Introduction

Wetlands include a wide range of aquatic habitats such as marsh, fen, peat land/open water, flowing water (rivers and streams) or static (lakes and ponds). These ecosystems being the transition zone between land and water are ecologically important in relation to stability and biodiversity of a region and also in terms of energy and material flow. These ecosystems perform a vital function of uptake of nutrients and bioremediation of heavy metals, volatile organics and other xenobiotic compounds and are aptly referred as “Kidneys of the landscape”. They also aid in recharge of groundwater aquifers and stabilization of shorelines (Ramachandra and Bharath 2015). These transitional zones or ecotonal region are repository of rich biodiversity and support food chain. Wetlands act as giant sponges, which help in slowing runoff, lower flood heights, reduce shoreline and stream bank erosion. The functional ability of wetlands is dependent on the type of trophic structure and material exchange. The trophic structure includes various trophic levels as producers (algae, etc.), primary consumers (zooplanktons and grazers), secondary consumers (small fish), tertiary (large fish, birds, etc.). Algae being the primary producers synthesize carbohydrates during photosynthesis and give out oxygen along with the production of other essential metabolites. Bulk of the CO2 gets sequestered into algal biomass in these wetlands systems that aids in combating global warming through reductions of GHG (Greenhouse gases) in the environment. However the stability of every system depends upon the balance between production and consumption of energy and matter at different trophic levels in any system. The functional aspects of wetlands are tied to the tradeoff between the ecosystem function and the anthropogenic impact that makes it very sensitive and delicate. Human impacts include altering the catchment (changes in land cover), encroachment, solid waste disposal in lake beds, sustained inflow of untreated sewage from urban localities, etc. (Ramachandra and Bharath 2015; Ramachandra 2001, 2009a, 2009b, 2010; Ramachandra et al. 2003, 2018a).
Bangalore had flourished during 19th and early 20th century owing to a salubrious microclimate and abundance of water in the city of lakes. Globalisation, liberalization, privatization are the agents fuelling urbanization in most parts of India during early 1990’s. Consequences of the unplanned urbanisation are enhanced pollution levels, lack of adequate infrastructure and basic amenities. This is evident in Bangalore with severe scarcity of water, frequent flooding, enhanced pollution levels, uncongenial buildings, mismanagement of solid and liquid wastes, etc. Increased and unprecedented population growth has resulted in enormous stress on potable water from a daily consumption point of view and also in regards to increased wastewater generated by the city. Unplanned growth has led to radical land use conversion of forests, surface water bodies, etc. with the irretrievable loss of land resources (Ramachandra et al. 2013a). Land use analyses show 1028 percent growth in built-up area during the last four decades with the decline of vegetation by eight-eight percent and water bodies by seventy-nine percent (Ramachandra et al. 2017; Ramachandra and Bharath 2016). Analyses of the temporal data reveals an increase in urban built up area of 342.83 percent (during 1973 to 1992), 129.56 percent (during 1992 to 1999), 106.7 percent (1999 to 2002), 114.51 percent (2002 to 2006) and 126.19 percent from 2006 to 2010 (Ramachandra et al. 2009a, 2012a; Bharath et al. 2018).
Rapid urbanization in recent times has led to the large scale generation of wastewater. Untreated or partially treated wastewaters are fed to surface water that finds its way into ground water sources. The sustained inflow of untreated or partially treated sewage to wetlands leads to the enrichment of nutrients such as nitrogen (N) and phosphorus (P), evident from the algae bloom and profuse growth of macrophytes. This has led to the contamination of existing water resources with pathogens and nutrients resulting in algal bloom due to eutrophic status of surface water.
Cauvery River caters to only fifty-five percent of over 9 million population and balance is met through groundwater. Bangalore city is facing severe water shortages today due to insufficient piped supply coupled with the fast decline of groundwater table. Plummeting groundwater table is due to poor infiltration because of increasing paved surface and also over exploitation. Sustained inflow of untreated sewage to lakes has also contaminated nearby groundwater sources (Department of Mines and Geology 2011; Government of India 2008) affecting the human health. Nitrogen as nitrate-N pollution leads to physiological disorders including blue baby syndrome (methemoglobinemia) and the persistent assimilation of nitrate rich water leads to carcinogenic symptoms (as nitrates get reduced in the body forming nitrosamines, which are carcinogens). Macrophytes grow profusely in these nutrient rich environments and progressively cover the entire surface of the water body hindering the passage of sunlight and diffusion of gases to the underlying water layers. Absence of sunlight affects trophic levels with the reduced algae and photosynthetic O2 generation depleting the dissolved oxygen concentration and hence affects the local biota. This study explores the feasibility of bioremediation path to treat wastewater for reuse and mitigate the water crisis in the city (Mahapatra et al. 2018; Ramachandra et al. 2018a).
