Environ. Sci. Technol. 2006, 40, 1250-1256
Using Size Fractionation and Pb Isotopes to Study Pb Transport in the Waters of an Organic-Rich Upland Catchment M A R G A R E T C . G R A H A M , * ,† SUSAN I. VINOGRADOFF,† ALASTAIR J. CHIPCHASE,† SARAH M. DUNN,‡ JEFFREY R. BACON,‡ AND JOHN G. FARMER† School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, and The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland
Processes controlling Pb release from a small organicrich upland catchment in northeast Scotland were investigated via measurement of Pb concentrations and 206Pb/207Pb ratios in rainwater, throughflow, surface flow, and receiving streamwaters under storm and baseflow conditions. For this catchment, the output of Pb via streams was only 2.0 ( 1.2 kg year-1 (11.4 ( 6.8 g ha-1 year-1), much lower than the input of 7.5 ( 2.0 kg year-1 (42.6 ( 11.4 g ha-1 year-1), and so the catchment is still a sink for anthropogenic Pb. Most (68-87%) of the output, however, occurred under storm conditions. Size fractionation revealed that 5060% was in large particulate form (>25 µm) with a 206Pb/ 207Pb ratio of ∼1.16, similar to that of the surface soils. Some 30-40% of the storm Pb output was associated with dissolved organic matter in the 0.024 and >0.031 m3 s-1 at BB3 and CB3, respectively) during April 2002 to August 2003 (Table 1). Importantly, this included some events occurring immediately after prolonged dry periods (e.g., April 2003). Continuous 2 h sampling at BB3 was undertaken during October to November 2003, and one further storm event was recorded (Table 1). Throughflow on the Catchment Slopes. Two-weekly throughflow water samples were routinely collected throughout the sampling period from two sampling pits (pits 1 and 2, Figure 1) set up in a previous study (33). The samples were obtained by channelling the flows emerging from each pedological horizon in a profile face cut into the upslope side of a covered and protected pit (33). During the wettest period, October 2002 to February 2003, throughflow water samples (100 mL) were collected from each of the soil horizons at both pits 1 and 2. Surface Flow on the Catchment Slopes. At the beginning of some storm events (April to October 2003), 1 L samples were taken from transient surface flows that entered the Birnie Burn. No discernible surface flows could be found flowing into the Cairn Burn.
Sample Preparation All 2-weekly rainwater and daily streamwater samples (∼250 mL) were acidified with 0.5 mL 15.55 M HNO3 (Aristar nitric acid, VWR International Ltd, Poole, U. K.) and then concentrated 10-fold by evaporation on a hotplate. The resultant concentrated samples were 2% v/v (0.31 M) HNO3. “Sample blanks” of 250 mL of deionized water were similarly VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary Data for Storm Event Stream Water Samples Collected at BB3 on the Birnie Burna
event
date
pre-event flow rateb (m3 s-1)
1 2 3 4 5 6c 7 8c
May 23-24, 2002 June 16-17, 2002 July 20-21, 2002 October 24-27, 2002 January 27-28, 2003 April 21-22, 2003 May 2-19, 2003 November 3-6, 2003
0.007 0.009 0.009 0.034 0.023 0.004 0.004 0.004
flow max (m3 s-1)
pre-event Pb (µg L-1)
max Pb (µg L-1)
mean Pb (µg L-1)
206Pb/207Pb
ratio
weighted mean 206Pb/207Pb ratio
0.088 0.039 0.250 0.142 0.696 0.077 0.113 0.012
0.3 0.3 0.5 0.3 0.1 0.2 0.2 0.2
4.0 2.1 6.2 1.8 2.6 1.7 7.3 1.9
1.8 ( 1.1 1.0 ( 0.5 2.5 ( 1.8 0.8 ( 0.6 1.2 ( 0.6 0.5 ( 0.4 0.5 ( 0.7 0.6 ( 0.6
1.160 ( 0.003 1.156 ( 0.004 1.153 ( 0.003 1.157 ( 0.004 1.156 ( 0.002 1.157 ( 0.001 1.156 ( 0.006 1.147 ( 0.007
1.163 1.152 1.152 1.160 1.156 1.156 1.155 1.148
mean
a Samples were concentrated by approximately a factor of 10 and acidified with 2% v/v HNO prior to analysis. b Pre-event flow rate values were 3 sometimes greater than the baseline value of ∼0.004 m3 s-1 due to proximity to other events. c Samples from these events were subjected to size fractionation.
acidified and reduced in volume. The concentrated sample blanks were also 2% v/v HNO3. Selected storm streamwater (∼250 mL) and catchment water samples were filtered through 25 µm (hardened ashless paper, 541), 1 µm (cellulose nitrate), and 0.45 µm (cellulose nitrate) Whatman filter papers. This fractionation scheme gave the following fractions: very large particles (>25 µm), medium-to-large particles (1-25 µm), small particles (0.45-1 µm), and colloidal/truly dissolved (25 µm fraction (mean value of 1.166 ( 0.007; n ) 10) were very 1254
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FIGURE 4. Pb concentrations and 206Pb/207Pb ratios for throughflow water samples from S (surface), A, B, C, and D soil horizons at pits 1 and 2 similar to those for the unfractionated water samples (mean value of 1.164 ( 0.012; n ) 12). The 206Pb/207Pb ratio for the >25 µm fraction in the surface flows was very similar to that for the same size fraction isolated from the storm event streamwaters (Figures 3a and 3b). This was also very similar to the mean ratio for 0-10 cm catchment soils ( 400 m amsl) part of the catchment (2). The smaller amount of Pb present in the
