During construction, large areas of exposed soil are often vulnerable to erosion from stormwater runoff. Suspended sediment can significantly degrade water quality, not only by settling in downstream waterways but also by transporting pollutants that attach to soil particles.
During the construction process, bare soil tends to be left exposed to the elements. This often leaves soil subject to stormwater runoff from storm events, causing the soil to erode away. Erosion can lead to issues both on the construction site and on neighboring properties.
One side effect of erosion is colloidal water. Colloidal “muddy” water is when sediment, such as soil, is left suspended and floating around within the body of water, giving it a coffee-stained look. Until the sediment settles back down to the bottom, the water body is left polluted and an ecological dead-zone with reduced penetrating light for photosynthesis and subsequently less oxygen.
In a construction zone, sediment that is tracked or spilled onto impervious surfaces or runoff from the bare areas of an active construction site can enter the nearby inlets that connect and drain to stormwater basins. Some soil that has high amounts of clay or silt deposits can prevent the basin from infiltrating properly, leading to the creation of colloidal “muddy” water. When this happens, it can become an issue for offsite sedimentation concerns.
A stormwater basin usually will have an outlet control structure for water to safely convey out of the stormwater basin. This hole or “orifice” on the outlet control structure is the first outlet of the basin.
One way a project can combat colloidal issues in stormwater basins is by installing a “skimmer” or “floating riser” . This is a device that connects to that hole or “orifice” in the outlet control structure to skim off cleaner water from the water surface once it exits the basin. The flexible connection to the outlet control structure must be watertight. The skimmer shall not rest on the basin bottom when the basin is dry. If a basin does not have an outlet control structure or an orifice, please refer to Appendix 7-5 in the Standards. This picture shown is of a floating riser properly attached to the outlet control structure in a basin called a “Wet Pond” which holds water consistency.
If a basin is continuing to discharge colloidal “muddy” water off-site particularly in a nearby waterway, another avenue for the project can be to install a treatment called flocculent in the water. A flocculent such as “PAM” (Polyacrylamide) may be added to the basin in accordance with manufacturer’s instructions to remove fine suspended colloidal material prior to dewatering. For additional information about dewatering, visit our blog post. This will help the suspended material to fall out of suspension and ensure the cleaner water from the surface is existing the basin.
All dewatering discharges must be to a stabilized location. A source of free cationic ions (such as Ca 2+) may be required at a rate of 50- 60 gm Ca2+ /kg PAM to encourage bonding between colloids and PAM. Materials such as lime, CaCl, gypsum or flyash may be used to provide the cation component. The flocculent shall not cause adverse environmental conditions to develop in the area receiving the basin discharge. Flocculent may be added through the use of ‘logs’ or similar devices impregnated with PAM to dose inflow water prior to entrance to the sediment basin. Such devices shall be placed to allow complete passage of the design storm and shall not obstruct flow through storm sewer systems.
Having resources available for the worst-case scenario does not mean that this is the solution to erosion from stormwater runoff. The best method to combat erosion and its consequences is through prevention. Using simple barriers such as silt fence, stone pads, and inlet protection can all make all the difference when it comes to keeping stormwater basins and our waterways clean and healthy. For complete design criteria, installation requirements, and maintenance guidance, please refer to the New Jersey Standards for Soil Erosion and Sediment Control, Standard for Sediment Basin (24-1).