Environ. Sci. Technol. 2003, 37, 2389-2393
Forestry Practices Increase Mercury and Methyl Mercury Output from Boreal Forest Catchments P E T R I P O R V A R I , * ,† M A T T I V E R T A , † JOHN MUNTHE,‡ AND MERJA HAAPANEN† Finnish Environment Institute, SYKE, Research Department, P.O. Box 140, FIN-00251 Helsinki, Finland, and IVL Swedish Environmental Research Institute, P.O. Box 47086, SE 402 58, Gothenburg, Sweden
We observed significant increases in the runoff output of total mercury (TotHg) and methyl mercury (MeHg) from a small spruce forest catchment (0.071 km2) after clearcutting and soil treatment. Here we show that forest regeneration practices may act as an important additional source of TotHg and MeHg to forest lakes. TotHg and MeHg in runoff from two small forested catchments were monitored during the period 1994 to 2001. In the autumn of 1997, one of the catchments was clear-cut. Soil preparation (mounding) was carried out in the autumn of 1998 and replanting in the summer of 1999. During the 3 years after the silvicultural treatment, medians of monthly flowweighted TotHg and MeHg concentrations (12.02 ng L-1 and 0.35 ng L-1, respectively) and output loads (0.80-0.97 g km-2 a-1 and 0.011-0.036 g km-2 a-1, respectively) increased significantly compared to the 3 years calibration period (8.13 ng L-1 and 0.15 ng l-1; 2.0-5.3 g km-2 a-1 and 0.11-0.16 g km-2 a-1, respectively). These results indicate that clear-cutting and/or soil treatment significantly increases the mobility of TotHg and MeHg accumulated in forest soil and may thus be an important factor for the total input of Hg to boreal freshwater ecosystems.
for increased loading to surface waters, very little is known about the factors controlling the mobility of TotHg and MeHg in soils. Clear-cutting with site preparation has been shown to cause increased leaching of nutrients as well as washout of suspended solids and organic matter (e.g. ref 15). Both DOC and POC fractions are involved as carriers in the transport of TotHg from catchments to aquatic ecosystem (16). In Que´bec, it was observed that MeHg levels were significantly higher in zooplankton in lakes with partially clear-cut watersheds than in lakes with burned or undisturbed watersheds (17). Later the same authors found that the average TotHg level in northern pike (Esox lucius L.) was significantly higher in lakes with logged catchments than in reference lakes (18). Even though several studies of pathways of Hg and MeHg from catchments to lakes have been conducted (e.g. refs 4, 8, 9), specific investigations of the effects of forestry practices on Hg release from forest catchment have not been made. Soil treatment (e.g. harrowing, scarification, and mounding) is a common measure to improve survival and growth of planted or self-regenerated conifers. Different soil treatment methods disturb from 20 to 65% of the soil surface. After the treatment the soil usually consists of undisturbed soil, furrows, and mounds. Harvesting results in more precipitation reaching the ground, but at the same time dry deposition of pollutants is reduced. The logging residues left after harvesting are subject to mineralization. The organic matter that is already incorporated into the soil may be subject to increased mineralization as a result of changes in soil moisture conditions, temperature, and the enhancement of nutrient cycling after harvesting and soil preparation. Soil preparation leads to increased day temperatures and decreased night temperatures leading to higher heat summation. The objective of this work was to estimate the effects of current forestry practices (clear-cutting, soil preparation, and regeneration treatment) on TotHg and MeHg concentrations in outflow streamwater and on output fluxes in a boreal spruce forest catchment in southern Finland.
