The effect of differential oxygen inhibition in the exterior of aggregates is however secondary to the effect of diffusive mass transfer limitation in creating the observed gradients in reduced selenium. This is evidenced by the similarity in reduced selenium gradients observed under anoxic and oxic conditions. Another effect of oxygen is that by enabling aerobic respiration it increases the limitations imposed by the electron donor . Oxygen can thus indirectly inhibit selenium reduction at the aggregate core by limiting electron donor concentrations if the supply from the surrounding solution is small. In contrast to selenite concentrations, those of pyruvate decrease with increasing distance from the advective boundary. This explains why higher concentrations of pyruvate in the input solution lead to a much higher reduced selenium concentrations at the core of aggregates under oxic conditions, but not under anoxic conditions. The concentrations of solid phase elemental selenium inside aggregates mirror those of selenite because selenite is the primary limiting factor in the production of elemental selenium . The half saturation constant for selenite in selenite reduction by E. cloacae is 0.7 mM , thus at selenite concentrations representative of our experiments , selenite reduction rates depend almost linearly on selenite concentrations . Consequently, the rates of selenite reduction are very heterogeneous inside aggregates with faster rates at the core where selenite accumulates. Once produced,flower buckets wholesale elemental selenium will not move, and is thus not affected by transport, as selenite is. This also explains why elemental selenium concentrations in aggregates respond more strongly than selenite concentrations to conditions that promote the production of both forms of reduced selenium.

Increasing selenite concentrations inside the aggregate will also lead to an increase of diffusive flux out of the aggregate. This negative feedback equilibrates concentrations of selenite in aggregates, which is why selenite distributions reached a steady-state in all simulations. This is not the case for elemental selenium, which is not affected by diffusive transport and can thus grow boundlessly while selenite reduction occurs. The rates of elemental selenium production reach a selenite concentration dependent steady-state, but the concentrations of elemental selenium keep increasing for as long as chemical input and microbial activity remain stable.The same process that leads to the accumulation of reduced selenium at the core of aggregates causes the retention of reduced selenium to scale with aggregate size. Specifically, selenite accumulates inside aggregates by virtue of diffusive mass transfer limitations that are more pronounced in larger aggregates. Smaller aggregates have a shorter average diffusion path length to the surrounding advective boundary than larger aggregates. Consequently, selenite export from smaller aggregates to the surrounding solution is less limited than in the case of larger aggregates, and selenite will accumulate to a lesser extend at their cores. The impact of aggregate size is enhanced in the presence of oxygen, since oxygen inhibition of selenium reduction will be stronger in smaller aggregates . It is for this reason that under bulk oxic conditions the concentrations of reduced solid phase selenium scaled more strongly with aggregate size than under anoxic conditions. Additionally, under oxic conditions, the interaction between electron donor concentrations and aggregate size becomes more important. The electron donor is needed both for aerobic respiration, which reduces oxygen concentrations in aggregates, and for selenium reduction. Thus, rates of electron donor consumption are faster under oxic conditions and the electron donor concentrations inside the aggregate are lower than under anoxic conditions. At the core of aggregates, the electron donor can become limiting for selenium reduction, but less so in aggregates of smaller size where the average diffusion path to the advective boundary is shorter than for larger aggregates.

As a result, there is an interactive effect of electron donor concentrations and aggregate size under conditions that are oxic in bulk: The concentrations of reduced selenium retained in the solid phase will scale more strongly with aggregate size if the supply of electron donor is larger. In summary, all other things equal, the amount of reduced selenium retained by a soil is expected to scale with increased aggregate size or electron donor concentrations, and with decreased aeration. Additionally, there are important interactive effects between these variables since, in the presence of oxygen, organic carbon concentrations increase the impact of aggregate size on reduced selenium concentrations. With regards to selenium retention, the impact of aggregate size on elemental selenium concentrations is particularly relevant. Elemental selenium is insoluble and environmentally observed rates of oxidation are 3-4 orders of magnitude below those of reduction . Thus soil elemental selenium is a good pool for long-term selenium retention and the transformation of selenium oxyanions to solid elemental selenium has been suggested as a remedial strategy for contaminated sites . Average concentrations of both solid phase selenite and elemental selenium scale with aggregate size, but the effect is particularly pronounced for the elemental selenium fraction. Furthermore, in contrast to selenite, elemental selenium concentrations are not expected to reach a steady-state according to our reactive transport model. It can therefore be expected that, as long as conditions are favorable to selenium reduction persist, the impact of aggregate size on elemental selenium content will keep increasing over time as the concentrations of elemental selenium build up. Increased soil aggregation may thus lead to a significant increase in long-term selenium retention. Further studies are needed to establish whether the impact of aggregate size on selenium retention described here is significant at the field scale. A single, artificial aggregate surrounded by saturated flow cannot capture the full complexity of a structured soil and relevant processes are likely to have been excluded. For example, selenite diffusing from aggregates may build up in macropores along the diffusion path, thereby decreasing the concentration gradient at the aggregate-macropore interface and reducing the impact of aggregate size on intra-aggregate selenite concentrations.

