Environmental Pollution 148 (2007) 729e738www.elsevier.com/locate/envpol

Hydrodynamic and geochemical constraints on pesticideconcentrations in the groundwater of an agricultural

catchment (Brevilles, France)

N. Baran*, C. Mouvet, Ph. Negrel

BRGM, 3 avenue Claude Guillemin, BP 6009, 45060 Orleans Cedex 2, France

Received 29 January 2007; accepted 31 January 2007

We present an integrated approach combining geochemistry and hydrogeology that leads to a better understandingof the spatial and temporal fluctuations of the pesticide concentrations in groundwater of a pilot agricultural catchment.

Abstract

The monitoring of a spring and seven piezometers in the 3 km2 Brevilles agricultural catchment (France) over five and a half years revealedconsiderable spatial and temporal variability in the concentrations of atrazine and its metabolite deethylatrazine (both systematically quantifiedat the outlet spring): maximum 0.97 and 2.72 mg L�1, mean 0.19 and 0.59 mg L�1, respectively. Isoproturon, the pesticide applied in the greatestamount, was detected in only 10 of the 133 samples. These observations can only partly be explained by land use and intrinsic pesticide prop-erties. Geochemical measurements and tritium dating showed the importance of the stratification of the sandy saturated zone and the bufferfunction of the unsaturated limestone. Principal component analysis on 39 monthly data series of atrazine, deethylatrazine, nitrate, chlorideand piezometric levels revealed a temporal structuring of the data possibly reflecting the existence within the aquifer of two different reservoirswith time-variable contributions.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Pesticide; Catchment; Groundwater; Water Framework Directive; Tritium

1. Introduction

Enhanced monitoring of groundwater quality over severalyears has revealed a pesticide contamination of aquifers inNorth America and Europe (Barbash et al., 2001; EEA,1999; Gilliom et al., 2006; IFEN, 2004). The observed concen-trations are in places higher than the European drinking-waterlimit, 0.1 mg L�1 per substance except for aldrin, dieldrin andheptachlor, 0.03 mg L�1.

The objective set by the Water Framework Directive (WFDe 2000/60/EC, OJEC, 2000) is for ‘‘all groundwater bodies toachieve the good quantitative and chemical status . at the lat-est by 2015’’. The Directive demands that European Union

* Corresponding author. Tel.: þ33 (0) 2 3864 3271; fax: þ33 (0) 2 3864

3446.

E-mail address: n.baran@brgm.fr (N. Baran).

0269-7491/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2007.01.033

Member States not only characterize their levels of groundwa-ter contamination, but also that they study the evolutionarytrends of their pollutant concentrations. This means that theymust be in a position to explain possible cases of non-achieve-ment. In France, 99% of the groundwater bodies risk non-achievement due to either nitrate or pesticide contaminations(Normand and Gravier, 2005). Monitoring groundwater qual-ity for these two parameters is thus particularly relevant.

Several countries have approached the stage of character-ization of their groundwater bodies either by using data de-rived from various measurement networks, as in France(IFEN, 2004), or by establishing specific sampling and analy-sis protocols (NAQUA network in Switzerland e OFEFP andOFEG, 2004; NAWQA network in the United States e USGS,2006). Pesticide monitoring networks, where they exist, are of-ten less than 10 years old with a fairly low measurement fre-quency (1e4 analyses per year). Trend interpretations are thus

730 N. Baran et al. / Environmental Pollution 148 (2007) 729e738

difficult and limited. The adopted approach is generally sim-ple, often only considering the frequency at which thresholdvalues (e.g. potability limit or maximum acceptable concentra-tion limit) are exceeded.

Characterizing an entire groundwater body from observa-tions limited in time and space remains a challenge. Little pub-lished data exist concerning intensive monitoring over severalyears, whether at the catchment outlet (Morvan et al., 2006;Rowden et al., 2001) or at observation points spread over a ba-sin (De Guzman et al., 2005; Lapworth et al., 2006). Onlya few studies have been published in which hydrogeologicalcontexts and hydrodynamic have been considered (Kolpinet al., 1997; Lapworth and Gooddy, 2006). By taking hydrody-namic into consideration by using the age of the water deter-mined from tritium concentrations, it is possible to showspatial and temporal trends of herbicide concentrations ingroundwater at both an aquifer scale (Trojan et al., 2003)and state- or nationwide scale (Kolpin et al., 2004, 1995; Millset al., 2005).

Notable changes in the use of pesticides generally resultfrom the development of regulations. In Europe, the herbicidesatrazine and isoproturon have been classified as priority sub-stances (2455/2001/EC, OJEC, 2001). As of September 2003all use of atrazine was forbidden in France following restric-tions already in force since 1991. In January 2004 the maxi-mum permitted application of isoproturon was reduced from2500 to 1800 g ha�1.

