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Fast estimation of pesticide risk potential on groundwater through the use of a dye tracing technique S. SCHUMANN Institute of Geoecology, Department of Hydrology and Landscape Ecology, Technical University Braunschweig, Langer Kamp 19c, 38106 Braunschweig, Germany
Abstract
Within the frame of the Research Project on IPMS Nepal (Herrmann, & Schumann 1999) a
series of dye tracing experiments was carried out, in order to identify preferential flow paths
from ponded agricultural fields towards groundwater and to estimate the transport of
pesticides along these paths.
The experiments were performed on the Nepalese governmental Horticulture Farm
Thamaghat in Jhikhu Khola Catchment (JKC), Kabhre District. There, the soil substrate is
loamy. A perched, isolated groundwater table establishes during monsoon time in a depth of
about 2,30 m below surface. This hydrogeologic situation is representative for khet areas
under cultivation in the JKC. As common within the whole area, the experimental fields were
terraced and ponding irrigation was used.
As a tracing substance the food-colour dye Vitasin Blue AE 90 was applied in combination
with a pesticide mix of Malathion, Metalaxyl, Dimethoate und Fenvalerate.
Results of the experiments are shown, and conclusions drawn on the usefulness of this type of
experiments for a first estimation of groundwater contamination risks through pesticide use in
agricultural activities.
Introduction
Application of pesticides in agricultural areas generally comes along with a potential risk of
groundwater contamination through the applied pesticides and their derivatives. In most
developing countries, little is known about the present state of groundwater contamination or
Landschaftsökologie und Umweltforschung 38 p. 178-189 Braunschweig 2002
possible risks. In Nepal, being one of these countries, an interdisciplinary research project on
environmental risks of pesticides (Environmental risks of pesticides and sustainable
development of integrated pesticide management for mountain areas of developing countries
considering socio-economic conditions and taking Middle Mountains, Central Nepal as an
example) was set up under the sponsorship of Volkswagen Stiftung Foundation (Herrmann &
Schumann 2001). Investigating the risk of a groundwater pollution was one aim within the
project's framework.
The project's field site was situated at approximately 50 km east of Kathmandu, at Tinpiple
and Tamaghat in Kabhre district, latter shown in Figure 1. Since the area is linked to
Kathmandu through the Arniko highway market orientated crops are grown, mainly rice,
potatoes and maize on khet (irrigated) land and tomatoes and maize on bari (rainfed) land.
The use of pesticides in Nepal has reached within the last decades a worrying level (Baker &
Gyawali, 1995) and a survey in the project area carried out by CEAPRED has shown high
application rates, more especially in khet, being among the highest in Nepal (CEAPRED
2000).
Fig. 1 Geographical situation of pilot test plots. Tinpiple is situated approximately 4 km from Tamaghat
0 10 20 30 km
Thamaghat:HorticultureFarm Panchkal
Despite high and very frequent pesticides application rates, samples of groundwater taken
during the project duration did not show contamination through recent applications. The
vadose zone showed a quasi non-detectable presence of pesticides. Possible reasons for the
non-detection of pesticides in the groundwater could be: (a) volatilisation of pesticides, (b) all
pesticides accumulate within the first few centimetres of soil, (c) the mean transit times
through the vadose zone are high, or (e) the pesticides are subject to such dilution in the
aquifer that they fall under limit of detection.
Hydrology and formulation of the problem
The hydrological regime of the area is controlled by monsoon with an average precipitation of
1276 mm for 1976-2000 at Tamaghat (Kansakar et al 2002). Monsoon, including pre-
monsoon, lasts roughly from April to September with it’s maximum normally in July.
Irrigation pattern is also controlled by monsoon since reservoirs do not exist.
