Environ. Sci. Technol. 2003, 37, 2626
Response to Comment on “Anthropogenic Sources of Arsenic and Copper to Sediments in a Suburban Lake, Northern Virginia” Saxe and Beck (1) raise two groups of questions regarding the mass-balance approach in our paper. (i) Only some of the data and calculations used for the mass balance were provided; the apparent number of samples collected is not sufficient to support a reliable mass balance; measurements were not made on all tributaries. The entire data set collected for Lake Anne for this study, including the total number and frequency of samples collected, concentrations, stream discharge, and QA/QC data, are detailed in Conko et al. (2), previously referenced in Rice et al. (3). Because the data are in the public domain, we did not reproduce them in this Journal. The important point in calculating stream loadings is not the number of samples collected but rather the range of streamflow sampled and the resulting relation with concentration. We sampled throughout the full range of discharge and found increased concentrations with increased discharge. There are only two perennial tributaries to Lake Anne, one of which drains an upstream lake (Lake Newport, see Figure 1 of ref 3). The upstream lake serves as a sediment trap; thus, the stream draining the lake is not representative of the Lake Anne catchment. In addition, this tributary is piped for most of its length and flows beneath roads and parking lots before discharging into Lake Anne. Therefore, we sampled the other tributary because it was more representative of the catchment. The arsenic and copper loads calculated from the input tributary were scaled up proportionally by area, excluding the Lake Newport catchment, to represent the total load by streamflow to Lake Anne. As Saxe and Beck point out, nearshore samples of sediment were not collected. Sediment samples collected from the deepest portion of the lake are most representative of the overall input to the lake because these sediments are less likely to be disturbed by wave action and erosive forces. Sediments in the deepest portion of the lake tend to be thicker because of sediment focusing (4, 5), increasing the accuracy of the chemical analysis and improving the resolution of the analysis on a per time basis. Furthermore, the nearshore sediments have been dredged several times in the past to improve boat access (Reston Association, 1998, written communication), destroying the nearshore record of trace element deposition over time. Nearshore samples of water were not collected in order to avoid bias of possible increased arsenic concentration by CCA-treated structures. Because the lake is well-mixed, water samples from the middle of the lake represent inputs from all sources and the overall water quality of the lake. The lake water samples were used only as an indication of the relative concentration of the different types of waters sampled and not in the mass balance calculations. The sensitivity analysis presented in our paper shows that even if all of the concerns that Saxe and Beck (1) raise about our mass balance were significant, leaching of CCA-treated wood must be invoked to balance the arsenic contribution to the lake sediments. (ii) Identification of CCA-treated wood; should have measured arsenic concentrations over time and alongside CCA-treated wood structures; alternative arsenic sources were not investigated. Newer CCA-treated wood frequently has a telltale greenish stain (indicating migration of copper from the wood), which 2626
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
is easily identified in the field. In addition, CCA-treated wood has been the dominant form of wood used for residential construction for decades because of its ready (and in many cases, exclusive) availability and its low cost relative to other types of wood. Furthermore, Reston Association (written communication) maintains records of dock and deck replacement activity, with building plans that specify use of CCA-treated wood. Painted structures were excluded from the wood-area estimates on the basis of the results of Stilwell and Gorny (6), who found that soils beneath painted CCA structures contain lower concentrations of these elements than soils beneath unpainted structures. As Saxe and Beck (1) point out, the ideal study would have been to measure arsenic concentrations over time and alongside CCA-treated wood structures. The next best line of investigation, of course, was to conduct leaching experiments with woods of different retention factors and of different ages to account for the likely decreases in leaching over time (see Table 1 of ref 3). The possibility of alternative arsenic sources was investigated, as the streams transport any and all contaminants within the catchment, including pesticides and fertilizers. Arsenical pesticide historical use is unlikely in this catchment, as evidenced by aerial photography, which shows land use change from 1964 to 1997 from nearly 100% forested (except for a small percentage of cattle grazing land) to nearly 100% developed. Agricultural cropping practices are not evident in any of the 26 aerial photographs (dated from 1949 to 1997) examined. In addition, background soil samples were collected randomly (spatially and with depth, n ) 21) and analyzed for arsenic and copper concentrations, which ranged from 2.9 to 13 µg/g dry weight arsenic and from 11 to 77 µg/g dry weight copper (7). The corresponding concentrations at the top of the lake sediment cores were 5 times greater for arsenic and 3 times greater for copper than the median concentrations of the 21 soil cores. If there were no additional sources of these elements to the lake, concentrations at the top of the sediment cores (28 µg/g As; 137 µg/g Cu) should be no greater than the greatest concentrations in the soil cores.
Literature Cited (1) Saxe, J. K.; Beck, B. D. Environ. Sci. Technol. 2003, 37, 2625. (2) Conko, K. M.; Kennedy, M. M.; Rice, K. C. Open-File Rep.sU.S. Geol. Surv. 2000, No. 00-481. (3) Rice, K. C.; Conko, K. M.; Hornberger, G. M. Environ. Sci. Technol. 2002, 36, 4962-4967. (4) Dillon, P. J.; Evans, R. D. Hydrobiologia 1982, 91, 121-130. (5) Hakanson, L.; Jansson, M. Principles of Lake Sedimentology; Springer-Verlag: New York, 1983; p 319. (6) Stilwell, D. E.; Gorny, K. D. Environ. Contam. Toxicol. 1997, 58, 22-29. (7) Rice, K. C. Ph.D. Dissertation, University of Virginia, 2001.
Karen C. Rice* U.S. Geological Survey P.O. Box B Charlottesville, Virginia 22903
Kathryn M. Conko U.S. Geological Survey MS 432, National Center Reston, Virginia 20192
George M. Hornberger Department of Environmental Sciences University of Virginia Charlottesville, Virginia 22903 ES030050E 10.1021/es030050e CCC: $25.00
2003 American Chemical Society Published on Web 05/06/2003
