Groundwater resources are an important water resource for approximately one billion smallholder farmers in Africa and Asia, but increasing water scarcity jeopardizes these people's livelihoods and food security . To make things worst, groundwater in nearly two-thirds of India and 40% of sub-Africa are located within fractured hard rock aquifers. Standard methodologies to quantify groundwater in fractured hard rock aquifers are often cost-prohibitive or inaccessible in these regions. As a result, groundwater is often not measured, monitored, and sustainably managed. Hard rock aquifers provide a limited - yet, important - water supply for these regions. Therefore, there is a need for tools that can be used in fractured hard rock aquifers that can enable routine monitoring of these critical groundwater resources.
Earlier this week, my colleagues and I published a paper in Frontiers in Environmental Science where we report a simple, easy-to-use method to measure groundwater recharge in a fractured hard rock aquifer located in the Gangeshwar watershed. The Gangeshwar watershed is located in rural India within Rajasthan's Jaisamand Lake Basin - a UNESCO G-WADI pilot site. This work builds upon previous work in the region  to measure groundwater recharge using chemical-based approaches, such as the Chloride Mass Balance (explanation on this from my last blog post). In an effort to simplify the Chloride Mass Balance, our most recent work investigates the use of electrical conductivity (EC) meters as a substitute for measuring chloride in the Chloride Mass Balance approach. The main benefit of using EC meters is that it is very easy-to-use (works just like a thermometer!). The simplicity of EC meters would increase the opportunity for more field studies by water users and provide real-time data collection. For this reason, EC meters are already being used to monitor drinking water quality standards , seawater intrusion [4,5], and rainfall runoff into streams within forested watersheds [6-8] and urban watersheds . And, until now, have yet to be used to estimate groundwater recharge.
Groundwater recharge essentially measures how much water enters an aquifer within a given year, and is a useful metric for developing a groundwater balance. Groundwater can be recharged from rain and nearby surface waters, but in this part of India, groundwater recharge is largely influenced by the summer monsoon rains. This means that the underlaying aquifers are replenished by rainfall during the monsoon season and then the groundwater slowly interacts with the hard-rock which alters the chemistry. For this reason, groundwater recharge measurements using chemical approaches, should be done after the monsoon season to increase accuracy.
Our findings demonstrated that EC and chloride were strongly correlated, particularly in the samples taken after the monsoon rains. This enabled us to estimate groundwater recharge using EC measurements in place of chloride in the Chloride Mass Balance method. This is good news because EC meters can be used to monitor groundwater: the storage properties of shallow groundwater and changes in groundwater quality caused by extreme events, anthropogenic pollution, or land-use changes. Despite all its benefits, the use of EC meters to monitor groundwater works best in conjunction with other hydrogeological tools. Nevertheless, it remains a basic approach to enhance routine groundwater monitoring in fractured hard rock aquifers.
1. Shah, T., Burke, J. & Villholth, K. in Water for Food, Water for Life (ed. Molden, D.) 395–423 (International Water Management Institute, 2007).
2. Rohde, M. M., Edmunds, W. M., Freyberg, D., Sharma, O. P. & Sharma, A. Estimating aquifer recharge in fractured hard rock: analysis of the methodological challenges and application to obtain a water balance (Jaisamand Lake Basin, India). Hydrogeology Journal 1–14 (2015). doi:10.1007/s10040-015-1291-9
3. World Health Organization. Guidelines for drinking-water quality: Recommendations. 515 (World Health Organization, 2008).
4. Rhoades, J. D., Kandiah, A. & Mashali, A. M. The use of saline waters for crop production. (1992).
5. Singhal, B. B. S. & Gupta, R. P. Applied Hydrogeology of Fractured Rocks. (Springer Science & Business Media, 2010).
6. Matsubayashi, U., Velasquez, G. T. & Takagi, F. Hydrograph separation and flow analysis by specific electrical conductance of water. Journal of Hydrology 152, 179–199 (1993).
7. Laudon, H. & Slaymaker, O. Hydrograph separation using stable isotopes, silica and electrical conductivity: an alpine example. Journal of Hydrology 201, 82–101 (1997).
8. McDonnell, J. J., Stewart, M. K. & OWENS, I. F. Effect of Catchment-Scale Subsurface Mixing on Stream Isotopic Response. Water Resources Research 27, 3065–3073 (2008).
9. Pellerin, B. A., Wollheim, W. M., Feng, X. & Vörösmarty, C. J. The application of electrical conductivity as a tracer for hydrograph separation in urban catchments. Hydrol. Process. 22, 1810–1818 (2007).