Ditches lining agricultural fields are used to convey runoff water and are typically mowed to reduce vegetative growth. If agricultural ditches are viewed as a wetland, ditch vegetation might be utilized to remove excess nutrients in runoff water. It was hypothesized that particular species of ditch vegetation would be more effective at removing nutrients in runoff water. Replicate 379 L Rubbermaid ® troughs (132 cm ×70 cm × 66 cm) were planted with individual species of soft rush (Juncus effusus), yellow primrose (Ludwigia peploides) and cutgrass (Leersia oryzoides), common wetland macrophytes found in Mississippi agricultural drainage ditches. Nutrient-enriched water (target concentrations = 5 mg L! 1 nitrate, 5 mg L! 1 orthophosphate, and 5 mg L! 1 ammonia) was pumped in at a 4 h hydraulic retention time at one end of the tub and discharged at the far end. Water samples were collected from discharge at 1 h intervals for 9 h and analysed for nutrient concentrations. Nutrient removal rates were compared for all plant treatments and unplanted sediment-water controls. Results indicated that no single species was most effective at removing both nitrogen species and orthophosphate, although all three plant treatments lowered nutrient concentrations in water relative to unplanted controls. Ammonia and nitrate concentrations in water were most decreased in Ludwigia peploides tubs (83±3% and 40±8%, respectively) and total orthophosphate was decreased most by Leersia oryzoides (29±7%). Leersia oryzoides was least effective at removing nitrate and ammonia concentrations. By determining specific plant retention of various nutrients, improved planning can be accomplished for best management practices and remediation techniques such as constructed wetlands or vegetated agricultural drainage ditches.

The myths and misconceptions about silver bioavailability and toxicity in aquatic environments do not outnumber the facts but perhaps persist and receive more recognition and credence because the fiction is more interesting. The myths range from those that encompass all "toxic heavy metals" to those that are specific for silver. Some of the myths are embodied in our current regulatory framework while others are apparently slated for use in future decision making. By presenting some of these issues as myths, perhaps we can focus some attention on areas where there are currently critical uncertainties regarding silver, its toxicity and bioavailability in aquatic systems.

Sediments that represented a wide range of characteristics were amended with silver compounds to observe partitioning and bioavailability. In laboratory studies, silver partitioning to particulates, sediment pore water, and overlying water was measured and bioavailability of silver was determined using Hyalella azteca in 10-day sediment toxicity tests. Three silver compounds were used as sources of silver for this study: silver nitrate, silver chloride, and silver thiosulfate complex. Sediment amendment procedures were adjusted as necessary depending on the characteristics of the individual compounds. Several sediment characteristics such as organic carbon, pH, redox, and acid volatile sulfides regulated silver partitioning and bioavailability. Bioavailability of silver was correlated with the overlying water concentration of silver, Ten-day LC50 values ranged from 1.62 to 379.7 mg Ag/kg for H. azteca exposed to sediments amended with AgNO3. In laboratory experiments, silver chloride and silver thiosulfate mere orders of magnitude less toxic and bioavailable than silver nitrate, with 10-day LC50 values greater than the highest concentrations of AgCl and silver thiosulfate complex amended to sediments (2560 and 1125 mg Ag/kg, respectively). (C) 1997 Academic Press.

The effects of exposure duration, test organism, and test endpoint on the toxicity of cadmium to a variety of freshwater species were evaluated. Toxicity of cadmium was assessed by monitoring the survival and reproduction of Ceriodaphnia dubia Richard; the survival of Daphnia magna Straus; and the survival and growth of Hyalella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promelas Rafinesque. Organisms were exposed in static systems for 48 h, 96 h, 7 d, 10 d, and 14 d to determine acute and chronic toxicity. Relative sensitivities of test organisms exposed to aqueous cadmium varied with test duration and test endpoint. In general, H. azteca was the most sensitive organism tested, followed in decreasing sensitivity by P. promelas, C. dubia, D. magna, and C. tentans. Mortality of C. dubia and D. magna was consistent up to 7 d, after which little additional mortality occurred. Effects of test duration on cadmium toxicity were most pronounced for H. azteca and C. tentans, with mortality and growth becoming increasingly sensitive with increasing test duration.

