Geo environmental indicators of vellayani lake of Thiruvananthapuram district, Kerala

Vellayani lake with a spread area of about 3.98 km2
is the second largest fresh water lake
in the Kerala state. The lake is located in the outskirts of Thiruvananthapuram District and lies
between the North latitudes, 8024'90" and 80
26'30" and East longitudes, 760 59'08" and 76 59'47".
It is getting polluted consequent to the discharge of toxic contaminant from urban and agricultural
areas. Lake of adequate baseline information is one of the major constrains challenging
conservation & efficient use of freshwater resources. At present, the the lake Vellayani is
disappearing in an unpredictable pace. Several reports have been published on the present
condition of Vellayani lake. The area of the lake has been reduced to a little less than half the area
of what previously existed (750 hectares in 1926). The scenic fresh water lake, at present is being
strangulated by the invasive plant species. A thick layer of grass has made some portions near the
banks as marshy grassland. Over the years, the lake has been filled with invasive plants such as
Eichhornia crassipes, Limnocharis flava, Salvinia molesta, Salvinia auriculata, Cabomba
Caroliniana, Lemna Minor and lotus (Cultivated for temple uses). All these indicates that the lake
is under heavy threat mostly due human intervention.
The present study was conducted as an attempt to evaluate the present geo-environmental
status of the lake. Several physico-chemical parameters were analysed, which included
Temperature, pH, EC, TDS, Turbidity, Salinity, TSS, Calcium, Magnesium, Sodium, Potassium, DO, CO2, Cl-, NO3, NO
2, NH3, SO42-, PO4, SiO4,
Alkalinity as well as heavy metals like Iron,
Copper, Cadmium, Lead and Zinc. The Chl-a concentration was also estimated in the lake water,
those concentrations showed that the lake was in oligotrophic. category.
The physico-chemical parameters like EC, TDS, TSS, DO, CO2, NO2, NH3, PO4 as well
as the heavy metal iron and cadmium. Highly significant correlation was found in the study, where
EC strongly correlated with TDS & Salinity, pH strongly correlated with DO, PO4 with NO3, Mgwith HCO3
and TA and HCO3 with TA. SiO4 showed a strong negative correlation with K. As per
the Gibb’s diagram, the samples fall in the rock-water interaction dominant area. Irrigation
suitability of the samples were analyzed, various indices were considered to find the suitability and
they were sodium percentage, sodium adsorption ratio (SAR), Kelly’s Ratio, Residual Sodium
Carbonate, Potential salinity, Permeability Index and Mg Hazard. As per the Kelly’s ratio about
64.31 percentage of samples were unsuitable for irrigation. All the samples belonged to the
excellent category of sodium absorption ration classification. Salinity and alkalinity hazard class
diagram (USSL) showed that all lake water samples fall under high salinity with low sodium (C3– S1) class.
The Sodium percentage ratio index showed that almost 64% of the lake water samples
fall under excellent category. The Wilcox diagram showed that about (78.57%) fell under
permissible to doubtful. RSC values of the samples ranged from1.045to 0.0592. Based on the US
Salinity Laboratory (1954), all the samples have values <1.25 meql-1 and are safe for irrigation
purposes. Based on the Doneen criteria, all the collected samples came under Class ΙΙ which is
categorized as good for irrigation. High values of potential salinity in the area can be ascribed to
due to comparatively high values of chloride and sulphate content in the study area. About 99% of
the samples were within the recommended limits and are suitable for irrigation purposes. As per
WQI, only 14 % of samples came under excellent category.
The higher values of nitrites, ammonia, phosphate, TDS and TSS along with the organic
carbon content in the sediments (Krishnakumar and Baijulal, 2006), are suspected to be the reason
for the growth of aquatic floating plantation in the lake. These parameters indicate that the water is
mainly polluted by the agricultural and urban runoff.
From the overall analysis, it was found that the lake water of Vellayani is portable for
drinking water purposes only after pre-treatment. The overall irrigation suitability of the lake was
found to be moderate.
This study recommends to take proper action against unscientific agricultural activities
and also recommends for the proper treatment of the lake water before using for irrigational as
well as drinking water purposes.

MPhil EI 2018-2019 INT Dr. R Jaishankar