Development of a plant-based screening method for assessment of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) bioavailability, bioconcentration, and phytotoxicity from contaminated soils

Millions of acres of land on military installations, manufacturing, storage, and disposal sites, are contaminated with the high explosive, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The limited amounts of published information on terrestrial plant responses to RDX contributes to the high level of uncertainty and costs associated with environmental risk assessments on RDX. The objective of this research was to develop a shortterm (<12 days) screening method to assess RDX bioavailability and phytotoxicity, as well as estimate plant responses to long term (2, 4, and 6 weeks) RDX exposure. Short-term (<12 days) screening experiments were developed to evaluate the growth and developmental responses of fifteen terrestrial plants to RDX amended (0–4000 ppm) Grenada, Memphis, and Bowdre soils. Sunflower plants were identified as the most RDX and selected for use during the long-term (2, 4, and 6 weeks) experiments to assess the effects of life stage and exposure duration on growth, development, and RDX bioconcentration in above ground tissues. RDX amended (0–100 ppm) Grenada soil was used during long-term experiments. Random but statistically significant differences were measured in growth responses following the short and long term experiments, however there were no consistent patterns. The most consistent indicators of detrimental impacts following both short and long term experiments were the observed adverse developmental effects, regardless of soil type, life stage, or exposure duration, including: underdeveloped roots, bifurcated or fused leaves, atypical bilateral symmetry, atypical pigmentation, atypical seedling emergence, curled or irregular leaf margins, delayed leaf expansion, and necrotic or yellow spots. The highest coefficients of determination for total bioconcentrated RDX in sunflower leaves were calculated using data for the embryo life stage, six weeks exposure (R2 = 0.80) and the two-week old seedling life stage, four weeks exposure (R2 = 0.73). The traditional approach which uses adverse impacts on growth responses to assess the impacts of anthropogenic compounds on plants was not adequate for assessing the potential for adverse impacts from RDX exposure. The toxicological approach evaluating a spectrum of plant responses to RDX exposure was effective. The efficacy of the short-term (<12 days) screening method was validated during the long-term experiments.