Stidson R T, Dickey C A, Cape J N, Heal K V, Heal M R
School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK.
Environ Sci Technol. 2004 Mar 15;38(6):1639-47. doi: 10.1021/es034736i.
The concentrations and input/output fluxes of trichloroacetic acid (TCA) were measured in all relevant media for one year at a 0.86 km2 upland conifer plantation and moorland catchment in SW Scotland (n > 380 separate samples analyzed). Annual wet precipitation to the catchment was 2.5 and 0.4 m for rain and cloud, respectively. TCA input to the catchment for the year was 2100 g, predominantly in rainwater (86%), with additional input via cloudwater (13%) and gas plus particle dry deposition (1%). There were no seasonal trends in TCA deposition, and cloudwater concentration was not enhanced over rainwater. TCA in precipitation exceeded concentrations estimated using currently accepted routes of gas-phase oxidation from anthropogenic chlorinated hydrocarbon precursors, in agreement with previous studies. Export of TCA from the catchment in streamwater totalled 1970 g for the year of study. The TCA concentration in streamwater at outflow (median 1.2 microg L(-1)) was significantly greater than that before the stream had passed through the conifer plantation. To well-within measurement uncertainties, the catchment is currently at steady-state with respect to TCA input/output. The catchment reservoir of TCA was dominated by soils (approximately 90%), with the remainder distributed in forest litter (approximately 9%), forest branchwood and stemwood (approximately 0.7%), forest foliage (approximately 0.5%), and moorland foliage (approximately 0.1%). Although TCA is clearly taken up into foliage, which consequently may be important for the vegetation, this was a relatively minor process for TCA at the catchment scale. If it is assumed, on the basis of laboratory extraction experiments, that only approximately 20% of "whole soil" TCA measured in this work was water extractable, then total mass of TCA in the catchment is reduced from approximately 13 to approximately 3.5 kg. Comparing the latter value with the annual flux yields an average steady-state residence time for TCA in the catchment of approximately 1-2 y, if all TCA is involved in catchment turnover. Considering that other evidence indicates the lifetime of TCA in soil and biota is considerably shorter than this (weeks rather than years), the magnitude of the TCA reservoir is suggested to be strong evidence for net natural TCA production in soils and/or that the majority of TCA in the reservoir is not involved with external fluxes.
在苏格兰西南部一个面积为0.86平方千米的山地针叶林种植园和荒地集水区，对所有相关介质中的三氯乙酸（TCA）浓度及输入/输出通量进行了为期一年的测量（分析了超过380个独立样本）。该集水区的年降雨和云量分别为2.5米和0.4米。当年进入该集水区的TCA总量为2100克，主要通过雨水输入（86%），另有通过云水输入（13%）以及气态和颗粒态干沉降输入（1%）。TCA的沉降没有季节性趋势，云水浓度也未高于雨水。降水中的TCA浓度超过了根据目前公认的人为氯化烃前体气相氧化途径估算的浓度，这与之前的研究结果一致。在研究当年，集水区通过溪水输出的TCA总量为1970克。溪水流出时的TCA浓度（中位数为1.2微克/升）显著高于溪水穿过针叶林种植园之前的浓度。在测量不确定度范围内，目前该集水区在TCA的输入/输出方面处于稳定状态。TCA在集水区的储存库中，土壤占主导（约90%），其余分布在森林凋落物（约9%）、森林枝桠材和主干材（约0.7%）、森林树叶（约0.5%）以及荒地树叶（约0.1%）中。尽管TCA显然被植物叶片吸收，这对植被可能很重要，但在集水区尺度上，这对TCA来说是一个相对较小的过程。如果根据实验室提取实验假设，在这项工作中测得的“全土”TCA中只有约20%可被水提取，那么集水区中TCA的总量将从约13千克降至约3.5千克。将后一个值与年通量进行比较，如果所有TCA都参与集水区的周转，TCA在集水区的平均稳态停留时间约为1 - 2年。鉴于其他证据表明TCA在土壤和生物群中的寿命比这短得多（以周而非年计），TCA储存库的规模被认为有力地证明了土壤中天然TCA的净生成，和/或储存库中的大部分TCA不参与外部通量。
