Impact of rainwater harvesting in India on groundwater quality with specific reference to fluoride and micropollutants.

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Groundwater is the main source of fresh water in many parts of the world however excessive abstraction is causing a continuous decrease in water tables in some places. In Rajasthan on the Western side of India, methods are being utilised to replenish groundwater and provide a reliable water supply. This is achieved in several ways including the use of Rainwater Harvesting (RWH) systems. These structures capture rainwater and runoff and allow it to infiltrate into the subsurface and subsequently aquifers. In India this has been achieved both through traditional approaches such as storing water in percolation ponds and in check dams. The use of more novel emerging approaches such as sand dams has also been explored. A sand dam is a concrete wall built across a seasonal riverbed. During the rainy season, a seasonal river forms and carries sand downstream. The sand accumulates behind the dam and is filled with water providing storage, this water can then be abstracted or percolate into the ground water.Although these techniques increase water availability it is unclear as to their effect on groundwater quality. Depending on the scale and location of RWH structures, rainwater contained within them may contain a range of harmful substances. These pollutants could travel through the RWH structures and contaminate the groundwater.Additionally, in Rajasthan, high fluoride concentrations in the groundwater are a major health concern. Excessive fluoride in drinking water causes dental and skeletal fluorosis. This problem may be worsened as dissolved organic matter (DOM) present in harvested rainwater which has been found to increase fluoride levels during recharge.The analysis of the transport of pollutants and DOM in RWH structures is thus of crucial importance in ensuring groundwater quality. This transport can be effected by a number of different factors most notably design and location of these structures.The fieldwork will be carried out to monitor the water quality used for recharge and groundwater in the vicinity of the three RWH structures over a period of two years. This will include topographical surveys, groundwater level monitoring, water sampling, tracer testing, weather recording and obtaining soil samples to provide information on the mineral characteristics of the aquifer material.In addition laboratory testing will be completed on water samples obtained from the field. The quality of the rainwater and groundwater will be assessed using a variety of techniques. Parameters tested will include nutrients, e.coli, heavy metals and pharmaceuticals amongst others. To enhance our understanding of the impact of DOM present in rainwater on Fluoride levels in groundwater, fluorescence excitation-emission matrix (F-EEM) will be used.Pollutant transport models which simulate pollutant transport and DOM interaction with fluoride in RWH structures and across the whole catchment will be created. These will be coded in open-source software that is commonly used in India. Being open source, these codes can be easily modified to add new or modify existing processes to investigate particular scenarios which may impact water quality. They can also be used by anyone so other practitioners or academics can build on the work of this project.Utilising field work, laboratory analysis and modelling simulations. Recommendations based on this research will include:- Depth and size of structure and abstraction and extraction points.- Timing, duration and rate of groundwater abstraction events after monsoon events.