DEATH BY 1,000 CUTS - IDENTIFICATION OF REHABILITATION PROBITIES THROUGH MODELING, MASS BALANCE AND FIELD INVESTIGATIONS

摘要

Washoe County Nevada (County) owns and operates a wastewater collection system and treatment plant located in the southern portion of the City of Reno in Northern Nevada. The South Truckee Meadows Water Reclamation Facility (STWRF) treats flow from the County's interceptor system as well as sewers that serve the City of Reno. The County has been proactive in the development and implementation of a reclaimed water treatment and distribution system, but in the last three years STMWRF reclaimed water has violated the State irrigation arsenic limit of 100 μg/L. With areas of geothermal influence in the south Truckee Meadows Region it was determined that the source of arsenic was from groundwater infiltration. The County developed a plan to identify the arsenic sources and develop a plan for mitigation. Mitigation could come from I/I source reduction or treatment. The County working with a consultant and a local hydrogeologist to develop an innovative plan to identify target areas of the collection system for focused investigation. The experimental plan included wastewater grab sampling in the interceptor flow as well as known locations of leaking manholes to quantify arsenic strength in the collection system and in the infdtrating groundwater. A mass balance approach was developed using measured concentrations, flow data from the County's ongoing flow monitoring program, and modeled flows from a recently calibrated hydraulic model to determine the highest priority target areas of the collection system for pipeline and manhole rehabilitation. The groundwater investigation, combined with the mass balance results identified high priority areas of the collection system to focus rehabilitation and additional field investigation efforts. Approximately 85,000 linear feet of pipeline were identified as high priority, indicating that the I/I contributions causing the water quality challenges at the STWRF were limited to less than 30 percent of the STWRF service area. Because of the nature of the issues caused by I/I in the STWRF system and because previous attempts at finding I/I sources using traditional CCTV resulted in limited successful I/I reductions, the County developed a mitigation plan that included additional long term sampling, manhole rehabilitation and Electro Scan inspection of the interceptor system to identify interceptor reaches for lining and rehabilitation. The cause of the water quality challenges were due primarily to groundwater infiltration during dry weather and during times when the groundwater table was elevated; therefore the County wanted an inspection technology that could identify defects that either may not be seeping at the time of inspection or that are in the portions of the sewer below the water surface. The County is currently in the process of full implementation of the mitigation plan, and has already identified 14 thousand feet for targeted inspection and rehabilitation.