Regarding the preparation of the USWR samples, a portion of the samples was thoroughly mixed and homogenized in 4 pre-cleaned PE buckets, with a volume of 10 L each, in order to ensure an appropriate homogeneity and create a single representative sample of USWR from each sampling site.
Prior to analysis, the USWR samples were separated into solid and liquid phases.
Regarding the concentration of metals adsorbed onto SS, the insoluble fraction of USWR on the filter was digested prior to flame atomic absorption spectrophotometry (FAAS) or graphite furnace (GFAAS) analysis.
As regards water chemistry characterization, laboratory tests of USWR samples were performed in the laboratories of the Research Institute of Environmental Protection (Vilnius Gediminas Technical University).
EC and pH were determined by analysing unfiltered USWR samples.
The DOC in USWR samples filtered through a syringe (pore size 0.45 [micro]m, Pall Corporation) was determined by employing a total organic carbon (TOC) analyser (Shimadzu-V CSN) at 900 [degrees]C.
For each sampling site, metal speciation in USWR was simulated using the chemical speciation program WHAM/ Model VII based on WHAM (Tipping 1994).
The speciation of the detected trace metals (Cd, Cu, Mn, Pb, and zn) and major elements (Mg, K, and Ca) in USWR was calculated using WHAM7 (Tipping et al.
The model outputs were analysed to provide the following results for each USWR type and metal: fractions of free metal ions relative to total dissolved metals, fraction of truly dissolved metal, fraction of metal bound to colloids (represented by FA).
For the descriptive statistics of element concentrations in the USWR, values of the mean, median, standard deviation and coefficient of variance (CV), were calculated for each element detected from the three sites.
Comparisons of the roof USWR data showed that the sample with a mass of SS from 4 to 9 times smaller than the other three samples had much higher metal concentrations, up to one order of magnitude.
Compared with the Lithuanian MAC limits for the environment, none of the USWR samples collected from the three sites contained amounts of dissolved Cd, Cr, Cu, Pb, Ni, and Zn (Table 3) in harmfully high concentrations.