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

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.

To date, in Rajasthan, the driest Indian state, more than 93,000 low-cost water harvesting structures have been erected across 3,529 villages. Furthermore, under a major initiative taken by the Chief Minister of Rajasthan (Mukhya Mantri Jal Swavlamban Abhiyan - MJSA), a large number of RWH structures are being built across 295 blocks of 33 districts, with the aim of making about 21,000 villages self-reliant for water and raise the groundwater level.

Nevertheless, there are uncertainties and gaps in information when it comes to demonstrate that the quality of the groundwater resulting of MAR is adequate for either direct drinking or productive uses. Hence, the major impact of this project will be to inform management practice by improving the scientific knowledge about the relationship between RWH, pollutants' fate and aquifer recharge, and in particular the interaction between pollutants, especially DOM.

Data collected during this project (see WP1 and WP2) will be useful for regulators at the local and state level to identify supply settings where water quality does not meet Indian or WHO standards. In this regard, site-specific solutions can be implemented to target pollutants and their corresponding sources. Based on the extent and severity of the contamination, regulators and end-users will be able to establish priorities for the protection of the environment and population. Furthermore, distinct water uses can be allocated based on actual quality status so that the different needs of industry, farmers and households can be supported without compromising human health and economic activities.

In the medium term, groundwater models derived from this research (WP3) will help define strategies to operate current MAR structures in a sustainable way. Understanding the relationship between groundwater quantity and quality along with their corresponding seasonal variations and land use practices will promote good operational water management. Stakeholders will be able to balance abstraction and demand in relation to recharge rates. For instance, abstraction scheduling might be planned in connection with seasonal patterns to avoid exposure to fluoride at harmful levels, which will clearly improve public health in those communities whose use aquifers for water supply.

In addition, the models can also be used to investigate the effect of a variety of forcing factors on groundwater quality and quantity. This can be used to analyse the effect of climate and land use change before any decisions are made for redevelopment of the catchment area. This will assist local engineers and planners in their decisions.

Globally, including in the UK, our research will also influence long-term policy and strategic development of RWH for MAR, particularly in areas where fluoride and micropollutants are a concern. When planning for new MAR structures, stakeholders can use our findings so that optimal infrastructure design and favourable geographical location can be defined based on the quality of harvested water and the fate of pollutants of concern during infiltration. Current RWH schemes will also benefit since a better knowledge on recharge mechanisms will help to improve operational conditions to satisfy water demands.

In addition, the fluorescence excitation-emission matrix analytical protocol developed during this project can be used by regulators and researchers to easily and rapidly detect micropollutants in water bodies, as the analysis uses less than 5ml of sample and takes less than two minutes. As different sources of DOM have a different fingerprint, it can be used to identify sources of pollutant in any surface or groundwater catchment.