Chemical Characteristics of Buffer Zone Sediments And Implications on Adjoining Water in Diyawanna Lake

Abstract

Diyawanna Lake is one of the major fresh water body located in the middle of Sri
Jayewardenapura Kotte. Most of surrounding area of the lake consists of marshlands. During
the recent past several reclamation and development activities going on around the lake. In
addition, sediments from surface runoff are finally accumulating in the lake margin (buffer
zone). However, there is no any study done on sediment quality and impacts of them on
adjoining water, especially in the buffer zone of such a fresh water body. Therefore, main aim
of this study is to investigate quality of the sediment accumulated in the buffer zone of the
lake and also to study the possible impacts on lake water from the sediments due to
biogeochemical reactions in the zone. Thirty three water samples were collected during dry
period and samples were analysed for pH, electrical conductivity (EC), oxidation reduction
potential (ORP), salinity, total dissolved solid (TDS), NO3
-, PO4
3-, SO4
2-, Cl-, alkalinity,
hardness, Na, K, Ca, Mg, Cd, Cr, Cu, Fe, Pb, Mn, Zn and Ni. In addition, black organic
sediment samples were collected from selected locations and tested for pH, ORP, EC, organic
carbon, total organic matter, NO-3, PO4
-3, Na, K, Ca, Mg, Cd, Cr, Cu, Fe, Pb, Mn, Zn and Ni.
Average values of pH, EC, ORP, TDS, NO3
-, PO4
3-, SO4
2-, Cl-, alkalinity, hardness, Na, K,
Ca, Mg, Cu, Zn and Fe for the lake water are 7.4, 222 μS/cm, -37 mV, 222 mg/l, 0.11 ppm,
15.00 ppm, 1.43 ppm, 20.00 ppm, 0.0002 ppm, 772 ppm, 16 ppm, 10 ppm, 36 ppm, 21 ppm,
0.001 ppm, 0.009 ppm and 0.14 ppm respectively. However, element Cd, Pb, Mn and Ni
were not detected. Piper classification for the lake water indicate CaSO4 type, which reflect
typical gypsum type of waters with impact of mine drainage due to mineral pyrite in soil.
Also, sulfur emitted by the vehicles may react with water to form sulfuric acid. In addition,
gypsum type of water may also due to accumulation of building materials such as cement.
Therefore reason for the CaSO4 type of water may be due to several sources in the area. In
addition, compared to WHO guidelines only Cr (0.58 ppm) present in the water is
considerably high, this may be due to direct discharge of urban dust into the lake.
Average values of pH (6.1), ORP (54 mV), EC (122 μS/cm), organic carbon (3.1%), total
organic matter (18%), nitrate (0.93 ppm), phosphate (12 ppm), Na (3.5 g/kg), K (11 g/kg), Ca
(73 mg/kg), Mg (84 mg/kg), Cd (40 mg/kg), Cr (391 mg/kg), Cu (134 mg/kg), Fe (44 g/kg),
Pb (833 mg/kg), Mn (157 mg/kg), Ni (196 mg/kg) and Zn (33 g/kg) in the sediments are
considerably different value than the water. Compare with Canadian Environment Quality
guidelines average values of Cd, Cr, Pb and Zn are higher in the sediments. This is mainly
due to accumulation of those elements from runoff water. In general, it can be concluded that
prevailing physical conditions of the lake water is controlling leaching of heavy metals from
sediments to water. Conversely, anthropogenic sources seem to be increase accumulation of
heavy metals in the buffer zone sediments.

Keywords: Diyawanna Lake, Water quality, Sediment quality