The California Department of Transportation (Caltrans) is conducting a multi-year study in Los Angeles and San Diego to examine the technical feasibility, costs, and operation and maintenance requirements of retrofitting structural Best Management Practices (BMPs) into existing highway and related infrastructure. Thirty-three locations are being retrofitted with thirty-nine BMPs using twelve different types of BMP technology. Automated monitoring stations have been installed upstream and downstream of each BMP to determine removal efficiencies from flow weighted composite samples. Constituents monitored in the runoff include: suspended solids (e.g., sediment), metals, nutrients, and organics (e.g., gasoline).To date, most projects have been sited, designed, constructed and monitored for at least one year. The purpose of the program is to identify the problems and solutions that occur with structural BMP retrofit, and to collect operation, maintenance, and performance data for the BMPs. Results to date indicate that there are substantial construction, maintenance, and cost challenges in retrofitting existing infrastructure with conventional structural BMP technology. Water quality monitoring results to date indicate that average pollutant removal efficiencies are 2 consistent with published values. Upon completion of the study, the information collected will enable more accurate prediction of cost and performance of BMPs for treating highway runoff.
This paper reports on experiences with infiltration basin and trenches constructed and operated as part of the Caltrans Best Management Practice (BMP) Retrofit Pilot Study Program. Fifty-two sites adjacent to highways and maintenance facilities in the Los Angeles and San Diego areas were evaluated and two infiltration basins and two infiltration trenches were constructed. One basin and one trench successfully met design expectations, which in California includes a 3-day drain time to prevent mosquito breeding. The drain time of the trench at the Carlsbad Maintenance Station in San Diego County was about twice as long as intended. The infiltration basin at I-5 at La Costa Ave., also in San Diego County, required months to drain. Based on post-construction analysis, contributing causes of these problems were inaccurate estimates of infiltration rates and groundwater depth, and application of inappropriate siting and design criteria. The short-term in-hole permeability tests resulted in higher estimates of infiltration rates than were later observed. Fluctuations in groundwater depth were larger than anticipated. Based on this experience and on computer modeling of one site, Federal Highway Administration recommendations for minimum infiltration rate and depth to groundwater can result in infiltration BMPs that do not meet performance requirements in California.
The performance of Drain Inlet Inserts (Fossil Filteρο.σι and StreamGuarδο.σι) in treating runoff from three California Department of Transportation (Caltrans) maintenance stations was evaluated as part of the Best Management Practice (BMP) Retrofit Pilot Program. Additionally, the effectiveness of an oil/water separator was evaluated at one Caltrans maintenance station. The study included 1) retrofitting the structural BMPs in existing maintenance stations and documenting those costs; 2) estimating percent pollutant removal efficiencies; 3) assessing the causes and frequency of flow bypass; and 4) documenting the type and level of effort required to maintain the structural BMPs. Drain Inlet Insert results to date show that reductions in metals, hydrocarbons, and solids are consistent with expectations for the technology; however, frequent flow bypass required more maintenance than anticipated. Oil/water separator results show no discernable difference between influent and effluent hydrocarbon concentrations at the low levels measured.
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