Dewatering: Simple Steps Can Save Time and Money

By Christopher Martell, Project Manager.  Published in Commerce Magazine, June 2007.

So often during the performance of a remedial action, the strategy of “dig and haul”, which involves the excavation of soils and the dewatering of these excavations, is chosen as the appropriate remedial alternative.  Dewatering facilitates the removal of affected media below the water table and, while relatively simple in concept, without adequate site characterization and a full understanding of all project parameters, including local geology and hydrogeology, dewatering can prove to be one of the most arduous tasks an environmental professional can undertake.  Improper planning and “surprise” conditions can delay site construction activities and increase costs by an order of magnitude.  Below are some simple steps which can save an environmental professional time and money during this process.

Geology/Hydrogeology: Inadequate characterization of local geologic and hydrogeologic conditions can cause countless delays in dig and haul remediation projects.  Some common questions which should be addressed prior to initiating any dewatering project are: What are the vertical and horizontal hydraulic gradients across the site?  What is the lithology?  Is the material at the base of the excavation acting as a confining layer?  What is the ground water recharge in the area of the excavation?  Are there seasonal and/or tidal influences which may affect the project?  What effect will precipitation have on the project?

The installation of soil borings to below the anticipated depth of your excavation will determine soil type, identify confining layers (if any), locate the depths of local aquifers in the area, and will also assist in determining if shoring will be required.  While the installation of shoring is commonly an OSHA compliance issue for worker safety, the presence of loose, granular soils in excavations may actually require shoring, even without an OSHA concern, to keep the excavation from collapsing.  Local groundwater data should also be reviewed to establish local hydrogeologic conditions.

Clean Equipment: A common pitfall of dewatering activities is increased silt/sediment loading of the dewatered material.  This affect is caused by the mobilization of soil particles and their subsequent suspension in the groundwater which is being removed from the excavation.  Dewatering pumps come equipped with screens which prevent large debris from entering the pumping chamber; however, they have no effect on suspended particles.  These fine particles accumulate in the pump housing, bind the internal pump mechanisms, and lead to pump failures.  This process can account for days, sometimes weeks, of delay depending on the duration of the project.

A simple and effective solution to this problem is to create a permeable barrier which allows ponded groundwater to pass through to a “sump” while leaving the suspended particles behind.  This can be accomplished by installing a berm of crushed stone between the pump and the ponded groundwater.  Often, concrete sewer basins are temporarily installed beneath the water table to act as sumps which facilitate the pumping process.  As the stone screening material becomes clogged, it can be easily removed and replaced using excavation equipment.  Geotextile membranes can also be used; however, given their cost, they are not always a viable alternative in preventing the heavy particle loading of effluent streams.  Landscaping fabric is a commonly used, less expensive alternative.

Onsite Storage:  One mistake, which is often made by first-time dewaterers, is the issue of onsite storage.  Will the dewatered material require pretreatment prior to discharge or will it be discharged directly to a stream, river or publicly operated treatment works?  If your answer is affirmative, do you have adequate onsite storage capacity to combat the dewatering activities, including your anticipated groundwater recharge rate, while being able to properly treat the dewatered material prior to discharge?  Typically when projects of this sort hit a snag, it is because the dewatering rate from the excavation exceeds the discharge rate, thus causing a backup of effluent which requires onsite storage in above ground tanks.  The rental of these tanks is not cheap and can quickly send your project spiraling out of control, especially if the dewatering is to continue for any length of time.  To further complicate this issue, many municipalities place maximum daily discharge rates on dewatering activities and may charge disposal fees per gallon discharged.  It is essential to the success of any project that a true understanding of anticipated discharge rates is acquired before any discharges take place. 

Another issue which is commonly overlooked is the cleaning of storage tanks upon completion of dewatering activities.  As designed, most tanks function as settling basins for the afore-mentioned suspended particles; therefore, over the course of a project, storage tanks commonly accumulate several feet of sediment, even when equipped with inline sediment filters, which requires removal and proper disposal prior to the return of the storage tank.  If the tank is an open-top, non-weir type tank, conventional excavation equipment may be used to remove the majority of sediment within the tank.  If, however, the tank is of the more common closed top variety, the removal of this material requires that confined space personnel enter the tank and physically remove the sediment via shovels, vacuum hoses, etc.  The cost to perform this task is exorbitant; however, necessary.  Knowing your tank options prior to pumping can save you potentially thousands in labor and disposal charges.

As with any project, proper research and preparation prior to initiating a dewatering project can make the difference between a successful dewatered excavation and a complete failure.  Several vendors provide pre-packaged dewatering systems which offer several advantages; however, when these off-the-shelf remedies do not fit your scope of work, a complete and proper investigation is recommended before attempting a project with as many variables as a dewatered excavation.

Christopher Martell, a Project Manager with Environmental Waste Management Associates, has more than 12 years of experience in the environmental industry and is responsible for the supervision of the environmental investigation and remediation of industrial, commercial and residential properties.  He earned his Bachelor of Science in Environmental Engineering from Roger Williams University, Rhode Island.