Bearing in mind the gaps in scientific knowledge on the onehand and the implementation of the WFD on the other, theaims of our study were to (i) characterize the spatial and tem-poral variability of groundwater contamination by differentpesticides with varied physical and chemical characteristics,(ii) identify possible temporal trends, and (iii) seek explana-tions for the former two points. To attain these objectives,a 5.5 year monitoring program, in particular of atrazine, iso-proturon and their metabolites, but also other geochemicaland hydrodynamic parameters combined in an integrated ap-proach, was conducted in the small (3 km2), well-instrumented(seven piezometers, one gauging station at the dischargingspring) Brevilles catchment in France (Morvan et al., 2006).

2. Materials and methods

2.1. Site location and hydrogeological context

The main spring is the outlet of a 3 km2 catchment (70 km west of Paris e

49�1003700N, 1�4102000E e Fig. 1). In 2000, the established atrazine (a herbi-

cide for corn) and deethylatrazine (its main metabolite) contamination of the

spring led the farmers to modify their cultivation practices by halting all use

of atrazine 3 years before the national ban on this product.

The unconfined aquifer is mainly in the 15-m thick Cuise Sands that are

overlain by variably calcareous and clayey Lutetian carbonates. The Lutetian

is more than 30 m thick in the upper part of the basin where it is overlain by

Bartonian limestone. The impermeable substratum of the aquifer is Sparnacian

clay.

Forest covers about 10% of the area, mainly in the upper part of the catch-

ment. The remainder of the catchment is covered by agricultural land. The

catchment contains no farm buildings and no pesticide storage sites or landfill

areas that could cause point pollution. There is also no railway and only

a single secondary paved road. The observed contamination is derived exclu-

sively from diffuse sources.

2.2. Agricultural practices

The gathering of information regarding agricultural practices for over a de-

cade was facilitated by the fact that the agricultural catchment is managed by

only seven farmers, one of whom already covers 40% of the surface area.

Wheat is the main crop (about 50% of the farmland), and corn represents about

15%. The remaining 35% corresponds to peas, oilseed rape, barley or fallow

land, in variable proportions according to the years. The average amount of ni-

trogen (ammonitrate, urea and manure) applied to the catchment over the years

for which we have information for more than 85% of the catchment (1999e

2003) was 12.84 t (�2.14).

Isoproturon and chlortoluron, herbicides for cereals, were the active ingre-

dients used in the largest quantities (a total of 694 and 352 kg, respectively,

during the period 1994e2004). This was not surprising, considering that ce-

reals have represented around 50% of the crop for at least the last decade.

For the considered period (1994e2004), atrazine, the only active ingredi-

ent systematically detected at the spring, ranked 15th in total mass applied

(125 kg), which is less than acetochlor (212 kg), the substance used as an at-

razine substitute since 2000. Fig. 1 shows the areas of atrazine application de-

clared by the farmers, and the plots of corn cultivation with no declared

atrazine application; nonetheless, these plots have very likely been treated

with atrazine since this herbicide was classically used on corn throughout

the region. The small amount of atrazine applied over the period 1994e

2004 has to be placed in the context of the major change in the agricultural

practices since 2000. The last atrazine applications in April 1999 used

1 kg ha�1 and no further applications occurred for the period 2000e2005.

2.3. Piezometer drilling and equipment

Seven piezometers (Pz), located throughout the catchment (Fig. 1), were

drilled and equipped down to the Sparnacian clay at the substratum of the

aquifer in early 2001. The piezometers were screened (0.7 mm slots) over

the full thickness of the Cuise Sands and locally over the lower part of the

limestone to enable tapping the entire aquifer. Pz4 was an exception with

only the deepest part of the Sands being screened (7 m out of 10 m). Piezomet-

ric levels were measured at each field visit, i.e. at least once a month.

A monitoring well cluster (Pz17A, Pz17B and Pz17C) was drilled in sum-

mer 2005 in a cultivated plot (wheat) close to Pz7. The three drillings of this

cluster, 2 m apart, are of different depths to enable studying the groundwater

stratification. Pz17A taps the upper part of the aquifer from 7.8 to 10.8 meters

below surface (mbs); Pz17B taps the upper part of the Sands from 12 to

15 mbs; Pz17C taps the lower part of the aquifer from 17 to 20 mbs.

2.4. Sampling and analytical methods

Sampling campaigns were conducted until August 2005, monthly for the

seven piezometers from March 2001, and bi-monthly in the spring from Octo-

ber 1999 to August 2004 after which the frequency became monthly. The

piezometers were sampled after pumping three purge volumes to stabilize the

groundwater’s physical and chemical parameters, such as pH and conductivity.

The pump in each piezometer was located at the same depth for each sampling

campaign.

