Time should be considered in developmental ecotoxicity test

Developmental toxicity tests are often used for the hazard assessment of chemicals and environmental media. One of the most widely used is the oyster embryo larval test (OEL), in which the development of oyster larvae is arrested at a single fixed time (e.g. 24 or 48 h) of toxic exposure, and the proportion of normal larvae measured. However, a major problem with this conventional approach is the lack of information on temporal trends in development. In this study, Pacific oyster Crassostrea gigas embryos were exposed to nominal concentrations of copper (CuSO₄) of <0.001 (control), 0.60, 1.25, 2.5, 5.0, 10.0 and 20.0 μg l⁻¹ (at 20 °C, salinity 35‰ and pH 8.1). Three replicates from each group were arrested and examined every 8 h during 24–72 h of exposure, and the number of viable larvae developed to D-shape was determined. The results revealed that the number of viable D-shape larvae in the control increased rapidly and reached an optimum at 32 h, before declining gradually due to starvation. Analysis of covariance (ANCOVA) showed that larval developmental rates during 0–32 h were significantly inhibited by Cu at all concentrations. This paper demonstrates that arrest and measurement at different time periods are important and should be incorporated into the OEL test. This would maximise the sensitivity of the test in detecting developmental effects in spiked or environmental samples.     

Gas-partitioning tracer test to quantify trapped gas during recharge

Dissolved helium and bromide tracers were used to evaluate trapped gas during an infiltration pond experiment. Dissolved helium preferentially partitioned into trapped gas bubbles, or other pore air, because of its low solubility in water. This produced observed helium retardation factors of as much as 12 relative to bromide. Numerical simulations of helium breakthrough with both equilibrium and kinetically limited advection/dispersion/retardation did not match observed helium concentrations. However, better fits were obtained by including a decay term representing the diffusive loss of helium through interconnected, gas-filled pores. Calculations indicate that 7% to more than 26% of the porosity beneath the pond was filled with gas. Measurements of laboratory hydraulic properties indicate that a 10% decrease in saturation would reduce the hydraulic conductivity by at least one order of magnitude in the well-sorted sandstone, but less in the overlying soils. This is consistent with in situ measurements during the experiment, which show steeper hydraulic gradients in sandstone than in soil. Intrinsic permeability of the soil doubled during the first six months of the experiment, likely caused by a combination of dissolution and thermal contraction of trapped gas. Managers of artificial recharge basins may consider minimizing the amount of trapped gas by using wet, rather than dry, tilling to optimize infiltration rates, particularly in well-sorted porous media in which reintroduced trapped gas may cause substantial reductions in permeability. Trapped gas may also inhibit the amount of focused infiltration that occurs naturally during ephemeral flood events along washes and playas.