Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

Monitoring Case Study-Two Exfiltration Trenches Located Near Miami International Airport in Dade County, Florida

This case study is based on an evaluation of the effects of two exfiltration trenches on groundwater by McKenzie and Irwin (1988).

This study was part of a continued effort to monitor the effects of urban stormwater recharge on the water quality of the Biscayne Aquifer, a shallow aquifer in Dade County, Florida. Two exfiltration trenches were investigated at two small commercial areas near the Miami International Airport. The first study area was located in an asphalt parking area adjacent to the airport with a drainage area of about 3 ha (8 ac) overlying a sandy soil. This exfiltration trench network is designed for stormwater retention with no outflow pipe. The capacity of the parking lot is about 1,000 vehicles. The second study site was located at the Miami International Free Trade Zone. It had a drainage area of about 4 ha (10 ac), consisting of asphalt-covered parking lot, with an exfiltration network in predominantly limestone rock. The capacity of the parking lot is several hundred vehicles. Figure 49 shows the locations of the two study areas in Dade County, Florida. The subsurface exfiltration trench is a stormwater management practice that is commonly used in south Florida. Exfiltration trenches in south Florida are usually constructed near or beneath the water table to induce artificial recharge (exfiltration) of stormwater. Some exfiltration trenches are designed to retain all stormwater from a drainage basin with no outflow other than recharge; others are designed to detain the first inch of stormwater before overflow begins by means of a discharge pipe.

Figure 49. Locations of the two study areas in east-central Dade County (McKenzie and Irwin, 1988)

Study Objectives

• Monitor stormwater runoff pollutant concentrations and their impact on aquifer water quality at two different depths for two exfiltration trenches overlayed by two different types of soil.

Design and Operation

Exfiltration trenches can be designed as single stand-alone units or connected to other catch basins to form a series of drainage networks. Figure 50 shows a schematic cross-section of a typical exfiltration trench and catch basin. The catch basin is the point of entry of the stormwater runoff into the trench system. The catch basin of the exfiltration trench not only passes stormwater runoff to the trench, but also functions as an initial sediment trap. The catch basin contains a perforated pipe, generally 900 mm (36 in) in diameter, which extends longitudinally into the trench from the catch basin. The pipe functions as an exfiltration conduit for the stormwater runoff and provides a secondary sediment trap. The area between the pipe and the trench walls is filled with a coarse aggregate, which prevents trench side wall collapse and plugging of pipe perforations, as well as serving as a conveyance to distribute exfiltrated stormwater to the trench walls. The trench is generally 1.5 m to 1.8 m (5 ft to 6 ft) wide, with the base typically 0.76 m (2.5 ft) beneath the water table. A nonwoven filter fabric is used along the periphery of the trench to deter filling in of the voids in the coarse aggregate by fine soils during reverse flow conditions that result from high groundwater levels. The filter fabric also prevents the migration of native soil into the trench, which would cause a reduction in infiltrative capacity. A 15.2 cm (6 in) layer of pea gravel is placed over the coarse aggregate and covered with builders' felt to prevent vertical infiltration of silts and sediment and to prevent subsidence.

Figure 50. Schematic of section of a typical exfiltration trench (adapted from McKenzie and Irwin, 1988)

Monitoring Program

At each trench site, two wells were placed about 0.3 m (1 ft) outside the exfiltration trench perimeter, and two additional wells were placed at 6.1 m (20 ft) outside the trench perimeter. For each cluster of two wells, one well was drilled to a bottom depth of 30.5 cm (1 ft) and the other was drilled to a bottom depth of 4.6 m (15 ft). Polyvinyl casing was seated in the borehole and the annular space grouted, leaving the bottom of the well exposed to the soil. This well configuration was designed to estimate both horizontal and vertical variation of water quality within the immediate zone of stormwater exfiltration. To measure the water levels in the exfiltration trench and immediate surrounding perimeter, two water-level recorders were installed, one in the catch basin and one immediately outside of the trench. An automated rain gage was installed at each site to measure the amount of rainfall for sampling events.

Water quality sampling at the two sites was conducted during five storms and one nonstorm period from April 1985 through May 1986. Stormwater samples were collected using grab-sampling techniques. Eleven water quality variables were targeted for collection: total Kjeldahl nitrogen (TKN), nitrate, ammonia nitrogen, orthophosphate, total phosphorus (TP), iron (Fe), lead (Pb), zinc (Zn), chemical oxygen demand (COD), total organic carbon (TOC), and color.

Conclusions

• In general, the pollutant parameters were greater in the stormwater sampled from the exfiltration trenches than in the ground water from the wells located 30.5 cm (1 ft) from each trench. The reduction in concentration suggested the possibility that some target variables in stormwater were partially removed within the exfiltration trench prior to recharge.

• Of all of the pollutant parameters, lead and zinc were the only target variables that indicated a significant difference between stormwater and adjacent groundwater monitoring at both the airport and free trade zone study areas. This suggests the possibility that the exfiltration trenches might function to some extent as traps for heavy metals found in stormwater.

• Aerobic microbial activity might have accounted for reductions in concentrations of nitrate and phosphorus.

• Comparison of the concentrations of some of the pollutant parameters in stormwater from the trenches and stormwater sampled in the wells located 6.1 m (20 ft) from the trenches indicated some vertical removal. However, except for ammonia nitrogen, the concentrations were greater in the 4.6 m (15 ft) sampled depth. This might have resulted from dilution in the upper zone by stormwater recharge.

References

McKenzie, D.J., and G.A. Irwin. 1988. Effects of Two Stormwater Management Methods on the Water Quality of Water in the Upper Biscayne Aquifer at Two Commercial Areas in Dade County, Florida. U.S. Geological Survey Water Resources Investigations Report 88-4069, Tallahassee, FL.