Lecture 29

Lecture 29

Specific Capacity

Fetter 7.6, 7.7, 7.8, 7.9

Pre-Test Calculations and Data Preparation (Continued)

Data Preparation

Components of drawdown

Aquifer losses

Well interference

Geohydrologic Boundaries

Partial Penetration

Dewatering

Well Losses

Barometric Efficiency

Tidal Efficiency

River Efficiency

Trend

Non-pumping water levels measured prior to pumping.

Length of test determines duration of trend measurements.

Man-made influences should be controlled.

Step Test

Used to determine pumped well efficiency.

Three successive periods of pumping.

Pumping relate increases each period.

Rates are normally 1/3, 2/3 and equal to that of the constant rate test.

Constant Rate Test

Production rate held constant for the duration of test.

Water level measurement frequency decreases with time.

Water level responses are used to determine aquifer properties.

Recovery

Water level measurements taken immediately after test.

Water levels not influenced by erratic pumping rates.

Recovery analysis compared to constant rate test.

Check valve on pump essential to eliminating slugging effects.

Observation Wells

Minimum of 3 wells preferred.

Alignment varies with problem.

Spacing increases with distance from the pumped well.

Generalized Design Features

Logarithmic spacing.

Parallel to boundary to minimize boundary effects.

Avoid partial penetration impacts.

Distance between PW and OW = 1.5m (pH/Pv)^1/2

where m = aquifer thickness in feet

pH = aquifer horizontal K in gpd/sq ft

Pv = aquifer vertical K in gpd/sq ft

Pumping tests are commonly 8 hours to 5 days in duration.

Pumping tests schedule p. 14, Walton (1987).

Specific Capacity

may not always have an aquifer test.

Specific Capacity - Yield of well divided by drawdown.

	Use Theis equation

	T = ® Specific Capacity
	(1.) Initially, guess T
	(2.) Solve for Q/h₀-h
	(3.) Adjust T until reasonable Q/h₀-h
		® Have to establish S
		® Assume well is 100% efficient, therefore, typically under predicts Specific Capacity.

Empirical Relationship-

	Razack and Huntley (1991) - measured T and Specific Capacity at 215 Wells

	T = 33.6

	T transmissivity (ft²/d)
	Q discharge (ft³/d)
	h₀-h drawdown (ft)

Well Interference

	What if more than one well pumps in an Aquifer?
	At a given point- Total drawdown is the sum of individual drawdown for each well.
	Laplace equation linear ® Superposition of drawdown is additive.
	(1.) Only valid in confined aquifer where T does not change.
	Or Unconfined aquifers if dewatering not significant.
	s = total drawdown
	s = s₁ + s₂ + s₃ .....s_n
	s_n = drawdown in individual wells.

Confined Aquifer

	s =
	Where W(u₁) = u₁ =

Figure 7.28 Composite pumping cone for three well tapping the

same aquifer. Each well is pumping at a different rate; thus the

pumping level of each is different.

Important to know if other wells pumping during an aquifer test.

If wells aligned parallel to a line source of recharge, effects of well

interference would not be as great.

ENV 302 - Lectures