Optimizing Turbidity Reduction in Construction Site Runoff.

A new EPA requirement for large construction sites is for the runoff to meet a specific turbidity limit.  One method to achieve this limit (280 nephelometric turbidity unit, NTU) is to apply a flocculant to water conveyances prior to the typical sediment basin.  We conducted tests of a variety of check dams for their ability to reduce sediment and turbidity in simulated runoff, both with and without added polyacrylamide (PAM).  The tested materials were rock, rock wrapped in either a excelsior or coir blanket, excelsior wattles, or coir logs installed in a series of three locations in the test ditch.  Samples were taken every two minutes after each check dam to determine changes in water quality.  The coir log, which was the least permeable of the dams, was the most effective in retaining sediment, while the rock was the least.  When PAM was applied to the check dams, turbidity was significantly reduced for all materials used, but the lowest turbidities were  achieved for the excelsior wattle and the coir log.  Further settling  post-treatment brought the turbidities to <20 NTU when PAM was used.  While there were differences in turbidity reduction for PAM on the five check dams, all produced turbidity levels which would be well within EPA guidelines.