Application of stable hydrogen and oxygen isotopes on groundwater resources assessment

Effective use of limited water resources is often difficult in Taiwan due to the steep topography, rapid steam flows and uneven spatial-temporal distribution of precipitation. In addition, the climate changes alter the precipitation pattern of the local hydrologic condition, which creates significant impacts on the groundwater recharge. Therefore, it is critical to accurately assess the interactive recharge from the upstream mountain blocks to the downstream alluvial fan for groundwater resources management. In-situ measuring of stable oxygen and hydrogen isotopes, chemical ion tracer method and numerical groundwater model were applied to determine the relationship between the spatial-temporal variation in rainfall, river water, and groundwater in the upstream mountain blocks and proximal fan area of the Choushui river and the relative influence mechanisms. The results showed that the groundwater mainly recharging from river water both in high and low flow periods and more significant in low flow period. Lighter δ18O discovered in groundwater at certain local regions indicated that recharge sources may come from the lateral recharge of the Choushui river and the upstream mountain blocks. Annual groundwater recharge amount estimated from stable base flow analysis and numerical simulation of Modflow model were 1.095 billion tons/year and 249 million tons/year for the upstream mountain blocks and proximal fan, respectively. Moreover, based on the oxygen isotope material balance result, the percentages of annual groundwater recharge from rainwater and river water in the upstream mountain blocks were 25.9 % and 74.1 %, respectively, and those in the proximal fan area were 37.3 % and 62.7 %, respectively. The quantity of groundwater recharge in the upstream mountain blocks was estimated to be 283 and 812 million tons/year from rainwater and river water, respectively, and that in the proximal fan area was 93 and 156 million tons/year, respectively. The results may serve as a reference for formulating a regional groundwater resources management plan. © 2019, Taiwan Agricultural Engineers Society. All rights reserved.