Material and Methods Introduction Pollution of freshwater ecosystems by mercury (Hg) has been a prioritized environmental issue in Sweden and many other countries for over 20 years. Large research efforts have been made to explain the links between emissions to the atmosphere and accumulation in freshwater fish. A number of research projects on TotHg and MeHg dynamics in forested catchments and wetlands have been performed in Sweden and Finland (1-7), North America (8, 9), and Germany (10, 11). Most of these studies have been focused on input/output budgets and relationships between TotHg and MeHg behavior and hydrology or water chemistry, e.g. TOC (total organic carbon), DOC (dissolved organic carbon), POC (particulate organic carbon), and pH. Hg accumulates efficiently in boreal forest soils. Increased concentrations in the mor layer is a result of atmospheric deposition as seen in gradient studies in Sweden (12) and elsewhere (13, 14). Although many investigations have identified the accumulation in forest soils as a potential risk * Corresponding author phone: +358-9-403000; fax: +358-940300 490; e-mail:
[email protected]. † Finnish Environment Institute, SYKE. ‡ IVL Swedish Environmental Research Institute. 10.1021/es0340174 CCC: $25.00 Published on Web 04/18/2003
2003 American Chemical Society
The treatment site was a small (0.071 km2) Norway spruce (Picea abies Karst.) forested catchment situated in southern Finland (61°01′N, 24°45′E). The soil of the catchment is dominated by coarse sand moraines with a thin humus layer (ca. 5 cm) on the top and only ca. 5% peatland of the area The stand mainly (98% of growing stock volume) consisted of mature Norway spruce and the rest of deciduous tree species (2%). The mean age of trees was 76 years in 1995. The upper part of the catchment (8%) had a seedling stand of Norway spruce planted in 1990. The catchment (excluding the seedling stand) was clear-cut in August 30-September 10, 1997, and the soil was treated (mounding with excavator) in September 1998 and afforested in June 1999. A buffer zone of 20 m breadth and 50 m length was left on both sides of the outflow stream. TotHg and MeHg concentrations in soil and deposition are shown in Table 1, Supporting Information. The reference catchment was located 3 km east from the treatment catchment. The most common soil texture types at the reference catchment were silty-sand moraine and sand till with a thin humus layer. Some 30% of the area is covered with peat (mean depth 40 cm). The forest cover (70 m3 ha-1) consists of Scots pine (Pinus sylvestris L.) 62%, Norway spruce 26%, and deciduous tree species 12%. The mean age of the trees was 42 years in 1995. The peatland part of the catchment VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Monthly flow-weighted TotHg and MeHg concentrations in runoff waters at the treatment and reference catchments. The arrows indicate the clear-cut and the soil treatment occasions. was drained in the 1960s. The eastern part was logged and planted in the1960s, and in the western part timber harvesting was carried out by thinning in 1978. Although the soil properties and forest cover differed from those of the treatment catchment, the hydrology of the outflowing water was very similar (see Figure 1, Supporting Information and Figure 3). Flow volumes were estimated either from monitored flow records from weirs or extrapolated from another monitoring station (location 61°40′N, 24°21′E, area 1.5 km2). At the treatment and reference catchments, flows were measured continuously by a v-notch weir with a water level recorder from the beginning of spring runoff until freeze-up. Winter visits were made each month to check the presence of flow and to measure if necessary. In the absence of monitored record during frozen conditions, the flow was estimated by interpolation from the records of the other monitoring station which was operating also during winter. Samples for TOC, TotHg, and MeHg analysis in runoff water were collected from weirs at the outflow of the catchments. Samples were taken with a 1-month interval, with more frequent (weekly) sampling during periods of high outflow. Samples from the catchments were collected from 1994 to 2000 and analyzed at the IVL Swedish Environmental Research Institute laboratory. TotHg and MeHg samples collected in Teflon bottles were shipped to the IVL laboratory and preserved with 0.5 mL 30% HCl (Merck, Suprapur). For analysis of TotHg the samples were oxidized using BrCl for at least 8 h. Excessive BrCl was removed by addition of 0.5 mL of NH2OH. A sample aliquot was transferred to a bubbler flask where SnCl2 was added. The produced Hg0 was purged using purified N2 and collected on a gold trap. After 20 min the trap was transferred to the analytical system where it was connected to an argon gas 2390
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stream leading to the CVAFS detector. Collected Hg was desorbed by heating the gold trap. Samples for analysis of MeHg were distilled using a procedure where 80% of the original volume was distilled and collected in a cooled receptor vessel. This solution was then transferred to a reaction bottle where an ethylating reagent was added (sodium tetraethylborate). The formed ethylated Hg species were purged from the reaction bottle by a N2 gas stream and collected on Carbotrap adsorbents. The collected Hg species were then thermally desorbed on to a GC column. After separation, the organo Hg species were pyrolyzed and detected using CVAFS. A detailed description of the analytical procedures for MeHg is presented elsewhere (19). The IVL laboratory is accredited for analysis of TotHg and MeHg in water (Swedac no 1213). The QA/QC procedures include daily