Analyzing Recharge Dynamics and Storage in a Thick, Karstic Vadose Zone. Issue 7 (7th July 2022) - Spellman, Patricia
Abstract: Karst vadose zone heterogeneity creates complex transmission and storage dynamics that affect the timing and magnitude of aquifer recharge. Young, high‐matrix permeability eogenetic karst aquifers may have significantly higher matrix storage than older, lower matrix permeability counterparts. In vulnerable and water‐limited karst regions, the timescales of storage may be important for seasonal and sub‐seasonal water resource management. We create a framework to quantify storage dynamics in karst aquifers using high‐resolution precipitation and groundwater levels from the Northern Guam Lens Aquifer in the US territory of Guam. We estimate recharge using the Water Table Fluctuation method, and then develop transfer functions between precipitation and recharge to quantify storage and release of water from the vadose zone. The transfer functions are partitioned into different flow pathways including conduit, conduit/matrix, and slowly draining matrix. Probability distributions are fit to each pathway to determine the average travel times of infiltrated waters. The results show that aquifer recharge through secondary porosity features typically occurs within a few hours of a rainfall event, but this rapid recharge accounts for only 12%–28% of total recharge. The majority of aquifer recharge (>70%) occurs within a month of a contributing storm event. An additional 10% of recharge, on average, took longer than a month to reach the water table. The framework established can help improve the hydrological modeling and freshwater management for karst aquifers. Plain Language Summary: Recharge is a critical component of water resource allocation as it governs the availability of water for public, agricultural, and industrial supply. Therefore, understanding how precipitation becomes recharge to the aquifer is an important step in water resource management. However, in young, minimally evolved (eogenetic) karst that maintain significant primary porosity, appreciable vadose storage may occur. In island settings where water resources are volatile, the timing of precipitation to recharge significantly impacts groundwater stores and water resource allocation. We develop transfer functions between precipitation and recharge and use analytical methods to quantify (a) timescales of vadose storage, (b) estimates of storage and (c) magnitudes of recharge in a thick, karst vadose zone on Guam. Our work aligns with previous observations and provides a new method to estimate storage and transport through the vadose zone to karst aquifers. Key Points: Vadose storage in eogenetic karst aquifers is appreciable, but most recharge typically occurs within a few months Our method can be applied to other areas where high‐resolution data exists, as our results align with previous observations and models