Study area and data collection

The subsurface of the province of Noord-Brabant (5100 km2_{), in the south of the Netherlands,} consists of fluvial unconsolidated sand and gravel deposits from the Meuse River, overlain by a 2-30 m thick cover of Middle- and Upper-Pleistocene fluvio-periglacial and aeolian deposits consisting of fine sands and loam. Some sediments have a high organic matter content and in some areas pyrite occurs in the subsurface (Broers 2004b), both of which are capable of reducing nitrate.

Noord-Brabant is a flat area with altitudes ranging from mean sea level (MSL) in the north and west to 30 m above MSL in the southeast. Groundwater tables are generally shallow, usually

Table 5.1 Symbols and constants used in this chapter

Symbol Description Units

t_r time of recharge yr

td time of degassing yr

t_s time of sampling yr

h0 depth of the water table m

z_d depth of degassing m

zs screen depth m

P_i partial pressure of gas i Pa

Pg total pressure in gas phase Pa

P_hydr hydrostatic pressure Pa

Pcap capillary pressure Pa

P_TDG total dissolved gas pressure Pa

ρw density of water kg/m3

σ_w surface tension of water (= 72.8×10-3 _{N/m at 293°C (Reid et al., 1987)) N/m}

r radius of bubble m

λ decay constant of 3_{H 0.05626 y}-1

Vg volume of exsolved gas per volume of water cm3/cm3

C_w,i concentration of (noble) gas i in water cm3 _STP/cm3

Cg,i concentration of (noble) gas i in gas cm3 STP/cm3

R_w,ij ratio of isotopes i and j in water (≡Cw,i/Cw,j) -

R_w,ij,0 ratio of isotopes i and j in water before degassing -

Rg,ij ratio of isotopes i and j in gas (≡Cg,i/Cg,j) -

f_j fraction of species j remaining in water after degassing (≡Cw,j/Cw,j,0) -

K_H,i Henry’s law solubility coefficient (≡Cw,i/Pi) (cm3 STP/cm3)/Pa

H_i dimensionless Henry’s law partitioning coefficient (≡Cg,i/Cw,i) -

KH,i = 1/(44.65 R T Hi) = 9.513×10-6/Hi (at 10°C)

D_i molecular diffusion coefficient of species i m2_/s

αD,ij diffusive fractionation parameter between isotopes i and j (≡Di/Dj-1) -

αM,ij mass dependent fractionation factor between species i and j (≡Rw,ij/Rg,ij) -

σ standard deviation of measurement uncertainty …

di difference between duplicates of sample i …

n number of samples -

1 cm3 _{STP is 1 cm}3 _{gas under standard temperature and pressure (≡ 4.465×10}-5 _moles)

within 1-5 m below the surface. A natural network of brooks, which developed in equilibrium with a shallow groundwater table, drained the area before 1900. To allow for agricultural use of the poorly drained areas, the natural drainage was artificially extended during the 20th century, resulting in a dense network of ditches, drains and small watercourses.

The geohydrological situation was mapped to identify homogeneous recharge areas for the design of the groundwater quality network of Noord-Brabant (Broers 2002). Homogeneous recharge areas lack a superficial drainage network, have relatively deep groundwater levels, permeable soils and high topographical position. Groundwater flow in these homogeneous recharge areas can be simplified to a two-dimensional model with impermeable left and bottom boundaries and a fully penetrating drain on the right (Figure 5.1). In this model, groundwater flows continually downward and groundwater travel times increase logarithmically with depth (Vogel 1967; Raats 1981). Monitoring wells were placed within the largest of the geohydrological homogeneous areas, using stratified sampling (Broers and Van der Grift 2004), such that the land use in the catchment area of the monitoring well was also homogeneous. The groundwater quality is monitored because Noord-Brabant is one of the areas in Europe which is most affected by agricultural pollution. Intensive livestock farming in the area produces a large surplus of nitrate rich manure. The agricultural recharge areas of the province of Noord-Brabant are vulnerable to diffuse groundwater pollution with nitrate because groundwater and contaminants can leach deep into the subsurface.

We selected 14 monitoring wells in recharge areas of several watersheds located throughout the province of Noord-Brabant (Figure 1.2) with intensive livestock farming in each of the catchments. These wells serve as a random sample that represents the groundwater quality in the areas with the highest risk to deeper groundwater, because of high pollutant loadings and vulnerable soils. The wells consist of nested piezometers with a diameter of 5.2 cm and a screen length of 2 m. Most wells had two piezometers, with screens at about 8 or 25 m below surface (Van Duijvenbooden 1993; Broers 2002).

We took tritium and noble gas samples from these wells with a submersible pump (MP1, Grundfos). Before sampling, the wells were purged for approximately 30 minutes until field parameters (temperature, pH, electric conductivity and dissolved oxygen concentration) measured in a flow-through cell showed constant values. Nitrate concentrations were determined as part of the annual groundwater quality monitoring effort (Van Duijvenbooden 1993). The Bremen

7452 years 20 40 60 80 100 m 5 km Streamline

Catchment area of observation well Observation well

Isochrone N=300 mm/yr

Figure 5.1 Schematic representation of groundwater flow in recharge areas. Groundwater flow

is continuously downward in recharge areas and groundwater travel times increase with depth. Monitoring wells were placed such that the land use in the catchment area of the monitoring well was homogeneous.

Mass Spectrometric Facility (Sültenfuß et al. 2004) determined the concentrations of tritium, helium and neon and the 3_He/4_{He and} 22_Ne/20_{Ne ratios. We measured total dissolved gas (TDG)} pressure (Manning et al. 2003) with a TDG probe (T300E, In-Situ Inc.) at screen depth during a second sampling campaign in 2006.