Evaluation of aquatic pollution and identification of phytoremediators in Vellayani lake

Abstract

A study entitled “Evaluation of aquatic pollution and identification of phytoremediators in Vellayani lake” has been carried out at the College of Agriculture, Vellayani during the year 2009-2011 to assess the extent of pollution and to identify the phytoremediators in Vellayani lake. Vellayani wetland ecosystem comprising a fresh water lake and surrounding padasekharams, extends through three panchayaths viz., Kalliyoor, Pallichal, Venganoor and two zones of Thiruvananthapuram Corporation namely Nemom and Thiruvallam. It is the sole drinking water source for the above area. Vellayani and adjoining areas is undergoing drastic reduction in area and water quality. Samples of water, sediment and macrophytes found in the lake were collected during pre monsoon, monsoon and post monsoon seasons, from 15 sites within the lake and were subjected to physical, chemical and biological analysis.

The physical properties of the lake water viz., colour, turbidity and the amount of suspended solids showed significant variation among seasons and with locations showing higher values during monsoon season. All the chemical properties of lake water were also significantly influenced by the season and location. On evaluating the water as per ISI/WHO criteria, it was found that the chemical characteristics like EC, NO3-N, K, Ca, Mg, S and Zn were within the maximum permissible limits. P and Cu were not detected in water in any of the seasons. pH of the lake water was within safe limits only during the monsoon season and it was highly contaminated with organic load. The NH4-N content of water exceeded the MPL at certain locations during pre and post monsoon seasons. With regard to the heavy metals, one location was contaminated with Fe, six locations with Cd and eight locations with Pb. During monsoon season, the water quality is generally good. Al content of water was always above the safe limits fixed by ISI/WHO. Hence suitable treatments for improving water quality should be advocated for its use as drinking water/ for domestic purposes.

With regard to the microbiological properties, the bacterial count was lowest during monsoon and highest during post monsoon season. Regarding the Coliform count, a gradual increase in number from pre monsoon to post monsoon season was observed. Algal population was seen uniformly distributed in all the 15 sites throughout the three seasons.

All the chemical properties of lake sediment were significantly influenced by the season and location. Among the chemical characteristics organic carbon, extractable K, Ca, Mg, S, Fe, Cu, Zn and Pb were higher during pre monsoon season while pH and NH4-N were higher during monsoon season. Most of the chemical characteristics like EC, NO3-N, extractable K, S, Fe, Cu, Zn and Pb recorded their lowest values during monsoon season, while organic carbon, extractable Ca, Mg and Cd were at their lowest during post monsoon. NO3-N and extractable Al recorded their highest value during post monsoon season. Carbonates and bicarbonates were not detected in the sediment samples. The texture of sediment ranged from sandy to sandy clay loam.

Fourteen species of aquatic macrophytes were found within the lake. Maximum number of plant species (12) was found during pre monsoon and minimum (9) during monsoon season. All the macrophytes showed higher elemental composition in shoot compared to root. Among the macrophytes, Eichhornea crassipes was found to be a good phytoextractor for N, K, S, Al, Fe, Cu and Cd and Pistia stratiotes for N, K, S, Al, Zn, Cu and Pb. Nymphaea odorata was found to phytoextract Cu and Cd and Nymphoides indicus for Cd and Pb. For S, Limnocharis flava and Scirpus grossus can be used as phytoextractors.