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Reading the Median and Range: What Maricopa County’s Data Shows

Government sampling in Maricopa County’s aquifer 112BSFLU indicates that iron concentration in groundwater has a median of 7.4 micrograms per liter (ug/L) with a range from 6.7 up to 46.5 ug/L. Residents often mistake the presence of iron detections as a direct sign their well is troubled by rust, but these figures reflect the general area, not a specific household well. Understanding the median and range helps establish a context: for example, half of the detected values fall below 7.4 ug/L, which is quite low and well within federal thresholds.

Sample Size and Its Impact on Data Reliability

The data behind these figures come from nine samples taken from five separate wells, collected over a period from August 2010 to September 2018. These wells were between 100 and 600 feet deep. While this sampling gives a useful snapshot of local groundwater quality, the small number of samples means the data may not capture all local variations. Small sample sizes limit the ability to predict the exact iron level in any individual household well.

Interpreting the Range Relative to Federal Standards

None of the measured iron concentrations exceeded federal thresholds, and detections below these standards are not considered violations or unsafe. The range measures appearances only. Iron detected at these concentrations typically affects water aesthetics—such as color or minor staining—but does not indicate a health risk. Chloride and total dissolved solids govern taste and corrosivity, but are not part of this iron evaluation.

How Sampling and Well Depth Inform Expectations

Sampling depths from 100 to 600 feet mean that water quality varies with depth and geology. Shallower or deeper wells than those sampled might encounter different iron levels. Groundwater iron is influenced by local rock composition and oxygen exposure, which can vary widely. Residents with wells deeper than 600 feet or shallower than 100 feet should consider testing their own water to confirm conditions before sizing treatment equipment.

Distinguishing Rust and Hardness from Similar Water Issues

Well rust is caused primarily by iron, which precipitates and oxidizes to form reddish stains or sediments. Hardness relates to dissolved minerals like calcium and magnesium, which cause scale buildup but are chemically distinct from iron. Often, homeowners confuse rust coloring with hardness effects or other sediment-related problems. Proper identification requires specific tests for iron and hardness components.

Prioritizing Water Testing Steps

Start with a visible inspection of water clarity and staining around fixtures. Follow by measuring hardness and iron concentration through laboratory or certified testing. Testing enables accurate sizing of treatment equipment like the Pentair Well Rust + Hardness System, which has a grain capacity of 32,000 grains to address both issues effectively. This stepwise approach ensures treatment matches actual water conditions rather than assumptions based on area data.

What Not to Conclude from Area-Wide Data

Maricopa County’s aquifer measurements provide a helpful baseline but do not replace testing of your specific well. Detection in the area does not mean your water contains high iron or hardness, nor that those detections violate any safety limits. The data should inform, not determine, your treatment decisions. Proper water analysis of your home supply remains the only way to confirm water quality and correctly size treatment systems.

Where this information comes from

Source: USGS groundwater sampling. Sampled 2018-09-19. These figures describe groundwater sampled in this area. They are not a test of any individual private well - only a test of that well can establish its own water.

On record: U.S. Geological Survey sampling of Iron in the 112BSFLU aquifer (Maricopa County): 9 results from 5 wells, median 7.4 ug/L, range 6.7-46.5 ug/L; none above any federal threshold