Deciphering complex groundwater age distributions and recharge processes in a tropical and fractured volcanic multi‐aquifer system

Abstract

Groundwater recharge in highly fractured volcanic aquifers in the humid tropics remains poorly understood. In this region, rapid demographic growth and unregulated land use change are resulting in extensive surface water pollution and a large dependency on groundwater extraction. Here we present a multi‐tracer approach including δ18Oδ2H, 3H/3He dating, and noble gases (NG) within the most prominent multi‐aquifer system of central Costa Rica, with the objective to assess dominant groundwater recharge mechanisms and age distributions. We sampled wells and large springs across an elevation gradient from 868 to 2421 m asl. Our results indicate relatively young apparent ages ranging from 0.0 ± 3.2 up to 43.5 ± 7.6 years within the unconfined aquifer system. Helium isotopes (R/Ra up to 5.4) indicate a dominant signal from the upper mantle and preclude 3H/3He dating in 50% of the samples. Potential recharge elevations (based on NG and δ18O) ranged from ~1350 to 2670 m asl. NG‐derived recharge temperatures ranged from 11.0°C to 19.4°C. Recharge estimates varied from 129 ± 78 to 1605 ± 196 mm/yr with a mean value of 642 ± 117 mm/yr, representing 20.1 ± 4.0% of the total mean annual rainfall as effective recharge. The shallow unconfined aquifer is characterized by young and rapidly infiltrating water, whereas the deeper aquifer units have relatively older water (>60 years). These results are intended to guide the delineation and mapping of critical recharge areas in mountain headwaters to enhance water security and sustainability in the most important headwater dependent systems of Costa Rica.

Nitrate Legacy in a Tropical and Complex Fractured Volcanic Aquifer System

“…In Figure 3, these samples are within δ 18 O values of −8.0‰ and −9.5‰. In this multi-aquifer system, the Barva aquifer has been characterized by δ 18 O median values near −9.0‰, relatively high tritium, and younger water age (<40 years; Sánchez-Murillo et al, 2022). Most high-elevation springs (green circles in Figure 3) exhibit similar depleted δ 18 O compositions with lower nitrate concentrations.…”

“…Well samples from the deeper semiconfined Colima aquifer units were characterized by low nitrate concentrations and more enriched compositions. The Colima system has been characterized by low tritium, enriched δ 18 O values, and older groundwater age (>60 years; Foster et al., 2002; Sánchez‐Murillo et al., 2022). The isotopic difference between the Barva and Colima ayesianns will be further discussed in Section 5.…”

“…The increasing forested area in the high‐elevation sector is related to the establishment of the Braulio Carrillo National Park (1978; ∼500 km 2 ; SINAC, 2023) and the advent of the Payment for Environmental Services (PES; Pagiola, 2008) in the late 90s. The most prominent recharge area within the BCS located above ∼1,500 m asl includes over 1,000 private hectares under the PES scheme (Sánchez‐Murillo et al., 2022) (Figure 1). In summary, 20 years ago, nitrogen major contributions to the groundwater system could be described as soils > fertilizers > sewage/manure > precipitation (Reynolds‐Vargas et al., 2006), while recent and abrupt land use changes have also resulted in an isotopic and chemical nitrogen evolution toward sewage/manure > soils > fertilizers > precipitation.…”

“…(2006) suggested that recharge to the shallow unconfined aquifer is primarily by direct infiltration, whereas recharge for the deeper semiconfined aquifer is either via percolation from the upper aquifer through pyroclastic beds or via infiltration from precipitation in the eastern part of the Central Valley (eastern topographic depression in Figure 1a). A recent study has demonstrated that the combination of high relief topography and rainfall type dynamics is translated into distinct isotopic spatial patterns within the BCS (Sánchez‐Murillo et al., 2022). Basically, air masses traveling from the Caribbean Sea experienced a strong orographic effect, resulting in a notable depletion in rainfall isotope ratios across the central volcanic front, which in turn led to isotopically depleted signatures in groundwater and surface water within this high‐altitude area and shallow responsive aquifers such as the unconfined Barva aquifer unit (Figure S1 in Supporting Information ).…”

Preface special issue: “Processes and patterns in tropical hydrology”

“…The authors use a relatively simple tracer‐aided model to track the water age of each incoming flux through the modelled cascade of stores, resulting in water ages as short as 3 months. Water ages are also estimated by Sánchez‐Murillo et al (2022), but in a highly fractured volcanic aquifer system in Costa Rica using a multi‐tracer approach. Young ages and a quick recharge dominate in unconfined units, with the unconfined aquifer containing decades old groundwater.…”