Groundwater levels in the intermountain basins of Nevada vary from hundreds of feet deep on the alluvial fans to five feet deep or less in central portions of the valleys. There are areas where artesian pressures, coupled with the correct geologic setting, can cause groundwater to naturally seep out from the ground as springs. It is no wonder that groundwater dewatering and groundwater management can be key considerations for construction projects and can have a significant effect on budgets and schedules. Simply put, dewatering is required anytime that trenches, structural fill, and other building/development components will be installed below the groundwater table. Construction dewatering is mainly performed to stabilize the subsurface prior to construction, allow for compacted stabilized fill to be placed prior to installation of building/development components, and to provide dry working environments for laborers. Typically, to facilitate construction, groundwater should be lowered to at least two feet below the bottom of planned excavations. Unfortunately, dewatering is not as simple as just pumping water out of the ground and putting it into the gutter. Some key considerations when dewatering will be required for a construction project are: estimations of dewatering flow rates, estimations of wells (spacing, depth, size), groundwater quality, and obtaining the correct type of discharge permit.
A dewatering system should be designed to balance the amount of effluent produced to lower the groundwater table to a level sufficient to allow construction. While it is possible to lower the groundwater well below the construction elevation, that may not always be prudent as large amounts of effluent are charged higher permitting rates and settlement from the dewatering could affect nearby structures. The deeper you dewater, the larger your dewatering radius of influence will be and the more likely the project is to cause some sort of settlement to adjoining or nearby structures. Further, the deeper a well is installed, the more it will cost to install it.
Dewatering system estimations are made by testing the properties of the aquifer along with reviewing geologic data to obtain information necessary to create a groundwater flow model. Common aquifer tests are slug tests (used to measure hydraulic conductivity) and pump tests (used to measure hydraulic conductivity, transmissivity, and the Storage Coefficient). Generally, slug tests are less accurate and calculate variables on an order of magnitude scale, however, for less complex dewatering, this option is usually sufficient to create a good groundwater flow model. Pump tests produce more reliable and detailed data and are preferable for projects with atypical considerations such as when dewatering is very deep, when a permanent dewatering system will be required, and/or when there are adjoining developments that could be adversely affected by settlement. Slug tests are typically done quicker and more cost effectively than pump tests, however, it would be beneficial to discuss which test is right for your project with an experienced engineer or geologist prior to choosing which test to use.
After the aquifer properties are calculated, a groundwater flow model will be developed for your aquifer, which will then be utilized to estimate the amount of construction dewatering required to facilitate construction but minimize the radius of influence. Groundwater flow modelling will allow you to estimate dewatering well spacing, dewatering well depth, average flow rates of dewatering pumps, an average total flow rate for effluent produced during construction dewatering, and the amount of time it will take the system to drawdown the groundwater to a level that will facilitate construction.
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Water quality varies highly depending on the geographic location of a site. A site in downtown Las Vegas next to a gas station will likely have poorer water quality to a similar project going through a residential area in Henderson. Groundwater quality is a key component when deciding what type of discharge permit your project will require and where you will be allowed to discharge to. A project could even have to treat the groundwater prior to discharge if the water contains contaminants above regulatory action levels! A common contaminant associated with the relatively turbid waters of shallower aquifers is suspended solids. Often you will see settlement tanks being utilized to knock the solids out of suspension prior to discharge.
Typically, effluent water from construction dewatering will need to be permitted by a regulatory agency, which in Nevada is the Department of Environmental Protection (NDEP). There are several permits available however, for construction dewatering, the two main permit types are the De Minimis Permit and the NPDES Individual Permit. The Deminimis Permit is a general permit meant to capture projects where dewatering duration and pumping rates are relatively low. The De Minimis Permit has a short application, relatively quick turnaround time back from the NDEP, and less stringent discharge monitoring requirements, however, only allows for up to 250 gpm of flow and cannot be used to discharge impacted water. The NPDES Individual Permit, which is required for more complex dewatering applications, has a complex application, typically takes six months or more for approval, usually requires weekly sampling of effluent discharge, and can require treatment of groundwater prior to discharge. Just to reiterate, it can take more than 6 months to get a NPDES Individual Permit approved which can have implications on the overall construction schedule.
All in all, groundwater dewatering and groundwater management are key considerations for construction projects and can have a significant effect on budgets and schedules. GES has the right experience and tools to consult with you on your dewatering projects. At GES, not only do we make the ground work for you, we also make the groundwater work for you too!
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