calibrations, blank checks, and use of independent reference materials and/or independent reference solutions. The uncertainty is calculated for each individual analytical determination using a statistical method which takes into account the uncertainties of each individual step in the analytical procedure. Recovery of MeHg in the distillation step is calculated from standard addition experiments. The detection limit is around 0.05 ng L-1 for both TotHg and MeHg. Output loads of TotHg and MeHg were derived by interpolation. Daily concentrations for nonsampling periods were interpolated from weekly or monthly samples. Measured and interpolated daily concentrations were multiplied by daily discharge to calculate daily loads. Daily loads were added up to monthly and annual loads. To evaluate the surplus of Hg load due the forest treatment, regression curves between the monthly loads of the reference and treatment catchments were fitted using data from the pretreatment calibration period (June 1994-August 1997). Calculated regression equations were used to estimate the change in runoff, TotHg, MeHg, and TOC load caused by the clear-cutting and soil preparation (eqs 1-4). Linear regressions for runoff, TOC, TotHg, and MeHg load were calculated for the pretreatment period using simultaneous monthly load values from the treatment and control area. Monthly runoff, TOC, TotHg, and MeHg load values had skewed distributions and therefore were log transformed to make the regression linear (see Figure 1, Supporting Information). Using the regression equations (eqs 1-4), the predicted runoff and load values without forest practices (“natural”) were calculated for the post-treatment period. The difference between measured and predicted values was used as an indicator of the change caused by forestry treatments
log(Rt + 1) ) 0.97 log(Rr + 1) + 0.12,
r2 ) 0.96, N ) 39 (1)
log(TOCt + 1) ) 0.92 log(TOCr + 1) + 0.03, r2 ) 0.95, N ) 39 (2) log(TotHgt + 1) ) 0.98 log(TotHgr + 1) + 0.20, r2 ) 0.91, N ) 39 (3) log(MeHgt + 1) ) 0.63 log(MeHgr + 1) + 0.04, r2 ) 0.61, N ) 39 (4) where Rt is the runoff from the treatment catchment; TOCt, TotHgt, and MeHgt are the TOC, TotHg, and MeHg output loads from the treatment catchment; Rr is the runoff from the reference catchment; and TOCr, TotHgr, and MeHgr are the output loads from the reference catchment. We tested for statistically significant differences in median concentrations of TotHg and MeHg between pretreatment period and the time after the clear-cutting using the Mann-Whitney test. We then asked if there were any
TABLE 1. Medians, Quartiles (25th and 75th), and Ranges of Monthly Flow-Weighted TotHg and MeHg Concentrations in the Clear-Cut and Reference Catchments before treatment TotHg ng L-1 catchment median 25th 75th clear-cut reference
8.13 4.90
after treatment MeHg ng L-1
range
6.32 9.65 3.19-20.31 4.04 6.20 2.85-20.00
median 25th 75th 0.15 0.33
TotHg ng L-1 range
median 25th
75th
MeHg ng L-1 range
0.08 0.21 0.04-1.93 12.02 8.98 16.92 5.97-24.44 0.25 0.39 0.15-0.77 4.74 4.05 6.43 2.90-11.41
median 25th 75th 0.35 0.33
range
0.20 0.97 0.03-2.88 0.21 0.39 0.06-0.70
differences in median concentrations between hydrological years (from October 1 to September 30) and between seasons in each calendar year using the Kruskal-Wallis test. Due to the low number of observations in dry summer conditions (June-August) and frozen winter conditions (DecemberFebruary), only spring (March-May) and autumn (September-November) values could be compared. Monthly output loads of TotHg and MeHg and water runoffs before and after the clear-cutting were tested using the Wilcoxon test. Correlations among median values for runoff water TotHg, MeHg, and TOC were assessed using Spearman rank correlations.
Results and Discussion A total number of 167 runoff samples (102 and 65 from the treatment and reference sites, respectively) were collected and analyzed for TotHg, MeHg, and TOC. The runoff concentrations of TotHg varied between 2.14 and 27.9 and 2.58-24.1 ng L-1 in the treatment and the reference catchments, respectively. The MeHg concentrations varied between 0.03 and 3.6 ng L-1 and 0.03-0.79 ng L-1 in the treatment and the reference catchments, respectively. A summary of the monthly flow-weighted TotHg and MeHg concentrations is presented in Table 1 and a graphical presentation of the development of the concentrations during the study period in Figure 1. The increases in TotHg and MeHg concentrations were statistically significant after the clear-cutting (Mann-Whitney, p ) 0.0001 and p ) 0.0029, respectively). When testing the annual TotHg concentrations, only the values of the first monitoring year were significantly lower than the following 2 years after clear-cutting (Kruskal-Wallis, p ) 0.0005). The seasonal TotHg concentrations did not differ between different years. The MeHg concentrations of the third year after clear-cutting (second year after soil preparation) were significantly higher than in the first forestry treatment year (Kruskal-Wallis, p ) 0.0015). We also found significantly higher autumn MeHg concentrations in the second and third year after clear-cutting, compared with the year before the treatment (Kruskal-Wallis, p ) 0.0012). The monthly TotHg and MeHg output loads were significantly higher after clear-cutting (Wilcoxon, p ) 0.0016 and p ) 0.0027, respectively) (Figure 2). During the 3 years after clear-cutting, also the annual TotHg load (2.0-5.3 g km-2 a-1) and MeHg load (0.11-0.16 g km-2 a-1) were significantly higher than during the preceding 2-year calibration period (0.80-0.97 and 0.011-0.036 g km-2 a-1, respectively). The TotHg and MeHg concentrations and the output fluxes before silvicultural treatment were of the same level as in studies carried out in forest catchments with upland and wetland sites in the United States, Canada, and Sweden (4, 7, 8). The silvicultural treatment had a substantial effect on the runoff. The monthly water runoffs were significantly higher after clear-cutting (Wilcoxon, p ) 0.0103) (Figure 3). The mean annual runoffs for the pretreatment (including the treatment year) and post-treatment period were 106 mm a-1 and 234 mm a-1, respectively. It seems that the increased water runoff had a major impact on the TotHg output, as