Several samples were also taken to quantify tritium activities in the

groundwater (April 2001 and June 2002, only at the spring; February 2003

and August 2005, all piezometers and the spring). Samples for chloride, nitrate

and sulfate determinations were kept, after in situ filtration (0.45 mm), in poly-

ethylene bottles. They were analysed by ion chromatography (accuracy 5e

10%; quantification limit 0.5 mg L�1).

Samples for pesticide analysis were collected in 1 L pre-combusted brown

glass containers kept on ice in coolers for transport to the laboratory and then

stored at 4 �C until analysis. After spiking with deuterated atrazine-d5 as

surrogate, atrazine (At) and its metabolites (deethylatrazine e DEA and deiso-

propylatrazine e DIA), isoproturon (IPU) and its metabolites (monodesmethy-

lisoproturon e MDIPU and didesmethylisoproturon e DDIPU), chlortoluron

731N. Baran et al. / Environmental Pollution 148 (2007) 729e738

Fig. 1. Location of the spring and piezometers (Pz), of plots cultivated at least once with corn and values of the quantities of atrazine applied over the period 1994e

2004. (The size of the circles is proportional to the total quantity applied over the period 1994e2004 whereas each sector corresponds to a year given.)

(CTU), and acetochlor (ACT, a corn herbicide) were extracted by liquideliq-

uid separation. Acetochlor degradates (oxanilic acid, OA, and ethanesulfonic

acid, ESA) were extracted by solid phase extraction after spiking with deuter-

ated 2,4D-d3. Triazines, urea and acetochlor degradates were analysed by liq-

uid chromatography (ThermoHypersil column; 20 mL injected) coupled with

mass spectrometry (DECA XP and Thermo-Finnigan� ion trap mass spec-

trometer equipped with an Electro Spray Ionization source). A gradient of wa-

ter and acetonitrile as mobile phases (0.75 mL min�1) was used. Acetochlor

was analysed by gas chromatography (capillary column: 30 m, 0.25 mm i.d.,

0.25 m film thickness) coupled with mass spectrometry (VARIAN� GC 3400

Saturn 2000). The injection of 1 mL of extract was performed in splitless

mode. Pesticide quantification limits were at the most 0.05 mg L�1 for all pes-

ticides e a threshold that we were able to lower during the monitoring. In Feb-

ruary 2005, liquidesolid extraction with an OASIS HLB (Waters�) cartridge

was used, enabling recovery rates of 110% for DIA as against about 30% pre-

viously. The extraction yields for the other molecules were higher than 90%

and thus comply with the accepted standards (EPA, 1995). Uncertainties for

pesticide analysis were estimated by the GUM method (ISO, 1995). Expanded

uncertainties, obtained by multiplying the relative combined uncertainty by

a coverage factor (k¼ 2, which is related to a confidence level of 95%) varied

between 5 and 60% according to the concentration and the molecule studied;

they are lower than 20% for almost all the samples for At and DEA in our

study.

When drilling piezometers 2, 3, 5 and 6, solids of the unsaturated zone

were collected and kept in hermetically sealed glass bottles. The water from

these solids was collected by freeze-drying and used for tritium activity

measurements. Sample aliquots (water from solids or groundwater) of

10 mL were mixed with 10 mL of a scintillation compound (Pico-Fluor

LLT) in plastic vials and counted on a low-background liquid scintillation

counter (PACKARD 2250 CA). The detection limit of 10 TU (Tritium Unit)

was empirically defined by analysing IAEA (International Atomic Energy

Agency) standard solutions. Tritium activity below this limit was determined

for certain samples after electrolytic enrichment. Tritium measurements in the

groundwater and in water from solids of the unsaturated zone were used to es-

timate the residence time of the water (Clark and Fritz, 1997; Kolpin et al.,

2004) and the water velocity in the unsaturated zone, the tritium peak being

attributed to 1960s, the decade with the maximum emissions (Clark and Fritz,

1997). To allow comparison of samples taken and analysed at different dates,

natural radioactive decay was considered.

2.5. Weather data

A national meteorological network rain gauge, installed 3.5 km from the

site in 1993, records the daily rainfall (P) yielding an annual average of

759 mm (1993e2005). Years 1999, 2000 and 2001 (939, 978 and 932 mm, re-

spectively) and years 2003, 2004 and 2005 (535, 634 and 572 mm, respec-

tively) were particularly wet and dry, respectively.

Daily potential evapotranspiration (PET), calculated using the Penmanne

Monteith equation, was taken from the closest national weather station, located

about 40 km southwest of the catchment. Years 2000, 2001, 2002, 2004 and 2005

showed similar values (703, 744, 756, 760 and 703 mm, respectively), whereas

732 N. Baran et al. / Environmental Pollution 148 (2007) 729e738

year 2003 gave a higher value (827 mm). The P and PET data were used to

estimate the number of water infiltration events every year.

