Page 97 - Hydrogeology Principles and Practice

HYDC03 12/5/05 5:36 PM Page 80

80 Chapter Three

water occurs to produce a Na-Cl water type before pH paper can be applied to indicate if the pH after
a region of static water and aquifer diagenesis is acidification is adequate. Once collected, samples
reached. Either part or all of this classic sequence should be stored in a cool box and shipped as soon
of hydrochemical change is identified in a number as possible, ideally the same day, to the laboratory
of aquifers including the Great Artesian Basin of for storage at 4°C in order to limit bacterial activity
− 2−
Australia (see Box 2.11), the Floridan aquifer system and degradation of nutrient species (NO , SO 4 and
3
3−
(Box 3.2) and the Chalk aquifer of the London Basin PO ). Freezing of samples should be avoided as this
4
(Ineson & Downing 1963; Mühlherr et al. 1998). can lead to precipitation of some elements.
While at a sampling location, and as part of the
sampling methodology, the well-head chemistry
3.5 Groundwater sampling and graphical should be measured, again with the aim of collecting
presentation of hydrochemical data data representative of the hydrochemical conditions
in the aquifer. Parameters such as temperature, pH,
The aim of field sampling is to collect a raw water redox potential (Eh) and dissolved oxygen (DO) con-
sample that is representative of the hydrochemical tent will all change once the groundwater sample is
conditions in the aquifer. To obtain a groundwater exposed to ambient conditions at the ground surface,
sample for analysis some type of sampling device is and during storage and laboratory analysis. On-site
required, for example a bailer, depth sampler, gas lift measurement of electrical conductivity (EC) and
sampler or pump (inertial, suction or submersible) alkalinity should also be conducted. To limit the
in either an open hole or borehole, or from purpose- exsolution of gases from the groundwater, for ex-
designed piezometers or nested piezometers. Care ample the loss of CO which will cause the pH value
2
must be taken to first flush the water standing in of the sample to increase, and in order to prevent
the well or borehole by removing up to three or four mixing of atmospheric oxygen with the sample and
times the well or borehole volume prior to sampling. so affecting the Eh and DO values, measurements
Abstraction boreholes that are regularly pumped pro- should be made on a flowing sample within an isola-
vide a sample that is representative of a large volume tion cell or flow cell. The flow cell is designed with a
of aquifer, whereas depth-specific sampling from a plastic tube leading from the sampling tap into the
multilevel system, or from a section of borehole col- base of the cell and an overflow tube at the top of the
umn isolated by inflatable packers, provides discrete cell. The lid of the flow cell has access holes into which
samples that are representative of a small volume of the various measurement probes can be inserted. A
aquifer. For a comparison of the performances of vari- typical time to allow for the Eh and DO electrodes to
ous depth sampling methods, the reader is referred to stabilize in the flow cell prior to recording the final
Price and Williams (1993) and Lerner and Teutsch values is between 20 and 30 minutes. For further in-
(1995). formation on hydrochemical parameter measurement
Groundwater samples can be conveniently collected and sample collection, including sampling of springs
in new or cleaned screw-cap, high-density polyethy- and pore waters, the reader is referred to Lloyd and
lene bottles. When filling sampling bottles, the bottle Heathcote (1985) and Appelo and Postma (1994).
should first be rinsed two to three times with the Before attempting any hydrochemical inter-
water sample and, for samples containing suspended pretation, it is first necessary to check the quality of
sediment or particles, filtered using a 0.45 µm mem- laboratory chemical analyses and that the condition
brane filter. A 0.1 µm filter is recommended where of electroneutrality has been met whereby the sum of
trace metals are to be analysed. Typically, several the equivalent weight of cations equates to the sum
filled sample bottles are collected at one site, with one of the equivalent weight of anions. This check is com-
sample preserved to stabilize the dissolved metals in monly carried out by calculating the ionic balance
solution. The conventional method of preservation error of the major ions where:
prior to analysis is to add a few drops of concentrated
nitric or acetic acid after the sample has been col- Ion balance error (%) = {Σcations −Σanions)/
lected and filtered to lower the pH to 2. A test using (Σcations +Σanions)} × 100 eq. 3.4