passage
Southern California's water has always traveled. For a century the region grew by importing water from distant rivers and mountains, and for just as long, droughts have exposed how fragile those long supply lines can be. When the imported supply runs short, a region has few choices: use less, store more, or produce new supply from a source that does not depend on the year's rainfall. Two neighboring regions, San Diego County and Orange County, chose that third path in different ways, and their choices show how geography shapes policy.
San Diego County looked to the ocean. The Carlsbad desalination plant, the largest in the nation, takes in seawater and forces it through reverse osmosis membranes, which strip out the salt and pass fresh water through. The plant produces about 50 million gallons of drinking water per day, roughly 10 percent of the county's supply. The argument for the plant is that the ocean does not run short, so it yields the same supply in a dry year as in a wet one. The trade-off is energy. Seawater reverse osmosis uses roughly 11 to 15 kilowatt-hours of electricity for every 1,000 gallons produced, compared with about 1 to 2 kilowatt-hours for conventional treatment and delivery, so drought protection arrives with a permanently higher power bill and a higher price per gallon.
Orange County looked to the ground beneath it. The county sits over a large natural groundwater basin, a body of water-bearing sand and gravel that its cities have pumped from for generations. Its Groundwater Replenishment System takes wastewater that has already been treated once, purifies it through microfiltration, reverse osmosis, and ultraviolet light, and then sends it into the ground to refill the basin. Wells later pump it back out as drinking supply. At 130 million gallons per day, it is the largest facility of its kind in the world. The approach is called indirect potable reuse: indirect because the purified water passes through the basin before anyone drinks it, rather than flowing straight to taps.
That indirection is not an engineering requirement so much as a public one. Early proposals to recycle wastewater in other cities collapsed under a slogan, "toilet to tap," that made the idea sound unsafe regardless of the treatment involved. Sending the purified water through the ground first gave the public a natural buffer they could accept, and years of plant tours, published test results, and repeated public explanation built the rest of the support. Orange County's system is now studied by utilities worldwide as much for its public process as for its plumbing.
Geography set the limits on what each region could choose. San Diego County lacks a comparable groundwater basin, so it had nothing underground to refill, while the ocean on its coastline was a dependable local source. Orange County had the basin, which made reuse cheaper for it than desalting seawater, since its source water arrives with far less salt to remove. Each region built the project its landscape allowed.
The choices also loop back on the regions that made them. Reliable supply supports population and economic growth, and growth raises demand and energy use, which shapes the next round of water decisions. Neither approach is cheap, and each region had to decide what its residents would pay for a supply that holds up in a drought, in money, in energy, and in willingness to accept the source.
Questions
- What does the Carlsbad plant produce each day, and about what share of San Diego County's supply is that?
- According to the passage, how does the energy use of seawater reverse osmosis compare with conventional treatment and delivery?
- What does "indirect" mean in indirect potable reuse, as the passage explains it?
- Why did sending purified water into the groundwater basin, rather than straight to taps, help the public accept Orange County's project?
- Using the passage, explain why San Diego County did not simply copy Orange County's reuse approach.
- The passage says supply choices "loop back" on a region. Describe that loop in your own words, naming at least two links in it.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: About 50 million gallons of drinking water per day, roughly 10 percent of San Diego County's supply. The second paragraph states both figures.
- Answer: Seawater reverse osmosis uses roughly 11 to 15 kilowatt-hours per 1,000 gallons, several times (roughly 5 to 15 times) the 1 to 2 kilowatt-hours for conventional treatment and delivery.
- Answer: The purified water passes through the groundwater basin before anyone drinks it, rather than flowing directly from the treatment plant to taps. The third paragraph defines this.
- Answer: The basin acted as a natural buffer the public could accept, countering the "toilet to tap" perception. Combined with tours, testing, and explanation, it made the recycled source feel safe.
- Answer: San Diego County lacks a comparable groundwater basin, so the indirect reuse model had nothing underground to refill, and its dependable local source was the ocean, which pointed to desalination.
- Answer: Reliable supply supports population and economic growth; growth raises water demand and energy use; higher demand and energy use then shape the next round of supply decisions. Any two or more of these links, clearly connected, earn credit.
Differentiation
Support
Provide a two-column comparison chart labeled source, process, output, and main trade-off for students to complete while reading, and let questions 1 through 3 be answered from the chart.
Extension
Ask students to write a short recommendation for a rain-poor inland city, stating which of the two strategies transfers there and which cannot, with the geographic reason for each.
ELL supports
Pre-teach "membrane," "basin," and "recharge" with a simple diagram showing seawater desalting on one side and purified water soaking into the ground on the other. Accept short-phrase answers for questions 1 and 2.