Gooddy D C, Mathias S A, Harrison I, Lapworth D J, Kim A W
British Geological Survey, Wallingford, Oxfordshire, OX10 8BB, UK.
Sci Total Environ. 2007 Oct 15;385(1-3):262-71. doi: 10.1016/j.scitotenv.2007.06.043. Epub 2007 Jul 27.
Agrochemical contamination in groundwater poses a significant long term threat to water quality and is of concern for legislators, water utilities and consumers alike. In the dual porosity, dual permeability aquifers such as the Chalk aquifer, movement of pesticides and their metabolites through the unsaturated zone to groundwater is generally considered to be through one of two pathways; a rapid by-pass flow and a slower 'piston-flow' route via the rock matrix. However, the dissolved form or 'colloidal species' in which pesticides move within the water body is poorly understood. Following heavy rainfall, very high peaks in pesticide concentration have been observed in shallow Chalk aquifers. These concentrations might be well explained by colloidal transport of pesticides. We have sampled a Chalk groundwater beneath a deep (30 m) unsaturated zone known to be contaminated with the pesticide diuron. Using a tangential flow filtration technique we have produced colloidal fractions from 0.45 microm to 1 kDa. In addition, we have applied agricultural grade diuron to a typical Chalk soil and created a soil water suspension which was also subsequently fractionated using the same filtration system. The deep groundwater sample showed no evidence of association between colloidal material and pesticide concentration. In comparison, despite some evidence of particle trapping or sorption to the filters, the soil water clearly showed an association between the <0.45 microm and <0.1 microm colloidal fractions which displayed significantly higher pesticide concentrations than the unfiltered sample. Degradation products were also observed and found to behave in a similar manner to the parent compound. Although relatively large colloids can be generated in the Chalk soil zone, it appears transport to depth in a colloidal-bound form does not occur. Comparison with other field and monitoring studies suggests that rapid by-pass flow is unlikely to occur beneath 4-5 m. Therefore, shallow groundwaters are most at risk from rapid transport of high concentrations of pesticide-colloidal complexes. The presence of a deep unsaturated zone will mean that most of the colloidal-complexes will be filtered by the narrow Chalk pores and the majority of pesticide transport will occur in a 'dissolved' form through the more gradual 'piston-flow' route.
地下水中的农用化学品污染对水质构成了重大的长期威胁，受到立法者、供水企业和消费者的共同关注。在双重孔隙度、双重渗透率的含水层（如白垩含水层）中，农药及其代谢物从不饱和带进入地下水的运动通常被认为是通过两种途径之一：快速的旁通流和通过岩石基质的较慢的“活塞流”路径。然而，人们对农药在水体中移动时的溶解形式或“胶体形态”了解甚少。暴雨过后，在浅层白垩含水层中观察到农药浓度出现极高的峰值。这些浓度可能可以通过农药的胶体运移得到很好的解释。我们在一个已知被除草剂敌草隆污染的深度为30米的不饱和带下采集了一份白垩地下水样本。我们使用切向流过滤技术制备了粒径范围从0.45微米到1千道尔顿的胶体级分。此外，我们将农用级敌草隆施用于典型的白垩土壤，并制备了一种土壤水悬浮液，随后也使用相同的过滤系统对其进行分级。深层地下水样本没有显示出胶体物质与农药浓度之间存在关联的迹象。相比之下，尽管有一些颗粒被截留或吸附在过滤器上的迹象，但土壤水明显显示出粒径小于0.45微米和小于0.1微米的胶体级分之间存在关联，这些级分中的农药浓度明显高于未过滤的样本。还观察到了降解产物，发现其行为与母体化合物相似。虽然在白垩土壤层中可以产生相对较大的胶体，但似乎不会以胶体结合的形式向深部运移。与其他实地和监测研究的比较表明，在4 - 5米以下不太可能发生快速的旁通流。因此，浅层地下水最容易受到高浓度农药 - 胶体复合物快速运移的影响。存在较深的不饱和带意味着大多数胶体复合物将被狭窄的白垩孔隙过滤，并且大多数农药将通过更缓慢的“活塞流”路径以“溶解”形式运移。
