Construction dewatering causes challenges with discharge standards
With the ongoing recovery of the American economy, construction projects are rebounding in the new residential and non-residential sectors. A major hurdle that will quickly, and sometimes permanently, derail new construction is the presence of shallow groundwater, which must be addressed on a temporary and often permanent basis to ensure foundations and building integrity will not be compromised by the shallow groundwater table.
What is Dewatering?
Dewatering is a common practice in construction where groundwater is extracted via a series of well points to lower the groundwater table and provide a dry environment upon which to build foundations. It often is utilized in building projects that are below a perched or shallow water table, such as basements, parking garages and other sub-surface structures. It also can be used with construction foundations in locations with particularly shallow water levels. The dewatering process typically involves intercepting the shallow aquifer surrounding the structure and pumping water continuously from a series of groundwater extraction wells to lower the overall water table and allow the construction area to remain dry. How and where this water is then disposed depends on federal and state regulations, NPDES and construction permits, and the quality of the water.
Because of this, successful dewatering projects must account for myriad variables, including the removal of regulated contaminants in subsurface water; seasonal variations in the water table; and other factors leading to fluctuations in flow, soil composition, weather and permitting considerations. Geotechnical firms often are employed by developers early in the process to evaluate subsurface hydrogeological conditions, obtain water quality levels, and make predictions for sustained pumping rates given the site-specific local subsurface hydrogeology.
Dewatering Regulatory Requirements
In the past, water from these construction dewatering projects could simply, upon extraction or removal, undergo minimal treatment, such as the removal of suspended solids via filtration with subsequent discharge to a local body of water; however, these days, state regulatory officials give more attention to the chemical makeup and nature of the water, and the suitability of these discharges for either onsite or offsite disposal or discharge.
Depending on how, how much and where one is discharging, there likely are water quality standards that will need to be met prior to the release of the water. Federal regulations for controlling discharges of pollutants from municipal separate storm sewer systems (MS4s), construction sites, and industrial activities were incorporated into the NPDES permit process by the 1987 amendments to the Clean Water Act and the subsequent 1990 promulgation of federal storm water regulations by the U.S. Environmental Protection Agency (EPA). The EPA regulations require construction and storm water discharges to comply with an NPDES permit. In California, EPA delegated its NPDES permitting authority to the State Water Resources Control Board (SWRCB).
In cases where a nearby local sanitary sewer is available and allowable for discharge, those local authorities (i.e., the publicly owned treatment works [POTW] or municipality) will allow or disallow discharge via a permit. NPDES permits or the equivalent state version, referred to as the SPDES permits, typically are not needed if the water is discharged to a sanitary sewer, reused on the construction site, discharged to adjacent land, used at an adjacent facility or treated off site. Therefore, these disposal options are preferred.
The acceptability of this wastewater by a municipality is ultimately the decision of the local authority and dictated by the POTW’s own discharge permit or hydraulic limitations, which also may apply. If the POTW or local treatment authority is not suitable or deemed capable of treating for the contaminants found in the influent, the generator of the water may be required to pretreat it. It is not uncommon in some cases for the discharge to be required to meet EPA’s drinking water maximum contaminant levels (MCLs) for instance.
Direct discharges to a waterbody, storm drain, or MS4 often requires extensive analytical data, money and time. A typical NPDES permit application requires detailed information about the chemical constituents in the proposed discharge, as well as a description of any treatment techniques used. Given the sensitivity of these receiving bodies of water and the aquatic resources within, discharge standards often are at or below the levels required in drinking water. Standards are very localized and specific to the receiving body of water.
Water Quality Issues
Dewatering typically faces three types of water quality issues: high levels of sediment, high pH (often from grouting or concrete work happening nearby) and naturally occurring contaminants. High levels of sediment and elevated pH commonly are problematic in temporary dewatering operations, where water is only removed during active construction. Temporary dewatering often involves the pumping of water accumulated in construction trenches and pits. Once construction is completed, the dewatering process may no longer be needed.
For temporary and simple dewatering treatment needs, often particle filtration or other filtration techniques are utilized to remove sediment. Simple, mobile temporary treatment systems can be an effective option for treating the extracted water during the active construction phase. Sometimes they are inadequate if additional chemical contaminants are present, such as iron, manganese, heavy metals, arsenic or other constituents that require a more sophisticated treatment train to meet discharge standards.
Permanent dewatering operations have longer-term considerations and involve extraction and treatment (often indefinitely) to provide support for underground structures. Prior to discharge, water quality considerations associated with groundwater removal need to be accounted for if they represent potential harm to human health, environmental health concerns for aquatic life or aesthetic issues in drinking water.
Common water quality contaminants that primarily affect aesthetic drinkability but are regulated by EPA include turbidity, iron, manganese, hardness (calcium and magnesium), odor, color and taste. Other water-quality contaminants are regulated for their direct impact on human health, such as arsenic, hexavalent chromium, selenium, radionuclides, nitrates, volatile organic carbons, lead and other heavy metals.
In situations where dewatering activities produce water with elevated levels of contamination, a customized approach to treatment is needed to lower long-term costs. The capability to customize treatment methods that will best serve the site needs is an important factor to consider when designing a dewatering treatment facility. The quantity and quality of the water and a clear understanding of discharge objectives is essential.
One example of a common groundwater contaminant combination that needs to be treated prior to discharge is elevated levels of suspended solids, iron and manganese. These contaminants can be reduced to EPA’s MCL limits (0.30 mg/L, 0.05 mg/L, and less than 5 ntu respectively) through a three-stage treatment train using an automated backwashing pre-filter followed by an oxidation/filtration system and a polishing stage of granular activated carbon filtration. With any treatment, residuals often are generated and must be managed properly. When dealing with elevated iron and manganese concentrations in the extracted water, backwashing of the oxidation/filtration system can occur daily. Special design techniques can be employed to capture, filter and recycle backwash water to provide a zero-liquid discharge option. Not only are solid waste residuals greatly minimized, but backwash water (i.e., wastewater) also can be recycled or even eliminated, lowering corresponding monitoring and disposal costs. The iron/manganese residuals generated from the backwashing operations can be dewatered to form non-hazardous solids that can be transported and disposed of periodically in a local area sanitary landfill at a relatively low cost.
Properly designed ex-situ treatment has become a familiar companion to construction dewatering. As the list of EPA-regulated contaminants grows, more situations are occurring in which these contaminants or co-contaminants, either naturally occurring or anthropogenic in origin, are found in construction-generated water that must be addressed on temporary or, more commonly, on a permanent ongoing basis. Understanding the regulatory constraints, geotechnical aspects of extraction (quantity), water quality, and corresponding treatment needs are all important considerations for developers and contractors undertaking these projects. Formation and selection of an experienced, qualified team early is critical for designing and implementing projects with such long-term implications. Developing a customized solution for managing wastewater discharges can greatly impact long term costs for permanent dewatering ventures.