A simplified hydrological model as valid for water-bound pesticide transport in khet is shown
in Fig. 2. Spraying of pesticides takes place such that amounts of the chemicals reach the soil
surface, i.e. the upper limit of the vadose zone. The soil substrate (at the site under
investigation) is loamy, with 45% sand, 37% silt and 18% clay in the first 20 cm. Organic
carbon amounts to about 1%. With depth soil composition varies between sand fraction by
40-58%, silt by 30-37% and clay by 12-22%, sand always dominating the substrate. When the
lands are irrigated, here always with the system of ponding irrigation, infiltration starts. The
infiltration flux (Q0’) is controlled through matrix flow and flow following preferential flow
paths. Infiltration determined with a double-ring-infiltrometre, shows rates as high as 150
mm/h during the first 5 minutes. It stabilises at 2 mm/h after 3.5 hours and drops finally to 0.6
mm/h after 22.5 hours. The classical sampling methods for pesticides in the vadose zone
consists of sampling with a N-min drill at depths until 90 cm below surface. Results of blank
samples which were taken in the area showed no presence of pesticides; samples taken during
monitoring after known application rates showed often pesticide concentrations close to
determination limit and of unusual distribution pattern (Vinke, 2002 personal
communication). During degradation experiments at the field site uncommon distribution
pattern with soil depth occurred (Interim Report 2001).
The phreatic zone of the area consists firstly of perched, isolated, aquifers which depend in
their thickness and spatial extension mainly on monsoon and irrigation pattern. The perched
aquifers may dry out completely during dry season, and be recharged again during wet season
and/or irrigation (Schumann et al. 2002). Therefore, the flux Q0’ ’ must be active during
Fig. 2 Simplified hydrological model of khet land as relevant for pesticide transport under ponding irrigation
INPUT
INTERCEPTIONVOLATILE LOSSES
QIRR
QIRR (IMP)
QR
ET IL
P PESTICIDE
IS
PONDIRR
EQO
QOI
SSZpor PFP USZpor PFP
QOII
AQUpor(frac) TEMP - FREE
AQUpor(frac) PER - CONF
QOIV
QOIII
PONDINGIRRIGATION WATER
VADOSE ZONESOIL WATER
PHREATIC ZONEtemporary, free GW
PHREATIC ZONEpermanent, confined GW
EXPORT
QIRR
QD
QIRRI
QSSZ
QUSZ
QPFP
QAQU
QAQUI QR
AQUpor(frac) Aquifer, porous (fractured) QAQU Groundwater FlowCONF Confined (AQU) QD Surface RunoffE Evaporation QFP Flux from Preferential Flow PathET Evapotranspiration QIRR Irrigation WaterFREE Free (AQU) QIRR(IMP) Irrigation Water Channel, importedGW Groundwater QM Flow by matric suctionIL Interception Loss QO Infiltration\Seepage FluxIS Interception Storage QR River WaterP Precipitation QSSZ Flux from Saturated Soil ZonePESTICIDE Pesticide Application QUSZ Flux from Unsaturated Soil ZonePER Permanent (AQU) SSZpor Saturated Soil ZonePFP Preferential Flow Paths TEMP Temporary (AQU)PONDIRR Irrigation Water, ponded USZpor Unsaturated Soil Zone
QM
irrigation recharging the aquifer. Below the perched, isolated aquifers a second, permanent
and confined aquifer lies. Classical sampling for pesticides in the phreatic zone is done by
sample withdrawal through wells or piezometres.
Since no pesticides could be detected in the aquifer and results of pesticide detection in the
soil were dissatisfying, a study on the vadose zone was set up, aiming on the quantification of
the infiltration flux into the vadose zone (Q0II) and from the vadose zone into the groundwater
(Q0III), and on the description of transport and transit flow paths and accumulation sites for
water and pesticides within the soil.
Experimental set-up
In principle, the set-up of the experiment aimed on a visualisation of flow pattern in the
ground and a sampling for pesticides along the visualised flow paths as well as in no-flow
sectors.
