Point and non-point source leaching of pesticides in a till groundwater catchment

3. Materials and methods

3.1 Boreholes and screens

Angled monitoring boreholes

Five 45^o angled boreholes (15 - 25 m deep) were installed with multi-level water sampling systems. Three boreholes are situated at the point source site and two boreholes are situated in the orchard. During augering, casing and telescoping were applied to minimize vertical transport of pollutants down in the boreholes. This risk of artifact contamination was further minimized by using the angled augering technique, which allows installations of filters underneath a "hot-spot" area in ground surface without penetrating the "hot-spot" itself.

Installations in boreholes

Installation of water samplers, standpipes for monitoring hydraulic head and bentonite sealing between the individual screened intervals are displayed in Figure 3 and 4. Depth of screens and stratigraphic formation of water sampling in the monitoring programme are shown in Table 1. Applied water sampling equipment in contact with the groundwater during sampling was made of glass, stainless steel or teflon to minimize chemical interaction between possible contaminants and sampling equipment.

Dybde af filtre i monitoringsboringer og strategrafisk oprindelse af vandprøver

Skrå monitoringsboringer og lokalisering af udtagede intakte søjler og jordprøver på punktkilden i, a) geologisk snit og b) situationsplan

Figure 4 Look here
Angled borings of the orchard monitoring section shown in, a) vertical view and b) horizontal view

3.2 Soil and groundwater sampling

3.2.1 Excavation and sampling of large undisturbed till columns

Point source site and orchard

Undisturbed columns were sampled from profiles excavated at the point source site (4 columns) and in the orchard (3 columns). The columns were circular with a diameter of 0.5 m and a height of 0.5 m, being large enough to represent fractures and macropores in the till. Sampling was preferably

Sampling depths and method

made in fracture zones. Sampling depth at the point source site was 2.6 - 4.6 m below ground surface and in the orchard 2.0 - 6.0 m. During sampling the columns were embedded in a fluid rubber casing which fixed the columns after hardening in a combined mould and transport steel cylinder. Before hardening, the fluid rubber enters a few millimetres into the till matrix. Thereby the outer surface of the columns is sealed, and problems with flow along this boundary during the experiments is eliminated. After fixation, the columns were detached from the till formation. The steel cylinder was removed after transport of the columns. During installation of the columns in the laboratory, they were operated using vacuum corresponding to an external pressure of 30 to 60 kPa to avoid disturbance.

3.2.2 Soil sampling for chemical analyses

Shelby tube cores (length 0.5 m) from the boreholes 2, 3 and 4 were sampled every second vertical metre and between these tubes, ordinary soil samples were collected. At the point source site, soil sampling (1 - S8) for

pesticide analyses was made in the profile along with the columns sampling, Figure 3b.

3.2.3 Groundwater sampling

Time series of water samples

A time series of 6 groundwater sampling events over a period of approximately 1 year was performed from the 15 screens of the 5 angled monitoring wells. Before each sampling event, the screens were emptied 1-5 times. Sampling was carried out applying a closed glass/teflon vacuum system (Prehnard-system, Prehnard aps.) to prevent possible atmospheric contamination during sampling. Samples were injected directly from the wells into the extraction liquid (dichloromethane) in the sampling bottles. The samples were stored at 2^o until analyses.

3.2.4 Chemical analyses

Non-reactive tracer concentrations in the influent and effluent were continuously monitored with an electric conductivity meter (Radiometer A/S) and data logger. In another experiment (Hindsby et.al., 1996) it was shown that the conductivity meter measurements could be correlated to the breakthrough of chloride (as measured by specific analyses). Recorded effluent conductivities (e_e) were normalized (e*) to influent conductivities (e_i) with the expression: e* = (e_e-e_background)/(e_i-e_background). Influent and effluent pH were monitored at the beginning and end of each experiment using a Radiometer pH-meter. Extraction of pesticides in water and till samples was made with dichloromethane. The extracts were refrigerated until analysis described in the following:

Solvents Dichloromethane and acetonitrile (HPLC-grade) were from Rathburn (Walkenburn, Scotland). HPLC-grade water was purified in a Milli-Q (Millipore, Bedford, MA, USA) filtration system. PIC-A low UV-ionpairing reagent was from Waters Associates (Milford, Ma, USA). Propylene glycol was from Fluka Chemie (Buchs, Switzerland). Disoduimhydrogenphosphate was from May and Baker LTD (Dagenham, England).

