Initial considerations of trace metal bioavailability: some regulatory experiences

E. Unsworth1, A. Peters1, J. Comloquoy2, M. Campbell2

1 Scottish Environment Protection Agency (SEPA), East Kilbride, G74 5PP

2 Scottish Environment Protection Agency (SEPA), Dingwall, IV15 9XB

Introduction

Under the Water Framework Directive (WFD) water bodies which fail to achieve good ecological status as a result of discharges of chemical substances may be identified for remedial action. It is important that this is both prioritised and appropriate to the risks posed.

Environmental Quality Standards (EQS) are derived in such a way as to minimise false negatives. One consequence of this is that failure of an EQS does not necessarily imply that unacceptable effects are likely to occur in the water course.

The measurement of more ecologically relevant forms of trace metals (for example by DGT) could lead to an improved understanding of the risk posed by trace metals at individual sites. This could provide better information on which decisions regarding the appropriate course of action could be based.

Experimental

The DGT units were placed in-situ in the river being monitored and left in place for one week (see photographs). Water temperature and pH were measured by a sonde (hourly frequency) over the whole deployment period. Water samples were taken at the start and end of the deployment to measure the total dissolved metal concentrations and water hardness.

After deployment the metal was extracted from the resin gel using 1 M nitric acid. The concentration of metal in the extraction solution was then measured by ICP-MS. The concentration of DGT labile species was then calculated using the diffusion rate of metal ions through the hydrogel and the time spent deployed in the river.

Background

This investigation arose out of an action plan to study minor water bodies on the Spey. Failure to meet the EQS for copper had been recorded in some burns flowing into the river Spey and it was noted that for the burns with distilleries sited next to them the copper concentration was greater downstream of the distilleries than upstream. The Green Burn which flows past the Glenfarclas and Dailuaine distilleries (see map) was chosen for this initial study with DGT.

Copper concentrations (both dissolved and DGT labile) were measured at a number of sites during this study. The Lade provided the feedstock water used by the Glenfarclas distillery and provided a background copper concentration for this study.

The distillery cooling water was discharged into the Green Burn. A Green Burn sampling point upstream of the distillery was not expected to be effected directly but could be subject to diffuse pollution from the effluent spreading on the surrounding fields. The spent lees (residue from the distillation process) were treated at the bioplant, after which the solids were spread on the surrounding fields and the liquor and wash waters were discharged in to the Green Burn.

Two sampling points directly downstream of the Glenfarclas distillery and the bioplant were monitored along with one final point above the confluence of the Green Burn with the Spey. This final point was downstream of both the Glenfarclas and Dailuaine distilleries, being 4.4 Km from the first upstream sampling point on the Green Burn.

Map showing DGT deployment locations on the Green Burn

Results and discussion

Copper concentrations in the Lade were below the limit of detection by ICP-MS analysis. This confirmed previous work that there was a negligible geochemical background input of copper to the water course. The copper concentrations (>EQS) in the Green Burn upstream of Glenfarclas were higher than in the Lade indicating that there was a source of copper entering this water course even upstream of the distillery. This was probably due to diffuse pollution from the distillery waste which was spread on the fields above the Green Burn. The DGT labile copper concentration (<EQS) measured at this site was ca. one fifth of that of the total dissolved copper concentration, the ratio of DGT labile/total dissolved copper concentration being 0.24. This result would be consistent with all the copper being present as copper-dissolved organic carbon (Cu-DOC) species. Previous studies have shown that copper-fulvic acid species have a diffusion coefficient one fifth of that of the free Cu2+ ion through the hydrogel used in these DGT devices and therefore a correspondingly lower DGT labile copper concentration would be measured.

Downstream of the Glenfarclas distillery the copper concentration increased, both total dissolved and DGT labile (>EQS). However the ratio of DGT labile to total dissolved copper concentration also changed to 0.4 This indicated that a larger fraction of the total dissolved copper concentration was in a labile form. This also indicated that the copper speciation had changed when the distillery outflow was added to the Green Burn.

Results and discussionThe final sampling point on the Green Burn (4.4 Km downstream of the first Green Burn sampling site) showed a decreased copper concentration (>EQS), compared to that immediately downstream of the distillery. This was due to a dilution effect from additional water input into the Green Burn during its passage to the Spey. The water volume in the Green Burn had doubled based on a visual estimate. The fraction of the total Cu concentration that was DGT labile had also changed (<EQS), becoming closer to that seen at the first Green Burn sampling site upstream of the Glenfarclas distillery, all be it with a higher absolute copper concentration. This indicated that the copper speciation had once again changed.

Acknowledgments

The authors would like to acknowledge the work carried out by the SEPA field and chemistry units.

Summary

The DGT deployments worked well. The DGT labile copper concentrations measured were lower than the total dissolved (labile plus non-labile) copper concentrations. The fraction of the total dissolved copper concentration that was DGT labile increased moving down stream from the site upstream of the Glenfarclas distillery to the sites just down stream of the Glenfarclas distillery. This had then decreased back to the lower level by the time the river has passed both distilleries at a point just above the confluence with the river Spey.

This study showed that although the dissolved copper concentrations measured were above the EQS the DGT labile, bioavailable, concentrations were not, at some sites. The ecological implications of this need to be taken into account when deciding if remedial action should be taken. This demonstrates that where copper concentrations exceed the EQS further measurements of the labile, bioavailable, copper concentration may need to be obtained.

Piston

Outer sleeve

Resin gel

Diffusive gel

Filter membrane

Figure 1. DGT holder and gel discs (not to scale)

Environmental Quality Standards

Dangerous Substances Directive (DSD) 76/464/EEC (to be superseded by the Water Framework Directive (WFD) categorizes copper as list 2, which is a substance to be controlled.

The Freshwater Fisheries Directive (FFD)78/659/EEC (to be superseded by WFD) categorizes copper as guideline control.

These control/monitoring levels are implemented by the use of Environmental Quality Standards (EQSs).

A quality standard defines an acceptable level or concentration of a substance; exceedence of which should not necessarily result in adverse effects.

EQS for copper 1µg/l, water hardness up to 10 mg/l CaCO3. EQS for copper 6µg/l, water hardness up to 50 mg/l CaCO3.

The guidance notes also state that this level may be significantly higher in the presence of organic matter.

Diffusive Gradients in Thin-films (DGT)

This technique involves the diffusion of metal ions from the water source, through a diffusive hydrogel and then accumulation on a resin (chelex) gel layer. Knowing the diffusion rate of the metal through the diffusive gel and the deployment time in the water source the concentration of labile mobile metal species can be calculated.

Only metal species which can diffuse through the diffusive gel (are mobile) and dissociate within the time taken to pass through the gel (are labile) then bind to the resin gel are measured by this technique. This is termed the DGT labile concentration.

Dissolved and DGT concentrations week 1

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Cu

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Figure 3. Fraction of copper measured as DGT labile

Figure 2. Dissolved and DGT labile copper concentrations

Background (in Lade)

Upstream of distillery

Downstream of distillery

Downstream of bioplant

Further downstream of both distilleries

Direction of flow

LadeUpstream distillery

Downstream distillery

Downstream bioplant

Daluaine