Question: A hydrology researcher tests 9 groundwater samples, 5 from a contaminated well, 3 from a nearby monitoring well, and 1 from a control well. If 4 samples are randomly chosen, what is the probability that exactly 2 are from the contaminated well and 1 is from the nearby monitoring well? - Treasure Valley Movers

Understanding Groundwater Sampling and Probability: Why Data Matters for Environmental Health

How do scientists ensure our drinking water remains safe? One key method involves analyzing groundwater samples to detect contamination. Recent studies highlight a method where hydrology researchers collect and examine samples from multiple sources—some contaminated, some monitored, and some controlled—to assess environmental risk. With growing awareness of water quality challenges across the U.S., from industrial runoff to aging infrastructure, tools like statistical sampling help interpret complex data. This article explores a real-world scenario: a researcher testing 9 groundwater samples—5 from a contaminated well, 3 from a nearby monitoring well, and 1 from a control sample—and asks: What’s the chance that if four samples are randomly selected, exactly two come from the contaminated well and one from the monitoring well?

Understanding these probabilities builds transparency and supports informed decision-making—critical in fields tied to public health and environmental responsibility.

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Why This Question Matters in Environmental Science and Public Trust

Groundwater contamination detection requires careful sampling and statistical rigor. When researchers randomly choose samples, they model real-world variability and assess risk with precision. The probability question posed reflects a practical, methodological challenge faced by scientists and regulators: They don’t test every drop, but instead use representative samples to infer broader conditions.

This line of inquiry supports better environmental monitoring, regulatory compliance, and public awareness. Finding ways to interpret such data clearly helps bridge the gap between technical research and community understanding—especially during emerging water quality concerns.

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How to Calculate the Probability: Breaking Down the Sample Query

Prime among statistical questions is determining how likely a specific pattern emerges from a defined group. In this case, we examine 9 groundwater samples:

• Contaminated well: 5 samples
• Nearby monitoring well: 3 samples
• Control well: 1 sample

Total: 9 samples. Choose 4 at random. We want exactly:

• 2 from the contaminated well
• 1 from the monitoring well
• 1 from the control well (no monitoring well sample left)

Note: No sample combination violates the total selection. This constraint shapes the calculation.

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Step-by-Step Breakdown: Combinatorics Behind the Probability

To compute this probability, use foundational combinatorics—specifically, combinations, which count how many ways to choose samples without regard to order.

The total number of ways to select any 4 samples from 9 is:

[ \binom{9}{4} = \frac{