Pesticide substances MCPA, 2-4-D, mecoprop, dichlorprop and dinoseb were obtained from KVK (Køge, Denmark), simazine and atrazine were from Fisons (Cambridge, England). DNOC were from Fluka Chemie (Buchs, Switzerland). For quality control custom made pesticide ampoules from Supelco (USA) were diluted and used to verify the standards.

Apparatus LC system consisted of two Waters model 510 pumps, a Water WISP 712 Autosampler and a Waters model 440 ultraviolet absorbance detector with extended wavelength module (229 and 405 nm). A Waters Novapak C-18 4 mm column (150 x 3.9 mm) with a Supelguard - column (Supelco, USA) was used. System handling, gradient control and data treatment was carried out with Maxima 820 software from Waters.

Water samples - liquid extraction 2 L water samples were placed on a magnetic stirrer and extracted with three times (15 min each) 100 ml dichloromethane. The organic phases were combined and dried (anhydrous sodium sulphate). The organic phase were concentrated after adding 50 mg/l propylene glycol as keeper on a rotary evaporator at 35 ^oC. Remaining dichloromethane was removed by evaporation at 40 ^oC under nitrogen flow. The residue was redissolved in 1 ml of A-eluent (See HPLC procedure).

Soil samples - Soxhlet extraction 50 g soil sample was mixed with 50 g anhydrous Na₂SO₄ (which had been cleaned up by Soxhlet extraction following the same procedure as described below). The total amount was mixed and homogenised in a mortar and transferred to a 33 x 18 mm Soxhlet tube. The sample was covered with precleaned cotton. Soxhelt extraction was carried out on a water bath with dichloromethane/acetone (4:1) for 24 hours at 72 ^oC.

The organic phase was reduced on a rotary evaporator at 31 ^oC and 290 mBar to 2 ml. 50 mg/l propanediol was added as a keeper and the reduced phase was transferred with dichloromethane to a vial.

The residual dichloromethane/acetone was removed under nitrogen flow at 40 ^oC and the sample was redissolved in 1 ml A-eluent (See HPLC procedure).

HPLC procedure The chromatography was performed with gradient elution at a flow rate at 1 ml/min at 25 ^oC in a column oven. A-eluent was prepared by dilution of the content of one PIC-A low UV reagent bottle in 1 L of Millepore filtrated water and by adding 300 ml acetonitrile. B-eluent was pure acetonitrile. The eluents were filtered through 0.22 mm Millepore filter and degassed during elution with helium sparging. The gradient profile was as follows: Initial conditions, 93 % A-eluent, after 7 minutes at initial conditions linear gradient for 4 minutes to 60 % A-eluent which was hold for 2 minutes returning in 1 minute to initial conditions which was hold in 5 minutes to restablize the system.

Detection limits were for all compounds in the order of 0.01 mg/l for water samples and 0.4 mg/kg for soil samples defined as three times the standard deviation for the total method.

3.3 Experimental set-up of undisturbed columns

In-situ pressure/temperature

The undisturbed till columns were installed in large flexible-wall pressure cells and connected to a percolation system by influent and effluentteflonlines, Figure 5. The experimental system enables realistic conditionsfor solute transport experiments and hydraulic measurements in the laboratory. In the cells the in-situ pressure and temperature of the till formation were restored during the hydraulic and solute transport experiments. Once installed in the pressure cell, the columns were water saturated by slowly pumping solution into the bottom of the column. Flow in the system was driven by hydrostatic pressure defined by the hydraulic head difference of the influent system and the effluent system. The flow was continuously monitored with weight transducers and a data logging system. Effluent from the pesticide experiments was injected directly into bottles prepared with extraction liquid (dicloromethane). The effluent samples were stored at 2 ^oC until analysis.

Applied permeameter pressure for the individual columns is shown in table 2.

