Monitoring and sampling of groundwater is a complex process. This complexity stems mainly from substantial spatial variability of groundwater composition, limited access to the system and lack of simple hierarchy of flow such as drainage pattern of surface water systems (cf. Figure D16.6). In some instances also temporal variability of groundwater quality has to be taken into account. It has to be emphasized that even in favourable situations the sampling process comprises only a small part of monitored GWB, whereas conclusions drawn from this sampling necessarily relate to the entire system. Therefore, representativity of the collected samples is of utmost importance. Here, only the most important issues associated with this problem are outlined. More comprehensive discussion can be found in D7 (Witczak et al, 2006) and in D17 (Scheidleder et al., 2006).
The installation of a monitoring well or a series of wells should always be preceded by careful assessment of the purposes and objectives of the monitoring system. The objectives will in many cases dictate the design parameters for the well, including well diameter, well casing and screen materials, well screen length and placement, and well screen slot size and open area. For instance, when the objective is to monitor the extent of three-dimensional contaminant plume, the well screen length should be short enough to conduct sampling of discrete intervals (typically between 0.5 to 2 meters). Moreover, the diameter of the well may need to be large enough to accommodate a pump for sampling, of sufficient capacity. The types of monitoring well completions range from single screened interval or open-borehole bedrock wells to more complex multiple -casing or multiple -screen wells Each type of well completion has its applications, advantages and disadvantages. General recommendations for the application of each well completion type are given by Nielsen ed. (2005).
2.1 Spatial representativity
Spatial representativity is straightforward only in simple situations when individual samples taken from well-defined location in an aquifer with determined interval of depth and in determined moment of time, is considered. Defining a representative monitoring network at a regional scale (GWB, aquifer) is the task which requires adequate hydrogeological knowledge of the system (Foster et al., 2004). Essential step here is establishing a conceptual model of the monitored GWB (see Figure D16.1 and D16.2). An example of such an approach is given in the guidelines of WFD implementation (WFD CIS Guidance Document No. 7, 2003)
After establishing a conceptual model of the monitored system, the next step is to define zones most suitable for monitoring. Selection of such zones will be guided by several criteria such as representativity:
(i) for specific part of the studied system (e.g. recharge/discharge zones), (ii) with respect to certain receptors (e.g. human health, surface water ecosystems, etc.),
(iii) with respect to expected anthropogenic load.
Different approaches towards establishing representative monitoring zones within the GWB have been proposed but up to now no generally accepted methodology exists (Nielsen ed., 2005;
Jousma and Roelofsen, 2004; Grath et al., 2001). For instance, a representativity index (RU) was developed as a tool for assessing the homogeneity of a network (Grath et al., 2001). A certain degree of homogeneity of the network is a statistical prerequisite for applying the arithmetic mean as preferred aggregation method, as proposed in WFD.
• A conceptual model is a simplified representation, or working description, of how the real hydrogeological system is supposed to behave.
• It describes how hydrogeologists assume a groundwater system behaves.
Figure D16.2. Conceptual model of the monitoring system (after WFD CIS Guidance Document No. 7, 2003).
Depth or depth interval(s) of the monitoring wells should take into account spatial structure of groundwater flow and objectives of the monitoring network. In unconfined systems the screen length, and especially the depths of the observation wells should be carefully chosen, depending on the transit time of water from the surface to the monitoring well and the degradation and retardation rates of contaminants in question.
2.2 Temporal representativity
Temporal representativity is related to minimum frequency of sampling which is required to detect trends or trend reversals of groundwater quality changes in the investigated GWB (Grath et al., 2001).
Detection and understanding of groundwater quality changes with time requires combining time serie s information, concentration–depth profiles, and age dating. In most cases, simple statistical evaluation of the available groundwater quality data restricted to a single well is not sufficient for effective detection of trends. Also information about spatial structure of groundwater flow and spatial distribution of hydrochemical zones in the system is required. Other complicating factors for trend analysis are long travel times to observation wells, spatial and temporal variations of anthropogenic load, groundwater age (especially deeper groundwater), reactive properties in the subsurface and finally temporal variations caused by meteorological effects (e.g. infiltration changes).
Transit time–based approach for monitoring design in case of unconfined systems is proposed by Broers (2004), Broers and Van der Grift (2004) and Broers and Van Geer (2005). In this approach, information about the transit time is based on flow patterns and simple formula (see Figure D16.3). This information can also be derived from tracer data.
It should be emphasized that temporal changes of groundwater composition observed at the given monitoring site may not only reflect varying anthropogenic load of contaminants but may also be a consequence of response of the given GWB to pumping (upconing by pumping, sea water intrusion) or due to physical handlings on groundwater such as flow cycles due to irrigation in phreatic aquifers (Walraevens et al., 2003).
Figure D16.3. Groundwater flow and isochrones patterns in a homogeneous unconfined aquifer with constant groundwater recharge, N. (a) elementary concept with formula to calculate transit time of water, tz , through the aquifer with the porosity ε (b) concept used for the set-up of the monitoring networks, (c) hypothetical case with drainage system. Local flow systems in (c) result in distortion of the vertical pattern of isochrones and larger variations in groundwater age in the drained areas (after Broers and Van der Grift, 2004).
Frequency of monitoring should be tuned to phys ical and chemical characteristics of the system, such as groundwater flow conditions, recharge rates, groundwater flow veloc ities and reactive processes (Zhou, 1996). Frequency of sampling during initial stages of monitoring should be higher than that adopted for routine operation of the monitoring network in order to characterize short-term (seasonal) changes of the monitored parameters which can be superimposed on general trends. Frequency of sampling should be higher also in the case of low precision of analyses associated with specific contaminants. In general, sampling frequency should be tailored to the properties of the system being monitored. Too rigid rules are not recommended.