Contamination of groundwater resources by diffuse, surface related sources is a serious problem in the Netherlands. Contamination by agricultural sources has especially increased during the last 40 years due to intensive livestock farming and the use of pesticides. In order to assess and quantify the human impact on groundwater quality in time and space, a national monitoring network for groundwater quality was established between 1979 and 1992 (Van Duijvenbooden et al. 1985, 1993). Since 1989, regional monitoring networks have also been installed as an addition to the national network. The monitoring wells of the national and provincial networks were installed using standardised dimensions and well completion. The wells were screened at about 10, 15 and 25 m depth . The shallow screens (10 m) and the deep screens (25 m) are sampled annually and analysed for inorganic macro and micro constituents.
The screen depths of the monitoring wells were chosen using an elementary concept of the groundwater flow and groundwater age distribution in an aquifer characterized by ground- water recharge due to precipitation (Van Duijvenbooden et al. 1985, Snelting et al. 1990). For groundwater flow to a fully penetrating drain or watercourse, the following travel time distribution is used (Eldor & Dagan 1972, Ernst 1973, Raats 1977):
(2.1) where tz= age at depth z [day], D = aquifer thickness [m], ε = porosity, N = groundwater
recharge [m day-1] and z = depth below land surface [m]. This equation is valid under the
following assumptions:
1. the aquifer is homogeneous, isotropic and has constant thickness 2. groundwater flow is steady
3. the rise of groundwater table is small compared with the aquifer depth 4. the horizontal fluxes are constant over depth z (Dupuit assumption).
Equation (2.1) yields a horizontal pattern of isochrones (lines of equal groundwater residence time) which is shown in Figure 2.1a. The equation has proved useful for a range of Dutch conditions, because the Netherlands has a flat topography and thick, permeable aquifers (see also Chapter 6). For typical Dutch conditions, the equation predicts groundwater ages of 12-13 year and 33-40 years at 10 and 25 m depth, respectively, assuming N = 300 mm/year, ε = 0.35 and D = 50 to 100 m, (e.g. Meinardi 1994). Thus, the established monitoring depths seem to be suitable to determine the effects of diffuse groundwater contamination that was introduced during the last 40 years. The locations of the observation wells were chosen to guarantee homogeneous land use in the upstream catchment area (Figure 2.1b). In this way, the observation well should yield a vertical pattern of groundwater quality of increasing age with depth for a specific land-use. Although the elementary concept seems suitable for the overall design of the monitoring locations and depths, deviations in the groundwater age distribution are to be expected due to aquifer heterogeneity and when a more complicated superficial drainage system exists. For example, one would expect that groundwater ages in regional discharge areas to deviate from the concept.
This chapter investigates the effects of a superficial drainage network and aquifer heterogeneity on the groundwater age distribution in aquifers in flat areas and presents the consequences for the monitoring of contaminants from diffuse sources. First, the effects are assessed using simulations of groundwater flow and groundwater age in different geohydrological situations. Second, the groundwater age distribution is evaluated for the two regional monitoring networks of Noord-Brabant and Drenthe using tritium measurements.
Geology and hydrogeology
Figure 2.2 summarises some relevant information on the geology and hydrology of the Netherlands and shows the positions of the monitoring wells of the Drenthe and Noord- Brabant regional networks. Geologically, the Netherlands is subdivided into a Holocene and a Pleistocene part. The low western part of the Netherlands comprises shallow Holocene marine and peri-marine deposits as well as fluvial deposits from the Rhine and Meuse rivers. The Pleistocene part of the Netherlands comprises older fluvial deposits and glacial and peri-glacial deposits at or near the surface. The provinces of Noord-Brabant and Drenthe are mainly located within the Pleistocene part of the Netherlands.
The altitude of Noord-Brabant ranges from 30 m above MSL (Mean Sea Level) in the south-east to 0 m above MSL in the north and west. The topography is determined by a buried horst-and-graben structure. The subsurface consists of older fluvial sand and gravel deposits from the Meuse river, overlain by a 2-30 m thick cover of fluvio-periglacial and eolian
32 t1 t2 t3 t4 N z A D streamline isochrone zero flux boundary
years 60 80 40 20 B N = 300 mm/yr 100 m streamline isochrone catchment area of observation well well 5 km
Figure 2.1 - Groundwater flow and isochrone patterns in a homogeneous aquifer with constant groundwater recharge drained by parallel fully penetrating ditches (after Ernst 1973) A. Elementary concept, B: Concept used for the set-up of the monitoring networks.
Pleistocene eolian and fluvio-periglacial deposits (cover sands) 0 40 km B D
Holocene marine, peri-marine, fluvial and eolian deposits
Pleistocene glacial till and ice pushed ridges Pleistocene fluvial deposits
Tertiairy and older deposits
Location of observations wells in Noord-Brabant and Drenthe
Simplified geology Average depth of water table
below surface < 1.2 m 4.0 - 10.0 m > 10 m 1.2 - 4.0 m Noord-Brabant Drenthe Watercourses A C
Figure 2.2 - Maps of (a) simplified geology, (b) depth of groundwater level, (c) watercourses in the Netherlands and (d) monitoring networks of Noord-Brabant and Drenthe
deposits consisting of fine sands and loam. In the western part of Noord-Brabant the top 20 m consists of estuarine clay and sand deposits from the Schelde estuary.
The province of Drenthe is situated on a glacial plateau, which is drained by several brooks. The topography of Drenthe ranges between 0 and 15 m above MSL. The subsurface mainly consists of heterogeneous glacial till (Figure 2.2a) underlain by a 200 m thick series of sandy fluvial deposits.
The provinces of Noord-Brabant and Drenthe are drained by a series of brooks. The extent and the position of these natural surface drainage networks are strongly related to the presence of local and regional groundwater flow systems (de Vries 1977, 1994, 1995). De Vries has shown that the stream systems in the Netherlands developed in equilibrium with the groundwater systems, and that the stream spacing and channel dimensions are controlled by subsurface permeability, rainfall characteristics and large-scale topography. Thus, the drainage network density partly reflects the permeability of the subsurface, and areas with shallow low-permeable layers have denser drainage networks than areas with shallow permeable layers. In both Noord- Brabant and Drenthe the original natural drainage network was artificially extended during the 20thcentury, to allow for agricultural use of the poorly drained areas. This resulted in a dense
network of ditches, drains and small watercourses (Figure 2.2c).
2.2 Simulation of the effects of drainage and heterogeneity on the groundwater age