Key groundwater-well terms during pumping: Which combination of statements correctly describes water-level behavior, depression head (drawdown), cone of depression, and radius of influence around a pumped well?
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AThe stilling well level represents the groundwater table level
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BDifference between original water table and pumped water level is depression head (drawdown)
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CAround a pumping well the water table forms a cone of depression
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DDistance from well center to where drawdown becomes negligible is the radius of influence
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EAll the above
Answer
Correct Answer: All the above
Explanation
Introduction / Context:Pumping tests and well hydraulics use several standard terms to quantify aquifer response: drawdown (depression head), cone of depression, and radius of influence. A stilling well or observation well is used to monitor groundwater levels under near-still conditions to avoid dynamic fluctuations.
Given Data / Assumptions:
- Unconfined aquifer with an initially horizontal water table.
- Steady pumping causes water-level decline near the well.
- Observation points record levels for analysis.
Concept / Approach:Drawdown equals static water level minus pumping water level. Spatial variation of drawdown creates a characteristic cone-shaped surface known as the cone of depression. Radius of influence is the distance from the well at which drawdown becomes negligible for practical purposes. A stilling/observation well reflects the ambient groundwater level (absent rapid dynamic disturbances).
Step-by-Step Solution:Identify monitoring: stilling well tracks groundwater level reliably.Define drawdown: s = h_static - h_pumped.Conceptualize the cone of depression around the well.Define radius of influence: distance to near-zero drawdown.
Verification / Alternative check:Log-log and semi-log analyses (e.g., Theis, Cooper–Jacob) utilize observation well drawdowns to estimate transmissivity and storativity, consistent with these definitions.
Why Other Options Are Wrong:Each individual statement (A–D) is correct; the most complete choice is “All the above.” Selecting only one omits the integrated description of well hydraulics terms.
Common Pitfalls:
- Equating radius of influence with aquifer boundary; it is a practical extent of measurable drawdown.
- For confined aquifers, the potentiometric surface responds similarly even though the water table is not exposed.
Final Answer:All the above