Recent investigations have suggested that parameters such as sorption and biodegradation exhibit considerable spatial variability both horizontally and vertically across the landscape. Estimates of spatial variability is essential for developing predictive chemical fate models. With this goal in mind, the research in this study was initiated to evaluate the variability of sorption and biodegradation of atrazine and alachlor in surface, vadose zone and aquifer sediments underlying the Ohio MSEA site.
Contamination of surface and ground water by non-point sources of pollution is an issue of both regional and national concern. Off-site migration of herbicides is of particular concern in the mid-west due to their extensive use in the region. To effectively evaluate the risk of herbicides on our water resources it is necessary to have a thorough understanding of the parameters which affect the environmental fate of these chemicals. Perhaps the two most critical processes affecting the mobility and persistence of these chemicals are sorption and biodegradation. These processes have received considerable attention; however, their interaction and spatial variability has not been adequately addressed in soil and sediment studies. This information is necessary to accurately predict the persistence and transport of herbicides in natural environments.
Determination of the Spatial Variability of Atrazine and Alachlor Sorption. Ten surface soil and eighty five subsurface sediment samples ranging in depth from 0-21 m were collected from the MSEA site in Piketon, Ohio. Atrazine and alachlor sorption was measured in 95 and 64 of the samples respectively with the batch equilibration technique over an initial concentration range of 0-5 mg L-1. The sorption of atrazine and alachlor by soil and sediment samples was typically non-linear. Sorption of atrazine and alachlor could be described by Freundlich isotherms with 1/n values ranging from 1 to 0.47 and 1 to 0.62 respectively. Though highly variable, sorption of atrazine decreased with depth from 0-1 m, but increased with increasing depth in the sediment samples (Fig. 1 and 2). Though the extent of alachlor adsorption was not linear with increasing depth, the data were clustered within subsurface strata. Comparison of alachlor sorption with atrazine sorption on the same materials revealed that the alachlor isotherms exhibited less curvature; however, the extent of sorption of both compounds on the same material was generally in agreement within a factor of two. These results indicate that the sorption of pesticides varies significantly both vertically and horizontally over a given landscape even within a given soil type. Estimates of sorption variability are essential for predicting the fate and transport of pesticides in the environment. This information will make it possible to improve existing transport models that do not currently include estimates of sorption variability.
Biological degradation of Atrazine and Alachlor. An inital survey of the biodegradation potential of atrazine and alachlor by the native microbial populations in soils and sediments at the Ohio MSEA site was conducted by measuring the disappearance of parent substrates in enrichment cultures inoculated with materials collected during the installation of the ground water monitoring system. The results indicated that the degradation of both herbicides was extremely slow in vadose zone and subsurface environments leading to extended half-lives in these environments. The limitations to biodegradation in the subsurface appeared to be due to the lack of active atrazine and alachlor degraders in these samples. In the surface soils, however, moderate degradative activity, consistent with other reported degradation rates for these two compounds, was observed. A later study designed to measure the variability in 14C-atrazine mineralization in the soils collected from the conventional management plot, the reduced tillage, rotational treatment plot, and a non-agricultural reference soil was conducted. Considerable variation in atrazine mineralization was observed both within and across treatments. Atrazine mineralization rates were significantly higher in soils from the conventional till continuous corn management plot relative to the other two sites. These results suggest that management practices have a significant influence on pesticide persistence in agricultural soils. Furthermore, variations in mineralization rates within treatments strongly suggests the need for estimates of spatial variation in biodegradation rates for the development of predictive models involving the fate of pesticides.
Sorption-limited biodegradation in subsurface sediments was investigated at 25 and 10 degrees C in 14C-atrazine amended sediment microcosms inoculated with an atrazine-degrading bacterial isolate capable of atrazine mineralization via ring cleavage. Initial equilibrium solution phase atrazine concentrations were calculated from previously measured atrazine sorption coefficients measured in separate experiments. First-order mineralization rate constants were positively correlated to initial equilibrium atrazine concentrations at 25 degrees C but not at 10 degrees C. The results of a representative experiment are presented in figure 3.
Our investigation of pesticide sorption and biodegradation involves laboratory bench scale measurements of relevant parameters such as soil sorption coefficients and pesticide mineralization rates in the soils and subsurface sediments collected at the Ohio MSEA. Biological degradation rates of atrazine are determined through the collection of 14CO2 evolved from radiolabeled atrazine amended soil and sediment microcosms. The interaction between sorption and biodegradation has been studied with an atrazine-degrading microbial isolate which we have used as a probe to investigate the bioavailability of sorbed 14C-atrazine. Estimates of the spatial variability in these parameters should prove useful in the development of predictive pesticide fate and transport models for agricultural systems.
The results of this study have shown that considerable variability exists in both the sorption and biodegradation of atrazine and alachlor in the soils and sediments at the MSEA site. In addition, agricultural management practices appear to have a significant influence on atrazine biodegradation. Biodegradation rates in the soils under continuous corn production showed higher rates and extents of atrazine mineralization when compared to soils collected from the low input, reduced tillage, rotational management systems. These results may be due to the maintenance of active atrazine-degrading microbial populations sustained by regular atrazine treatments. Based on increased mineralization rates, atrazine leaching to vadose zone and aquifer sediments should be reduced under conventional management practices. Finally, atrazine and alachlor reaching the subsurface zones beneath the MSEA are predicted to persist for extended periods based on negligible degradation rates in enrichment experiments. The primary limitation to herbicide degradation in these environments appears to be the lack of an active herbicide degrading microbial population. However, atrazine sorption can limit biodegradtion in subsurface environments when a large active atrazine-degrading population is present.