3. Results and discussion

3.1. Pesticides

3.1.1. At the catchment outlet (the Brevilles spring)Atrazine and its main degradate, deethylatrazine, were sys-

tematically detected in the spring (Fig. 2), even though the lastatrazine application in the catchment was in April 1999. Themean concentrations (n¼ 133) and standard deviation were0.19� 0.7 mg L�1 and 0.59� 0.18 mg L�1 for At and DEA, re-spectively. Maximum concentrations (0.43 and 1.16 mg L�1)were observed on 9th October 2000 and 19th January 2001for At and DEA, respectively. This continued detection sug-gests a stock of these compounds in the soil and/or unsaturatedzone that is partly mobilized during each infiltration event. Asecond hypothesis to explain the atrazine persistence is thatthe groundwater renewal time is longer than the monitoringperiod of 5.5 years and that there is limited pesticide degrada-tion within the groundwater body (Albrechtsen et al., 2003).

The atrazine and deethylatrazine concentrations showeda strong temporal variability, more particularly marked at thebeginning of the monitoring. The values, notably for atrazine,were more constant after April 2003. A hypothesis to explainthe stabilization of the concentrations is a reduction in thenumber of infiltration events after this date. Indeed, a simplemonthly calculation of the hydrological balance, (PePET),showed that the recharge events were much greater (in bothnumber and volume) in 2000, 2001 and 2002 than in the fol-lowing years (Fig. 2).

The time-series of isoproturon and chlortoluron concentra-tions were very different to that of atrazine (Fig. 2). The twosubstituted ureas were only very rarely quantified (once andtwice on 133 samples for IPU and CTU, respectively) through-out the monitoring and a few times detected at low

concentrations that could not be accurately quantified (10and 3 detections, respectively). These rare detections in com-parison to those of atrazine could appear surprising since IPUand CTU applications in the catchment over the period 1994e2004 were 5.5 and 2.8 times higher than that of atrazine. How-ever, it is in agreement with earlier studies confirming the raredetection of these molecules in groundwaters at national scale(IFEN, 2004), a phenomenon probably due to the molecules’physical and chemical properties and notably to the rapid for-mation of large amounts of bound residues (Perrin-Ganieret al., 1996; Pieuchot et al., 1996). The greater risk of Atleaching compared to IPU has been demonstrated by compar-ison of sorption, volatilization and mineralisation processes ofboth molecules (Boivin et al., 2005; Mordaunt et al., 2005). Inaddition, Walker et al. (2005) highlighted the reduced leachinglosses with increased delay between application and signifi-cant rainfall events. The results of Walker et al. (2005) couldexplain why IPU and CTU have only been quantified here dur-ing year 2001, the wettest year of the monitoring period witha very rainy month of March (three times the average rainfall)closely matching the period of phenylurea applications.

MDIPU and DDIPU were never detected at the spring. Theabsence of DDIPU detection could be due to the fact that thisproduct does not appear in the first stages of biodegradation.The absence of MDIPU, a metabolite that appears in the firststage of biodegradation in the surface layers (Alletto et al.,2006), is more likely to be due to its rapid degradation thanto an absence of production (Sorensen and Aamand, 2001).

After the solideliquid extraction method was adopted, DIAwas detected more frequently (four times out of the ninesamples analysed since February 2005) although the concen-trations were very low and close to the 0.025 mg L�1 quantifi-cation limit. These observations are in agreement with theresults obtained for different groundwater measurementprograms in which DIA is detected less frequently thanDEA or atrazine (Kolpin et al., 2000, 2004; Pucarevic et al.,2002).

Fig. 2. Time-series of atrazine, deethylatrazine (DEA), isoproturon (IPU), and chlotoluron (CTU) concentrations (mg L�1) at the spring and monthly precipitation

minus potential evapotranspiration (mm).

733N. Baran et al. / Environmental Pollution 148 (2007) 729e738

3.1.2. Spatial and temporal variability at thecatchment scale

Acetochlor and its two main metabolites, ESA and OA,have so far not been quantified at any of the eight spatially dis-tributed observation points (including the spring). However,a risk of metabolite leaching does exist as they have been de-tected in other aquifers (De Guzman et al., 2005; Kolpin et al.,2000) and at depth in the soils of the present catchment (Baranet al., 2004). The absence of detection could result from (i) thesmall amounts applied to date over the whole of the catch-ment, (ii) the location of the treated plots, preferentially inthe upper part of the catchment, thus far from the spring andin areas where the unsaturated zone is thicker than 15 m,and (iii) little recharge since 2003, the two last points suggest-ing that the time lag necessary for the molecules to reach thegroundwater could be longer than the duration of ourmonitoring.

IPU was not detected frequently in any of the piezometers(as observed for the spring), generally less than twice over theentire monitoring period, except for Pz5 (38 detections for 44samples, maximum concentration: 0.13 mg L�1). The degrada-tion products MDIPU and DDIPU were never detected in anyof the piezometers, whilst CTU was detected only a very fewtimes and, like IPU, at periods close to the applications, sug-gesting that a fraction of these pesticides applied in certainparts of the catchment may rapidly reach the groundwater.

