The subject property was a recently constructed apartment building and associated parking garage located in downtown Denver, Colorado. During the course of the Phase I ESA Targus identified documentation of prior injection of dewatering liquid during construction. One authorized injection well was retained for potential future sump service in the event of elevated groundwater.
During a subseqent high snowmelt season groundwater levels rose beyond historical levels and the design water level. As a result of sump and drain configuration and high water levels, the capacity of the injection well was exceeded and discharge to stormwater was required. The previously-existing dewatering and “remediation” stormwater discharge permit remained in place and was utilized. During the course of discharge monitoring the concentration of naturally occurring selenium was found to periodically exceed newly established Total Maximum Daily Load (TMDL) surface water criteria. Targus conducted a pilot scale treatability evaluation and found that removal of fine particulates using sub-micron bag filters was sufficient to meet applicable criteria. Targus installed sequentially finer bag filters and demonstrated achievement of monitoring criteria – until the permit limits were further reduced.
Thereafter the owner was able to get a separate opinion from a structural engineer other than the original designer. With that information it was decided to replace the prior pumps (1,000 gallons per minute) with a small pump that was served only by the injection well and that, in the event of unseasonably high water levels, allowed some accumulation in the lower parking level. In the few years since transition, water levels have remained below the lower basement floor level and periodic filter replacement is expected to extend the functional life of the injection well.
Targus’ assisted the client in exploring options that would help facilitate regaining consistent compliance with the discharge permit. Targus’ efforts included interaction with the Colorado Department of Public Health & Environment (CDPHE), interaction with the consultant conducting routine monitoring, and gathering and reviewing information regarding reported success in maintaining compliance. Targus conducted weekly testing that included characterization of water samples from the discharge stream for the purpose of checking whether solids removal may be effectively applied at the property.
