Using oxygen, hydrogen, and tritium isotopes to assess pond water’s contribution to groundwater and local precipitation in the pediment tableland areas of northwestern Taiwan

Environmental isotopes including stable hydrogen and oxygen, and radioactive tritium isotopes are employed to assess groundwater sources in the Taoyuan–Chungli Tableland (TCT), northwestern Taiwan. We evaluate the contribution to groundwater resources of copious ponds distributed throughout the TCT. Results indicate that both precipitation and pond water are the end sources of proximal shallow groundwater (P-SGw) in the TCT. Recharge of P-SGw to deep groundwater in the proximal-fan region (P-DGw) is obvious, because δ values of P-DGw are similar to those of P-SGw and no barrier restricts P-SGw entering P-DGw. However, the infiltration of precipitation or pond water to shallow groundwater in the mid- and distal-fan regions (MD-SGw) is restricted somewhere because of impermeable confining layers overlying the MD-SGw. In addition, apart from the upstream recharge of the P-SGw, MD-SGw overlying the deep groundwater in the mid/distal-fan regions (MD-DGw) contributes to MD-DGw through barrier-layer leakage between MD-SGw and MD-DGw. Overall, δ values indicate that pond-sourced moisture accounts for about 6% of precipitation in the studied tableland. This contribution fraction is larger than that in the western plains or reservoir catchments of Taiwan. The calculations also indicate that precipitation and pond water respectively contribute 51% and 49% to P-SGw. Among the 49% contribution from pond water to P-SGw, 46% is from direct infiltration and the remaining 3% is via the evaporation–precipitation process. The fractional results clearly indicate that pond water is a vital source of both local precipitation and groundwater.