A single well pumping and recovery test to measure in situ

acrotelm transmissivity in raised bogs

S. van der Schaaf*

Department of Environmental Sciences, Sub-department of Water Resources, Wageningen University,

Nieuwe Kanaal 11, 6709PA Wageningen, The Netherlands

Received 8 April 2003; revised 14 November 2003; accepted 10 December 2003

Abstract

A quasi-steady-state single pit pumping and recovery test to measure in situ the transmissivity of the highly permeable upper

layer of raised bogs, the acrotelm, is described and discussed. The basic concept is the expanding depression cone during both

pumping and recovery. It is shown that applying this concept yields comparable results from pumping test and recovery,

although the flexibility of the acrotelm matrix may cause considerable differences during individual tests.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Raised bogs; Acrotelm; Transmissivity; Pumping tests; Peat hydraulics

1. Introduction

The acrotelm is the shallow top layer of a living

raised bog. It includes the living peat moss at the

surface. In bogs of north-western Europe it is usually

between 10 and 40 cm deep. In its original concept, it

contains the oscillating water table (Ivanov, 1953;

cited by Ingram, 1978) and hence has rapidly

changing moisture conditions. Thus it is periodically

aerated, which causes a relatively rapid downward

increase in the degree of decomposition of its material

(Romanov, 1968). The term acrotelm was proposed

by Ingram (1978) and has become generally accepted

since. The peat body below the acrotelm is termed

catotelm. This so-called diplotelmic approach does

not imply a division into two soil horizons in the strict

sense, because the boundary between acrotelm and

catotelm is not defined in a measurable and repro-

ducible way. However, the diplotelmic approach has

proved a useful concept in understanding the

hydrology of raised bogs (Ingram and Bragg, 1984;

Van der Schaaf, 1996, 1998, 1999).

A hydrologically relevant feature of the acrotelm is

the downward increase in the degree of decompo-

sition, which implies a downward decrease in fiber

elasticity, an increase in the volume fraction of small

particles and hence a transition from large pores at the

surface to smaller pores downwards. Consequently, a

transition occurs from a large hydraulic conductivity

near the surface to considerably smaller values at

some decimeter depth. The difference may be up to

several orders of magnitude (Balyasova, 1979;

Ivanov, 1981). Some rather common values are

105 m d21 or larger close to the surface and

110 m d21 at some decimeters below it. The average

Journal of Hydrology 290 (2004) 152160

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0022-1694/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jhydrol.2003.12.005

* Fax: 31-317-484885.E-mail address: sake.vanderschaaf@wur.nl (S. van der Schaaf).

hydraulic conductivity of the catotelm is usually some

orders of magnitude smaller (Van der Schaaf, 1999).

Thus, in spite of its shallowness, the acrotelm is the

one and only aquifer in a raised bog. Because of the

sharp downward decrease in the hydraulic conduc-

tivity, the transmissivity of the acrotelm strongly

depends on the level of the water table and is a

regulating system for the outflow of water from a

raised bog (Verry et al., 1988; Van der Schaaf, 1999).

Hence, in hydrological studies of raised bogs,

information on transmissivity characteristics of the

acrotelm is almost indispensable.

The method to measure acrotelm transmissivity

described here was developed because classic

methods, such as Hooghoudts and Ernsts augerhole

method (Van Beers, 1963) could not be used because

the auger hole often filled up with water within a

second after water removal. Therefore a method was

developed, which is based on pumping during the test

instead of one where water is removed or added at the

start only.

2. Description of the tests

Square pits of approximately 25 25 40 cm3deep were cut with a spade at different locations on

the bogs Raheenmore Bog and Clara Bog, both in

Co. Offaly, Ireland. The pits were left to settle for at

least a day before tests were carried out. At each test,

the pit was pumped during 20 s to 5 min at a

constant rate of 110 l min21, depending on the

expected transmissivity and the expected drawdown

during the test. The small pump was a 12 V battery-

powered centrifuge type and a sieve was used to

prevent large peat particles in the pit from clogging

the pump (Fig. 1).