The atrazine and deethylatrazine concentrations in the pie-zometers varied from not detected (<0.025 mg L�1 for atrazinein Pz4) to several mg L�1 (0.97 and 2.72 mg L�1, respectivelyfor atrazine and deethylatrazine in Pz5). Fig. 3 gives whiskerplots of the observed data for each piezometer throughoutthe monitoring and shows that the spatial and temporal vari-abilities of the contamination are large at any given point.No overall clear logic in the upgradientedowngradient distri-bution of the contamination was found. The lower At andDEA concentrations observed for Pz7 and the spring com-pared to Pz5 can nevertheless be tentatively explained. SincePz5 is upgradient from Pz7 and the spring, but also in themost downgradient plot known to have had a corn crop, re-charge water infiltrating plots of no corn cultivation locatedbetween Pz5 and Pz7 and the spring could lower the At andDEA concentrations in Pz7 and the spring.

The pesticide concentrations were not directly linked withthe amounts applied in the corresponding plot over the last12 years for any of the piezometers. Pz6, which was in oneof the two plots that received the most atrazine over the period1994e2004, did not show the highest atrazine or DEA concen-trations. Neither did the intensity of contamination at anygiven point appear to be directly linked to the thickness ofthe unsaturated zone above that point; Pz4 and Pz5, with sim-ilar thicknesses of unsaturated zone, showed very different de-grees of contamination.

Piezometer 4 appeared to be uncontaminated by both pes-ticides and nitrate (Fig. 3). It lies 200 m upgradient from thespring the concentrations of which were close to 0.2 and0.6 mg L�1 for atrazine and deethylatrazine, respectively. Noapplication of atrazine has been declared for this plot

(Fig. 1) and this could explain the fact that the parent moleculeand its degradates were not detected in Pz4. Nevertheless, sev-eral plots upstream were treated with atrazine and could haveinduced contamination of Pz4 via transfers in the saturatedzone, as was probably the case for Pz5 and Pz8 which werealso in plots untreated with atrazine but downstream of severaltreated plots (Fig. 1). Pz4 was in a plot of cultivated land withnitrate loading comparable with the other plots, unlike applica-tions of pesticides that are variable from one plot to another.Consequently, the lack of nitrate contamination compared toother piezometers remains unexplained. We therefore needto improve our knowledge of the hydrodynamic of the systemso as to better understand the temporal and spatial variabilitiesof the pesticide concentrations in the groundwater.

3.2. Hydrodynamic of the system: clues fromgroundwater chemistry

Nitrate data, alone or combined with chloride or sulfate andtritium data, are among the geochemical parameters that mayimprove the understanding of aquifer hydrodynamic (Negreland Pauwels, 2004). Examination of the chloride vs. sodiumconcentrations (Fig. 4a) showed that Pz3 and Pz4 water wasclose to the seawater dilution curve (SW) and thus onlyslightly modified by human activity, such as the input of min-eral fertilizers or organic fertilization (Negrel and Pauwels,2004). Anthropic impact was, however, perceptible for theother piezometers where the chloride/sodium ratios movedaway from the SW dilution curve (Fig. 4a).

The low chloride concentrations measured for Pz3 and Pz4,(median values of 10.3 and 13.8 mg L�1, respectively), the sul-fate concentrations of the two piezometers being similar tothose of the other piezometers, plus the unconfined characterof the groundwater, suggested that denitrification cannot be in-voked to explain the low observed nitrate concentrations forPz3 and Pz4 (Pauwels and Talbo, 2004). One hypothesis isthat the solutes have not yet reached the groundwater due toa slow infiltration rate above these points. Consequently, tri-tium measurements were made at the spring and all piezome-ters to estimate the residence time of the water at eachobservation point (Clark and Fritz, 1997; Kolpin et al.,2004). Tritium activities below <0.8 TU correspond to sub-modern water recharged before 1952 (first thermonucleartest), high activities (>30 TU) to water with a considerablecomponent of recharge from the 1960s to 1970s, while inter-mediate values are a mixture of sub-modern and recent re-charge (Clark and Fritz, 1997).

