A Unified Assessment of Hydrologic and Biogeochemical Responses in Research Watersheds in Eastern Puerto Rico Using Runoff–Concentration Relations

Abstract

An examination of the relation between runoff rate, R, and concentration, C, of twelve major constituents in four small watersheds in eastern Puerto Rico demonstrates a consistent pattern of responses. For solutes that are not substantially bioactive (alkalinity, silica, calcium, magnesium, sodium, and chloride), the log(R)-log(C) relation is almost linear and can be described as a weighted average of two sources, bedrock weathering and atmospheric deposition. The slope of the relation for each solute depends on the respective source contributions to the total river load. If a solute were strictly derived from bedrock weathering, the slope would be -0.3 to -0.4, whereas if strictly derived from atmospheric deposition, the slope would be approximately -0.1. The bioactive constituents (dissolved organic carbon, nitrate, sulfate, and potassium), which are recycled by plants and concentrated in shallow soil, demonstrate nearly flat or downward-arched log(R)-log(C) relations. The peak of the arch represents a transition from dominantly soil-matrix flow to near-surface macropore flow, and finally to overland flow. At highest observed R (80 to [90 mm/h), essentially all reactive surfaces have become wetted, and the input rate of C becomes independent of R (log(R)-log(C) slope of -1). The highest R are tenfold greater than any previous study. Slight clockwise hysteresis for many solutes in the rivers with riparian zones or substantial hyporheic flows indicates that these settings may act as mixing end-members. Particulate constituents (suspended sediment and particulate organic carbon) show slight clockwise hysteresis, indicating mobilization of stored sediment during rising stage.

Geomorphic regime modulates hydrologic control of chemical weathering in the Andes–Amazon

“…The inset shows a linear regression of the CICRA data with the shaded 95% CI and the degrees-of-freedom adjusted r 2 value. Stallard and Murphy (2014) to explain the dilution in dissolved organic carbon at high runoff in streams in Puerto Rico, although they did not see the same dilution of other elements (e.g., Na and Si), which we see in the SP catchment. Nonetheless, a switch to fast, near-surface flow paths at SP is consistent with the observation from this site that the fraction of particulate organic carbon derived from recently living biomass also increases at high runoff rates, because the erosion of surface soils by fast, near surface flow would be expected to mobilize modern particulate organic carbon (Clark et al, 2013;Clark, 2014).…”

“…In many catchments, the concentrations of base cations (i.e. Ca, Mg, Na, and K) and silicon are highly correlated with runoff and exhibit a "chemostatic" response to flow whereby the variation in solute concentrations is substantially smaller than the variation in runoff (Godsey et al, 2009;Clow and Mast, 2010;Stallard and Murphy, 2014). However, concentration-runoff relationships for base cations and silicon vary from one river to another, ranging from effectively chemostatic behavior to significant dilution with increasing runoff (Godsey et al, 2009;Moon et al, 2014).…”

“…As a result, concentration-runoff relationships may show temporal variability within a single catchment. For example, runoff generation during large storms may dominantly result from flow through near-surface flow paths where fluids interact minimally with primary reactive minerals (e.g., Bonell and Gilmour, 1978;Calmels et al, 2011;Stallard and Murphy, 2014). In contrast, river water during base flow conditions may dominantly be sourced from deeper subsurface flow paths with longer transit times (Tipper et al, 2006;Beven, 2011;Calmels et al, 2011).…”

Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed, Luquillo Mountains, Puerto Rico

“…1b), flow through alluvium, or longer groundwater flowpaths and therefore longer contact time with saprolite or bedrock. Dissolved Cl, which originates from seasalt, and SO 4 and Na, which have both seasalt and bedrock sources (McDowell and Asbury, 1994;Shanley et al, 2011;Stallard and Murphy, 2014), had relatively small changes in concentration with elevation in the watershed, and Cl concentrations increased less than other solutes as elevation decreased (Fig. 2e, f and h).…”

Linking silicate weathering to riverine geochemistry—A case study from a mountainous tropical setting in west-central Panama

“…Rivers on the volcanic islands of the Lesser Antilles and in Costa Rica are also comparatively well studied (McDowell et al, 1992(McDowell et al, , 1995Rad et al, 2007Rad et al, , 2011Rad et al, , 2013Goldsmith et al, 2010;Gaillar det et al, 2011a, b;Jones et al, 2011;Lloret et al, 2011Lloret et al, , 2012. Riverine geochemistry at the watershed scale has been studied in Puerto Rico by McDowell and Asbury (1994), McDowell (1998), and Stallard and Murphy (2014). Newbold et al (1995) examined the solute chemistry and determined nutrient export for six streams in northwestern Costa Rica, and several studies at the La Selva Tropical Forest Research Station in the coastal lowlands of eastern Costa Rica have investigated the processes that control water compositions, regulate nutrient transport and cycling, affect surface watergroundwater interaction, and influence interbasin groundwater flow (Pringle et al, 1993;Genereux and Pringle, 1997;Genereux and Jordan;Porder et al, 2006;Wiegand and Schwendenmann, 2013).…”