Furthermore, the trends here discussed were obtained with selenate supplied in excess, while diffusive limitations in selenate supply may reduce selenium reduction in larger aggregates under field conditions. On the other hand, the dynamic saturation conditions in a natural surface soil may serve to increase the impact of aggregate size on selenium reduction through steeper redox gradients, as the macropores surrounding aggregates are expected to be filled with air rather than water part of the time. The advantage of simplified models in studying complex natural systems is that they allow for the isolation of individual processes that may be inextricable in nature. The results here described, point to general reactive transport mechanisms that may lead a soil with a larger mean aggregate size to retain more selenium. This would have implications for the management of irrigated seleniferous soils,flower harvest buckets since the fraction of macro-aggregates in a soil is sensitive to agricultural management on time scales of 2 years or less . Notably, conservation tillage has been found to lead to significant increases in both mean soil aggregate size and organic matter content of surface soils . Similarly, the addition of manure can result in a rapid increase of mean aggregate size within the first two years of application . It is particularly interesting to note that organic carbon input can increase aggregate size, since the concomitant increase in electron donor availability and aggregate size are expected to synergistically enhance selenium retention according to our model.If there were to be additional external benefit to enhancing soil structure for seleniferous soils, in the form of reduced selenium exports, conservation tillage and organic amendments may prove to be an attractive management technique in regions prone to irrigation-induced selenium contamination. Viewed in this context, our results suggest that field studies investigating the effect of conservation tillage and organic matter applications on selenium retention are may be warranted in the search for management strategies to reduce selenium export. In conclusion, our reactive transport model suggests that enhanced soil aggregation may increase the fraction of elemental selenium in aggregates and consequently increase the degree to which selenium is retained. This effect could be useful in the prevention of irrigation-induced selenium contamination. Managing seleniferous agricultural soils in a way that optimizes aggregation may reduce the amount of exported selenium and thus alleviate the impact of selenium pollution for aquatic ecosystems downstream of such soils.For millennia, agriculture has had far-reaching impacts on human society and natural systems. However, the development of modern pesticides, alongside other technological advances of the green revolution, caused dramatic production increases but also concomitant increases in environmental and health concerns. Numerous studies have documented negative effects of pesticides on a wide range of organisms as well as ecosystem services such as water and air quality. However, documentation of direct negative effects of non-occupational pesticide exposure on human health has proven much more elusive, despite substantial public apprehension.

Reproductive harm tied to chemical exposure is of particular concern, because health at birth is correlated with both adult health and non-health attributes . Further, negative birth outcomes, such as low birth weight, preterm birth, and birth abnormalities, have been causally associated with other environmental conditions during pregnancy, including air pollution, extreme heat, and maternal residence in proximity to toxic release sites. Nevertheless, evidence linking residential agricultural pesticide exposure with adverse birth outcomes remains equivocal. The absence of conclusive evidence of the health impacts of agricultural pesticide use may be due in part to the logistical challenges of health research. Since controlled studies are clearly unethical, much of the available evidence relating pesticides to adverse health outcomes comes from occupationally exposed groups, such as certified pesticide applicators, which may not reflect exposures that are relevant for the broader agricultural community. Due to a lack of refined pesticide use data for most regions, studies of non-occupational pesticide exposure either use broad, correlative measures of pesticide use at large scales or seek detailed measures of individual exposure via blood/urine samples or interviews. While such refined measures provide a valuable metric of exposure at multiple snapshots during gestation, the costs and logistical challenges associated with such approaches often constrain sample sizes to between 100–2000 births making statistical analyses of rare outcomes difficult. These logistical challenges have resulted in creative attempts to tie proxies of pesticide exposure to adverse birth outcomes. For instance, at the broadest scale, Winchester et al. associated seasonal differences in pesticide concentrations in surface waters with national rates of birth defects. Similarly, Schreinemachers investigated birth defects as a function of county wheat acreage— which they used as a proxy for herbicide exposure, while Petit et al.investigated birth weight and fetal growth as a function of crop composition—which they interpret as a proxy for insecticides. These studies suggested significant negative effects of pesticides on birth defects and fetal growth. However, studies with more refined measures of agricultural pesticide exposure and/or birth outcomes have generally reported null or inconsistent effects of exposure on birth defects, low birth weight, and gestational length, in part due to the consistently low number of comparisons. Previous studies using blood serum, urine samples, or interviews have often focused on specific chemicals or classes of chemicals for feasibility. Yet, in most agricultural communities a great diversity of chemicals are applied daily, making it difficult to isolate the effects of any specific toxic agent due to other chemicals in the environment and the understudied synergistic or antagonistic interactions among them. Even if one could isolate exposure to a specific chemical group, pesticide metabolites in blood serum and urine still reflect exposure to different chemicals across a range of toxicities within a given chemical group. Further, half-lives range from a few hours to weeks for some chemicals of high concern, such as organophosphorous insecticides, making it difficult to accurately capture exposure to even one chemical group over gestation. The confounding effects of unobserved exposures or other unobserved factors may underlie some of the counter intuitive results in the literature. For example, in an innovative pooled cohort study evaluating pesticide exposure and birth outcomes in an agricultural community in the Salinas Valley, CA as well as urban populations in New York and Cincinnati, Harley et al.reported no significant effects of OP metabolite levels on birth weight overall or for the agricultural population in particular. Yet, the same study reported a negative effect of OP metabolite levels in urban minority populations, despite their lower average OP metabolite levels relative to the agricultural group.