The influence of de-icing salts on the remobilization of heavy metals in an artificial wetland

The influence of de-icing salts on the remobilization of heavy metals in an artificial wetland

De Koning, M.K.

Van de Giesen, N.C. (mentor)

Civil Engineering and Geosciences

Water Management

Water Resources

2012-10-18

Artificial wetlands are used at different locations in the world as a treatment facility for wastewater of different composition. In order of Rijkswaterstaat several artificial wetlands are constructed for road runoff treatment. In all cases the artificial wetland is preceded by a sedimentation basin / storage buffer. One of them is located along the A1 near ‘t Gooi. A research on the purification efficiency of this wetland shows higher metal concentrations in the effluent in wintertime (Tromp, Helofyteninfiltratiesystemen voor zuivering wegwater, 2005). This is probably due to de-icing salts. The given solution is to bypass the artificial wetland in wintertime to avoid metal flush out. This research strives to deepen the insight into metal mobilisation in artificial wetlands under the influence of de-icing salts. The main question of this research is worded: “What is the influence of de-icing salts on the remobilization of heavy metals in an artificial wetland?” For this research six artificial wetlands located at traffic junction Raasdorp, A5 and A9, are available for measurements. Visual inspection reduced this amount to one due to various deficiencies in the other five. This resulted in a sub question: Does the filter function hydraulically as it is designed for? By use of a water balance, visual inspection and gathered data by use of divers an indication of the hydraulic functioning is made. Since high iron concentrations were measured it is concluded that groundwater is leaking into the artificial wetland. The other option, iron is leaching out of the filter substrate, is rejected by the column test results. Here the iron concentrations are decreasing which should not be the case if it is originating from the substrate. Artificial wetlands are characterised by enhancement of treatment performances because of the presence of vegetation. The main processes which are introduced by vegetation are summarized by the term phytoremediation. Vegetation is able to bind heavy metals in the root zone and also storing them into their biomass. To get insight into the heavy metal concentrations in several parts of the treatment facility water samples were took. The question in relation to the water samples is: “Do heavy metals and PAH occur in the influent of the artificial wetland and is there a difference with the effluent of the artificial wetland?”. Elevated levels for heavy metals were observed at the influent of sedimentation basin and artificial wetland. The highest concentrations were measured for copper and zinc. The other four measured heavy metals were: cadmium, chrome, nickel and lead. These metals did not show significant higher concentrations. In most cases here, the concentration was below the detection limits of the laboratory analysis.PAHs were also measured but in most cases the concentrations were below the detection limits of the laboratory analysis. Because of this less attention is paid to the behaviour of PAHs in comparison to heavy metals. The results of the measurements display a pattern where in general the pollutant concentrations are highest in the influent of the sedimentation basin, followed by the influent of the artificial wetland. The lowest concentrations were in general measured in the effluent of the artificial wetland. This proves the expected behaviour of the treatment facility. The goal of measuring the conductivity at location is to get an answer onto the following question: “Can the presence and movement of de-icing salts be quantified and evaluated based on the measured conductivity into the treatment facility?”. An increase of dissolved salt causes an increase of the conductivity. In wintertime rise of the conductivity in the treatment facility was measured and the results are providing insight in the movement of deicing salt through an artificial wetland. As a last step a column test is carried out to investigate the influence of de-icing salt on a possible flush out of heavy metals. The column test is filled with a soil sample what is took out of the original artificial wetland. Four different runs are carried out where for each consecutive run the de-icing salt concentration in the influent is increased. The question on which an answer should be found here is: “Is for the column test effluent an increase of heavy metal concentrations measured by increasing conductivity of the influent per run?”. The results of the column test did not show the expected increase of heavy metal concentrations into the effluent of each run. The first two runs did show elevated concentrations of copper and zinc but for the last two runs concentrations were considerably lower. For the third run an significant elevated concentration of lead is measured where this concentration is for the other three runs near the detection boundary. The effluent of the fourth run should according to the hypothesis contain the largest heavy metal concentrations, but this was not the case. The measured concentrations of the de-icing salt related ions did show the expected behaviour for the four runs. The concentrations in the effluent were increasing for each consecutive run as was also expected based on the conductivity measurements.

artificial wetland
heavy metals
de-icing salt
remobilization

http://resolver.tudelft.nl/uuid:3fb56858-db8a-4712-b337-01f218c81d5f

2012-10-19

Student theses

master thesis

(c) 2012 De Koning, M.K.