Which of the following statements is true for groundwater self-purification?

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BlackTom AI · Accepted Answer

To evaluate groundwater self-purification, we need to consider how contaminants behave underground. 
Option 1 suggests contaminants are diluted easily but not washed out effectively. In reality, groundwater transport is slow and dilution alone does not remove contaminants quickly; this option exaggerates easy dilution as a primary cleansing mechanism, which is misleading because removal also depends on advection, dispersion, and reactions.
Option 2 claims contaminants are washed out rapidly but warm temperatures slow reactions. In groundwater systems, reactions often proceed slowly due to limited mixing and low temperatures; yet the statement about rapid washout is not generally true because many contaminants persist and are not rapidly flushed out by groundwater flow.
Option 3 states: contaminants are not washed out or rapidly diluted and lower concentrations of dissolved oxygen exist for decomposition. This aligns with groundwater self-purification, where slow groundwater movement means contaminants are not rapidly removed or diluted, and reducing conditions (lower dissolved oxygen) favor anaerobic decomposition pathways that can break down contaminants over longer timescales.
Option 4 says contaminants wash out rapidly but lower dissolved oxygen exist for decomposition. The rapid washout claim conflicts with typical groundwater behavior, where flow is gradual and mixing is limited, making rapid clearance unlikely.
Option 5 asserts contaminants are easily volatilized and diluted but not washed out rapidly, and temperatures are too warm. Volatilization is generally negligible for many dissolved groundwater contaminants and underground temperatures are not typically described as "too warm" for self-purification; moreover, this option misstates the primary processes involved in groundwater cleanup.

Putting these pieces together, the statement in option 3 best captures the standard understanding of groundwater self-purification: contaminants are not quickly washed out or diluted, and decomposing conditions often involve low oxygen that enable certain breakdown processes over time.

11:375:101:01 INTRO ENV SCIENCE Lecture #16 Quiz

Which of the following statements is true for groundwater self-purification?

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