As a first approximation to enable a direct comparison ofall observation points, it was possible to consider that, cumu-lated over several years, nitrate loadings were relatively simi-lar throughout the cultivated sector because of its small size,the few farmers concerned, the crop rotations and the inputsnecessary for all crops. Considering the small size of thecatchment, the tritium input function via rainfall could be con-sidered homogeneous on the basin scale, allowing comparisonof nitrate/tritium pairs between different points (Fig. 4b). Thelowest nitrate contamination was clearly in the oldest water,

734 N. Baran et al. / Environmental Pollution 148 (2007) 729e738

Fig. 3. Box-whisker plot of atrazine, DEA, chloride, nitrate, piezometric level and depth of unsaturated zone. Median, 25th and 75th percentiles (1st and 3rd quar-

tiles corresponding to the bottom and the top of the box), minimum and maximum (whiskers) and outliers (any point that falls below lower quartile minus 1.5 times

the distance between the quartiles or above upper quartile plus 1.5 times the distance between the quartiles) are represented.

some older than 1963 (Pz4 and Pz17C) and thus prior to theagricultural intensification of the 1970s. Pz3 showed a concen-tration of 4 TU (a value lower than actual concentrations inprecipitations of the Orleans area; unpublished data, BRGM)that could correspond to a mixture of pre- and post-1953 wa-ters, leading to a weak contamination of the groundwater atthis point. For a same tritium activity (Fig. 4b), the nitrate con-centration could nevertheless appear as variable (Pz7 and Pz5).This offset may result from local differences in agriculturalpressure (total amounts of fertilizer, fertilizer type, organic in-puts), which we did not take into account in our first simplistichypothesis of homogeneous loadings at the catchment scale.The spring was close to the curve linking the oldest and youn-gest waters (Fig. 4b), confirming that it was indeed a mixtureof waters of different age and quality, as illustrated by the re-sults from the monitoring of the piezometers (Figs. 3 and 4a).

Differences in the residence time of the water could merelybe due to differences between the piezometers of the unsatu-rated zone thicknesses if transfers were homogeneous withinthe unsaturated zone at the catchment scale. However, thiswas not the case because the water in Pz3 was older thanthat in Pz2, which has a thicker unsaturated zone. The notionof transfer rate in the unsaturated zone therefore also needs tobe examined in more detail.

The tritium profiles for the unsaturated zones of Pz2 andPz3 indicated different transfer processes in the unsaturatedzone above the water table of the two piezometers (Fig. 5).The profile in Pz3 showed a single symmetrical and relativelybroad peak indicating that the main transfer mechanism wasconvectionedispersion. Assuming that this peak correspondsto 1963 (Smith et al., 1970), the average rate would be ofthe order of 0.5 m per year, a velocity that explains why the

735N. Baran et al. / Environmental Pollution 148 (2007) 729e738

solutes currently being transferred between the soil and thewater table at Pz3 have yet to reach the groundwater. Pz2,however, showed several narrower peaks of tritium, of whichthe main ones were at 11 and 35 m depth (Fig. 5). This profilesuggested the existence of several infiltration rates, whichcould be due to fractures in the limestone of the unsaturatedzone. The infiltration differences in the unsaturated zoneabove Pz3 and Pz2, as well as the presence of a forestedarea upstream of Pz3 (Fig. 1), could explain the low contam-ination of Pz3 vs. Pz2.

The tritium profile with several peaks at Pz6 (Fig. 5) againsuggested several infiltration rates due to fracturing; part of thesolutes applied at the surface could thus reach the groundwaterrelatively rapidly and cause the relatively high pesticide andnitrate concentrations that were recorded. Since the profilefor Pz5 is very short (12.4 m) compared to the three others(38, 35.5 and 22.5 m), it was more difficult to pronounce on

Fig. 4. Nitrate vs. sodium (a) from the monthly analysis (March 2001 to

August 2005) of groundwater in the piezometers (Pz) and bi-monthly analysis

of the spring (SW: seawater dilution line) and (b) nitrate vs. tritium (measured

value and error) for piezometers and the spring (August 2005).

the transfer mechanisms occurring in the unsaturated zoneabove the water level of this piezometer. The four tritium pro-files illustrated the heterogeneity of the water (and associatedsolutes) infiltration rate within the unsaturated zone at thecatchment scale. Such heterogeneity could in part explainthe spatial variability of the contamination.

The groundwater tritium measurements at Pz4, suggestingwater older than 1963, confirmed another characteristic ofthe hydrogeological system. Geophysical surveys have re-vealed the presence of faults in the vicinity of Pz4, whichmay have led to its hydraulic isolation (unpublished data).Coupled with a very slow infiltration in the unsaturatedzone, this hydraulic isolation would result in low contamina-tion at this point. Unfortunately we cannot pronounce on thehypothesis of slow transfer because no tritium profile wasmeasured in the unsaturated zone of Pz4. A second hypothesisto explain the absence of contamination is that of groundwaterstratification resulting from clay enrichment in the CuiseSands with depth (Gutierrez, 2002). In addition, Pz4 isscreened only in the lower part of the Sands, which could ex-plain why its sampled waters were old and contamination free.The hypothesis of groundwater stratification was validated bythe results from the Pz17 well cluster tapping different ground-water levels. The measurements made on 10th May 2005showed contamination diminishing with depth, both for nitrate

Fig. 5. Tritium profiles obtained for the water recovered from the unsaturated

zone of Pz2, Pz3, Pz5 and Pz6.

