Groundwater monitoring system design criteria

Monitoring and sampling of groundwater is a complex process. This complexity stems mainly from large spatial variability of groundwater composition (Figure 10.1), limited access to the system and lack of simple hierarchy of flow such as drainage pattern of surface water systems.

Very low velocity of groundwater (except of flows in karstic channels), measured in years or decades, should be taken into account. This leads to often encountered cases when a given contaminant is not detected in a well, even though its source is located nearby. The absence of a contaminant may be due to its chemical transformations in the system or migration induced time lag between its release from the source and its appearance in the well. However it should not be due to faulty or improper analytical techniques used.

A special problem is posed by sampling of water from the unsaturated zone that is a source of important data but needs specialized methods and equipment (Nielsen, 2005). One has to keep in mind that different properties of some contaminants such as DNAPL and LNAPL organics require relevant methods of monitoring (Figure 10.2). Good practice groundwater monitoring implies adequate knowledge of groundwater flow and contaminant behaviour.

Figure 10.1. Natural hydrogeochemical field may be spatially variable, for instance due to variable redox (Ratajczak and Witczak, 1984).

Figure 10.2. Contaminants in groundwater system. Organic liquids less dense than water (LNAPL) tend to float on the water table (after Fetter, 2001).

10.2.1 Development of conceptual model for groundwater body

Good practice in groundwater sampling depends on monitoring objectives (monitoring of Natural Background Level-NBL (Bedessem et al., 2005)), monitoring of chemical status of GWB (WFD CIS WG_C, 2005), extend of contaminant plume (Nielsen, 2005; etc.). From this perspective, two general aspects of sampling and monitoring should be considered:

• spatial representativity,

• temporal 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. 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 (Figure 10.3). An example of such approach is given in the guidelines of WFD implementation (WFD CIS Guidance Document No. 7 (2003)).

Figure 10.3. Conceptual model of the monitoring system (WFD CIS Guidance Document No. 7 (2003)).

Temporal representativity is related to minimum frequency of sampling which is required to detect trends or reversal trends of groundwater quality changes in the investigated GWB (Grath et al., 2001).

10.2.2 Selection of monitoring zone

After establishing the conceptual model of the system, the next step is to define the zones most suitable for monitoring. Selection of such zones will be guided by several criteria such as: (i) representativity for specific part of the studied system (e.g. recharge/discharge zones), (ii) representativity with respect to certain receptors (e.g. human health, surface water ecosystems, etc.), (iii) representativity 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, 2005; Jousma, Roelofsen, 2004, Grath et al., 2001). The representativity Index was developed as a tool for assessing the homogeneity of the network. A certain degree of homogeneity of the network is a statistical prerequisite for the admissibility of applying the arithmetic mean as aggregation method as proposed in WFD (Grath et al., 2001).

Figure 10.4. 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).

10.2.3 Depth of monitoring wells

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 lengths, 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. In general, the larger the degradation rates or the retardation, the more shallow should the monitoring effort be for effective early warning, prediction, and calibration. Shallow monitoring using multilevel wells is frequently the best option for a variety of objectives and conditions. 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 transit time is based on flow patterns and simple formula (see Figure 10.4). This

information can also be derived from tracer data.

10.2.4 Detection of trends, frequency of sampling

Detection and understanding of groundwater quality changes with time requires combining time series information, concentration–depth profiles, and age dating. In most cases, straightforward statistical evaluation of the available groundwater quality data 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 (concept of hydrogeosomes according to Stuyfzand, 1999). 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) (WFD CIS Guidance Document No. 7, 2003, WFD CIS WG_C, 2005).

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 (e.g. flow cycles due to irrigation in phreatic aquifers) – Walraevens et al. (2003).

Frequency of monitoring should be tuned to physical and chemical characteristics of the system, such as groundwater flow conditions, recharge rates, groundwater flow velocities and reactive processes (Zhou, 1996). Frequency of sampling in the case of shallow groundwater (0-10m) should be approximately every 6 months (DVWK, 1992), Similar frequency for slow flow in unconfined aquifer is recommended during operational monitoring for trend assessment (Grath et al., 2001). Karstic aquifers need more frequent sampling. 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 (see Chapter 9 – measurement uncertainty in relation to threshold value). In general, sampling frequency should be tailored to the properties of the system being monitored. Too rigid rules are not recommended.