The Influence of Regional Groundwater on the Dissolved Organic Matter in Costa Rican Streams

Abstract:

The aim of this study was to better understand the role of regional groundwater in stream organic matter in two tropical watersheds in Costa Rica. Dissolved organic matter (DOM) concentration, stable C isotopes, absorbance, and fluorescence were used to distinguish DOM sources in two adjacent watersheds, the Arboleda and the Taconazo, which are similar in size, soils, rainfall, and vegetation, yet differ because the Arboleda receives a significant input of regional groundwater that is chemically distinct from the local groundwater, whereas the Taconazo does not. Characteristics of the DOM differed largely among the two streams. Fluorescence and isotope data suggested that more degraded DOM was predominant during dry periods in the Arboleda due to the influence of regional groundwater in that stream. During wet periods, fresher soil-derived DOM was predominant in both streams, similar to the quality of DOM in the Taconazo during base flow. DOM δ¹³C values of the Arboleda during baseflow ranged from -23 to -34‰, while in the Taconazo values ranged from -25 to -34‰. During storm flow the ranges were smaller and more similar. The Taconazo ranged from -23 to -29‰ and the Arboleda ranged from -23 to -31‰. These differences were more noticeable at baseflow and during the dry season (December to April), suggesting that the DOM from regional groundwater entering the Arboleda had less of an impact when heavily diluted by soil DOM mobilized by precipitation.

In both streams, during periods of heavy precipitation, DOM concentrations were much higher than during base flow. Further, at baseflow the Arboleda showed generally lower concentrations of dissolved organic carbon (DOC) than the Taconazo, yet export of DOC from the Arboleda watershed was far higher than from the Taconazo due to the large contribution by regional groundwater to the Arboleda water export. Results from this research emphasize the role of regional groundwater in shaping the quality of the organic matter in receiving streams, and their importance as conduits for surface transport of subsurface carbon.