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Influence of purging the surrounding monitoring wells for groundwater sampling during a hydraulic conductivity test

in a monitoring well.

Hafeez Chishti, Ph.D., P.Geol.PHH ARC Environmental

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SLUG TEST is a common and reliable way of determining the lateral hydraulic conductivity of local and distinct geologic horizons under in-situ conditions.

Slug tests are often used at hazardous waste sites, since large volume of contaminated water do not have to be dispersed, as in a pump test.

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Technique

An instantaneous drop or rise of water level is created in a monitoring well and the recovery of water levels back to normal is recorded with respect to time. The time it takes for getting back to original static water level actually represents the ease of flow of groundwater through the soil pores measured as Hydraulic Conductivity (K). The numerical value of “K” is calculated using certain formulae.

The technique used to create a sudden drop in water level and measuring its rise back with time is known as Rising Head Method, and the technique used to create a sudden rise in water level and measuring its fall back with time is known as Falling Head Method.

SWLBailer

Logger

t0

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The simplest interpretation of piezometer recovery is that of Hvorslev (1951). The analysis assumes a homogenous, isotropic medium in which soil and water are incompressible. Hvorslev's expression for hydraulic conductivity (K) is:

where:K = Hydraulic Conductivity (m/s)r = Radius of the well casing (m)R = Radius of the borehole (well casing plus sand pack) (m)Le = Length of well screen (m)T0 = Time for the water level to rise or fall 37% of the initial change (s).

oe

e

TLRL

rK

2

ln2 ⎟⎠⎞

⎜⎝⎛

=

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PREFERRED PRACTICESfor conducting a slug test

1) Selection of the monitoring well with soil profile that best represents the siteIf soil around the well is not representative of the site, value of K will not be the representative of the site.

2) Preference of using rising head rather than the falling head methodFalling head method employ rise of water in the well which entails: a) Water going into unsaturated zone relatively quickly compared to saturated zoneb) Risk of spreading of contamination in the unsaturated zone above water table.

3) Creating instant draw down rather than using methods of continuous pumpingCreating instant draw down using a bailer allow the measurement of TimeZero (t0) reading, compared to using a pump which pumps water out but at the same time letting formation water into the well.

4) Recording monitoring well dimensionsOmitting to record radius of well standpipe, borehole annulus, and the height of well standpipe above grade may pose problems in slug test analyses.

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Borehole Log

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Recommended Time Intervals (Minutes)

00.05

123456789101214162025304050607590

105120150180210240

For manual measurementsof drawdown

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9

A Typical Drawdown Curve

2.250

2.300

2.350

2.400

2.450

2.500

2.550

2.600

2.650

2.700

2.750

2.800

0 20 40 60 80 100 120 140

Time (Minutes)R

ecov

ery

(Met

ers) Bailer

Logger

SWL

t0

100 5 10 15 20 252.5

Site: 424

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Borehole Log

12

2.000

2.050

2.100

2.150

2.200

2.250

2.300

2.350

2.400

0 50 100 150 200 250 300

Time (min)

Dra

wdo

wn

(m)

Time – Drawdownwith Purging

Test: 424-B

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Time-Drawdownwithout purging

2.200

2.300

2.400

2.500

2.600

2.700

2.800

0 50 100 150 200 250 300

Time (min)

Test: 424-B

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2.000

2.050

2.100

2.150

2.200

2.250

2.300

2.350

2.400

0 50 100 150 200 250 300

Time (min)D

raw

dow

n (m

)

During purging

Test: 424-B

K = 1.45x10-7 m/s

2.240

2.260

2.280

2.300

2.320

2.340

2.360

0 10 20 30 40 50

Time (min)

15

2.000

2.050

2.100

2.150

2.200

2.250

2.300

2.350

2.400

0 50 100 150 200 250 300

Time (min)

Dra

wdo

wn

(m)

Test: 424-B

2.200

2.300

2.400

2.500

2.600

2.700

2.800

0 50 100 150 200 250 300

Time (min)

K = 9.7x10-8 m/s

K = 1.4x10-7 m/s

Test: 424-B

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K = 9.7x10-6

K = 1.4x10-7

Non-purging

Purging

2.250

2.300

2.350

2.400

2.450

2.500

2.550

2.600

2.650

2.700

2.750

2.800

0 20 40 60 80 100 120 140

Time (Minutes)

Rec

over

y (M

eter

s)

Hvorslev Analyses

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3.200

3.300

3.400

3.500

3.600

3.700

3.800

0 50 100 150 200 250

Time (min)

2nd Set of Similar Data from a Different SitePurging was in progress when test started

3.490

3.500

3.510

3.520

3.530

3.540

3.550

3.560

3.570

0 5 10 15 20 25 30

Time (min)

3.000

3.100

3.200

3.300

3.400

3.500

3.600

0 50 100 150 200 250

Time (min)

Test 296-D

Test 296-D

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Purging was in progress when test started

2.950

3.000

3.050

3.100

3.150

3.200

3.250

3.300

0 50 100 150 200 250

Time (min)

3.200

3.300

3.400

3.500

3.600

3.700

3.800

0 50 100 150 200 250

Time (min)

Test: 296-D

Test: 296-D

K = 3.42 x10-7

K = 1.23 x10-8

Without Purging

Recovery after 25 minutes: 64%

Recovery after 240 minutes: 97%

With Purging

Recovery after 25 minutes: 14%

Recovery after 240 minutes: 91%

Test completed without purging

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Purging started while test was in progress

Test completed without purging

3.000

3.100

3.200

3.300

3.400

3.500

3.600

0 50 100 150 200 250

Time (min)

2.750

2.800

2.850

2.900

2.950

3.000

3.050

3.100

0 50 100 150 200 250

Time (min)

Test: 470-D

K = 3.56x10-7

Test: 470-D

K = 1.67x10-7

3rd Set of Data From a Different SitePurging started when test started

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K = 1.67 x10-7

K = 3.56 x10-7

K = 3.41x10-8

Non-purging Purging

Hvorslev Analyses

These two are comparable

210 4 8 12 16 202

Site: 472

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1.750

1.800

1.850

1.900

1.950

2.000

2.050

2.100

2.150

0 20 40 60 80 100 120 140

Time (Minutes)R

ecov

ery

(Met

ers)

1.750

1.800

1.850

1.900

1.950

2.000

2.050

2.100

2.150

0 20 40 60 80 100 120 140

Time (min)

Rec

over

y (m

)

1.750

1.800

1.850

1.900

1.950

2.000

2.050

2.100

2.150

0 20 40 60 80 100 120 140

Without Purging

During Purging

Test: 472-A

Test: 472-A

Test: 472-A

Without Purging

Recovery after 25 minutes: 34%

Recovery after 120 minutes: 89%

With Purging

Recovery at 25 minutes: 13%

Recovery at 120 minutes: 65%

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K = 1.32x10-7

K = 8.13x10-9

Hvorslev Analyses

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Coarse grained soils especially gravel and very coarse to coarse sand, do not appear to have any significant effect of purging (for water sampling) in the vicinity on the test run.

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CONCLUSIONS

Purging of water for groundwater sampling or for any other reason, must be avoided in the vicinity of a monitoring well at which a slug test is in progress. This is because purging creates drawdown which may interfere with drawdown levels being recoded at the test well.

Such an effect of purging is significant in soils with fine to medium grain, and is minimum in coarse grained soils.

Preferably, slug test should be performed as a standalone activity.

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Thank You