FIGURE 2. The monthly TotHg and MeHg loads (mg km-2) from the treatment and reference catchment. Open bars represent the calculated Hg load without silvicultural treatment, and closed bars indicate the measured increase in the Hg load. The line with dots is the reference catchment. Regression curves were fitted for the pretreatment period using simultaneous monthly load values from the silvicultural treatment and control area (see Figure 1, Supporting Information). The regression equations were used to calculate the predicted loads without the silvicultural measures for the posttreatment period (eqs 3 and 4). The difference between the measured and predicted values was used to indicate the change caused by the silvicultural treatments. there was no significant increment in concentrations of TotHg. The median TotHg concentrations were 1.5-1.8 times higher after the treatment. The same effect was seen as significant increase in MeHg output during the first year after clear-cutting, when the median MeHg concentrations were 1.9 times higher than before. Total organic carbon (TOC) concentrations in runoff water in the treatment and reference catchment varied between 7.8 and 84 mg L-1. In the treatment catchment the concentrations of TOC peaked during the first year after clear-cut and then returned to the same level as during the calibration period. The annual mean TOC concentrations before and after the clear-cutting were 20 ( 7.3 and 32 ( 15 mg L-1, respectively. The TOC concentrations of the first year after clear-cut differed significantly from the calibration period (Kruskal-Wallis, p ) 0.0001). TOC concentrations of runoff water correlated positively with TotHg (r ) 0.66, p ) 0.0001) but did not correlate with MeHg (r ) 0.12, p ) 0.241). Consequently, TOC may explain much of the increase in concentrations and outflow of TotHg but cannot explain the increase in MeHg. The elevated MeHg concentrations together with declining TOC and TotHg concentrations VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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that former aerobic sites became anaerobic and thereby stimulated Hg methylation. Because these saturated sites are located near the outflow stream (and in the buffer zone on both sides of the stream), they could directly contribute MeHg load to the stream. However, no data on this exist, and therefore the mechanisms for increased mobility and production of MeHg in the catchment require more research in order to be defined. The increased MeHg concentrations in the first and second year after soil treatment may indicate that the soil treatment had the major effect on Hg methylation and MeHg mobility. The behavior of TotHg seems to be closely connected to organic carbon (humic substances) and is mobilized earlier together with TOC and is more affected by clear-cutting than MeHg. Increased anthropogenic emissions of Hg into the atmosphere have led to deposition well in excess of natural levels in northern Europe and have enhanced the store of Hg in soil and vegetation in recent years (21).
FIGURE 3. Monthly runoffs and flow-weighted TOC concentrations in the treatment and reference catchments. during the second and third year after clear-cutting may indicate enhanced Hg methylation activity in the catchment. Summary of the statistical tests is shown in Table 2, Supporting Information. It is probable that changed hydrological pathways due to soil treatment cause increased leaching of TOC and DOC. Elevated amounts of particulate and dissolved humic matter may have led to increased TotHg associated with humic substances. This change seems to be temporary and only lasts for 1-2 years. It is not possible to distinguish between permanent changes in hydrological pathways (causing longterm increases in MeHg leaching) and methylation of soil bound Hg. In any case, since the MeHg effect is more long lasting, the MeHg apparently either has a higher mobility in the soil water or the enhanced environmental conditions (humidity, temperature increase, etc.) favor Hg methylation. The impact of disturbing the forest soil and water flow paths on MeHg transport was recently observed in Sweden (20). In this case, only a small area of the catchment was disturbed and no clear-cutting was made. MeHg concentrations and transport increased markedly, but no significant change in TotHg was observed. It was suggested that the increased MeHg output may be explained by the changing water flowpaths mobilizing the MeHg already present in the soil, but MeHg production may also contribute. Further, it was concluded that the removal of the trees is not an important factor. The present study showed that the clear-cutting of a mature spruce forest clearly increased the TOC and TotHg concentrations and TotHg and MeHg loads already in autumn 1997 before the soil treatment. Each effect of the different forest treatments (clear-cutting, mounding, afforestation) could not clearly be distinguished. However, decreased transpiration and hence increased soil water content of the treated watershed may have led to an increased area of saturated soils. Increased water in the catchment may also have raised the water level in the minor areas of wetland (
