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Colloid migration in subsurface environments has attracted special attention lately because of its suspected role in facilitating transport of contaminants to groundwater. This study investigated the potential role of water‐dispersible soil colloids (WDCs) with variable composition in transporting Cu and Zn through undisturbed soil columns. Copper or Zn solutions without colloids (controls) and combined with suspensions of montmorillonitic, mixed, illitic, and kaolinitic colloids with a range of surface properties were applied at a constant flux into undisturbed soil columns. The soil columns represented upper solum horizons of Maury (fine, mixed, mesic Typic Paleudalf) and Loradale (fine‐silty, mixed, mesic Typic Argiudoll) soils with contrasting porosities and organic C (OC) contents. Colloid and metal recoveries in the eluent varied with metal, colloid, and soil properties. The presence of colloids typically enhanced metal transport by 5‐ to 50‐fold over that of the control treatments, with Zn being consistently more mobile than Cu. The greatest metal transport potential was shown by colloids with high negative surface charge and OC content and the lowest by colloids with large particle size, low negative surface charge, and high Fe‐ and Al‐hydroxyoxide contents. Although the dominant transport mechanism was metal sorption by colloids and cotransport, the additional soluble Cu and Zn transported in the presence of colloids suggests involvement of physical exclusion, competitive sorption, or increased metal solubilization processes. Increased amounts of OC content in the soil column appeared to overshadow the effects of macroporosity on the transport of both metals, especially Cu. These findings have important ramifications on the use of contaminant transport prediction models and the application of efficient remediation technologies.

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Conservation tillage practices have been shown to affect a number of soil physical properties, but less is known about their effect on the size, shape, and arrangement of soil constituents. To evaluate soil structural differences between tillage treatments, six paired conventionally tilled and no‐tilled plots of Maury silt loam soil (fine, mixed, mesic Typic Paleudalf) were analyzed. Oriented undisturbed samples (16 by 9.5 by 5.5 cm) were taken from each plot at depths of 0 to 5, 10 to 15, and 20 to 25 cm. The samples were impregnated with polyester resin and two vertical thin sections were prepared from each sample. The micromorphic features were described, and the size (equivalent spherical diameter), shape, and orientation of pores and aggregates were measured using an interactive image analysis system. The no‐tilled plots had platy structure near the surface and interconnection of fine pores (50–100 µm) throughout the profile. Eartbworm channels with excrement infillings were abundant in the no‐till plots at all depths, but absent in conventionally tilled plots. The conventionally‐tilled soil was composed of granular and fragmented structural units with no evidence of earthworm activity. The no‐till plots contained about 11% pore area >50 µm, compared with about 16% for conventional tillage management. However, the no‐till plots have an average pore size (0.18–0.26‐mm diam.) that is greater than the conventionally tilled plots (0.12–0.17‐mm diam.). The finer aggregate size of the conventionally tilled plots (0.25–0.39‐mm diam.) compared with the no‐till plots (0.41–1.08‐mm diam.) is probably due to plowing and disking.

Colloid migration in subsurface environments has attracted special attention lately due to its suspected role in facilitating transport of contaminants to groundwater. This study was conducted to evaluate the stability and potential transport of water‐dispersible colloids (WDC) through intact soil columns, and the properties of colloids and soil columns facilitating or retarding colloid stability and transportability. Water‐dispersible colloids were fractionated from six representative soil samples with diverse mineralogy and physicochemical characteristics. Their stability was evaluated from settling‐rate experiments at different pH levels. The results demonstrated that colloid stability was pH dependent. Colloid transportability was assessed by introducing colloid suspensions at a constant flux into intact soil columns representing upper Bt horizons of a Maury (fine, mixed, mesic Typic Paleudalf) and a Loradale (fine‐silty, mixed, mesic Typic Argiudoll) soil and evaluating characteristics of the suspensions that were eluted. After five pore volumes of leaching, colloid recovery in the eluents ranged from 35 to 90% depending on type of colloid, initial concentration in the influent, and soil column. The mineralogical composition of the colloid, which was correlated with particle size, appeared to have a profound effect on colloid transportability, following the sequence smectitic > mixed > kaolinitic. Total exchangeable bases (TEB) and pH of WDC also significantly influenced colloid transport. Increasing colloid concentration in the influent slightly increased colloid transportability. Soil columns with better macroporosity and less surface charge (Maury) transported more colloids than soil columns with less macroporosity and higher surface charge (Loradale).

Chewing coca in South America began by at least 8000 cal BP: our authors found and identified coca leaves of that date in house floors in the Nanchoc Valley, Peru. There were also pieces of calcite — which is used by chewers to bring out the alkaloids from the leaves. Excavation and chemical analysis at a group of neighbouring sites suggests that specialists were beginning to extract and supply lime or calcite, and by association coca, as a community activity at about the same time as systematic farming was taking off in the region.