O papel dos drenos interceptores na contenção do espalhamento dos contaminantes nos aquíferos rasos.

Abstract

The interceptor drains are useful in dilimiting the polluted water and these may be of fully or partially penetranting type. Such interceptor drains are easy for construction and installation, and demand minimum maintenance. However, their effective use depends, in most of the cases, on a deeper knowledge about their geological situation and existing hidrologic conditions. The interceptor drains considered here, which are in fact treated as open drains, which are commonly used in the interior in agricultural fields are supposed to drain away rapidly the upper part of the aquifer, to facilitate effect drawdown of the freatic surface. The purpose of this study is to optimize the depths and spacings of such interceptor drains, by filling in the same with proper material, packed within the drain, for effectively removing the contaminated water of the aquifer. The mode of propagation of subsurface water in the viscinity of the interceptor drains was simulated in terms of potencial functions, for use in agricultural lands, with a view to control the spread of agricultural chemicals in shallow aquifers. The concept of subsurface water divide is used here-in for quantifying the efficiency of the interceptor drains in delimiting the spread of contaminants that are potentially dangerous. The results of such simulations suggest that interceptor drains might create strong effective hidraulic barrier that serves in the prevention of movement of agricultural chemicals entering subterranean waters of the aquifers. For such a barrier to be effective and not to depend on seasonal hydrological changes that occur in the locality, inclusive of dependence on regional gradients of the freatic line and on the recharge that occurs in the said locality, functional relationships were established between the effectiveness of the interceptor drain and the existing hydraulic conditions. It was established in this study that the interceptor drains may as well be used to capture or even wash off the contaminants in shallow aquifers and thus containing the spread of agricultural chemicals. The subteranean contamination that is becoming the biggest problem of the day in devoloped, as well as in developing countries, may be effectively controlled and even eliminated by use of such interceptor drains, which act as means for rapid removal of water polluted by agricultural chemicals, as well as by herbicides and pesticides. Although these contaminants are principally detected in shallow parts of aquifers, such agricultural chemicals might get transported to deeper strata of aquifers. The degradation of subsurface water quality may thus be minimized and even prevented by way of effective control of such chemical agents in subterranean waters. These drains extended to desired depths might prove to be an effective solution to the pollution problem, by way of capturing major part of the polluted flow through flow tubes that are captured by interceptor drains. Various simulations that were made to enable study the effect of recharge on the efficacy of interceptor drains showed that with more of concentrated recharge, the number of flow lines and flow tubes that enter the interceptor drain increase, which shows that a greater recharge into the drain facilitatesd faster removal of polluted water from the aquifer. The ground water divides that are witnessed in these cases of recharge show that both increase in gradient of the terrain, as also induced recharge, cause corresonding increases in the flow tubes that enter such drains, thus showing the efficacy of such drains. Recharge in general causes a point of inflexion on the down-stream side of the drain, while the major potential of 101.4 m lies on the upstream side of the drain , the point of inflextion being more predominant for greater inclinations of the freatic line and for greater rates of recharge. The present study helps in introducing the concept of quantifying the capacity of interceptor drains that are partially penetrating in terms of effective removal or reduction of poluents in subsurface waters, in a system that is steady state and stationary. The results so obtained were presented in the form of figures and tables, with useful anexes that furnish the needed theory behind the interceptor drains, that are auto-explicative.