Visualisation was attained by the use of a dye tracer, Vitasin Blue FCF 90. The colour index
(C.I.) of the substance is 42090, it’s aqueous solubility is greater than 50 kg⋅m-3 (Clariant
Fig. 3 Experimental set-up at tracer experimental site 1 at Tamaghat Horticulture Farm. Section for sampling after 3 and 14 days, and after 3 month respectively
3. Ponding
4. Sampling with Nmin-sampler
2. Application of dissolvedVitasin Blue FCF (blue)
1. Spraying of:Malathion,Meatlaxyl,DimethoateFenvalerate
5. Vertical transection ofthe vadose zone with aimon sampling
PONDIRR
SSZpor PFP USZpor PFP
AQUpor(frac) TEMP - FREE
QOIII
PONDINGIRRIGATION WATER
VADOSE ZONESOIL WATER
PHREATIC ZONE temporary,free GW
1999). According to a study by Flury & Flühler (1995) the adsorption coefficient (K) ranges
from 0.16 to 5.95 dm3⋅kg-1. K varies in the lower values for CaCO3-free soils. The soil found
at the experimental site was carbonate-free.
The experimental set-up is shown in Fig. 3, with running-phases indicated. For the
experiment, four experimental plots (2x1 metres) were prepared with Nepalese workers as
common for rice production. Then, in step 1 the four target compounds (Vinke et.al. 2002)
Malathion, Dimethoate, Fenvalerate and Metalaxyl were sprayed onto the top soil, using
common concentrations. They were equal to the concentrations applied during the
degradation experiments mentioned before. Thereafter, dissolved Vitasin Blue was applied
using a watering can aiming on a concentration of 8g⋅l-1 after ponding the plots with 10 cm
water column (step 3).
After a waiting period of three and 14 days and three month respectively steps 4 and 5 were
carried out. Step 4 consisted of conservative sampling for pesticides with the N-min drill at
three places within the test plots, while step 5 included cutting a section at the centre part of
the test plots. This vertical section had a width of 1 m and a depth of up to 2.60 m. Besides
Fig. 4 Application of Vitasin Blue and ponding at experimental site
photographing, samples for pesticides and Vitasin Blue were taken along and besides the
indicated flow pattern.
The experiment was run on the grounds of Horticulture Farm, Thamaghat.
Fig. 4 shows the application of Vitasin Blue and ponding, Fig. 5 the sections after 3 days in
the part of conservative standardised sampling using a N-min drill.
Results and discussion
Here, the results valid for the first experimental plot shall be presented. Sampling for the plot
took place three days after the application of pesticides and Vitasin Blue. Field situation of the
section for the first 1.20 metre below surface is presented in Fig. 6. Here, bold outlined labels
indicate samples for pesticides and Vitasin Blue, and labels without margin samples for
Vitasin Blue only. In Table 1 the results for the pesticide concentrations of Fenvalerate,
Metalaxyl and Dimethoate are shown in µg⋅kg-1. Generally the sample number increases with
depth in the soil profile.
Fig. 5 Sections after 3 days. View of standardised sampling technique holes (N-min drill)
It is remarkable that in great depths (2.20 and 2.40 m below surface) strong Vitasin Blue
concentrations were observed, and come together with concentrations of Metalaxyl and
Dimethoate. At 2.40 m also a non-quantifiable amount of Fenvalerate was detected.
Dimethoate and Metalaxyl appear in quantifiable amounts in the uplifted groundwater. This
Fig. 6 Selective sampling at a section up to a depth of 120 cm, three days after application. Numbers with margins stand for sampling for pesticides and Vitasin Blue, without margin for sampling for Vitasin Blue only. Dark soil parts indicate the presence of Vitasin Blue. Sample 12 (cf. Table 1) was taken at a blue spot in a depth of 2.40 m below surface
O 12: 2,40 m b.s.
groundwater corresponds to the water of the perched, isolated, aquifer which depends on
irrigation pattern. Accordingly, the flux Q0III
must have been active during the experiment
recharging the aquifer and transporting pesticides. Transport pattern do not exactly
correspond to water solubility, since after Vinke et al (2002) Metalaxyl (with 8 mg/l) is less
soluble than Dimethoate (with 25 g/l). However, Metalaxyl has proved to be more persistent
than Dimethoate (28:8 DT50 under laboratory conditions) and it could be, that degradation had
already diminished the amounts of Dimethoate under detection limits (samples 8, 9, 10) or
below quantification limit (sample 12). The lower concentrations of Dimethoate than
Metalaxyl in the upper two samples (1 and 2) support the assumption that degradation had
already started, i.e. especially that directly after spraying the concentration of Metalaxyl was
about 1/6 of the concentration of Dimethoate in the top soil.
