Dewatering Groundwater in SD Pond

[Will007]

I am trying to design a dewatering system to existing groundwater at bottom of pond. County requires bottom of pond to be at least 1ft higher than water table with underdrain system. How do I design the underdrain system?

 

[bortzman]

you can't "dewater" groundwater by gravity (unless it's perched); you would need to pump.
The bigger question is why would you want to? The NPDES regulations require that you infiltrate water into the groudwater as a first alterative. Dewatering or removing ground water to a surface water is in direct violation.

 

[bltseattle]

Will007 it is not clear what you are trying to achieve. The 1' groundwater separation criteria is typically not a license to lower the groundwater, but triggers a larger, shallower pond or a pond liner. To design a pond liner or to lower the groundwater, if allowed at all, you should work with your project geotech as they will have the best understanding of your site soil.

I have seen a "wet pond" design that used perimeter subsurface drains to lower the groundwater, the collected groundwater was drained into the SD system downstream of the pond, but that was a rare situation. It is more typical in my experience to line a pond to prevent stormwater-groundwater interaction, in which case preventing buoyant uplift of the liner is a major design element.

Related to Bortzman's comment, if you do need to dewater the excavation you would need to treat that water to comply with surface water standards before you release it. This can be done in a temporary pond or Baker tank. However to my knowledge NPDES does not prohibit excavation or groundwater dewatering. We dewater excavations all the time, in many jurisdictions, in fact, wihout triggering any NPDES concerns, so long as we address the dewatering as part of the Temporary Erosion and Sedimentation Control Plan / Stormwater Pollution Prevention Plan.

 

[cvg]

permanent dewatering at the bottom of the pond would require a permit - probably would be considered a well. But not NPDES. Depending upon where you are, even temporary dewatering of excavation might require a permit and measurement of the amount of water removed. Records of the pumping would be provided to the department of water resources