Abstract

Organic and inorganic carbon reservoirs are investigated in surface and groundwater in the semi to hyper-arid Rio Loa Basin in Northern Chile. Part of the study area is within the Andean volcanic arc, and volcanism is responsible for large inputs of C-14-depleted CO2 which hampers groundwater residence time estimations using dissolved inorganic carbon (DIC). To better understand the area's carbon system, we investigated C-14 and delta C-13 in dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in groundwater and surface water. DOC concentrations are low in all water samples analyzed, 16-53 mu M. Fluorescence and delta C-13 measurements on DOC indicate that dissolved organic matter (DOM) entering groundwater has characteristics of humic compounds derived from terrestrial plants in the recharge area. Fluorescence is lost in groundwater in the lower part of the system due to microbial or abiotic degradation. Changes in C-14(DOC) and delta C-13(DOC) also indicate that microbial activity contributes to DOC, possibly from geogenic CO2. While the range of C-14(DIC) is very large, 1.6 to >50 pmC, that of C-14(DOC) is restricted between 53 and 72 pmC. We estimated C residence times in the volcanic arc, based on corrected C-14(DOC), are 3,200 years compared to C-14(DIC) estimates that range from 2,000 to 15,000 years. While uncertainties in the estimate due to sources and sinks of DOC within the different aquifers exist, the range in age is somewhat restricted, despite the large distances that separate different sampling points and the different types of aquifers sampled. This leads us to conclude that wetter climate conditions in the past increased recharge and storage of the aquifers, and DOC inputs to recharging water was enhanced by more widespread plant growth during those past wet climates.