The relations between Vitasin Blue and the pesticides in the 12 samples are shown in Fig. 7.
Fits are not too bad and show for Metalaxyl the best correspondence. Dimethoate, since
Tab. 1 Concentrations (µg⋅kg-1) of pesticides in 12 soil samples, three days after application following selective sampling. Residue analysis by Bajrachaya K. & Vinke C.
Sample Fenvalerate Metalaxyl Dimethoateµg/kg µg/kg µg/kg
1 53 62 132 14 29 213 n.q. n.q. n.q.4 n.d. n.q. n.d.5 n.d. n.d. n.d.6 n.d. n.q. n.q.7 n.d. n.q. n.q.8 n.d. n.q. n.d.9 n.d. n.q. n.d.
10 n.d. n.q. n.d.11 n.d. 11 1112 n.q. 14 n.q.
[ng/L] [ng/L] [ng/L]uplifted GW n.d. 1 4
n.d.: not detectablen.q.: not possible to be quantified
Solubility +
already quite degraded shows the worst relation. In the section cut 14 days after application
the correspondence from Vitasin Blue to Metalaxyl is no longer given (not shown) since
degradation had also here too strong influences. Hence, no conclusions should be drawn from
Vitasin Blue concentrations to pesticide concentrations. For this detailed studies on
correspondence under different conditions, and specific for each pesticide would need to be
carried out.
However, selective sampling along flow pattern allows pesticide tracking. This has also been
shown by Reichenberger et al (2001) for pesticide tracking in Brasilian Oxisols using Vitasin
Blue as tracing substance.
Analysis of the samples taken by the N-min drill show for Fenvalerate and Metalaxyl
quantifiable amounts only up to 5 cm below surface and a non-quantifiable amount up to 20
cm. For Dimethoate only a non-quantifiable amount in the first 5 cm. In Fig. 5 can be seen
that the N-min drill did not remarkably hit the blue parts of soil where, according to the
selective sampling technique, the transported pesticides are found. Depths greater than 90 cm,
where high concentrations of pesticides still occur are not captured at all.
Conclusion
The presented experimental set-up discloses a rather fast and easily carried out first indication
for possible pesticide transport through the vadose zone. It furthermore reveals flow pattern,
-100
10203040506070
0.00 20.00 40.00 60.00
Vitasin Blau (mg/l)
Pes
tici
de
con
cen
trat
ion
(µµ µµg
/kg
)Dimethoate
Metalaxyl
Fenvalerate
Linear(Dimethoate)
Linear (Metalaxyl)
Linear(Fenvalerate)
Fig. 7 Relation between Vistasin Blue concentration and pesticide concentrations three days after application. A linear fit was applied
and allows indication for presence and importance of preferential flow in the substrate.
However, repetitions of sections longer than 14 days after application are not useful since
then degradation or leaching of pesticides might prevail. This, of course depends on applied
pesticides. Furthermore, pesticides seem to be transported mainly through preferential flow
paths, which are predominantly active after dry periods, when soils are dry, and become less
active with growing water content (Schumann et al 2002). Also Flury et al. (1995) observed a
slower transport of herbizides with growing soil water content in loamy soils as then matrix
flow outweighs preferential flow.
The method is extremely useful if depth of water table below surface is small. Using the
method might save one from carrying out medium-term observations concerning groundwater
pollution without results. It furthermore could explain pesticide whereabouts if balancing of
pesticides in field trials fails. The easy set-up for the method and the possibility that soil
samples can be deep frozen and then be transported to a pesticide analysis laboratory makes
the method useful for remote areas, developing countries or countries without a pesticide
laboratory.
The method should follow a first sample survey for pesticides in groundwater wells.
