Investigation of Automated Sampling Techniques to Measure Total Mercury in Stream-Water During Storm-Events

Department of Environmental Sciences, University of Virginia

Introduction

High-flow events (storms, snowmelt) are a dominant transport mechanism for total mercury (HgT) from the terrestrial to the aqueous environment. High-gradient headwater catchments are a primary source of downstream contamination because they store large pools of Hg in soils and sediments. Consistent, high-frequency event-sampling of headwater streams is rare, however, because of the unpredictability of high flows, remoteness of sites, and the difficulties associated with the ultra-clean sampling procedures (Shanley et al., 2008). The use of automated sampling techniques with an ISCO® sampler has been demonstrated in several studies for trace metals and two studies for Hg (Nelson et al., 2007; Lawson and Mason, 2001). The errors associated with using automated techniques for HgT sampling, however, has not been systematically evaluated in the literature. This research focuses on quantifying the relative errors associated with using an automated sampling system for streamwater HgT analysis and establish quality assurance guidelines for future sampling campaigns.

Objectives

Methods

Conclusions

Acknowledgements

Ami L. Riscassi*, Todd M. Scanlon

Results

Materials

What’s next?

• FP/Teflon® lined caps

• FP/Teflon® tubing (30 ft.)

• FP /Teflon® strainer (made from tubing)

• FP /Kynar® connectors/fittings

• C-flex pump tubing for peristaltic pump

• ISCO® 2900 automated sampler

• glass bottles for ISCO samples

• FP/Teflon® bottles for grab samples

Literature Cited

Amber ‘Dirty Hands’ Converse (UVA master’s student) provided analytical assistance and Kelly Hokanson (UVA undergraduate) provided field assistance. Susie Maben and Frank Deviney, of the Shenandoah Watershed Study (SWAS), provided historical streamwater chemistry data. Funding was provided through the NSF Career Grant, the DOE Graduate Assistantship in Areas of National Need (GAANN) fellowship, and the EPA Science to Achieve Results (STAR) fellowship.

Figure 1. Grab sampling streamwater for Hg analysis (USGS photo taken from Shanley et al., 2002).

Figure 2. ISCO automated sampler (right) at Paine River in Shenandoah National Park, Virginia.

Figure 3. Teflon sample line and strainer.

1) Quantify the cumulative errors associated with automated sampling for HgT analysis.

Potential errors are determined fora) sequential sampling with the same strainer, sample & pump tubingb) empty sample bottles left open c) samples left open in an ISCO

2) Determine when it is appropriate to use automated sampling techniques

* Corresponding author: Ami Riscassi (alr8m@virginia.edu)

Lawson, N. M., and R. P. Mason (2001), Concentration of mercury, methylmercury, cadmium, lead, arsenic,and selenium in the rain and stream water of two contrasting watersheds in Western Maryland, Water Research, 35(17), 4039-4052.

Nelson, S.J. et al. (2007), Mass balances of mercury and nitrogen in burned and unburned forested watersheds at Acadia National Park, Maine, USA, Environmental Monitoring and Assessment, 126, 69-80.

Shanley, J. B., et al. (2008), Comparison of total mercury and methylmercury cycling at five sites using the small catchment approach, Environmental Pollution, 154, 143-154.

All experiments were conducted with water from a local stream (Staunton River) during base flow conditions. HgT concentration of streamwater used in the experiments was 0.29 ppt.

Analysis of major cations/anions has not yet been completed, but mean concentrations calculated from 10 yrs of weekly baseflow samples indicate basic parameters are as follows: 6.7 pH, 18 μS conductivity, 86 μeq/L alkalinity, 44 μeq/L SO4, 23 μeq/L Cl, and DOC 0.7 mg/L.

Equipment test (water 5° C) :

Empty bottle test:

All samples were preserved within one hour of collection with 100% BrCl solution (5 ml/L). Samples were analyzed within a week of preservation for unfiltered HgT on a Tekran 2600 Cold Vapor Atomic Fluorescence Spectrophotometer (CVAFS) at the University of Virginia Lab. The relative percent difference (RPD) for field duplicates was <15%.

Equipment testHgT concentrations of streamwater sampled with an ISCO were elevated compared to ‘grab’ samples. HgT contamination ranged from 0.00 to 0.84 ppt. The relative percent difference (RPD) between grab and ISCO samples ranged from 0.2 to 94%. The largest RPD’s are associated with baseflow samples immediately following a highly concentrated sample (25 or 50 ppt), a situation which is atypical in the natural environment.