Wetlands are the regional ecological barometers reflecting the health of a region due to the ecosystem services such as regulating the regional micro-climate (Ramachandra and Kumar 2010; Ramachandra and Bharath 2014), recharging groundwater aquifers (Government of India 2008), thereby influencing the life of the people adjacent to it. The city landscape (in the current spatial extent of 741 sq.km) had enjoyed 1452 interconnected water bodies in 1800 catering the domestic and irrigation water demand and now there were 193 wetlands (Ramachandra et al. 2017; Ramachandra and Kumar 2008, 2010). Urban water bodies are prone to an increased anthropogenic stress in recent times due to dumping of solid waste, encroachment of wetlands, sustained inflow of domestic sewage and industrial effluents leading to poor water quality with a very frequent algal blooms and rapid macrophyte growth with periodic successions (Mahapatra et al. 2011a, 2011b, 2018; Raj 2013). Influx of partially treated and untreated sewage has resulted in overgrowth, ageing, and subsequent decay of macrophytes creating anoxic conditions and devouring the system from life giving oxygen. This has impacted the food chain and hence the ecological integrity of the system.
Bangalore city is located on two ridges (North-Northeast and South-Southwest) with three watersheds (Hebbal-Nagavara, Koramangala-Bellandur, Vrishabhavathi). Northern and eastern parts of the city are with gentle slopes, while southern and western parts are very rugged undulating terrain of the region has helped in the creation of interconnected water-bodies to meet the domestic and irrigation requirements during the pre-colonial period. These interconnected drainage system is supposed to transfer the storm water from one water-body to another, started receiving sewage with rapid population growth and lack of appropriate sewage treatment systems. Population in Bangalore has increased from 5.6 million (2001) to 9.5 million (2011). Population increase has led to large quantum of sewage influx into wetlands leading to contamination of wetlands and associated groundwater systems.
Collapse of land regulation is evident during the past two decades due to large scale unauthorized occupation of open spaces (wetlands, grasslands, parks) by the influential section of the society in collusion with the bureaucracy. Large scale land conversion of common lands to built-up in recent times further substantiates the nexus (Ramachandra et al. 2017; Ramachandra and Bharath 2016; Ramachandra et al. 2007; Ramachandra and Sudhira 2007). Changes in the land cover have altered the regional hydrology evident from frequent floods, conversion of perennial wetlands to seasonal wetlands and decline of groundwater table. However, authorities have kept some wetlands alive by diversion of sewage, which flows consistently and maintains the water levels in the system of interconnected lakes.
Water Supply in Bangalore:
Water is being pumped from Cauvery River ~100 km from the city with an electricity requirement of 75-100 MW. Bangalore is located at higher elevation (900 m above mean sea level) and Cauvery river courses are at 500 m above mean sea level. This exercise suffices the need for approximately fifty-five percent of Bangalore city dwellers, while the rest are dependent on ground water and unauthorized drinking water supplies. Arkavathy River, with two reservoirs at Hesaraghatta (built in 1894, now dry) and Tippagondanahalli (built in 1933) insignificantly and irregularly contribute to a small fraction of the demand (30 MLD). The Chamrajasagar reservoir at Thippagondanahalli (or TG Halli reservoir), located at the confluence of the Arkavathy and Kumudavathy rivers, receives inflow mostly from the Kumudavathy but with a low flow rate (Lele et al. 2013; Ramachandra and Solanki 2007; Sawkar 2012). Water demand in Bangalore is roughly about 150 liters per day (lpd) per person and the total water requirement for domestic purposes is about 1, 400 million liters per day (MLD). Water available from Cauvery (Stages I to IV, Phase I) and Arkavathy (Hesarghatta and Tippagondanahalli reservoirs) rivers is about 975 MLD. The loss of water during transportation and distribution is about thirty percent. The surface meets about sixty percent of the city demand while the significant portion is met from groundwater sources (39%) and zone –wise details are indicated in Table 1 and Fig. 1. Zone wise water share from Cauvery (compared to groundwater usage) ranges from forty-three percent (south Bangalore), fifty-five percent (west Bangalore), sixty percent (south east Bangalore), sixty-three percent (central Bangalore), seventy percent (north Bangalore) and seventy-seven percent (east Bangalore).