Since speciation can affect bioavailability and toxicity of copper in aquatic systems, accurate predictions of effects of bioavailable forms require detection and/or measurement of these forms. To develop an approach for measurement of bioavailable copper, a copper sulfate solution (CuSO4 . 3Cu(OH)(2) . H2O) was used in 10-d aqueous and sediment toxicity tests with Hyalella azteca Saussure. These tests encompassed ranges of pH (6.5 to 8.1), alkalinity (10 to 70 mg/L as CaCO3), hardness (10 to 70 mg/L as CaCO3), and conductivity (30 to 300 mu mbo/cm). Changes in copper speciation were measured using atomic absorption spectroscopy (AA) for dissolved copper and differential pulse anodic stripping voltammetry (DPASV) for labile copper, and concentrations were evaluated relative to amphipod survival. Ten-day LC50s based on AA-measured aqueous copper concentrations ranged from 42 to 142 mu g Cu/L, and LC50s based on DPASV-measured copper concentrations ranged from 17.4 to 24.8 mu g Cu/L. In 10-d tests using copper-amended sediments with diverse characteristics and AA-measured copper concentrations spanning an order of magnitude, total copper concentrations were not predictive of sediment toxicity, but H. azteca survival was explained by DPASV measurements that varied by less than or equal to 4%. In order to make defensible estimates of the potential risk of metals in sediments or water, it is essential to identify the fraction of total metal that is bioavailable. In these experiments, DPASV was useful for measuring bioavailable copper in aqueous and sediment tests with H. azteca.

Because of recent concerns regarding the ability of acute (48–96 h) sediment toxicity tests to accurately assess the potency of sediment-bound contaminants, effects of exposure duration, test organism selection, and test endpoint on the observed toxicity of aqueous phase copper and a copper-contaminated freshwater sediment were evaluated. Toxicity of sediment-bound copper was assessed by monitoring survival and reproduction of Ceriodaphnia dubia Richard, survival of Daphnia magna Straus and Hyalella azteca Saussure, and survival and growth of Chironomus tentans Fabricius and Pimephales promelas Rafinesque. Organisms were exposed in static systems for 48 h, 96 h, 7 d, 10 d, and 14 d, which enabled measurement of acute and chronic toxicity. Relative sensitivities of test organisms exposed to copper in water and copper-contaminated sediment varied with test duration and test endpoint. In general, C. dubia was the most sensitive organism tested, followed in decreasing sensitivity by D. magna, P. promelas, H. azteca, and C. tentans. A temporal mortality threshold was demonstrated by C. dubia and D. magna when exposed to copper, with little mortality occurring after 96 h of exposure. Effects of test duration on copper toxicity were most pronounced for H. azteca and C. tentans, with mortality and growth effects becoming increasingly sensitive with increasing test duration. Formulated sediment served as a suitable control and reference sediment in this study, and matched a variety of field-collected test sediment characteristics. In tests utilizing copper-contaminated sediments, observed responses (mortality, growth, reproduction) corresponded with overlying water concentration of copper rather than concentration of copper in bulk sediment or pore water.

To accurately determine the relative potency of contaminated sediments that exhibit toxicity in screening tests, definitive toxicity tests are conducted by diluting test sediment with a nontoxic reference sediment of similar characteristics. To demonstrate the usefulness of formulated sediment as a reference and dilution sediment, the relative potency of a copper-contaminated sediment was assessed by diluting with a formulated sediment. Toxicity of sediment-bound copper was assessed by monitoring the survival of Ceriodaphnia dubia Richard, Daphnia magna Straus, Hyalella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promelas Rafinesque. Organisms were exposed in static systems for 10 d using test sediment dilutions that produced mortality in range-finding tests. Formulated sediment served as a suitable control sediment, with organism survival ranging from 85–100%. Formulated sediment was prepared and matched ten of eleven characteristics of the field-collected test sediment. When used to dilute the copper-contaminated sediment, formulated sediment resulted in decreased concentrations of copper in sediment, pore water, and overlying water with concomitant increases in test dilution. Thus, formulated sediment served as a suitable reference and dilution sediment in this study. Evidence indicates that formulated sediments could be used in this capacity when suitable natural reference sediments are otherwise unavailable.