Empty bottle testMean HgT contamination of uncapped empty bottles ranged from 0.07 to 0.25 ppt for the 3 and 7 day periods tested. Greater contamination was observed in open bottles with other samples (0.29 vs 0.10 ppt).

Uncapped sample testAll uncapped samples lost HgT ranging from 0.1 to 4.8 ppt. The RPD of [HgT] between ‘grab’ and ISCO samples ranged from 10 to 30%.

Air temperature range: (25-58°F)

(1) Repeat the bottle experiments in a heated environment (‘summertime’). (2) Concurrent manual stream sampling (middle of water column) and ISCO automated stream sampling during storm event.

Figure 4. Staunton River, Va.

Figure 5. Set-up of ISCO line test.

Figure 6. Set-up of ISCO bottle test for contamination/evasion.

Figure 7. Tekran 2600 and Dr. Babi.

Uncapped sample test:

• ISCO programmed to rinse line 3X before sample collection• ISCO samples streamwater (white vat)• Manual sample poured from vat directly to bottle (‘grab’ sample)• ISCO samples Hg-spiked streamwater (red vat)• Manual sample poured from vat directly to bottle (‘grab’ sample)• Rinse off outside of strainer with 50 ml streamwater (black vat)• ISCO samples streamwater (white vat)...• Repeat for [Hg] spike: 0.5, 5.0, 25.0, & 50.0 ppt

• 2 sets of uncapped empty bottles in ISCO without samples• 2 sets of uncapped empty bottles in ISCO with samples (0.5- 50.0 ppt) • First bottle sets collected after 3 days and second sets after 7-days, and

all bottles filled with streamwater of known [HgT]

• 2 sets of ISCO bottles filled with streamwater, and streamwater spiked with 0.5, 5.0, 25.0, and 50.0 ppt Hg

• First bottle set collected after 3-days and second at 7-days.

Figure 8. HgT concentration of ‘grab’ and ISCO samples. The X-axis indicates the spiked HgT concentration in ppt. Absolute difference and (RPD) are labeled above the respective data.

Figure 9. HgT concentrations of streamwater poured into a unopened bottle, and poured into empty bottles that had been left open in an ISCO. Absolute difference (ppt) is labeled above the respective data.

Figure 10. HgT concentrations of samples left uncapped for 3 and 7-days in an ISCO. The X-axis indicates spiked [HgT]in ppt. Maximum absolute difference and RPD are labeled above the respective data.

The cumulative errors of using an ISCO automated streamwater sampler resulted in a RPD in HgT concentration ranging from: 8-10% for 50 ppt, 11-15% for 25 ppt, 2-20% for 5ppt, 10-71% for 0.8 ppt, and 5-120% for 0.3 ppt. ISCO automatic samplers can be used to measure streamwater for HgT when concentrations are above 5 ppt (2<RPD<20%). These concentrations are common during high-flow events. Streamwater with HgT concentrations in the 0.2 to 5.0 ppt range should be manually sampled (RPD>20%). The EPA Method 1631 specifies that the RPD between field duplicates should be less than 20%.

All materials used in automated sampling are fluoropolymer (FP) (except pump tubing) and are cleaned by soaking in a heated (75 °C) 4N HCl acid vat for 12 hrs., rinsed 3X with DI, dried under a clean bench, and double bagged until use. Sample bottles are cleaned according to EPA protocol.

0

10

20

30

40

50

60

stream 0.5 5.0 25.0 50.0

[Hg

T],

pp

t

actual

3-day open

7- day open

-0.04(10%)

-0.13(15%)

-1.4(30%)

-4.7(20%)

-4.8(10%)

0

10

20

30

40

50

60

stre

am 0.5

stre

am 5.0

stre

am 25.0

stre

am 50.0

stre

am

[Hg

T],

pp

t

'grab'ISCO

0.13(22%)

0.00(0.2%) -0.24

(39%)

0.13(3%) 0.02

(5.4%)

0.35(1%)

0.28(44%)

0.10(0.2%)

0.84(94%)

0

0.2

0.4

0.6

0.8

1

3-day 7-day

[Hg

], p

pt

unopened

open bottle with no samples

open bottle with samples

0.14

0.25

0.07

0.17