Contaminant Buildup and Distribution on Urban Impervious Surfaces at Parking Lots - page 01

Contaminant buildup on urban surfaces consists of simultaneous processes of contaminant accumulation and removal from a surface that occur during dry periods. The rate of contaminant buildup is influenced by factors such as climate, land-use, traffic volume, vicinity of roads and streets, population density and catchment surface conditions. Understanding the physical processes of buildup and wash off processes is fundamentally important when predicting the total stormwater pollutant loads and load variability from a catchment. In an effort to enhance our knowledge of site specific conditions and to test various urban water runoff control and treatment methods, an experimental catchment area of a total of 1810 m2 was established in Belgrade, Serbia, in the parking lot area of the Faculty of Civil Engineering, University of Belgrade. Contaminant buildup dynamics and spatial variations were investigated with the aim of estimating the range of contaminant content on impervious surfaces of the research site under site specific conditions, as a step towards proper selection of stormwater treatment methods and establishment of design criteria.

 

Experimental

The examination of temporal accumulation and spatial variation of contaminants accumulated on surfaces of the Faculty's parking lot was performed on six locations, as presented in Fig. 1. The experimental catchment includes local parking inter-connection roads (asphalt), parking lots (asphalt and cube stone), concrete pavements, a metal roof and green areas. The experimental site is located 50 m from the surrounding streets and separated from heavy traffic corridors by a 0.7 m high concrete wall and a line of planted trees that are intended to reduce the influence of the nearby city traffic.

Measurements performed over three sampling campaigns during dry periods in August 2012 (02, 10 and 17 of August) were used to estimate the range of contaminant concentrations and contaminant buildup in the parking lots. Previous studies (Deletić et al., 1998) proved that the vacuum sweep technique is more efficient in removing the fine materials from a typical pavement surface structure. Research literature indicates that a significant fraction of urban runoff contaminants are attached to the finer fraction of solids (particles <150 μm) (Zereini and Wiseman, 2010; Deletić and Orr, 2005). Therefore, the "wet" method, as defined by Deletić and Orr (2005), was used for sample collection. This method involved the encapsulation of a chosen area of the parking lot, simultaneous wetting of the area with deionised water and removal of the formed suspension with a Kärcher Puzzi 100 Super industrial vacuum cleaner. An square metal frame enclosure, 707 mm wide and 707 mm long (enclosed area of 0.5 m2) was constructed to ensure a consistent area was sampled on each occasion. During sample collection the metal frame was sealed to the asphalt surface with putty to retain water and sediment inside the frame during sampling (Fig. 2).

The vacuum sweeper contain a clean and a dirt water tank (capacity 10 L and 9 L, respectively) and an extraction pipe with a polyethylene brush and spray nozzles. The wet sweeping conditions were created by use of the spray nozzles. The spray nozzles are positioned under the head of the polyethylene brush, with a spray pressure of 1 bar and a 1 L/min spray rate. This water pressure improves the collection efficiency by dislodging fine particles bonded to the surface. At the start of vacuuming, about 50 mL of deionized water was initially sprayed over the framed area in order to avoid unintended re-suspension of finer particles during the sampling procedure. A total 4 L of deionized water was used from the clean water tank to collect samples at each sampling location. The water recovery rate during wet-sweeping (the ratio between the mass of water used in the clean water tank and the mass of water collected by wet-sweeping) was between 93% and 97%.

During the vacuuming process, the vacuum nozzle was moved across the surface in a parallel stripes at a rate of approximately 10 cm/s. During the movement, deionized water was continually sprayed while the formed aqueous sample suspension containing surface particles was simultaneously vacuumed (Fig. 2). This procedure was repeated four times, in all directions. The collected samples were transferred to pre-cleaned polyethylene and glass containers, sealed, labelled and temporarily stored at 4°C in a refrigerator until the daily sampling campaign was completed and the collected samples were transported to a laboratory for analyses. The vacuum cleaner's clean and dirty water tanks, as well as the extraction pipe and nozzle, were cleaned with 6 L of deionized water before each sampling.

Solid particles were collected by dry vacuum cleaning from 15 m2 of the parking lot (asphalt) area, and after drying (50oC) were dry sieved according to the national standard (SRPS U.Bl.026:1968) to the following particle size fractions: <63 μm, 63-90 μm, 90-125 μm, 125-250 μm and >250 μm.

Laboratory analyses of the collected surface water samples were performed in the City Institute of Public Health - Human Ecology and Ecotoxicology Department, Belgrade, Serbia. The laboratory is accredited in accordance with the recognized International Standard ISO/IEC 17025:2006. The following parameters were investigated: Chemical Oxygen Demand (COD), 5-day Biochemical Oxygen Demand (BOD5), Chlorides (Cl-), Sulphates (SO42-), Total Suspended Solids (TSS), Settleable Solids (SS), Total Solids (TS), Total Phosphorus (TP), Total Nitrogen (TN), Oils and Grease (Soxhlet extraction - Oils), Heavy Metals (HM): Pb, Zn, Cu, Ni, Fe, Cr. All parameters were measured following standard methods (Rice et al., 2012). The assessment of contaminant loads in the sieved solids fractions was performed in an accredited laboratory according to the following national standards: SRPS ISO 11261, ISO 11263 for nutrients and DML 5.1 (based on EPA 3051/7010:1994, ISO 11969:1996, EN 1483:2007) for heavy metals.

 

Fig01
Figure 1: Aerial view of the sampling sites.

 

Fig02
Figure 2: a) Setting the frame; b) Wet-vacuuming of a parking lot sampling site.