736 N. Baran et al. / Environmental Pollution 148 (2007) 729e738

(67.7, 64.4 and 0.5 mg L�1 for Pz17A, Pz17B and Pz17C, re-spectively) and chloride (36.6, 32.9 and 5.6 mg L�1). Tritiumactivity also decreased with depth (21� 3, 11� 2 and<1 TU) confirming that an older water (whose quality hasbeen less affected by human activities) exists at the base ofthe aquifer. The results equally indicated limited mixing be-tween the upper and deeper parts of the groundwater, the pre-cise thickness of each of these two compartments beingunknown presently. The finding of groundwater stratificationin Pz4 and Pz17, two piezometers located in different partsof the catchment, suggested this is not a localized phenomenonbut that it might extend over a large part, if not all, of thegroundwater body. This absence of mixing has an effect onpesticide concentrations along a vertical profile within the sat-urated zone, as shown by Pz17A, Pz17B and Pz17C where Atand DEA concentrations decreased with depth (0.16, 0.06,<0.025 mg L�1 for At and 0.2, 0.14, <0.025 mg L�1 forDEA, respectively).

3.3. Statistical analysis of the organic and inorganicparameters: hydrodynamic perspectives

To better understand the relationships between the piezom-eter and spring observations, principal component analysis(PCA) was carried out using XLSTAT Software�, with thecorrelation tests using a threshold value of alpha¼ 0.05;only the meaningful correlations at this level are discussed be-low. Thirty-nine variables (At, DEA, nitrate, chloride concen-trations, and piezometric level at each of the sevenpiezometers corresponding to 5� 7 variables, and At, DEA,nitrate, chloride concentrations at the spring) are considered,each with a series of data for the period April 2001 to October2005 (40 individuals).

Atrazine concentration and piezometric level correlatedpositively for Pz3, negatively for Pz5, Pz8 and Pz7, andwere not correlated for Pz2, Pz4 and Pz6. This variable linkbetween piezometric level and atrazine concentration possiblyresulted from a variable remobilization of atrazine. This coin-cides with atrazine inputs that were very heterogeneous spa-tially and temporally at the catchment scale (Fig. 1).

As regards piezometric levels, Pz4 correlated with no otherpiezometer, supporting the hypothesis of its hydraulic isola-tion. Pz2 and Pz3, the most upstream in the catchment, corre-lated positively with all the piezometers except Pz4 and Pz7,the most downstream piezometers. This absence of correlationbetween the upstream and downstream piezometers reflectedthe system’s inertia, which increased with the thickness ofthe unsaturated zone. With the climatic context showing a de-ficiency as of February 2003, the piezometric levels of Pz4 andPz7 stopped rising in MarcheApril 2003, whereas those ofPz2 and Pz3 continued to rise until SeptembereOctober.There was thus a major phase difference in piezometric evolu-tion between the upstream and downstream parts of the sys-tem. The piezometric levels of Pz5, Pz6, Pz7 and Pz8, allrelatively central in the catchment, correlated positively withone another.

Eight factors were needed to explain more than 80% of thevariance. This is a proof of the complexity of the catchment’shydrodynamic. Projection on the F1eF2 axes (explainingabout 47.4% of the variance e Fig. 6) showed a temporalstructuring on passing from the samples correlated negativelywith factors F1 and F2, through samples correlated positivelywith these factors (up to the end of 2003), to end with the sam-ples correlated negatively with factors 1 and 2. The two factorscould correspond to the contribution of two different reser-voirs within the aquifer. It was worth noting that the piezomet-ric levels of Pz2 and Pz3, and to a lesser extent Pz5 and Pz8,showed a strong correlation (0.96, 0.96, 0.78 and 0.74, respec-tively) with factor 1, in fact the strongest correlation of thesefour variables to any factor of the PCA. Factor 2 correlatedstrongly with the piezometric levels of Pz7 and Pz6 (0.82and 0.71, respectively). These observations highlight the im-portance of recharge on the groundwater quality at the catch-ment scale.

This conceptual model, considering a contribution of tworeservoirs within the aquifer with time-variable proportions,is in agreement with the climatic trend, at first very wet andthen very dry as of 2003. The rainfall deficit was reflectedthroughout 2003 by a staggered lowering of the piezometriclevels depending on the observation point’s location in thecatchment. All of the piezometers showed a drop in levelsbut this was not synchronous everywhere, since it dependedon the position of the piezometer in the catchment The con-ceptual model also agrees with the tritium concentrations mea-sured at the spring (5 TU on 27th April 2001, 11 TU on 18thJune 2002, 10 TU on 17th December 2003 and 9 TU on 8thAugust 2005, all data corrected on 8th August 2005) whichseemed to indicate a contribution of a more recent water

Fig. 6. Principal component analysis representation (projection on planes 1 and

2) obtained for the 39 variables (see Section 3.3). Each label corresponds to

one of the 40 sampling dates (mm/yy).

