7.9.1 Introduction
Groundwater is defined as subsurface water, which occurs in the water saturated zone of ground. It may lie near surface or deep in the bedrock. Groundwater is present everywhere and is, hence, one of the output media for elements in the terrestrial ecosystem. The monitoring of groundwater chemistry is dependent on the definition of the hydrological area. Usually it is monitored in open wells and observation tubes penetrating the loose overburden covering the bedrock. Monitoring may also take place in springs.
7.9.2 Methods
7.9.2.1 Sampling frequency
The frequency of water sampling should be adapted to the overall chemical composition of the groundwater in the particular sampling point. With high sampling frequency the annual variation will be well covered. On the other hand, it will increase the costs. So, a compromise will be necessary. Due to low temperature during the winter period it is often impossible to carry out sampling. Ground water should be sampled not less than 6 times a year, preferably more frequently in spring during the snowmelt period.
7.9.2.2 Allocation of groundwater tubes
Concentrate groundwater sampling to the discharge area of the catchment (Figure 7.9.1) where there is natural groundwater seeping belts or springs. Note that groundwater catchment usually differs from that of surface water. For ideal monitoring an additional line of groundwater tubes should be established covering both the recharge and discharge areas. The well line should run perpendicularly to slope contours (Figures 7.9.1 and 7.9.2). Placing of groundwater tubes in recharge areas should be planned in association with soil water samplers. In the ideal case monitored groundwater is found in a surficial aquifer composed of glaciofluvial material (sand and gravel) with relatively high hydraulic conductivity. In till deposits groundwater flow is often very slow and retention time is in the order of months or years. This hinders an appropriate temporal monitoring.
|
|
Figure 7.9.1 Siting proposal for groundwater monitoring within a catchment.
|
|
|
Figure 7.9.2 Illustration of a slope with positions for sampling tubes and lysimeters.
|
7.9.2.3 Groundwater sampling
Groundwater tubes installed in the terrain must be simultaneously strong and made of such material, that absolutely no contamination occurs (e.g. polyamide). All materials used in sampling should be analytically verified to eliminate contamination risks. For practical reasons, minimum inner diameter of the tube is ca. 30 mm. The top of the tube must be capped to avoid external dirt while allowing proper air circulation. Always use disposable gloves when handling instruments used in sampling. Smoking is forbidden and no petroldriven engine (e.g. snowmobile) is let to run in the vicinity of sampling site.
Sampling equipment
As an example of simple, manually operated, sampling equipment, used for soaking up groundwater through an observation tube from a maximum depth of 5-6 metres is shown in Figure 7.9.3. The equipment is adapted for use in areas where no electricity is available and where the sampling sites are far from roads. A hollow, cylindrical body, made of polyamide or other inert material is designed to be lowered into the sampling tube below the groundwater table. It is equipped with a weight embedded in the bottom part of the body. Water can pass through holes in the cylinder walls. An uncoloured silicon tube is connected to the top of the cylinder. These parts are kept during transportation in a protection tube manufactured of grey PVC. The protection tube is filled with deionized water, which is exchanged between the sampling occasions. The silicon tube is connected to a longer plastic tube, which is connected to 2-3 litre Pyrex or glass bottle, equipped with a polyethene or polyamide plug. The plug has two outlets, one for the tube from groundwater to the sampler and one for the air vacuum pump. When vacuum is created by the hand pump groundwater is sucked into the sampler bottle. When the bottle plug is not in use store it in an extra polyethene bag or similar clean place.
If groundwater is on such a deep level that it is impossible to suck up, submersible pumps should be used. Note that all the metal parts of the pump may create a definite risk of contamination. A weakness of the described sampling equipment is the unavoidable CO2 -escape.
Water pumped up from a groundwater tube often contains suspended clay mineral particles. Groundwater samples for heavy metals analysis should always be filtered through 0.45 µm membrane before acid conservation. If there are still clay particles left in the sample when the acid is added, metals in clay are released or metals in the groundwater may adsorb to negatively charged clay particles.
|
|
Figure 7.9.3 Simple equipment for groundwater sampling.
|
Sampling from wells and springs
If the sampling site is a well, water is percolating in a natural way and no flushing pumping is necessary. Be sure that water has not stayed unchanged for a long time (see observation tubes below). It is, however, desirable for the interpretation of the analysis results if the results can be correlated to either the well flow or to the groundwater level in some upstream observation tube. Water for the analysis of physical properties and primary constituents is sampled by filling the sample bottle directly from the well. The bottle can also be filled using a syringe (without filter). For metal and trace-element analysis filtering and preservation with acid is necessary.