Trykcelle til forsøg med strømning og transport af pesticider i intakte morænelersblokke

Triaxial tryk anvendt i permeabilitet- og udvaskningsforsøg med intakte søjler

3.4 Laboratory investigations of hydraulic conductivity and pesticide leaching

Pesticide leaching and hydraulic experiments were carried out with the large diameter (0.5 m) undisturbed till column specimens, sampled from various depths in the excavated profiles of the point source site and the monitoring section of the orchard.

Bulk permeability tests of the undisturbed till columns were performed by recording effluent yield from the columns at different hydraulic gradients. The type of flow system represented by the columns were analyzed by leaching the columns with CaCl₂ followed by flushing with groundwater.

Leaching with groundwater

The water used for leaching was obtained from a confined aquifer at a site that is geologically similar to the site where the columns were taken. The chemical composition (major ions in mg/l) of the water in the aquifer was: TDS 730; Ca²⁺ 120; Mg²⁺ 31; Na⁺ 31; HCO₃^- 408; SO₄^-2 73; Cl^- 72; and the pH was 7,4.

3.5 Pesticide adsorption experiments

Samples of experiments

Analysis of the till clay mineralogy was carried out using the Shelby tube core material collected from well 1 at the point source site at depths of 2.0, 2.3, 2.5, 2.8, 3.0 and 3.5 m and from well 4 in the orchard at depths of 2.1, 11.0, 22.2 and 30.3 m. Investigations of simazine adsorption were carried out using core till material from well 4 at depths of 2.1 m and 22.2 m.

The grain size fraction <30 mm was separated by sedimentation, and the fraction <2 mm was separated in a particle-size centrifuge. For X-ray diffraction, clay mineralogy was investigated by preparation of oriented slides of specimens saturated by K⁺ and analyzed air-dry or heated to 300 ^oC, or saturated by Mg²⁺ and analyzed air-dry or after treatment with glycerol.

X-ray Diffractometry

The mineralogy of the samples was investigated by X-ray diffraction using Coka-radiation (pulse height selection) on a Philips 1050 goniometer with fixed slits.

Adsorption experiments

In order to investigate the adsorption of simazine, two samples from well 4, one shallow and one deep (from 2.1 m depth and 22 m depth, respectively) were selected; pH (in water with a 1:2.5 sediment: water ratio) was 7.3 for the 2.1 m sample and 7.8 for the 22 m sample. 5 g of sample was dispersed in 10 ml of a 1 ppm solution of simazine in water. After centrifugation, the supernatant was decanted into a flask. For each sample, this procedure was repeated twice with the simazine solution and the three times with distilled water, the supernatant being added to the volume in the flask. The concentrations of simazine in the flask and in the prepared simazine solution were determined by HPLC (see section 3.2.4).

3.6 Mapping of pesticide used in the project area

Pesticides of mapping

Pesticides used in the project area were mapped including identification of what pesticides have been applied and calculation of applied amounts together with the area distribution of application. The following eight herbicides were selected for the mapping: Dichlorprop, mecoprop (MCPP), MCPA, 2,4-D, simazine, atrazine, terbuthylazine and amitrol.

In the mapping, distinction was made between pesticides used in the orchard and pesticides used in the agricultural areas adjacent to the orchard.

Subdivision of project area

Given the quality of information available, three different approaches of mapping were used with distinction between:

	Theagriculturalarea,1956-1994
	Theorchard,1956-1983
	Theorchard,1984-1992.

Quality of information

No information is available about the amounts and distribution of pesticides used in the agricultural sections of the project area. The amounts used were therefore estimated from the total agricultural consume in grain growing areas of Denmark. The sources used for estimation are the agriculture statistical review and the annual consume of pesticides reported by Kemikaliekontrollen and the Danish EPA.

Detailed spraying records

For the period before 1984 there are no relevant spraying records available from the orchard. The calculation of the use in the period up to 1984 was based on information of which products having been used, as informed by the former managers of Skælskør orchard, combined with standard spray treatment plans of the period.

For the period 1984-1992 detailed spraying records from the orchard were available. They contain in general all important information about the products used, the dose and the total calculated use in each treatment. Due to the high quality of this material it has been possible to calculate the load on plot level for all pesticides used in the period 1984-1992.

CNS report of mapping

For further details of methods used for mapping pesticide use, see Attachment I, CNS report.