The method, at present state, does not allow a quantification of water flux or pesticide
transport. It is not possible to draw from Vitasin Blue concentrations to pesticide
concentrations. Further detailed studies should follow in order to refine the method and hence
enable the quantification of water-bound transport. The use of bromide could be helpful in
this context since it allows a quantification of water flux.
Acknowledgement
Funding of the work by Volkswagen Stiftung is highly appreciated.
Thanks deserve all Nepalese workers for physical strength and K. Bajrachaya and C. Vinke
for analysis of the pesticide samples.
References
Baker, S. L. & Gyawali, B. K. (1994) Promoting Proper Pesticide Use in Nepal. HMG Ministry of Agriculture (Nepal)/Winrock Internat. Report Series, 28, Kathmandu, Nepal
Clariant (1999) Sicherheitsdatenblätter gemäss 91/155/EWG Vitasin Blue AE 90. CEAPRED (2000) Socio-economic survey of the Jhikhu-Khola watershed. Unpublished
report. IPMS-Project Nepal, CEAPRED, Kathmandu Flury, M., Leuenberger, J., Studer B. & Flühler H. (1995) Transport of anions and herbicides
in a loamy and a sandy field soil. Water Resources Research 31 (4), 823-835 Flury, M. & Flühler H. (1995) Tracer Characteristics of Brilliant Blue FCF. Soil Science
Society of America Journal 59 (1), 22-27 Herrmann, A. & Schumann, S. (2002) Collaborative project on environmental risk of
pesticides and development of an Integrated Pesticide Management System (IPMS) in Nepal considering socio-economic conditions: status and perspectives. Proc. Internat. Workshop on Environmental Risk Assessment of pesticides and Integrated Pesticide Management in Developing Countries, Kathmandu Nov. 2001. Landschaftsökologie und Umweltforschung 38, Braunschweig, 38-48
Herrmann, A. & Schumann, S. (1999) Ein Forschungsansatz zu Umweltrisiken von Pestiziden und der nachhaltigen Entwicklung eines Integrierten Pestizid-Management (IPM) unter Berücksichtigung der sozioökonomischen Verhältnisse in Nepal. In: Proc. Braunschweig Congress GEOöKon ’99 14-16 Oct. 1999 (ed. by M. Flake, R. Seppelt & D. Söndgerath), Landschaftsökologie und Umweltforschung 33, Braunschweig, 235-245
Interim Report 1 January 2000-31 December 2000 (2001) Environmental risks of pesticides and sustainable development of integrated pesticide management for mountain areas of developing countries considering socio-economic conditions and taking Middle Mountains, Central Nepal as an example. Unpublished report, Institute of Geoecology, Technical University Braunschweig
Kansakar, S., Prajapati, S. & Pokhrel A. (2002) Hydrological and Meteorological Components in the Integrated Pesticide Management Project at Jaisidhi Khola, Tinpiple. Proc. Internat. Workshop on Environmental Risk Assessment of pesticides and Integrated Pesticide Management in Developing Countries, Kathmandu Nov. 2001. Landschaftsökologie und Umweltforschung 38, Braunschweig
Reichenberger, S., Amelung, W., Laabs, V., Pinto, A., Totsche, K.U. & Zech, W. (2001) Pesticide displacement along preferential flow pathways in a Brazilian Oxisol. Geoderma (in print)
Schumann, S., Stichler, W. & Prajapati, S.B. (2002) Underground water passages and runoff formation pattern in an irrigated khet catchment. Proc. Internat. Workshop on Environmental Risk Assessment of pesticides and Integrated Pesticide Management in Developing Countries, Kathmandu Nov. 2001. Landschaftsökologie und Umweltforschung 38, Braunschweig,. 190-204
Vinke, C., Kreuzig, R. & Bahadir, M. (2002) Analytical investigations on the fate of pesticides in soils of the Jhikhu Khola catchment. Proc. Internat. Workshop on Environmental Risk Assessment of pesticides and Integrated Pesticide Management in Developing Countries, Kathmandu Nov. 2001. Landschaftsökologie und Umweltforschung 38, Braunschweig, 265-275
Vinke, C. (2002) Personal communication
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