Sampling from observation tubes
Establish the groundwater level by plumbing. Use plumbs made of inert material. Turnover pumping is necessary to avoid "stagnant" water, which has been in contact with both the atmosphere and the tube walls, and therefore differs chemically from the groundwater in the aquifer. If the sample should represent the groundwater closest to the screen of the sampling tube, the volume of water contained in the tube is pumped 1.5 - 2 times. If the sample should represent a larger part of the aquifer, the water is turned several times more.
When "fresh" water has reappeared, the actual sampling can begin. Put on disposable plastic gloves. Pump some water into the collection vessel and rinse it. Avoid touching the bottom with the flexible suction tube, since the water may become clouded. Fill the collecting vessel with pumped water. Lift the plastic plug and put in on the extra collecting vessel to ensure that it is not contaminated.
Rinse the 250-500 ml plastic bottle, intended for anion analyses, with water from the collecting vessel. Fill the bottle to the brim and screw on the lid to ensure that as few air bubbles as possible are left in the bottle. Samples for heavy metal determinations must be filtered and preserved by adding 0.5 ml of suprapur quality conc. HNO3 per 100 ml of sampling water. If possible use automatic dosing pipette with a disposable nose.
For details on collection and handling of water samples, please see Chapter 8.2.
7.9.3 Analyses
7.9.3.1 Field analyses
If field analyses are carried out these should be done immediately after the water has been pumped to minimize changes in quality. Pour water from the collecting vessel into a beaker for field analysis and measurements of certain parameters (e.g. pH, conductivity, dissolved oxygen). Never make measurements directly in the collecting vessel, except temperature.
7.9.3.2 Laboratory analyses
For a list of available standards, see Chapter 8.5
7.9.4 Quality assurance/Quality control
See data quality management in Chapter 8.
7.9.5 Data reporting
Mandatory parameters
|
list
|
|
unit
|
SO4S
|
DB
|
sulphate as sulphur
|
mg/l
|
NO3N
|
DB
|
nitrate as nitrogen
|
mg/l
|
NH4N
|
DB
|
ammonium as nitrogen
|
mg/l
|
NTOT
|
DB
|
total nitrogen
|
mg/l
|
CA
|
DB
|
calcium
|
mg/l
|
NA
|
DB
|
sodium
|
mg/l
|
K
|
DB
|
potassium
|
mg/l
|
MG
|
DB
|
magnesium
|
mg/l
|
CL
|
DB
|
chloride
|
mg/l
|
DOC
|
DB
|
dissolved organic carbon
|
mg/l
|
AL
|
DB
|
total aluminium
|
µg/l
|
ALL
|
DB
|
labile aluminium
|
µg/l
|
PH
|
DB
|
pH
|
|
COND
|
DB
|
specific conductivity at 25 oC
|
mS/m
|
ALK1)
|
DB
|
alkalinity, GRAN plot
|
mmol/l
|
FLOW
|
DB
|
groundwater flow
|
l/(s x km2)
|
WL
|
DB
|
groundwater level
|
cm from surface
|
Optional parameters:
|
list
|
|
unit
|
TEMP
|
DB
|
temperature
|
oC
|
STOT
|
DB
|
total sulphur
|
mg/l
|
PO4P
|
DB
|
phosphate as phosphorous
|
µg/l
|
PTOT
|
DB
|
total phosphorous
|
µg/l
|
SIO2
|
DB
|
silica (as silica)
|
mg/l
|
MN
|
DB
|
manganese
|
µg/l
|
FE
|
DB
|
iron
|
µg/l
|
AS
|
DB
|
arsenic
|
µg/l
|
CD
|
DB
|
cadmium
|
µg/l
|
CR
|
DB
|
chromium
|
µg/l
|
CU
|
DB
|
copper
|
µg/l
|
MO
|
DB
|
molybdenum
|
µg/l
|
NI
|
DB
|
nickel
|
µg/l
|
PB
|
DB
|
lead
|
µg/l
|
ZN
|
DB
|
zinc
|
µg/l
|
1) Please note the change of unit.
Example files
GW exmaple Excel file
GW example ASCII file
- File identifier SUBPROG states the subprogramme.
- MEDIUM refers to either tube sampling (TUBE) or spring sampling (SPRING).
- LEVEL is given as sampling depth (cm) from the ground (or spring water surface).
- Spatial pool SPOOL refers to the number of individual sampling points.
- If groundwater flow can be calculated and the sampling is done more than once a month, the chemical component values should be given as flow weighted means, status flag is W. Monthly values are reported without status. Groundwater flow is reported as a monthly mean. For calculation of flow weighted means, please see Annex 7. General information on flags is available in Chapter 4.
- Sampling year and month are given as YYYYMM, day field is left blank.