737N. Baran et al. / Environmental Pollution 148 (2007) 729e738

between the 2001 measurement and those of 2002 and 2003and, with less certainty, a lessening of its contribution duringthe last measurement. Projection onto the F1eF3 axes(37.4% of the variance e data not shown) also showed a tem-poral organization in the sample series, which passed froma negative correlation with F1 to a positive correlation, onwhich were superimposed fluctuations linked variably to pos-itive or negative correlations with F3. The contribution of F3could correspond to a water fraction circulating more rapidlyand to the highly varied chemistry between samples. Rapidcirculation of water and solutes in the unsaturated zone wouldbe favoured by the fracturing revealed notably by the tritiumprofiles and whose existence was also assumed from the rapiddetection of IPU after its applications in 2001.

4. Conclusions

The intensive monitoring of water quality over a long pe-riod (5.5 years) in a 3 km2 agricultural catchment (Brevilles)has made it possible to demonstrate and quantify high spatialand temporal variabilities in pesticide concentrations and moreparticularly as regards atrazine (At) and deethylatrazine(DEA). The variabilities remained high even though At hasnot been used in this catchment for 6 years. Comparison ofthe At and DEA time-series and those of the phenylureas(IPU and CTU) showed very strong differences that werenot related to the quantities applied, but attributed to the factthat the formation of bound residues is far greater for the latterthan the former.

The spatial variability of the At and DEA concentrations inthe groundwater could be explained only in part by the spatialand temporal variability of the atrazine applications. The pos-itive or negative correlations in some piezometers between Ator DEA concentrations and piezometric level highlighted theimportance of recharge on the temporal variability of the con-centrations, recharge water being contaminated to various de-grees by these molecules. However, as this type of correlationwas not found for all the piezometers, other factors must besought to explain the temporal variability in groundwaterquality.

The complexity of the system’s hydrodynamic has beenshown through the analysis of results from geochemical pa-rameters. The very high inertia of the unsaturated zone, dem-onstrated by tritium measurements and the absence of markedanthropogenic effects at certain points of the catchment (Pz3and Pz4), supported by the nitrate concentrations and so-dium/chloride ratios, buffered the system and limited the cli-matic effects on the groundwater hydrodynamic. Despite thisinertia, it is possible that a proportion (unquantified) of the wa-ter, and transported solutes, circulated more rapidly in theunsaturated zone along a set of fractures and probably also inthe least clayey parts of the saturated sand and in parts of thesaturated limestones.

The spatial distribution and temporal evolution of ground-water contamination were complex. The most important fac-tors involved included the stratification of groundwaterquality, and the spatial and temporal variability of the risk of

pesticide leaching associated with, on one hand the differentuses and physical and chemical properties of the molecules(degradation, sorption and formation of bound residues), andon the other hand the various geochemical and hydrodynamicproperties of the solids in the soil, the vadose and the saturatedzones.

The Brevilles spring served as an integrating point for allthe processes existing at the catchment scale. The observationsmade in the different piezometers enabled framing the obser-vations made at the spring (water that is younger or older, wa-ter that is more or less contaminated). However, the questionarose as to the representativeness of observation points suchas piezometers or wells at the scale of the groundwater body.

Although factors that explain the differences in concentra-tions observed both spatially and over time have been identi-fied, predicting the evolution of the quality of thegroundwater remains a challenge since the different factorsidentified superimpose each other. Modelling could be a solu-tion, providing that the way the catchment works is well un-derstood and that additional data relative to certain processesspecific to pesticides (degradation and desorption kinetics,for example) have been acquired.

The continuation of such integrated monitoring of ground-water quality coupled with the use of different tools (modelling,isotope geochemistry) will enable a better understanding of thehydrodynamic of the hydrogeological system and is a verypromising scientific approach. In addition, the response to therequirements of the Water Framework Directive (where to sam-ple, at what frequency, trend characterization, estimation of thelevel of confidence and precision of the results provided bymonitoring networks, etc.) will also require this type ofapproach on a detailed study scale.

Acknowledgements

The work was carried out initially under the European 5thFRDP PEGASE project (contract EVK1-CT1999-00028), thenwas supported by the European Union FP6 Integrated Aqua-Terra Project (project no. GOCE 505428) under the thematicpriority of ‘‘Sustainable development, global change and eco-systems’’. Financial aid was also received through BRGM re-search projects (POLDIF and QUALHYS projects) and anAgreement (012095) with the Seine-Normandy Water Agency.The authors would like to thank the farmers and the munici-pality whose help greatly facilitated the work for this studyand Monsanto for supplying the analytical standards for aceto-chlor OA and ESA degradates.

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