10. Proposed Groundwater Monitoring Network

10. PROPOSED GROUNDWATER MONITORING NETWORK

10.1 Selection of sites for groundwater monitoring network

There are two important features that distinguish groundwater from surface waters which need to be considered when designing a monitoring network for groundwater quality and quantity. These are:

• the slow movement in groundwater with relatively large residence times; and,
• the considerable degree of physicochemical and chemical interdependence between water and material of aquifer.

The spacing of the observation wells in the groundwater quality network will depend on the strategy for differentiation between diffuse and point pollution stations, between national and regional stations and with the differences between principal networks, specific networks and temporary networks. The criteria for a monitoring network are very important for the evaluation and comparison of the data. If there is a need to evaluate data from all over the EEA area, the sampling sites should be chosen by the same criteria. If they are not, a consistent report is probably impossible. While minimum densities for groundwater have not been developed, other guidance on station location and sampling may be provided. The density of observation wells in a groundwater network depends on:

• The size of the area or country.
• The geological and hydrogeological complexity of the area.
• The geological and hydrological setting and sizes of the main aquifers.
• The land use of the area.
• The admittance to the area and the possibilities for agreeing the establishment of the stations with the landowners.
• Existing monitoring systems.
• The objectives and time limits of the network.
• Financial limitations for establishment of the network and for routine groundwater sampling through time.

The main general demands of the network are:

• All main aquifers should be observed. These aquifers are defined according to the geological information and the known groundwater resources of the area.
• The distance between the observation wells will depend on the geological conditions.
• The network should be based on existing wells in the area from which, valid hydrogeological information can be extracted. Wells drilled for all different purposes can be used depending on the completion programme for the wells. The use of existing wells will reduce the cost of drilling and installation of observation devices.
• It is important also to monitor aquifers that at the time are not being used for groundwater abstraction, both shallow and deep aquifers.

Areas with a high rate of infiltration should be monitored more intensively.

A groundwater quality observation station should be able to monitor the general composition of the groundwater with different types of contaminants. Principally there are different sources of contaminants: diffuse sources arising from the atmosphere: diffuse sources from land use, (e.g. from farming); and, point sources such as landfills, contaminated sites and leaking sewer-systems. These three sources need a different approach to the design and establishment of suitable monitoring networks.

It is important to confirm that the monitoring wells which are chosen for the EEA network have been designed and constructed in the same way so it is possible to compare the results from all the Member States. It is also important that all the selected wells are described in detail both concerning the criteria and the technical design. Stations are considered similar whatever network they serve.

10.2 Proposed network

For groundwater the following is proposed.

1. Monitoring station selection should be based on existing national monitoring networks.
2. Where possible all nationally important aquifers (groundwater in porous media, karst groundwater and others) should be covered. The importance of aquifers could be defined with respect, for example, to quantity and/or quality, spatial extension, actual or planned use.
3. The selected monitoring stations within these selected aquifers should be distributed in a more or less regular geometric pattern and, as a rule, with a density of at least 1 site per 20 to 25 km² of aquifer.
4. In special cases a less intensive density of sampling stations is acceptable. This might be especially so in large similar hydrogeological structures with only low impacts (e.g. low density of population, small portion of arable land, mostly forests and grasslands, no serious point sources). In spite of their potential high vulnerability this may also be considered for karst areas.
5. The selected monitoring stations should not be exclusively based on drinking water abstraction points (drinking water abstraction is usually concentrated in a few least impacted areas) and on stations monitoring extremely local hotspots of contamination, as they would not provide comparable information on a Europe-wide scale.

Table 10.1 summarises (based on best available information) the extent of groundwater monitoring undertaken in EEA Member States (Koreimann et al. 1996). It indicates that for the interim network there may be major spatial gaps in the information available.

Table 10.1 Summary of the extent of groundwater monitoring undertaken in EEA Member States

The aim will be to develop a fully statistically representative network for groundwater quality and quantity assessment. In particular the potential gaps in information (Table 10.1) will need to be addressed in the development of the network and the statistical basis of the suggested optimum density of monitoring stations (1 per 20 or 25 km²) should be further statistically assessed. In addition the applicability of the concept of establishing reference stations in aquifers not affected by human activities will be assessed. Reference stations would be in areas not influenced by groundwater pumping and other anthropogenic activities. In some areas within the EEA (small countries or in densely populated areas) it will not be possible to establish such stations.

10.3 Sampling frequency

For the proposed general surveillance type network, a sampling frequency of twice a year should initially be assessed. One sampling would ideally have been conducted during a period of high ground water level, that is at the end or immediately after a period of high infiltration and recharge of the reservoir. The second sampling would be at a period of low ground water level, that is at the end of a period of minimum infiltration or maximum abstraction. This sampling schedule relates to groundwater reservoirs relatively close to the surface. When deeper reservoirs occur at the same sampling station they should be sampled at the same time as the shallower ones. In areas of very rapid infiltration like karst-areas other guidelines might be applied, e.g. modified according to seasonal exploitation in water supply areas of tourist centres.

10.4 Selection of determinants

10.4.1 Groundwater quality

The quality parameters on which information might be required can be divided into seven groups (Table 10.2).

1. Descriptive parameters;
2. Major ions;
3. Additional parameters;
4. Heavy metals;
5. Organic substances;
6. Pesticides;
7. Microbes.

Groups 1, 2 and 3 are, at present, measured in all countries with an observation network. Group 4, 5 and 6 are only measured in a few of the EEA countries. The types of parameters included in a programme depend on the purpose of the monitoring network. Group 4 and 5 are important in monitoring programmes on point pollution as landfills and contaminated sites, while Groups 4 and 6 are specially important for diffuse pollution from farming. The number of analysed compounds within each group depends on the purpose of the network and on the economy.

Table 10.2 List of suggested determinants required for the groundwater quality monitoring network

1	Descriptive determinants	Temperature, pH, DO, (EC).
2	Major ions	Ca, Mg, Na, K, HCO3, Cl, SO4, P04, NH4, NO3, NO2, Total organic carbon
3	Additional determinants	Choice depends partly on local pollution source as indicated by land-use framework
4	Heavy metals	Hg, Cd, Pb, Zn, Cu, Cr. Choice depends partly on local pollution source as indicated by land-use framework
5	Organic substances	Aromatic hydrocarbons, halogenated hydrocarbons, phenols, chlorophenols. Choice depends partly on local pollution sources as indicated by land-use framework.
6	Pesticides	Choice depends in part on local usage, land-use framework and existing observed occurrences in groundwater.
7	Microbes	Total coliforms, border="0" width="15" height="20">