Studies Indicate Turbidity-Reducing Polymer Product Safe for Aquatic Life
Employing polymers is a method that construction contractors’ environmental managers can use to comply with a new EPA final rule (http://www.epa.gov/waterscience/guide/construction) for construction sites of 10 acres or less, “Effluent Limitations Guidelines and Standards for the Construction and Development Point Source Category.” Recent research on one polymer product should allay fears that contractors might have about toxicity to aquatic life.
The new rule establishes a national technology-based turbidity limit of 280 nephelometric turbidity units (NTUs)—a measurement of light scattering as a beam of light passes through a water sample—in effluent running off from a construction site. Polyacrylamide (PAM) is one type of polymer that fits this application and the type used in EarthBound polymers developed by Earth Chem, Fort Collins, Colo. PAM is a chemical consisting of many subunits of acrylamide molecules. Polyacrylamide is a simple organic compound that replicates monomers to form long-chain polymer molecules. By increasing the cohesion of soil particles, PAM reduces the detachment and transport of sediments in runoff. PAM also flocculates particles in runoff, forming clumps that drop to the bottom. These clumps can slow the rate of runoff, reducing erosion capacity.
Two forms of the EarthBound product are available for use in construction site stormwater management: gel logs and tablets. Both forms work in the same way, suspending in water flow so that they can react with sediment and flocculating fine particles. Both forms are designed to dissolve at a targeted rate; the manufacturer adjusts the chemistry according to the soil type. They are also designed for simple use and eliminate the need for the contractor to precisely meter the polymer using a pump and other equipment.
Both EarthBound gel logs and tablets are designed to be placed at the upper end of a ditch system or near active earthmoving operations. The contractor hangs a gel log—which is packaged in a biodegradable net—from a stake set upslope, in the center of the ditch system. However, the gel logs are slow-dissolving and better suit more continuous runoff situations. The tablets, in contrast, dissolve much more quickly and better suit a flash storm event.
In February and March 2011, Earth Chem had Smithers Viscient, Wareham, Mass., test both EarthBound gel logs and tablets for toxicity to Daphnids and Rainbow Trout.
First, the lab tested both the gel logs and tablets on Daphnids per the Organisation for Economic Co-operation and Development’s (OECD) Guideline for Testing of Chemicals Daphnia sp., Acute Immobilization Test Guideline No. 202. The tests used fortified well water with total hardness by calcium carbonate of 180 milligrams per liter (mg/L), total alkalinity by calcium carbonate of 92 mg/L for the gel logs and 94 mg/L for the tablets, pH of 8.3 for the gel logs and 8.1 for the tablets, and specific conductance—a measurement of the amount of ions in water and thus the water’s ability to conduct electricity—of 740 micromhos per centimeter for the gel logs and 710 micromhos per centimeter for the tablets.
The lab prepared a 100-mg whole product/L primary stock solution by placing 0.2005 gram of the test substance from the gel log and 0.1998 gram from the tablets in a 2-liter beaker and bringing it to volume with freshwater that was reconstituted for hardness. Exposure solutions (0.40-, 1.0-, 2.6-, 6.4-, 16-, 40- and 100-mg whole product/L) were prepared by adding the appropriate volume of the primary stock solution to 1 liter of dilution water and mixing with a glass rod for one minute. Five Daphnids were present in 200 mL of solution volume within each of four 250-mL test vessels, meaning a total of 20 Daphnids were exposed. Testing occurred for 48 hours, with a photoperiod of 16 hours light at an intensity of 62 to 81 footcandles for the gel logs and 69 to 87 footcandles for the tablets, and eight hours of darkness. The test temperature was 19–21 degrees Celsius (66.2–69.8 degrees Fahrenheit).
According to the test criteria, for the EarthBound product to be deemed nontoxic to Daphnids, no more than 10 percent of the Daphnids at test termination would be immobile or trapped on the surface of the water. The results indicated that the 48-hour median effective concentration for immobilization (EC50) of at least 50 percent was estimated to be greater than 100 mg whole product/L, the highest concentration tested.
Smithers Viscient also tested the EarthBound gel log and tablet products for toxicity to Rainbow Trout per the OECD Guideline No. 203, Guidelines for Testing of Chemicals, Fish Acute Toxicity Test. The tests used laboratory well water with total hardness by calcium carbonate of 68 milligrams per liter (mg/L), total alkalinity by calcium carbonate of 22 mg/L, pH of 7.0, and specific conductance—a measurement of the amount of ions in water and thus the water’s ability to conduct electricity—of 310 micromhos per centimeter.
The lab prepared a 100-mg whole product/L primary stock solution by placing 3.0052 gram from the gel log and 3.0013 gram from the tablets in 30 liters of dilution water. Exposure solutions (0.40-, 1.0-, 2.6-, 6.4-, 16-, 40- and 100-mg whole product/L) were prepared by adding the appropriate volume of the primary stock solution to 15 liters of dilution water and mixing with a glass rod for one minute in a 39-by-20-by-25-cm aquarium. Ten Rainbow Trout were present in the aquarium. Testing occurred for 96 hours, with a photoperiod of 16 hours light at an intensity of 81 to 86 footcandles for the gel logs and 87 to 98 footcandles for the tablets, and eight hours of darkness. The test temperature was 13–15 degrees Celsius (55.4–59 degrees Fahrenheit) for the gel logs and 13–14 degrees Celsius (55.4–57.2 degrees Fahrenheit) for the tablets.
Under this test, for the EarthBound product to be deemed nontoxic to Rainbow Trout, the exposure solutions could not be lethal to more than 50 percent of the species. The results indicated that the 96-hour 50 percent median lethal concentration (LC50) was estimated to be greater than 100 mg whole product/L, the highest concentration tested.
The test results indicate that the product is not toxic to these species and thus aquatic life in general. The issue of precise dosing also has been addressed, setting the stage for contractors’ more widespread use of polymers for the purpose of complying with EPA’s new standard.