Wetlands In Bangalore
Eunice Beady
Wetlands being the transition zone of land and water plays a significant role in nutrient cycling, treatment of water, attenuation of floods, maintaining stream flow, recharge ground water, moderate local microclimate, provision goods (fish, fodder, fuel, drinking water, etc.) and services (regulating, cultural, etc.) to the dependent population [1]. Sustained discharge of untreated or partially treated sewage has been altering the chemical integrity of aquatic environment by enriching the system with nutrients, leading to the eutrophication of urban water bodies [2]. Wastewater generated in the domestic and industrial sectors consists of chemical ions, nutrients and heavy metals [3,4,5,6].
Plant communities (macrophytes) in wetlands [11] act as nutrient sink by uptake of elements released by sediment to water column, which will influence water chemistry. Assessment of the chemical composition in the macrophytes provides an information about the uptake ability of plants to nutrients [12], nutrients availability for metabolism and the nutrient value of the plants [13]. The ability of macrophytes to uptake nutrients and metals from soil and water forms the basis of phytoremediation [14]. Nutrient composition or accumulation in the tissues is an important feature for identifying the ecological strategy of the plant species, and this aids in predicting the competitive complex interactions among the plant communities [15, 16] and aboveground biomass stores higher proportion of nutrients [17]. Phytoremediation capability of aquatic macrophytes has been studied earlier by researchers [18,19,20,21,22,23,24,25,26,27], and hence, they are being used in monitoring the status of an ecosystem (biomonitoring).
Heavy metals have increased enormously in the environment from anthropogenic sources due to industrialization and enhanced agricultural activities (pesticides, etc.). Heavy metals in the environment have been posing challenges due to the hazardous properties such as toxicity, persistence, accumulation in the biological organism leading to biomagnification in food webs [28,29,30,31,32,33], which further get transformed into more toxic compounds [34] posing serious challenges to biotic health. The occurrence of toxic pollutants in water bodies (lakes, ponds, streams and rivers) would affect the health of population who depend on these water sources to meet their daily requirements (water, fish, food, etc.). Consumption of water and wetland goods laded with metals would lead to the accumulation in the kidneys, liver and bones of humans, resulting in chronic disruption of metabolic activities, and lead to cardiovascular, neurological and renal diseases [35, 36]. Table 1 provides the sources and toxic effects of heavy metals on plants and humans. Bottom sediments, plants and other organisms in polluted wetlands contain heavy metals [37] due to bioaccumulation. Analyses of spatial distribution of heavy metals in sediments and macrophytes of wetlands aid in tracing the sources and the extent of contamination, which is useful in remediation and prudent management of water bodies.
Bioconcentration factor (BCF) in macrophytes is the ratio of heavy metal concentration in the plant to that in the sediment at a sampling location (Eq. 1). Higher values of BCF indicate the easy assimilation of heavy metal by macrophytes from sediments and the higher possibility of heavy metal redistribution in the environment [59]. BCF expresses the ability of a plant to uptake a specific element from sediments and subsequent accumulation in its tissues. Higher BCF values imply of good bioaccumulation or accumulation capability of macrophytes. A BCF value higher than one indicates that a particular plant species is aiding as a hyper-accumulator of trace elements [60].
Translocation factor (TF) describes the efficiency of a plant to translocate metal from its root to shoot and is computed as the ratio of concentration (mg/kg) of metal in plant shoot to the concentration of the same metal in plant root (Eq. 2). Higher TF values indicate higher capacity of mobility [61].
Higher C values in sediment were in the northwest and northeast shoreline side of the lake, attributed to higher terrestrial C sources of domestic sewage from the urbanized pockets of the catchment. Lower C values in the lake sediments were observed at southern side and outlets of the lake, where depth is greater than 1 m. Similar C and N distributions were reported in the earlier studies [66]. Higher C:N in middle regions is due to the sustained inflow from the neighboring residential layouts on both sides and the middle regions are with stagnant water. This highlights the storm water drains are being misused with the discharge of sewage, contributing terrestrial organic matter into the lake.
