Stream Monitoring Below a Treatment Plant
A student team samples a stream at three sites on five July days at 07:00 and 14:00. Site S1 is 200 m upstream of a wastewater treatment plant outfall. Site S2 is 100 m downstream of the outfall. Site S3 is on a tributary that enters the main stream between S1 and S2; the tributary drains fertilized cropland and a parking lot. Mean results: nitrate 1.2 mg/L at S1, 6.8 mg/L at S2, 5.9 mg/L at S3; dissolved oxygen 8.5, 5.2, 5.4 mg/L; turbidity 4, 18, 15 NTU; temperature 18, 22, 21 °C. The team concludes that the plant is the main cause of elevated nitrate and low dissolved oxygen. Their report includes no flow measurements and no replicate samples at each site-time.
1. Identify two confounding variables in this design and explain, using the numbers, how each could produce the observed pattern without the plant being the main cause.
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Two confounders are tributary input and time-of-day warming.
- Tributary input: S3, on the tributary, already has 5.9 mg/L nitrate and 15 NTU turbidity. Because S2 is downstream of the tributary confluence, its 6.8 mg/L nitrate and 18 NTU could be mostly tributary water rather than plant effluent. The tributary drains fertilized cropland and a parking lot, both plausible nitrate and sediment sources.
- Time-of-day warming: S2 and S3 are warmer (22 °C and 21 °C) than S1 (18 °C). Warmer water holds less dissolved oxygen, and afternoon photosynthesis and respiration can also lower DO. Thus the 5.2 mg/L DO at S2 is not uniquely evidence of plant effluent.
A third problem is the lack of flow measurement: if the tributary contributes most of the flow at S2, dilution and load calculations change. Without discharge, concentration alone cannot identify the source.
2. Propose a revised sampling plan that could separate the plant's effect from the tributary's effect. Include at least three specific design features.
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Add a main-stem site between the outfall and the tributary confluence (S2a). Sampling S1 upstream, S2a in the plant-only reach, S3 on the tributary, and S2 in the mixed reach separates the two inputs.
- Measure discharge at every site and compute mass flux (concentration times flow), not just concentration.
- Collect depth-integrated samples in triplicate at each site and time; include field blanks and a duplicate.
- Sample during baseflow after at least 48 hours without rain, and repeat during a dry period and a wet period to test runoff effects.
- Sample at several times of day or standardize to early morning to reduce diurnal DO effects.
A conservative tracer or a before/after plant shutdown comparison would further isolate the plant.
3. The team's conclusion says the plant is the main cause. Based on the data, is that conclusion supported? Explain what additional evidence would strengthen or weaken it.
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Not supported as stated. The data are consistent with the plant contributing nitrate, but they do not show that it is the main cause. S3 shows the tributary is already high in nitrate and turbidity, and S2 lies below the confluence, so the tributary could dominate. The lack of flow data prevents a load balance; a small-volume high-concentration plant discharge could be less important than a large tributary load. Lower DO at S2 could be temperature-driven.
Evidence that would strengthen the claim: a significant increase in nitrate mass flux between S1 and S2a before the tributary enters, plus a tracer signal from the outfall. Evidence that would weaken it: tributary mass flux accounts for most of the nitrate at S2, or the plant effluent load is small relative to the tributary load.