What issue is indicated if higher BOD samples are yielding higher oxygen uptake readings?

When higher biochemical oxygen demand (BOD) samples yield higher oxygen uptake readings, this suggests the presence of a toxic substance in the sample. BOD measures the amount of oxygen that microorganisms require to break down organic matter in water. If there is a toxic substance present, it can disrupt normal microbial activity, potentially leading to an incorrect interpretation of the biological activity in the sample.

In situations where the microbial population is stressed or inhibited by toxic substances, the expected relationship between BOD and oxygen uptake may not hold true. Typically, an increase in BOD should correspond to a proportional increase in oxygen consumption, assuming the microbial community is healthy and active. However, if toxicity is present, it can inhibit some microorganisms while allowing others to thrive, complicating the interpretation of the BOD results.

The other answer choices address unrelated issues. Sample dilution typically results in lower readings of BOD and oxygen uptake rather than higher. An inaccurate testing method could yield inconsistent results but does not inherently link to increased oxygen uptake from higher BOD readings, while decreased sample size could lead to variability but would not specifically explain increased oxygen uptake in response to higher BOD.