Macrophyte sample analyses revealed that higher concentration of carbon was in Colocasia esculenta root and lowest in Eichhornia crassipes root, and these values were lesser compared to earlier study [55]. Emergent macrophytes had higher carbon concentration than floating plants because of fibers in developed support system [14]. Nitrogen was higher in Alternanthera philoxeroides shoot and lowest in Typha angustifolia shoot. The results of this study showed higher nitrogen content than earlier report [55].
Tables 6 and 7 compare metal concentrations in sediment and macrophytes of Varthur lake with various other studies. Cadmium is of most concern due to the greatest mobility in soil environment [69] and is widespread heavy metal, which is extremely toxic to humans and plants [70]. Cadmium enters aquatic environment through anthropogenic sources like industrial effluent and agricultural runoff [71]. Cadmium concentration in this study was higher compared to the earlier study [40] and above PEL and critical range [62, 63]. The highest concentration (23.7 mg/kg) was observed in the northwest shoreline sample (V12) and the lowest (1.4 mg/kg) in the middle sample (V36) (Fig. 5). These values were lower compared to the samples of Bellandur Lake [2]. The outlet and middle region had lower concentration of cadmium compared to other regions (Fig. 5). Chromium is toxic for plants as it alters N metabolism and impairs protein formation [27]. More than half of the sampling points had critical ranges of chromium. Sediments in the inlet and north shoreline regions had higher concentrations of chromium than other samples (Fig. 6). The present study values were 10 times higher than the earlier reports [40]. Copper concentrations in the current study were higher compared to the earlier study [40] and lower compared to Bellandur Lake [2]. Lead is one of the most toxic metals at low concentrations and non-essential element for plant [27, 72]. The main source of lead in the sediment is from lead pipes, mixing of gun powder, waste batteries, etc.[73]. Earlier studies [2, 40] recorded lower lead concentrations compared to the present study. Zinc in the sediment samples was little higher than the earlier reports [40]. Plant growth, metabolism and physiology are effected by toxic metal nickel. The concentration of nickel in sediments was` lower than PEL and critical values.
The Varthur lake catchment also receives surface runoff containing fertilizers and pesticides from agriculture and floriculture lands, which is contributing to Cd, Pb, Ni in the lake surface sediments. Untreated wastewater from industries such as electroplating, metallurgical, batteries manufacturing, vehicle garages, etc. has contributed to the accumulation of heavy metals (Cr, Cu. Cd and Zn), which is evident from the analysis of sediment samples at the north shoreline. Among macrophyte samples, Alternanthera philoxeroides and Eichhornia crassipes had higher concentration of all investigated heavy metals.
The study highlights the presence of organic and heavy metals in sediments and macrophytes, indicating contamination due to the sustained inflow of untreated sewage and industrial effluents. Environmental changes in the lake catchment during the past 5 decades due to rapid urbanization have been responsible for the lake contamination. The organic contamination is mainly due to untreated sewage and runoff entering the lake. The metal pollution is due to entry of industrial wastewater and agricultural runoff. The study recommends proper treatment of sewage before letting into the lake to prevent contamination and associated health hazards in the vicinity.
Wetlands act as natural water purifiers by removing contaminants, excessive nutrients, and suspended particles and absorbing many pollutants in surface waters. This enhances the quality of groundwater supplies and mitigates the negative effects of point and non-point sources of pollution.
Wetlands act like giant sponges, storing, then slowly releasing groundwater, and floodwater. The extent of groundwater recharge depends on the type of soil and its permeability, vegetation, sediment accumulation in the lakebed, surface area to volume ratio and water table gradient.
Wetlands downstream of urban areas perform valuable flood control services. Wetlands along rivers and streams store excess water during rainstorms. This reduces downstream flood damage and lessens the risk of flash floods. The slow release of this stored water to rivers and streams helps keep them from drying up during periods of drought.
Lakes provide areas for walking, jogging and exercise as well as small play areas for children. Many wetlands contain a diversity of plants and animals that provide beautiful places for sightseeing, fishing, hunting, boating, bird watching and photography.
Temperature effects various physical, chemical and biological reactions in the aquatic organisms. It influences water chemistry i.e. DO, solubility, density, pH, alkalinity, salinity, conductivity etc. Aquatic organisms have varying tolerance to temperature. An increase in the temperature speeds up the chemical reactions, increases the rate of metabolic activities, reduces the solubility of gases like dissolved oxygen and carbon dioxide in the water.
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