The discharge was controlled by connecting

different pre-calibrated lengths and diameters of

polythene tubing to the outlet of the pump. The

discharge was checked during tests with a calibrated

vessel and a stopwatch. The drawdown in the pit

should remain within a few centimeters from the

equilibrium level to prevent too much of the upper and

most permeable part of the acrotelm from being

excluded from the flow. Otherwise, a considerable

underestimation of the transmissivity might result.

Therefore, pumping was continued either until no

visible further drawdown occurred, or until the

drawdown had reached a value of 3 cm. In the latter

case the water table in the pit would often continue to

fall if the pumping was not stopped, because the flow

towards the pit would occur in less and less permeable

parts of the acrotelm. After the pumping had been

stopped, the water table in the pit would rise again and

a recovery test was done. Therefore, two sets of

equations had to be derived: one for the pumping test

itself and one for the recovery test.

3. Equations

3.1. The pumping test

Pumping until no visible further drawdown occurs

does not necessarily mean that steady state has been

attained. Therefore, the steady-state Thiem equation

was used as a basis to derive an equation to calculate

acrotelm transmissivity from the tests. Strictly speak-

ing, this may not be entirely justified, because during

any such test the depression cone will keep expand-

ing. However, close to the pit, the hydraulic gradient

does not change very much shortly after pumping

has started. This situation is termed transient steady

state by Kruseman and Ridder (1990). In this paper,

the usage of Thiems equation will be discussed later.

Fig. 1. Pumping test as carried out during the fieldwork.

S. van der Schaaf / Journal of Hydrology 290 (2004) 152160 153

At this stage, the equation is applied without further

comment. It reads

Q 2pTah2 2 h1ln

r2r1

1

where Q is well discharge (pumping rate) [L3T21], Tais acrotelm transmissivity [L2T21], h1 is phreatic level

[L] at a distance r1 [L] from the well and h2 is the

phreatic level at a distance r2 from the well.

Eq. (1) holds approximately if (Kruseman and

Ridder, 1990):

(a) the lateral extent of the aquifer is much larger

than the distance to which the phreatic level is

noticeably affected by the drawdown in the well;

(b) the aquifer is homogeneous in the horizontal

direction over the area in which the phreatic level

is noticeably influenced by the drawdown in the

well;

(c) the phreatic level was approximately horizontal

immediately before the test;

(d) the discharge rate was constant during the test;

(e) the well fully penetrates the aquifer;

(f) the flow is horizontal;

(g) the saturated depth of the acrotelm aquifer is

constant over the area in which the phreatic level

is noticeably affected by the drawdown in the

well.

Conditions (a), (c), (d), and (e) are normally

satisfied. Condition (b) is usually satisfied to a

reasonable extent if the site is properly chosen, e.g.

as much as possible in the middle of a microtopo-

graphical element, such as a hollow or a hummock.

Conditions (f) and (g) can be satisfied approximately

by keeping the drawdown in the well small, as

discussed in Section 2.

In Eq. (1), both r1 and r2 may be chosen arbitrarily,

as long as r1 r2 and at least equal to the well radiusrw: Hence, the radius r1 can be substituted by rw: If h2is the phreatic level immediately before the test, then

r2 is the distance to which the effect of the pumping

has extended. Theoretically this distance should be

infinite, but if the equation is combined with the

pumped volume of water, it will yield a finite value

for r2; as will be shown below. Then the difference

h2 2 h1 is the drawdown yw [L] in the well.

Substituting r1; h1 and h2 in Eq. (1) and writing Taexplicitly yields:

Ta Q ln

r2rw

2pyw2

Because the horizontal cross-section of the spade-dug

wells was approximately square, an effective rw was

to be found. This could be done by either calculating

the radius of a circle with the same area, which gives

an underestimation, or one with the same circumfer-

ence, which gives an overestimation. Averaging the

two yields

rw