FASES DEL CICLO

In the Netherlands, threats to groundwater quality have increased because agricultural practices have become more intensive after the Second World War. The introduction of new maize varieties in the early 70’s and the simultaneous fast increase of intensive livestock farming caused a large increase of manure loads (see also Chapter 3). Thus, diffuse contamination of young groundwater is likely in agricultural areas.

The effects of the groundwater age distribution on the proportion of contaminated groundwater were illustrated using the 1995 monitoring results of the Drenthe network. Based on local and regional groundwater studies in the Netherlands, groundwater pollution indices were defined to distinguish groundwater that shows signs of anthropogenic pollution. Two indices were used: a general pollution index (POLIN) that was proposed by Stuyfzand (1993) and the MANURE index that indicates agricultural pollution (appendix I).

Table 2.7 compares the proportions of young groundwater and the proportions of ground- water with indications of pollution for agricultural areas in Drenthe. The table shows that 60- 100% of young groundwater in agricultural areas showed signs of pollution. Especially for deep groundwater, a striking difference between the recharge and the intermediate areas was observed for both pollution indices, which coincides with the proportion of young ground- water. Although the intermediate areas have an overall lower risk for groundwater pollution compared with recharge areas; still a considerable proportion of young groundwater with signs of anthropogenic pollution was present due to the groundwater age variations that characterize them.

The simulated isochrone patterns were used to evaluate the advective transport of a block front

contamination input. Figure 2.13 shows the advective propagation of an imaginary pollution block front input of 20 year for scenarios with and without drainage (scenarios C1 and C2). The block front pollution was introduced homogeneously in the whole model. In the scenario without drains, the pollution front moved down vertically. The volume of contaminated groundwater increased during the first 20 years and decreased gradually because contaminated groundwater was removed at the major drain in the upper right quarter of the model. After 100 years a considerable amount of contaminated groundwater is still present at depth in the aquifer.

In the scenario with drains (C2) the contaminated groundwater is also removed from the local flow systems. This resulted in shallower contamination of the aquifer in the drained areas. Because of the shorter transit times in the local flow systems, the upper part of the aquifer became gradually decontaminated after 60 to 100 years in the drained areas. The pollution that originated from the regional recharge areas, however, was still in transit in deeper parts of the aquifer.

The model simulations and the proportions of young, contaminated groundwater in the Drenthe network both showed that the potential contamination of deep groundwater

resources is concentrated in the regional recharge areas. Ultimately, these contamination fronts proceed laterally under the local flow systems, but these are long-term effects that are less relevant for the time scales of groundwater quality monitoring.

However, the shallow groundwater in the intermediate, drained areas is also vulnerable for diffuse contamination and large spatial variations in groundwater age and risks for

contamination of deeper groundwater were simulated and observed. In part of the

intermediate areas, the groundwater contamination patterns resemble those in recharge areas, whereas in the other part old, uncontaminated groundwater dominates. Given the large spatial variability of the groundwater age in the intermediate, drained areas, the use of spatially averaged groundwater recharge fluxes as proposed by Meinardi (1994) is not appropriate for predicting the depth of groundwater contamination and the proportion of young and potentially contaminated groundwater. Spatially averaged values of groundwater recharge rates do not explain the observed variations in the proportions of young and old groundwater in the drained parts of provinces of Drenthe and Noord-Brabant.

Number of Post-1950 Indications for General observations water agricultural indications for

pollution pollution

(MANURE) (POLIN)

% % %

Shallow screens (5-15 m depth)

agriculture/recharge 15 100 100 85

agriculture/intermediate 26 75 51 34

discharge areas 11 18 18 0

Deep screens (15-30 m depth)

agriculture/recharge 16 100 65 83

agriculture/intermediate 26 39 20 23

discharge areas 11 0 9* 0

* due to one well with brackish water (Cl>50)

Table 2.7 - Proportion of post-1950 groundwater and proportions of two indicators of anthropogenic pollution in the Drenthe regional monitoring network

A groundwater monitoring program which aims to quantify the recent human impact on groundwater quality must account for the differences in groundwater ages and contamination risks for deep groundwater between the recharge areas and the intermediate, drained areas. It is advisable to differentiate sample size, monitoring depth and monitoring frequency to account for these differences. A proposition for a risk-based concept and area-specific monitoring objectives is presented in Chapters 3 and 4. Given the large spatial variability of groundwater ages in the drained, intermediate areas at the relevant monitoring depths, large variations in the concentration of dissolved solutes are anticipated. Contaminated and uncontaminated groundwater will both be present at similar monitoring depth, but the locations of

contamination are difficult to predict without modelling the local groundwater flow patterns. As a result, a relatively large sample size will be necessary to acquire precise statistics of contaminant concentrations for those areas with high variability (Chapter 4). This is especially important in the Netherlands, where drained, intermediate areas have large spatial extension, relative to the recharge and discharge areas.

50 t=20 yrs

t=40 yrs

t=60 yrs

t=100 yrs

Figure 2.13 - Propagation of a 20 year contaminant block front in the model scenarios C1 (moderately permeable cover layer without drains) and C2 (idem, with drains) after 20, 40, 60 and 100 years