INVESTIGATION ON AQUIFER RECHARGE POTENTIAL OF RAINWATER HARVESTING USING GEOINFORMATICS APPROACH: CASE STUDY OF PUNE CITY, INDIA

Abstract

Only about 2.5% of the total water available on earth is fresh water. Groundwater systems have always constituted a pre-dominant and strategic reserve of freshwater storage. In most urban areas, the demand-supply gap of fresh water is especially acute leading to indiscriminate abstraction of groundwater. To aggravate this, very little recharge happens in such areas due to increased run-off of the water due to urbanization, leaving little water available for infiltration. In such conditions, systematic rainwater harvesting and managed aquifer recharge play an important role in augmenting the local supply of water, both from unconfined as well as confined (deep) aquifers.

The main objective of this study was to understand the potential for recharge of the groundwater in the urbanized area of Pune city which is the eighth largest city in India and forms a part of the Pune metropolitan area. The study broadly covers the use of Rooftop Rainwater harvesting for recharge through gravity injection to confined aquifers or deep-seated aquifers and delineation of groundwater potential zones for recharge of surface aquifers.

Accordingly, the calculation of rooftop areas and the creation of spatial maps indicating the potential recharge zones, considering various factors such as the extent of urbanization, local geology, soil cover, slope, etc. was done. The acquired data was used for analysis to understand the areas with a potential for recharge and to formulate strategies for systematic recharge and scientific water management. The methodology employed a combination of remote sensing data, rooftop areas, geology maps, ward information, population data, well locations in the city, land use/land cover data and other thematic data.

Data for a total of 3074 wells was used to understand their use as potential wells to recharge the deep aquifers. The data was analyzed for several parameters viz. distribution across the zones (wards), status of the well (perennial, etc.), usage, diameter of the wells and the capacity of the pump being used. Further, the percentage of the wells for each of the zones was calculated for the study area. Frequency ratios for each of the considered thematic factors and their sub-classes were calculated for the wells – (both Dug wells and borewells).

An MCDA approach using AHP was used for the delineation of potential groundwater recharge zones for the surface aquifers. The potential recharge zones were identified and then ranked based on their suitability for groundwater recharge. Thematic factors, LULC, Slope, Rainfall, drainage density, type of soil and lithology were considered for the identification of potential recharge zones.

Increasing urbanization along with a burgeoning population has created challenges, mainly an increasing imbalance between demand and supply. The LULC analysis shows that the total built-up area in Pune has increased from 50.87 km2 in 2003 to 126 km2 in 2019 which is about half of the city indicating rapid urbanization. Data from Pune Municipal Corporation shows the population of Pune city has increased from 1.7 million in 1981 to 3.31 million in 2011 and is projected to rise to 6.09 million in 2031 and 8.26 million in 2041. According to the World Urbanization Prospects, the entire urban agglomeration of Pune is projected to have a population of 8.1 million by 2030. (WUP, 2014). Considering the above, the city must undertake a systematic groundwater recharge program to supplement the supply of water in the future.

The results of the study of rooftop run-off for rainwater harvesting indicate that an amount of 43.92 MCM (1.6 TMC) of water is available for harvesting. This water can be used for artificial recharge of the wells in the city. On a conservative estimate, even if half of this total rainwater is harvested, the recharge can replenish the groundwater annually by about 21 - 22 MCM (0.7 to 0.80 TMC). This is almost a third of the present-day extraction of groundwater and can help maintain the groundwater balance if used for recharge.

Further, the study of groundwater recharge potential zones shows that roughly 44% of the city has a good to high potential for recharge. The ‘groundwater recharge potential zones’ map of the study area generated through a weighted overlay analysis was categorized into five zones viz. low, moderate, good, high, and very high potential. The Groundwater potential map shows a coverage of 13.24 km2 (5%), 124.71 km2 (50%), 72.92 km2 (29 %), 8.11 km2 (3%), and 30.27 km2 (12%) for the above zones, respectively.

From a spatial perspective, the study reveals that high and good potential recharge zones lie mainly in the western part of the city. The central part of the inner city shows low potential for recharge. The analysis of the distribution of wells across the city shows that major extraction is happening towards the Central and Western parts of the city. Most of the wells fall in the Northwest (27%) and Central (26%) parts of the city followed by the West (16%). Another important finding is that only about 18 % of the yielding wells are presently being used for rooftop rainwater harvesting. This indicates a huge potential for using the wells for recharge.

Along with these studies, an experimental RWH system to harness the rooftop run-off was implemented in the field to check the efficacy of the technique. Two methods were used for implementation - Pit based, and PVC cylinder-based filters. These were used for recharge of the confined/deep aquifers in the area of study. Both, the above RWH techniques were found to be effective, though the PVC cylinder-based filter was found to be more cost-effective. This demonstrates that direct gravity injection using low-cost filtration techniques & clean filter mechanisms is a viable and low-cost method to harness the rainwater in the city.

Based on the present study and globally successful practices presented earlier, especially the Integrated Water Resource Management (IWRM) framework, the Integrated Urban Water Management (IUWM) framework and the Managed Aquifer recharge, it is suggested that a systematic groundwater management framework – the ‘SCARPE’ framework comprising of the following elements - Systems approach, Conjunctive use, Aquifer replenishment, Right recharge mechanism, Protection of Existing sources and community involvement and Effective monitoring and balancing demand and supply be used in Pune city. This will help to maintain the groundwater balance and augment the water resources in the city.

Specifically, it is suggested the three wards of Baner-Balewadi, Aundh and Kamla Nehru Park should initially be targeted for systematic recharge of the existing wells. Dedicated recharge zones should be created in the high-potential areas within the city, where rainwater can be collected and directed into the aquifer.

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