Abstract of the Master thesis:
The rainwater runoff from highly trafficked road surfaces is contaminated by a high concentration of pollutants such as heavy metals and hydrocarbons, and also by high de-icing salt concentrations in the winter months. Detention ponds along highways serve, among other things, to protect receiving water bodies (rivers and streams) from hydraulic short-circuiting and the risk of water quality impairment by pollutants, as well as the prevention of critical salt concentrations in the detention pond runoff.
As part of the thesis, a series of measurements for a detention pond along the highway A8 (Munich - Augsburg) were developed near Odelzhausen and operated in the period from the 4th of November 2014 to the 19th of January 2015. In addition, a self-sustaining station for flow measurement and rain gauge was installed.
Based on sediment samples from the detention pond, the effect of de-icing salt, sodium chloride (NaCl), calcium chloride (CaCl2) and different pH values on the process of sedimentation and thus on the cleaning performance of detention ponds was investigated. By faster sedimentation the cleaning performance of detention ponds increases. The investigations of the settling analysis in the laboratory were recorded with a video camera and were analyzed afterwards. The topic of the sedimentation process has been published in several studies in the area of estuaries (Portela et al. 2013; Sutherland et al. 2014), and is being extended to the field of detention ponds along highways in this thesis.
This thesis illustrates that salts lead to larger flocks and thus to a higher settling velocity according to Stokes, especially in the case of sediment particles less than 63 μm in size, which act as a transport matrix for almost all pollutants (Ball et al. 1998). It turned out that divalent salts and, in particular, an acidic pH range ensures the fastest settling. In the alkaline pH range, a concentration-front was measured with speeds of about 0.011 mm/sec (pH 10) and 0.015 mm/sec (pH 11).
In addition to the settling analysis, recorded with a video camera, the water and sediment samples of the detention pond were analyzed by an optical emission spectrometry with inductively coupled plasma (ICP-OES) at the University of the Bundeswehr Munich. The sediment samples were moreover investigated for the influence of de-icing salt with a scanning electron microscope (SEM) regarding the surface structure and a rotary rheometer for measuring the viscosity. Furthermore, the samples were analyzed for particle size distribution by laser light scattering.