Isotope-based mean recharge elevations across Costa Rica

Abstract:

In tropical regions, topography can impact the provision of water for domestic supply, which is complex and usually involves many sources (i.e., deep and shallow groundwater, surface water, and spring water). Overall, tap water at lowland urban settlements disproportionally depends on high elevation recharge from 'mountain water towers'. Isotopic metrics serve to better evaluate conservation and regulation needs in critical mountainous or plain recharge areas. Here, mean recharge elevations (MREs), obtained from isotopic lapse rates across Costa Rica, were used to categorized main spatial recharge patterns as: a) high altitude recharge sites are located across the main mountain range (1,976-3,218 m asl), b) main urban areas exhibited MREs between 1,331-1,975 m asl, and c) coastal and distant lowland regions resulted in lower elevations (<1,330 m asl). A large disproportionality exists between MREs and water use elevations (denoted by the sampling site elevation, SE). Median MRE exhibited a large spectrum with a median value >1,000 m asl; however, MRE-SE differences can reach up to 2,500 m asl, indicating in some cases regional groundwater flowpaths. In the tropics, significant temporal and spatial data gaps require more systematic efforts that account for better: i) sampling gradients to improve isotopic lapse rates; ii) surface water and groundwater sampling during baseflow regimes to characterize mean annual conditions; iii) isoscapes in order to trigger a positive feedback between prediction and observation efforts; iv) forest conservation and land use practices; and v) protection laws in critical recharge elevations.